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Tissue Definition and Examples in Biology

Types of Plant and Animal Tissues

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Bird bone tissue cross section
 Bone is a type of connective tissue in animals.Steve Gschmeissner / Getty Images


By Anne Marie Helmenstine, Ph.D.Updated on November 26, 2019

In biology, a tissue is a group of cells and their extracellular matrix that share the same embryonic origin and perform a similar function. Multiple tissues then form organs. The study of animal tissues is called histology, or histopathology when it is concerned with diseases. The study of plant tissues is called plant anatomy. The word “tissue” comes from the French word “tissu,” which means “woven.” French anatomist and pathologist Marie François Xavier Bichat introduced the term in 1801, stating that body functions could be understood better if they were studied at the level of tissues rather than organs.

Key Takeaways: Tissue Definition in Biology

  • A tissue is a group of cells with the same origin that serve a similar function.
  • Tissues are found in animals and plants.
  • The four main types of animal tissues are connective, nervous, muscle, and epithelial tissues.
  • The three main tissue systems in plants are the epidermis, ground tissue, and vascular tissue.

Animal Tissues

Muscle fibers
 Muscle is one of the types of animal tissue. Dlumen / Getty Images

There are four basic tissues in humans and other animals: epithelial tissue, connective tissue, muscle tissue, and nervous tissue. The embryonic tissue (ectoderm, mesoderm, endoderm) from which they derive sometimes varies, according to species.

Epithelial Tissue

Cells of epithelial tissue form sheets that cover the body and organ surfaces. In all animals, most epithelium derives from the ectoderm and endoderm, except the epithelium, which derives from the mesoderm. Examples of epithelial tissue include the skin surface and the linings of the airways, reproductive tract, and gastrointestinal tract. There are several kinds of epithelium, including simple squamous epithelium, simple cuboidal epithelium, and columnar epithelium. Functions include protecting organs, eliminating waste, absorbing water and nutrients, and secreting hormones and enzymes.

Connective Tissue

Connective tissue consists of cells and non-living material, called the extracellular matrix. The extracellular matrix may be either fluid or solid. Examples of connective tissue include blood, bone, adipose, tendons, and ligaments. In humans, cranial bones derive from the ectoderm, but the other connective tissues come from the mesoderm. Functions of connective tissue include shaping and supporting organs and the body, allowing body movement, and providing oxygen diffusion.

Muscle Tissue

The three types of muscle tissue are skeletal muscle, cardiac muscle, and smooth (visceral) muscle. In humans, muscles develop from the mesoderm. Muscles contract and relax to allow body parts to move and blood to pump.

Nervous Tissue

Nervous tissue is divided into the central nervous system and peripheral nervous system. It includes the brain, spinal cord, and nerves. The nervous system derives from the ectoderm. The nervous system controls the body and communicates between its parts.

Plant Tissues

Plant tissues
VectorMine / Getty Images

There are three tissue systems in plants: epidermis, ground tissue, and vascular tissue. Alternatively, plant tissues may be categorized as either meristematic or permanent.


The epidermis consist of cells that coat the outer surface of leaves and the bodies of young plants. Its functions include protection, waste removal, and nutrient absorption.

Vascular Tissue

Vascular tissue is akin to blood vessels in animals. It includes the xylem and phloem. Vascular tissue transports water and nutrients within a plant.

Ground Tissue

Ground tissue in plants is like connective tissue in animals. It supports the plant, manufactures glucose via photosynthesis, and stores nutrients.

Meristematic Tissue

Actively dividing cells are meristematic tissue. This is the tissue that allows a plant to grow. The three types of meristematic tissue are apical meristem, lateral meristem, and intercalary meristem. Apical meristem is the tissue at stem and root tips that increases stem and root length. Lateral meristem includes tissues that divide to increase the diameter of a plant part. Intercalary meristem is responsible for the formation and growth of branches.

Permanent Tissue

Permanent tissue encompasses all cells, living or dead, that have stopped dividing and maintain a permanent position within a plant. The three types of permanent tissue are simple permanent tissue, complex permanent tissue, and secretory (glandular) tissue. Simple tissue is further divided into the parenchyma, collenchyma, and sclerenchyma. Permanent tissue provides support and structure for a plant, helps manufacture glucose, and stores water and nutrients (and sometimes air).

What Is Cytosol? Definition and Functions

What Cytosol Is and How It Differs From Cytoplasm

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Cross section of an animal cell
Rasi Bhadramani / Getty Images


By Anne Marie Helmenstine, Ph.D.Updated on November 14, 2019

Cytosol is the liquid matrix found inside cells. It occurs in both eukaryotic (plant and animal) and prokaryotic (bacteria) cells. In eukaryotic cells, it includes the liquid enclosed within the cell membrane, but not the cell nucleus, organelles (e.g., chloroplasts, mitochondria, vacuoles), or fluid contained within organelles. In contrast, all of the liquid within a prokaryotic cell is cytoplasm, since prokaryotic cells lack organelles or a nucleus. The cytosol is also known as groundplasm, intracellular fluid (ICF), or cytoplasmic matrix.https://79cd629c8664801d8e621d9cd77f2b8b.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

Key Takeaways: What Is Cytosol?

  • The cytosol is the liquid medium contained within a cell.
  • The cytosol is a component of the cytoplasm. The cytoplasm includes the cytosol, all the organelles, and the liquid contents inside the organelles. The cytoplasm does not include the nucleus.
  • The main component of cytosol is water. It also contains dissolved ions, small molecules, and proteins.
  • The cytosol is not uniform throughout the cell. Protein complexes and the cytoskeleton give it structure.
  • The cytosol serves several functions. It is the site of most metabolic processes, transports metabolites, and is involved in signal transduction within the cell.

Difference Between Cytosol and Cytoplasm

Cytosol and cytoplasm are related, but the two terms are not usually interchangeable. The cytosol is a component of cytoplasm. The cytoplasm encompasses all of the material in the cell membrane, including the organelles, but excluding the nucleus. So, the liquid within mitochondria, chloroplasts, and vacuoles is part of the cytoplasm, but is not a component of the cytosol. In prokaryotic cells, the cytoplasm and the cytosol are the same.https://79cd629c8664801d8e621d9cd77f2b8b.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

Cytosol Composition

The cytosol consists of a variety of ions, small molecules, and macromolecules in water, however, this fluid is not a homogeneous solution. About 70% of the cytosol is water. In humans, its pH ranges between 7.0 and 7.4. The pH is higher when the cell is growing. Ions dissolved in the cytosol include K+, Na+, Cl-, Mg2+, Ca2+, and bicarbonate. It also contains amino acids, proteins, and molecules that regulate osmolarity, such as protein kinase C and calmodulin.

Organization and Structure

The concentration of substances in the cytosol is affected by gravity, channels in the cell membrane and around organelles that affect calcium, oxygen, and ATP concentration, and channels formed by protein complexes. Some proteins also contain central cavities filled with cytosol having a different composition from the outside fluid. While the cytoskeleton is not considered to be part of the cytosol, its filaments control diffusion throughout the cell and restrict movement of large particles from one part of the cytosol to another.https://79cd629c8664801d8e621d9cd77f2b8b.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

Cytosol Functions

The cytosol serves several functions within a cell. It is involved in signal transduction between the cell membrane and the nucleus and organelles. It transports metabolites from their production site to other parts of the cell. It is important for cytokinesis, when the cell divides in mitosis. The cytosol plays a role in eukaryote metabolism. In animals, this includes glycolysis, gluconeogenesis, protein biosynthesis, and the pentose phosphate pathway. However, in plants, fatty acid synthesis occurs within chloroplasts, which are not part of the cytoplasm. Nearly all of a prokaryote’s metabolism occurs in the cytosol.https://79cd629c8664801d8e621d9cd77f2b8b.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html


When the term “cytosol” was coined by H. A. Lardy in 1965, it referred to the liquid produced when cells broke apart during centrifugation and the solid components were removed. However, the fluid is more accurately called the cytoplasmic fraction. Other terms sometimes used to refer to cytoplasm include hyaloplasm and protoplasm.

In modern usage, cytosol refers to the liquid portion of the cytoplasm in an intact cell or to extracts of this liquid from cells. Because the properties of this liquid depend on whether or not the cell is alive, some scientists refer to the liquid contents of living cells as aqueous cytoplasm.

Karl Landsteiner and the Discovery of the Major Blood Types

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Karl Landsteiner
 11/1/30-New York: Dr. Karl Landsteiner, at his desk. Bettmann/Getty Images


By Regina BaileyPublished on February 21, 2019

Austrian physician and immunologist Karl Landsteiner (June 14, 1868 — June 26, 1943) is most noted for his discovery of the major blood types and developing a system for blood typing. This discovery made it possible to determine blood compatibility for safe blood transfusions.

Fast Facts: Karl Landsteiner

  • Born: June 14, 1868, in Vienna, Austria
  • Died: June 26, 1943, in New York, New York
  • Parent’s Names: Leopold and Fanny Hess Landsteiner
  • Spouse: Helen Wlasto (m. 1916)
  • Child: Ernst Karl Landsteiner
  • Education: University of Vienna (M.D.)
  • Key Accomplishments: Nobel Prize for Physiology or Medicine (1930)

Early Years

Karl Landsteiner was born in Vienna, Austria in 1868, to Fanny and Leopold Landsteiner. His father was a popular journalist and Viennese newspaper publisher and editor. The death of Karl’s father, when he was only six years of age, resulted in the development of an even closer relationship between Karl and his mother.

Young Karl was always interested in science and mathematics and was an honor student during his primary and secondary school years. In 1885, he began studying medicine at the University of Vienna and earned an M.D. in 1891. While at the University of Vienna, Landsteiner became very interested in blood chemistry. Upon earning his M.D., he spent the next five years doing biochemical research in laboratories of well known European scientists, one of whom was Emil Fischer, an organic chemist who won a Nobel Prize in Chemistry (1902) for his research on carbohydrates, specifically sugars.

Career and Research

Dr. Landsteiner returned to Vienna in 1896 to continue to study medicine at Vienna General Hospital. He became an assistant to Max von Gruber at the Hygiene Institute, where he studied antibodies and immunity. Von Gruber had developed a blood test to identify the bacteria responsible for typhoid and contended that chemical signals on the bacteria were being recognized by antibodies in the blood. Landsteiner’s interest in antibody studies and immunology continued to develop as a result of working with Von Gruber.

In 1898, Landsteiner became assistant to Anton Weichselbaum at the Institute of Pathological Anatomy. For the next ten years, he conducted research in the areas of serology, microbiology, and anatomy. During this time, Landsteiner made his famous discovery of blood groups and developed a system for classifying human blood.

Discovery of the Blood Groups

Dr. Landsteiner’s investigations of interactions between red blood cells (RBCs) and serum of different people were initially noted in 1900. He observed the agglutination, or clumping together, of red blood cells when mixed with animal blood or other human blood. While Landsteiner was not the first to make these observations, he is credited with being the first to explain the biological processes behind the reaction.

Landsteiner performed experiments testing red blood cells against serum from the same patient as well as serum from different patients. He noted that a patient’s RBCs did not agglutinate in the presence of their own serum. He also identified different patterns of reactivity and categorized them into three groups: A, B, and C. Landsteiner observed that when the RBCs from group A were mixed with serum from group B, the cells in group A clumped together. The same was true when RBCs from group B were mixed with serum from group A. The blood cells of group C did not react to serum from either groups A or B. However, the serum from group C caused agglutination in RBCs from both groups A and B.

Agglutination Type A Blood
 This image shows agglutination (clumping) of type A red blood cells when mixed with ANTI-A serum. No clumping occurs when mixed with ANTI-B serum.  Ed Reschke/Photolibrary/Getty Images

Landsteiner determined that blood groups A and B have different types of agglutinogens, or antigens, on the surface of their red blood cells. They also have different antibodies (anti-A, anti-B) present in their blood serum. A student of Landsteiner’s later identified an AB blood group that reacted with both A and B antibodies. Landsteiner’s discovery became the basis for the ABO blood grouping system (as the name of group C was later changed to type O).

Landsteiner’s work laid the foundation for our understanding of blood groupings. Cells from blood type A have A antigens on the cell surfaces and B antibodies in the serum, while cells from type B have B antigens on the cell surfaces and A antibodies in the serum. When type A RBCs contact serum from type B, A antibodies present in B serum bind to A antigens on the blood cell surfaces. This binding causes the cells to clump together. Antibodies in the serum identify the blood cells as foreign and initiate an immune response to neutralize the threat.

A similar reaction occurs when type B RBCs contact serum from type A containing B antibodies. Blood type O has no antigens on the blood cell surfaces and do not react with serum from either types A or B. Blood type O does have both A and B antibodies in the serum and thus reacts with RBCs from both A and B groups.

Landsteiner’s work made blood typing possible for safe blood transfusions. His findings were published in the Central European Journal of Medicine, Wiener klinische Wochenschrift, in 1901. He received the Nobel Prize for Physiology or Medicine (1930) for this life saving accomplishment.

In 1923, Landsteiner made additional blood grouping discoveries while working in New York at the Rockefeller Institute for Medical Research. He helped to identify blood groups M, N, and P, which were initially used in paternity testing. In 1940, Landsteiner and Alexander Wiener discovered the Rh factor blood group, named for research conducted with rhesus monkeys. The presence of the Rh factor on blood cells indicates an Rh positive (Rh+) type. The absence of the Rh factor indicates an Rh negative (Rh-) type. This discovery provided a means for Rh blood type matching to prevent incompatibility reactions during transfusions. 

Death and Legacy 

Karl Landsteiner’s contribution to medicine extended beyond blood groupings. In 1906, he developed a technique for the identification of the bacterium (T. pallidum) that causes syphilis using dark-field microscopy. His work with poliomyelitis (polio virus) lead to the discovery of its mechanism of action and development of a diagnostic blood test for the virus. In addition, Landsteiner’s research on small molecules called haptens helped to elucidate their involvement in the immune response and the production of antibodies. These molecules ramp up immune responses to antigens and induce hypersensitivity reactions.

Landsteiner continued researching blood groups after retiring from the Rockefeller Institute in 1939. He would later change his focus to the study of malignant tumors in an attempt to find a cure for his wife, Helen Wlasto (m. 1916), who was diagnosed with thyroid cancer. Karl Landsteiner suffered a heart attack while in his laboratory and died a couple of days later on June 26, 1943.

Rudolf Virchow: Father of Modern Pathology

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Pathologist Rudolf Virchow Observing Operation
Bettmann Archive / Getty Images


By Alane LimUpdated on December 26, 2018

Rudolf Virchow (born October 13, 1821 in Shivelbein, Kingdom of Prussia) was a German physician who made a number of strides in medicine, public health, and other fields such as archaeology. Virchow is known as the father of modern pathology—the study of disease. He advanced the theory of how cells form, particularly the idea that every cell comes from another cell.

Virchow’s work helped bring more scientific rigor to medicine. Many prior theories had not been based on scientific observations and experiments.

Fast Facts: Rudolf Virchow

  • Full name: Rudolf Ludwig Carl Virchow
  • Known For: German physician known as the “father of pathology.”
  • Parents’ Names: Carl Christian Siegfried Virchow, Johanna Maria Hesse.
  • Born: October 13, 1821 in Schivelbein, Prussia.
  • Died: September 5, 1902 in Berlin, Germany.
  • Spouse: Rose Mayer.
  • Children: Karl, Hans, Ernst, Adele, Marie, and Hanna Elisabeth.
  • Interesting Fact: Virchow was an advocate for government involvement in public health, increased education, and social medicine—the idea that better social and economic conditions could improve people’s health. He stated that “physicians are the natural advocates of the poor.”

Early Life and Education

Rudolf Virchow was born on October 13, 1821 in Shivelbein, Kingdom of Prussia (now Świdwin, Poland). He was the only child of Carl Christian Siegfried Virchow, a farmer and treasurer, and Johanna Maria Hesse. At a young age, Virchow already exhibited extraordinary intellectual abilities, and his parents paid for extra lessons to advance Virchow’s education. Virchow attended the local elementary school at Shivelbein and was the best student in his class in high school.

In 1839, Virchow was awarded a scholarship to study medicine from the Prussian Military Academy, which would prepare him to become an army physician. Virchow studied at the Friedrich-Wilhelm Institut, part of the University of Berlin. There, he worked with Johannes Müller and Johann Schönlein, two medicine professors who exposed Virchow to experimental laboratory techniques.

Rudolph Virchow, German pathologist, 1902.Artist: C Schutte
Print Collector/Getty Images / Getty Images


After graduating in 1843, Virchow became an intern at a German teaching hospital in Berlin, where he learned the basics of microscopy and the theories on the causes and treatment of diseases while working with Robert Froriep, a pathologist.

At the time, scientists believed that they could understand nature by working from first principles rather than concrete observations and experiments. As such, many theories were incorrect or misleading. Virchow aimed to change medicine to become more scientific, based on data gathered from the world.

Virchow became a licensed doctor in 1846, traveling to Austria and Prague. In 1847, he became an instructor at the University of Berlin. Virchow had a profound impact on German medicine and taught a number of people who would later become influential scientists, including two of the four physicians who founded Johns Hopkins Hospital.

Virchow also began a new journal called Archives for Pathological Anatomy and Physiology and Clinical Medicine with a colleague in 1847. The journal is now known as “Virchow’s Archives” and remains an influential publication in pathology.

In 1848, Virchow helped evaluate typhus outbreaks in Silesia, a poor area in what is now Poland. This experience impacted Virchow and he became an advocate for government involvement in public health, increased education, and social medicine—the idea that better social and economic conditions could improve people’s health. In 1848, for example, Virchow helped establish a weekly publication called Medical Reform, which promoted social medicine and the idea that “physicians are the natural advocates of the poor.”

In 1849, Virchow became the chair in pathological anatomy at the University of Würzberg in Germany. At Würzberg, Virchow helped establish cellular pathology—the idea that disease stems from changes in healthy cells. In 1855, he published his famous saying, omnis cellula e cellula (“Every cell comes from another cell”). Although Virchow was not the first to come up with this idea, it gathered much more recognition thanks to Virchow’s publication.

In 1856, Virchow became the first director of the Pathological Institute at the University of Berlin. Alongside his research, Virchow remained active in politics, and in 1859 was elected as the city councilor of Berlin, a position he held for 42 years. As city councilor, he helped improve, among other things, Berlin’s meat inspection, water supply, and hospital systems. He was also active in Germany’s national politics, becoming a founding member of the German Progressive Party.

In 1897, Virchow was recognized for 50 years of service to the University of Berlin. In 1902, Virchow jumped out of a moving tram and injured his hip. His health continued to deteriorate until his death later that year.

Personal Life

Virchow married Rose Mayer, the daughter of a colleague, in 1850. They had six children together: Karl, Hans, Ernst, Adele, Marie, and Hanna Elisabeth.

Honors and Awards

Virchow was given a number of awards during his lifetime for both his scientific and political accomplishments, including:

  • 1861, Foreign Member, Royal Swedish Academy of Sciences
  • 1862, Member, Prussian House of Representatives
  • 1880, Member, Reichstag of the German Empire
  • 1892, Copley Medal, British Royal Society

A number of medical terms have also been named after Virchow.


Virchow died on September 5, 1902 in Berlin, Germany, due to heart failure. He was 80 years old.

Legacy and Impact

Virchow made a number of important advances in medicine and public health, including recognizing leukemia and describing myelin, though he is most well known for his work in cellular pathology. He also contributed to anthropology, archaeology, and other fields outside of medicine.


Virchow performed autopsies that involved looking at body tissue underneath the microscope. As a result of one of these autopsies, he identified and named the disease leukemia, which is a cancer that affects the bone marrow and blood.


Virchow discovered that the human disease trichinosis could be traced to parasitic worms in raw or undercooked pork. This discovery, along with other research at the time, led Virchow to postulate zoonosis, a disease or infection that can be transmitted from animals to humans.

Cellular pathology

Virchow is most known for his work on cellular pathology—the idea that disease stems from changes in healthy cells, and that each disease only affects a certain set of cells rather than the entire organism. Cellular pathology was groundbreaking in medicine because diseases, which were previously categorized by symptoms, could be much more precisely defined and diagnosed with anatomy, resulting in more effective treatments.

Life and Work of Francis Crick, Co-Discoverer of DNA’s Structure

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Francis Crick
 Francis Crick is the co-discoverer of the structure of the DNA molecule. Bettmann/Getty Images


By Regina BaileyUpdated on October 29, 2018

Francis Crick (June 8, 1916–July 28, 2004) was the co-discoverer of the structure of the DNA molecule. With James Watson, he discovered the double helical structure of DNA. Along with Sydney Brenner and others, he demonstrated that the genetic code is composed of three base codons for reading the genetic material.https://77a5b0e3870888c6950255f661508880.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

Fast Facts: Francis Crick

  • Full Name: Francis Harry Compton Crick
  • Known for: Co-discovered the double helical structure of DNA
  • Born: June 8, 1916 in Northampton, England
  • Died: July 28, 2004 in La Jolla, California, United States
  • Education: University of Cambridge, Ph.D.
  • Key Accomplishments: Nobel Prize for Physiology or Medicine (1962)
  • Spouses’ Names: Ruth Doreen Dodd (1940–1947) and Odile Speed (1949–2004)
  • Children’s Names: Michael Francis Compton, Gabrielle Anne, Jacqueline Marie-Therese

Early Years

Francis Harry Compton Crick was born on June 8, 1916 in the English town of Northampton. He was the eldest of two children. Crick began his formal education at the Northampton Grammar School, then attended Mill Hill School in London. He had a natural inquisitiveness for the sciences and enjoyed conducting chemical experiments under the tutelage of one of his uncles.https://77a5b0e3870888c6950255f661508880.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

Crick earned his Bachelor of Science degree in physics from the University College London (UCL). He then started his Ph.D. work in physics at UCL, but was unable to finish due to the start of World War II. During the war, Crick worked for the Admiralty Research Laboratory, conducting research on the design of acoustic and magnetic mines.

After the war, Crick moved from studying physics to studying biology. He very much enjoyed pondering the new discoveries that were being made in the life sciences at the time. In 1950, he was accepted as a student at Caius College, Cambridge. He was awarded his Ph.D. in 1954 for his study of the X-ray crystallography of proteins.https://77a5b0e3870888c6950255f661508880.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

Research Career

Crick’s transition from physics to biology was critical to his work in biology. It has been said that his approach to biology was refined by the simplicity of physics, as well as his belief that there were still big discoveries to be made in biology.

Crick met James Watson in 1951. They had a common interest in discerning how the genetic information for an organism could be stored in the organism’s DNA. Their work together built upon the work of other scientists such as Rosalind Franklin, Maurice Wilkins, Raymond Gosling, and Erwin Chargaff. The partnership proved fortuitous to their discovery of DNA’s double helix structure.

For the majority of his career, Crick worked for the Medical Research Council at Cambridge in England. Later in life, he worked for the Salk Institute in La Jolla, California, in the United States.

The Structure of DNA

Crick and Watson proposed a number of significant features in their model of the structure of DNA, including:

  1. DNA is a double-stranded helix.
  2. The DNA helix is typically right-handed.
  3. The helix is anti-parallel.
  4. The outside edges of the DNA bases are available for hydrogen bonding.

The model consisted of a sugar-phosphate backbone on the outside and pairs of nitrogenous bases, held together by hydrogen bonds, on the inside. Crick and Watson published their paper detailing the structure of DNA in the science journal Nature in 1953. The illustration in the article was drawn by Crick’s wife Odile, who was an artist.

Crick, Watson, and Maurice Wilkins (one of the researchers whose work Crick and Watson had built upon) were awarded the Nobel Prize in Physiology for Medicine in 1962. Their discoveries furthered the understanding of how the genetic information from one organism is passed down to its progeny from generation to generation.

Later Life and Legacy

Crick continued to study other aspects of DNA and protein synthesis after the discovery of the double helical nature of DNA. He collaborated with Sydney Brenner and others to demonstrate that the genetic code is made up of three base codons for amino acids. The research demonstrated that, since there are four bases, there are 64 possible codons, and the same amino acid can have multiple codons.

In 1977, Crick left England and relocated to the United States, where he served as the J.W. Kieckhefer Distinguished Research Professor at the Salk Institute. He continued to research in biology, focusing on neurobiology and human consciousness.

Francis Crick died in 2004 at the age of 88. He is remembered for the significance of his role in the discovery of DNA’s structure. The discovery was pivotal to many later advances in science and technology, including screening for genetic diseases, DNA fingerprinting, and genetic engineering.

Biography of Rita Levi-Montalcini

Nobel Prize Winning Scientist

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Italian Scientist Rita Levi Montalcini at the celebrations of her 100 birthday.
Alessandra Benedetti / Corbis via Getty Images


By K. Kris HirstUpdated on February 06, 2020

Rita Levi-Montalcini (1909–2012) was a Nobel Prize-winning neurologist who discovered and studied the Nerve Growth Factor, a critical chemical tool the human body uses to direct cell growth and build nerve networks. Born into a Jewish family in Italy, she survived the horrors of Hitler’s Europe to make major contributions to research on cancer and Alzheimer’s disease.

Fast Facts: Rita Levi-Montalcini

  • Occupation: Nobel Prize-winning neuroscientist
  • Known For: Discovering the first nerve growth factor (NGF)
  • Born: April 22, 1909, in Turin, Italy 
  • Parents’ Names: Adamo Levi and Adele Montalcini
  • Died: December 30, 2012, in Rome, Italy
  • Education: University of Turin
  • Key Accomplishments: Nobel Prize in Medicine, U.S. National Medal of Science
  • Famous Quote: “If I had not been discriminated against or had not suffered persecution, I would never have received the Nobel Prize.”

Early Years 

Rita Levi-Montalcini was born in Turin, Italy, on April 22, 1909. She was the youngest of four children from a well-to-do Italian Jewish family led by Adamo Levi, an electrical engineer, and Adele Montalcini, a painter. As was the custom in the early 20th century, Adamo discouraged Rita and her sisters Paola and Anna from entering college. Adamo felt that the “woman’s role” of raising a family was incompatible with creative expression and professional endeavors.

Rita had other plans. At first, she wanted to be a philosopher, then decided she wasn’t logically minded enough. Then, inspired by Swedish writer Selma Lagerlof, she considered a career in writing. After her governess died of cancer, however, Rita decided she would become a doctor, and in 1930, she entered the University of Turin at the age of 22. Rita’s twin sister Paola went on to great success as an artist. Neither of the sisters married, a fact about which neither expressed any regret.


Levi-Montalcini’s first mentor at the University of Turin was Giuseppe Levi (no relation). Levi was a prominent neurohistologist who introduced Levi-Montalcini to the scientific study of the developing nervous system. She became an intern at the Institute of Anatomy at Turin, where she grew adept at histology, including techniques like staining nerve cells.

Giuseppe Levi was known for being something of a tyrant, and he gave his mentee an impossible task: figure out how the convolutions of the human brain are formed. However, Levi-Montalcini was unable to obtain human fetal tissue in a country where abortion was illegal, so she dropped the research in favor of studying nervous system development in chick embryos.

In 1936, Levi-Montalcini graduated from the University of Turin summa cum laude with a degree in Medicine and Surgery. She then enrolled in a three-year specialization in neurology and psychiatry. In 1938, Benito Mussolini banned “non-Aryans” from academic and professional careers. Levi-Montalcini was working at a scientific institute in Belgium when Germany invaded that country in 1940, and she returned to Turin, where her family was considering emigrating to the United States. However, the Levi-Montalcinis ultimately decided to remain in Italy. In order to continue her research on chick embryos, Levi-Montalcini installed a small research unit at home in her bedroom.

World War II

In 1941, heavy Allied bombing forced the family to abandon Turin and move to the countryside. Levi-Montalcini was able to continue her research until 1943 when the Germans invaded Italy. The family fled to Florence, where they lived in hiding until the end of World War II

While in Florence, Levi-Montalcini worked as a medical doctor for a refugee camp and fought epidemics of infectious diseases and typhus. In May 1945, the war ended in Italy, and Levi-Montalcini and her family returned to Turin, where she resumed her academic positions and worked again with Giuseppe Levi. In the fall of 1947, she received an invitation from Professor Viktor Hamburger at the Washington University in St. Louis (WUSTL) to work with him conducting research on chick embryo development. Levi-Montalcini accepted; she would remain at WUSTL until 1977. 

Professional Career 

At WUSTL, Levi-Montalcini and Hamburger discovered a protein that, when released by cells, attracts nerve growth from nearby developing cells. In the early 1950s, she and biochemist Stanley Cohen isolated and described the chemical which became known as the Nerve Growth Factor.

Levi-Montalcini became an associate professor at WUSTL in 1956 and a full professor in 1961. In 1962, she helped establish the Institute of Cell Biology in Rome and became its first director. She retired from WUSTL in 1977, remaining as emerita there but splitting her time between Rome and St. Louis. 

Nobel Prize and Politics

In 1986, Levi-Montalcini and Cohen were together awarded the Nobel Prize in Medicine. She was only the fourth woman to win a Nobel Prize. In 2002, she established the European Brain Research Institute (EBRI) in Rome, a non-profit center to foster and promote brain research. 

In 2001, Italy made her a senator for life, a role which she did not take lightly. In 2006, at the age of 97, she held the deciding vote in the Italian parliament on a budget that was backed by the government of Romano Prodi. She threatened to withdraw her support unless the government reversed a last-minute decision to cut science funding. The funding was put back in, and the budget passed, despite attempts by the opposition leader Francesco Storace to silence her. Storace mockingly sent her crutches, stating that she was too old to vote and a “crutch” to an ailing government.

At the age of 100, Levi-Montalcini was still going to work at the EBRI, now named after her.

Personal Life 

Levi-Montalcini never married and had no children. She was briefly engaged in medical school but had no long-term romances. In a 1988 interview with Omni magazine, she commented that even marriages between two brilliant people might suffer because of resentment over unequal success.

She was, however, the author or co-author of over 20 popular books, including her own autobiography, and dozens of research studies. She received numerous scientific medals, including the United States National Medal of Science, presented to her at the White House by President Ronald Reagan in 1987.

Famous Quotes

In 1988, Scientific American asked 75 researchers their reasons for becoming a scientist. Levi-Montalcini gave the following reason:

The love for nerve cells, a thirst for unveiling the rules which control their growth and differentiation, and the pleasure of performing this task in defiance of the racial laws issued in 1939 by the Fascist regime were the driving forces which opened the doors for me of the “Forbidden City.”

During a 1993 interview with Margaret Holloway for Scientific American, Levi-Montalcini mused:

If I had not been discriminated against or had not suffered persecution, I would never have received the Nobel Prize.

Levi-Montalcini’s 2012 obituary in the New York Times included the following quote, from her autobiography:

It is imperfection—not perfection—that is the end result of the program written into that formidably complex engine that is the human brain, and of the influences exerted upon us by the environment and whoever takes care of us during the long years of our physical, psychological and intellectual development.

Legacy and Death

Rita Levi-Montalcini died on December 30, 2012, at age 103, at her home in Rome. Her discovery of the Nerve Growth Factor, and the research that led to it, gave other researchers a new way to study and understand cancers (disorders of neural growth) and Alzheimer’s disease (degeneration of neurons). Her research created fresh pathways for developing groundbreaking therapies. 

Levi-Montalcini’s influence in nonprofit science efforts, refugee work, and mentoring students was considerable. Her 1988 autobiography is eminently readable and often assigned to beginning STEM students.

Euglena Cells

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PRINT Science

By Regina BaileyUpdated on January 26, 2018

What Are Euglena?

five euglena with red eyespots
Gerd Guenther/Science Photo Library/Getty Images

Euglena are tiny protist organisms that are classified in the Eukaryota Domain and the genus Euglena. These single-celled eukaryotes have characteristics of both plant and animal cells. Like plant cells, some species are photoautotrophs (photo-, -auto, -troph) and have the ability to use light to produce nutrients through photosynthesis. Like animal cells, other species are heterotrophs (hetero-, -troph) and acquire nutrition from their environment by feeding on other organisms. There are thousands of species of Euglena that typically live in both fresh and saltwater aquatic environments. Euglena can be found in ponds, lakes, and streams, as well as in waterlogged land areas like marshes.

Euglena Taxonomy

Due to their unique characteristics, there has been some debate as to the phylum in which Euglena should be placed. Euglena have historically been classified by scientists in either the phylum Euglenozoa or the phylum Euglenophyta. Euglenids organized in the phylum Euglenophyta were grouped with algae because of the many chloroplasts within their cells. Chloroplasts are chlorophyll-containing organelles which enable photosynthesis. These euglenids get their green color from the green chlorophyll pigment. Scientists speculate that the chloroplasts within these cells were acquired as a result of endosymbiotic relationships with green algae. Since other Euglena do not have chloroplasts and the ones that do obtained them through endosymbiosis, some scientists contend that they should be placed taxonomically in the phylum Euglenozoa. In addition to photosynthetic euglenids, another major group of non-photosynthetic Euglena known as kinetoplastids are included in the Euglenozoa phylum. These organisms are parasites that can cause serious blood and tissue diseases in humans, such as African sleeping sickness and leishmaniasis (disfiguring skin infection). Both of these diseases are transmitted to humans by biting flies.

Euglena Cell Anatomy

Euglena Cell
Claudio Miklos/Wikimedia Commons/Public Domain

Common features of photosynthetic Euglena cell anatomy include a nucleus, contractile vacuole, mitochondria, Golgi apparatus, endoplasmic reticulum, and typically two flagella (one short and one long). Unique characteristics of these cells include a flexible outer membrane called a pellicle that supports the plasma membrane. Some euglenoids also have an eyespot and a photoreceptor, which aid in the detection of light.

Euglena Cell Anatomy

Structures found in a typical photosynthetic Euglena cell include:

  • Pellicle: a flexible membrane that supports the plasma membrane
  • Plasma membrane: a thin, semi-permeable membrane that surrounds the cytoplasm of a cell, enclosing its contents
  • Cytoplasm: gel-like, aqueous substance within the cell
  • Chloroplasts: chlorophyll containing plastids that absorbs light energy for photosynthesis
  • Contractile Vacuole: a structure that removes excess water from the cell
  • Flagellum: cellular protrusion formed from specialized groupings of microtubules that aid in cell movement
  • Eyespot: This area (typically red) contains pigmented granules that aid in the detection of light. It is sometimes called a stigma.
  • Photoreceptor or Paraflagellar Body: This light-sensitive region detects light and is located near the flagellum. It assists in phototaxis (movement toward or away from light).
  • Paramylon: This starch-like carbohydrate is composed of glucose produced during photosynthesis. It serves as a food reserve when photosynthesis is not possible.
  • Nucleus: a membrane-bound structure that contains DNA
    • Nucleolus: structure within the nucleus that contains RNA and produces ribosomal RNA for the synthesis of ribosomes
  • Mitochondria: organelles that generate energy for the cell
  • Ribosomes: Consisting of RNA and proteins, ribosomes are responsible for protein assembly.
  • Reservoir: inward pocket near the anterior of the cell where flagella arise and excess water is dispelled by the contractile vacuole
  • Golgi Apparatus: manufactures, stores, and ships certain cellular molecules
  • Endoplasmic Reticulum: This extensive network of membranes is composed of both regions with ribosomes (rough ER) and regions without ribosomes (smooth ER). It is involved in protein production.
  • Lysosomes: sacs of enzymes that digest cellular macromolecules and detoxify the cell

Some species of Euglena possess organelles that can be found in both plant and animal cells. Euglena viridis and Euglena gracilis are examples of Euglena that contain chloroplasts as do plants. They also have flagella and do not have a cell wall, which are typical characteristics of animal cells. Most species of Euglena have no chloroplasts and must ingest food by phagocytosis. These organisms engulf and feed on other unicellular organisms in their surroundings such as bacteria and algae.https://02d12f14e64e78821c2af1c4f38fda29.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

Euglena Reproduction

Euglenoid Protozoans
 Euglenoid Protozoans. Roland Birke/Photographer’s Choice/Getty Images

Most Euglena have a life cycle consisting of a free-swimming stage and a non-motile stage. In the free-swimming stage, Euglena reproduce rapidly by a type of asexual reproduction method known as binary fission. The euglenoid cell reproduces its organelles by mitosis and then splits longitudinally into two daughter cells. When environmental conditions become unfavorable and too difficult for Euglena to survive, they can enclose themselves within a thick-walled protective cyst. Protective cyst formation is characteristic of the non-motile stage.

In unfavorable conditions, some euglenids can also form reproductive cysts in what is known as the palmelloid stage of their life cycle. In the palmelloid stage, Euglena gather together (discarding their flagella) and become enveloped in a gelatinous, gummy substance. Individual euglenids form reproductive cysts in which binary fission occurs producing many (32 or more) daughter cells. When environmental conditions once again become favorable, these new daughter cells become flagellated and are released from the gelatinous mass.

Biography of Robert Hooke, the Man Who Discovered Cells

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Drawing of a flea
Robert Hooke/Wikimedia Commons/Public domain


By Mary BellisUpdated on January 13, 2020

Robert Hooke (July 18, 1635–March 3, 1703) was a 17th-century “natural philosopher”—an early scientist—noted for a variety of observations of the natural world. But perhaps his most notable discovery came in 1665 when he looked at a sliver of cork through a microscope lens and discovered cells.

Fast Facts: Robert Hooke

  • Known For: Experiments with a microscope, including the discovery of cells, and coining of the term
  • Born: July 18, 1635 in Freshwater, the Isle of Wight, England
  • Parents: John Hooke, vicar of Freshwater and his second wife Cecily Gyles
  • Died: March 3, 1703 in London
  • Education: Westminster in London, and Christ Church at Oxford, as a laboratory assistant of Robert Boyle
  • Published Works: Micrographia: or some Physiological Descriptions of Minute Bodies made by Magnifying Glasses with Observations and Inquiries Thereupon

Early Life

Robert Hooke was born July 18, 1635, in Freshwater on the Isle of Wight off the southern coast of England, the son of the vicar of Freshwater John Hooke and his second wife Cecily Gates. His health was delicate as a child, so Robert was kept at home until after his father died. In 1648, when Hooke was 13, he went to London and was first apprenticed to painter Peter Lely and proved fairly good at the art, but he left because the fumes affected him. He enrolled at Westminster School in London, where he received a solid academic education including Latin, Greek, and Hebrew, and also gained training as an instrument maker.

He later went on to Oxford and, as a product of Westminster, entered Christ Church college, where he became the friend and laboratory assistant of Robert Boyle, best known for his natural law of gases known as Boyle’s Law. Hooke invented a wide range of things at Christ Church, including a balance spring for watches, but he published few of them. He did publish a tract on capillary attraction in 1661, and it was that treatise the brought him to the attention of the Royal Society for Promoting Natural History, founded just a year earlier.

The Royal Society

The Royal Society for Promoting Natural History (or Royal Society) was founded in November 1660 as a group of like-minded scholars. It was not associated with a particular university but rather funded under the patronage of the British king Charles II. Members during Hooke’s day included Boyle, the architect Christopher Wren, and the natural philosophers John Wilkins and Isaac Newton; today, it boasts 1,600 fellows from around the world.1https://b9182bbfafbb4fed391032daf2437235.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

In 1662, the Royal Society offered Hooke the initially unpaid curator position, to furnish the society with three or four experiments each week—they promised to pay him as soon as the society had the money. Hooke did eventually get paid for the curatorship, and when he was named a professor of geometry, he gained housing at Gresham college. Hooke remained in those positions for the rest of his life; they offered him the opportunity to research whatever interested him.

Observations and Discoveries

Hooke was, like many of the members of the Royal Society, wide-reaching in his interests. Fascinated by seafaring and navigation, Hooke invented a depth sounder and water sampler. In September 1663, he began keeping daily weather records, hoping that would lead to reasonable weather predictions. He invented or improved all five basic meteorological instruments (the barometer, thermometer, hydroscope, rain gauge, and wind gauge), and developed and printed a form to record weather data.

Some 40 years before Hooke joined the Royal Society, Galileo had invented the microscope (called an occhiolino at the time, or “wink” in Italian); as curator, Hooke bought a commercial version and began an extremely wide and varying amount of research with it, looking at plants, molds, sand, and fleas. Among his discoveries were fossil shells in sand (now recognized as foraminifera), spores in mold, and the bloodsucking practices of mosquitoes and lice.

Discovery of the Cell

Hooke is best known today for his identification of the cellular structure of plants. When he looked at a sliver of cork through his microscope, he noticed some “pores” or “cells” in it. Hooke believed the cells had served as containers for the “noble juices” or “fibrous threads” of the once-living cork tree. He thought these cells existed only in plants, since he and his scientific contemporaries had observed the structures only in plant material.

Nine months of experiments and observations are recorded in his 1665 book “Micrographia: or some Physiological Descriptions of Minute Bodies made by Magnifying Glasses with Observations and Inquiries Thereupon,” the first book describing observations made through a microscope. It featured many drawings, some of which have been attributed to Christopher Wren, such as that of a detailed flea observed through the microscope. Hooke was the first person to use the word “cell” to identify microscopic structures when he was describing cork.

His other observations and discoveries include:

  • Hooke’s Law: A law of elasticity for solid bodies, which described how tension increases and decreases in a spring coil
  • Various observations on the nature of gravity, as well as heavenly bodies such as comets and planets
  • The nature of fossilization, and its implications for biological history

Death and Legacy

Hooke was a brilliant scientist, a pious Christian, and a difficult and impatient man. What kept him from true success was a lack of interest in mathematics. Many of his ideas inspired and were completed by others in and outside of the Royal Society, such as the Dutch pioneer microbiologist Antoni van Leeuwenhoek (1632–1723), navigator and geographer William Dampier (1652–1715), geologist Niels Stenson (better known as Steno, 1638–1686), and Hooke’s personal nemesis, Isaac Newton (1642–1727). When the Royal Society published Newton’s “Principia” in 1686, Hooke accused him of plagiarism, a situation so profoundly affecting Newton that he put off publishing “Optics” until after Hooke was dead.

Hooke kept a diary in which he discussed his infirmities, which were many, but although it doesn’t have literary merit like Samuel Pepys’, it also describes many details of daily life in London after the Great Fire. He died, suffering from scurvy and other unnamed and unknown illnesses, on March 3, 1703. He neither married nor had children.

Learn About the 4 Types of Protein Structure

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The four types of protein structures
 Illustration by Nusha Ashjaee. ThoughtCo.


By Regina BaileyUpdated on May 04, 2019

Proteins are biological polymers composed of amino acids. Amino acids, linked together by peptide bonds, form a polypeptide chain. One or more polypeptide chains twisted into a 3-D shape form a protein. Proteins have complex shapes that include various folds, loops, and curves. Folding in proteins happens spontaneously. Chemical bonding between portions of the polypeptide chain aid in holding the protein together and giving it its shape. There are two general classes of protein molecules: globular proteins and fibrous proteins. Globular proteins are generally compact, soluble, and spherical in shape. Fibrous proteins are typically elongated and insoluble. Globular and fibrous proteins may exhibit one or more of four types of protein structure. 

Four Protein Structure Types

The four levels of protein structure are distinguished from one another by the degree of complexity in the polypeptide chain. A single protein molecule may contain one or more of the protein structure types: primary, secondary, tertiary, and quaternary structure.https://dc0ca4e72f942f171d9b7691c9fa1ea9.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

1. Primary Structure

Primary Structure describes the unique order in which amino acids are linked together to form a protein. Proteins are constructed from a set of 20 amino acids. Generally, amino acids have the following structural properties:

  • A carbon (the alpha carbon) bonded to the four groups below:
  • A hydrogen atom (H)
  • A Carboxyl group (-COOH)
  • An Amino group (-NH2)
  • A “variable” group or “R” group

All amino acids have the alpha carbon bonded to a hydrogen atom, carboxyl group, and an amino group. The “R” group varies among amino acids and determines the differences between these protein monomers. The amino acid sequence of a protein is determined by the information found in the cellular genetic code. The order of amino acids in a polypeptide chain is unique and specific to a particular protein. Altering a single amino acid causes a gene mutation, which most often results in a non-functioning protein.https://dc0ca4e72f942f171d9b7691c9fa1ea9.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

2. Secondary Structure

Secondary Structure refers to the coiling or folding of a polypeptide chain that gives the protein its 3-D shape. There are two types of secondary structures observed in proteins. One type is the alpha (α) helix structure. This structure resembles a coiled spring and is secured by hydrogen bonding in the polypeptide chain. The second type of secondary structure in proteins is the beta (β) pleated sheet. This structure appears to be folded or pleated and is held together by hydrogen bonding between polypeptide units of the folded chain that lie adjacent to one another.https://dc0ca4e72f942f171d9b7691c9fa1ea9.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

3. Tertiary Structure

Tertiary Structure refers to the comprehensive 3-D structure of the polypeptide chain of a protein. There are several types of bonds and forces that hold a protein in its tertiary structure. 

  • Hydrophobic interactions greatly contribute to the folding and shaping of a protein. The “R” group of the amino acid is either hydrophobic or hydrophilic. The amino acids with hydrophilic “R” groups will seek contact with their aqueous environment, while amino acids with hydrophobic “R” groups will seek to avoid water and position themselves towards the center of the protein. ​
  • Hydrogen bonding in the polypeptide chain and between amino acid “R” groups helps to stabilize protein structure by holding the protein in the shape established by the hydrophobic interactions.
  • Due to protein folding, ionic bonding can occur between the positively and negatively charged “R” groups that come in close contact with one another.
  • Folding can also result in covalent bonding between the “R” groups of cysteine amino acids. This type of bonding forms what is called a disulfide bridge. Interactions called van der Waals forces also assist in the stabilization of protein structure. These interactions pertain to the attractive and repulsive forces that occur between molecules that become polarized. These forces contribute to the bonding that occurs between molecules.

4. Quaternary Structure

Quaternary Structure refers to the structure of a protein macromolecule formed by interactions between multiple polypeptide chains. Each polypeptide chain is referred to as a subunit. Proteins with quaternary structure may consist of more than one of the same type of protein subunit. They may also be composed of different subunits. Hemoglobin is an example of a protein with quaternary structure. Hemoglobin, found in the blood, is an iron-containing protein that binds oxygen molecules. It contains four subunits: two alpha subunits and two beta subunits.

How to Determine Protein Structure Type

The three-dimensional shape of a protein is determined by its primary structure. The order of amino acids establishes a protein’s structure and specific function. The distinct instructions for the order of amino acids are designated by the genes in a cell. When a cell perceives a need for protein synthesis, the DNA unravels and is transcribed into an RNA copy of the genetic code. This process is called DNA transcription. The RNA copy is then translated to produce a protein. The genetic information in the DNA determines the specific sequence of amino acids and the specific protein that is produced. Proteins are examples of one type of biological polymer. Along with proteins, carbohydrateslipids, and nucleic acids constitute the four major classes of organic compounds in living cells.

Proteins in the Cell

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This is a molecular model of the protein hemoglobin. This molecule transports oxygen around the body in red blood cells. It consists of four globin proteins (amino acid chains; green, yellow, blue and pink).
Laguna Design / Science Photo Library / Getty Images


By Regina BaileyUpdated on January 23, 2020

Proteins are very important molecules that are essential for all living organisms. By dry weight, proteins are the largest unit of cells. Proteins are involved in virtually all cell functions and a different type of protein is devoted to each role, with tasks ranging from general cellular support to cell signaling and locomotion. In total, there are seven types of proteins.


  • Proteins are biomolecules composed of amino acids that participate in nearly all cellular activities.
  • Occurring in the cytoplasm, translation is the process through which proteins are synthesized.
  • The typical protein is constructed from a single set of amino acids. Every protein is specially equipped for its function.
  • Any protein in the human body can be created from permutations of only 20 amino acids.
  • There are seven types of proteins: antibodies, contractile proteins, enzymes, hormonal proteins, structural proteins, storage proteins, and transport proteins.

Protein Synthesis

Proteins are synthesized in the body through a process called translation. Translation occurs in the cytoplasm and involves converting genetic codes into proteins. Genetic codes are assembled during DNA transcription, where DNA is decoded into RNA. Cell structures called ribosomes then help transcribe RNA into polypeptide chains that need to be modified to become functioning proteins.

Amino Acids and Polypeptide Chains

Amino acids are the building blocks of all proteins, no matter their function. Proteins are typically a chain of 20 amino acids. The human body can use combinations of these same 20 amino acids to make any protein it needs. Most amino acids follow a structural template in which an alpha carbon is bonded to the following forms:

  • A hydrogen atom (H)
  • A carboxyl group (-COOH)
  • An amino group (-NH2)
  • A “variable” group

Across the different types of amino acids, the “variable” group is most responsible for variation as all of them have hydrogen, carboxyl group, and amino group bonds.

Amino acids are joined through dehydration synthesis until they form peptide bonds. When a number of amino acids are linked together by these bonds, a polypeptide chain is formed. One or more polypeptide chains twisted into a 3-D shape forms a protein.

Protein Structure

The structure of a protein may be globular or fibrous depending on its particular role (every protein is specialized). Globular proteins are generally compact, soluble, and spherical in shape. Fibrous proteins are typically elongated and insoluble. Globular and fibrous proteins may exhibit one or more types of protein structures. 

There are four structural levels of protein: primary, secondary, tertiary, and quaternary. These levels determine the shape and function of a protein and are distinguished from one another by the degree of complexity in a polypeptide chain. The primary level is the most basic and rudimentary while the quaternary level describes sophisticated bonding.

A single protein molecule may contain one or more of these protein structure levels and the structure and intricacy of a protein determine its function. Collagen, for example, has a super-coiled helical shape that is long, stringy, strong, and rope-like—collagen is great for providing support. Hemoglobin, on the other hand, is a globular protein that is folded and compact. Its spherical shape is useful for maneuvering through blood vessels.

Types of Proteins

There is a total of seven different protein types under which all proteins fall. These include antibodies, contractile proteins, enzymes, hormonal proteins, structural proteins, storage proteins, and transport proteins.


Antibodies are specialized proteins that defend the body against antigens or foreign invaders. Their ability to travel through the bloodstream enables them to be utilized by the immune system to identify and defend against bacteria, viruses, and other foreign intruders in blood. One way antibodies counteract antigens is by immobilizing them so that they can be destroyed by white blood cells.

Contractile Proteins

Contractile proteins are responsible for muscle contraction and movement. Examples of these proteins include actin and myosin. Eukaryotes tend to possess copious amounts of actin, which controls muscle contraction as well as cellular movement and division processes. Myosin powers the tasks carried out by actin by supplying it with energy.


Enzymes are proteins that facilitate and speed up biochemical reactions, which is why they are often referred to as catalysts. Notable enzymes include lactase and pepsin, proteins that are familiar for their roles in digestive medical conditions and specialty diets. Lactose intolerance is caused by a lactase deficiency, an enzyme that breaks down the sugar lactose found in milk. Pepsin is a digestive enzyme that works in the stomach to break down proteins in food—a shortage of this enzyme leads to indigestion.

Other examples of digestive enzymes are those present in saliva: salivary amylase, salivary kallikrein, and lingual lipase all perform important biological functions. Salivary amylase is the primary enzyme found in saliva and it breaks down starch into sugar.

Hormonal Proteins

Hormonal proteins are messenger proteins that help coordinate certain bodily functions. Examples include insulin, oxytocin, and somatotropin.

Insulin regulates glucose metabolism by controlling blood-sugar concentrations in the body, oxytocin stimulates contractions during childbirth, and somatotropin is a growth hormone that incites protein production in muscle cells.

Structural Proteins

Structural proteins are fibrous and stringy, this formation making them ideal for supporting various other proteins such as keratin, collagen, and elastin.

Keratins strengthen protective coverings such as skin, hair, quills, feathers, horns, and beaks. Collagen and elastin provide support to connective tissues like tendons and ligaments.

Storage Proteins

Storage proteins reserve amino acids for the body until ready for use. Examples of storage proteins include ovalbumin, which is found in egg whites, and casein, a milk-based protein. Ferritin is another protein that stores iron in the transport protein, hemoglobin.

Transport Proteins

Transport proteins are carrier proteins that move molecules from one place to another in the body. Hemoglobin is one of these and is responsible for transporting oxygen through the blood via red blood cells. Cytochromes, another type of transport protein, operate in the electron transport chain as electron carrier proteins.

What Is A Diploid Cell?

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Human karyotype
 This human karyotype shows the complete set of human chromosomes. Each chromosome pair represents a set of homologous chromosomes in each diploid cell. Credit: somersault18:24/Science Photo Library/Getty Images


By Regina BaileyUpdated on January 22, 2020

diploid cell is a cell that contains two complete sets of chromosomes. This is double the haploid chromosome number. Each pair of chromosomes in a diploid cell is considered to be a homologous chromosome set. A homologous chromosome pair consists of one chromosome donated from the mother and one from the father. Humans have 23 sets of homologous chromosomes for a total of 46 chromosomes. Paired sex chromosomes are the X and Y homologs in males and the X and X homologs in females.https://cff017bf1df3d5a2c312281d6ee9f32b.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

Diploid Cells

  • Diploid cells have two sets of chromosomes. Haploid cells have only one.
  • The diploid chromosome number is the number of chromosomes within a cell’s nucleus.
  • This number is represented as 2n. It varies across organisms.
  • Somatic cells (body cells excluding sex cells) are diploid.
  • diploid cell replicates or reproduces through mitosis. It preserves its diploid chromosome number by making an identical copy of its chromosomes and distributing its DNA equally between two daughter cells.
  • Animal organisms are typically diploid for their entire life cycles but plant life cycles alternate between haploid and diploid stages.

Diploid Chromosome Number

The diploid chromosome number of a cell is calculated using the number of chromosomes in a cell’s nucleus. This number is abbreviated as 2n where n stands for the number of chromosomes. For humans, the diploid chromosome number equation is 2n = 46 because humans have two sets of 23 chromosomes (22 sets of two autosomal or non-sex chromosomes and one set of two sex chromosomes).

The diploid chromosome number varies by organism and ranges from 10 to 50 chromosomes per cell. See the following table for the diploid chromosome numbers of various organisms.

Diploid Chromosome Numbers
OrganismDiploid Chromosome Number (2n)
E.coli Bacterium1

Table of the diploid chromosome number for various organismshttps://cff017bf1df3d5a2c312281d6ee9f32b.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

Diploid Cells in the Human Body

All of the somatic cells in your body are diploid cells and all of the cell types of the body are somatic except for gametes or sex cells, which are haploid. During sexual reproduction, gametes (sperm and egg cells) fuse during fertilization to form diploid zygotes. A zygote, or fertilized egg, then develops into a diploid organism.https://cff017bf1df3d5a2c312281d6ee9f32b.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

Diploid Cell Reproduction

Diploid cells reproduce through mitosis. In mitosis, a cell makes an identical copy of itself. It replicates its DNA and distributes it equally between two daughter cells that each receive a full set of DNA. Somatic cells go through mitosis and (haploid) gametes undergo meiosis. Mitosis is not exclusive to diploid cells.

Diploid Life Cycles

Most plant and animal tissues consist of diploid cells. In multicellular animals, organisms are typically diploid for their entire life cycles. Plant multicellular organisms have life cycles that vacillate between diploid and haploid stages. Known as alternation of generations, this type of life cycle is exhibited in both non-vascular plants and vascular plants.

In liverworts and mosses, the haploid phase is the primary phase of the life cycle. In flowering plants and gymnosperms, the diploid phase is the primary phase and the haploid phase is totally dependent upon the diploid generation for survival. Other organisms, such as fungi and algae, spend the majority of their life cycles as haploid organisms that reproduce by spores.

Golgi Apparatus

The Cell’s Manufacturing and Shipping Center

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Golgi apparatus
 The Golgi apparatus, or complex, plays an important role in the modification and transport of proteins within the cell. Encyclopaedia Britannica/UIG/Getty Images


By Regina BaileyUpdated on October 29, 2019

There are two major types of cells: prokaryotic and eukaryotic cells. The latter have a clearly defined nucleus. The Golgi apparatus is the “manufacturing and shipping center” of a eukaryotic cell.

The Golgi apparatus, sometimes called the Golgi complex or Golgi body, is responsible for manufacturing, warehousing, and shipping certain cellular products, particularly those from the endoplasmic reticulum (ER). Depending on the type of cell, there can be just a few complexes or there can be hundreds. Cells that specialize in secreting various substances typically have a high number of Golgi.

Italian cytologist Camillo Golgi was the first to observe Golgi apparatus, which now bears his name, in 1897. Golgi used a staining technique on nervous tissue that he called “internal reticular apparatus.”

While some scientists doubted Gogli’s findings, they were confirmed in the 1950s with the electron microscope.

Key Takeaways

  • In eukaryotic cells, the Golgi apparatus is the “manufacturing and shipping center” of the cell. The Golgi apparatus is also known as the Golgi complex or Golgi body.
  • A Golgi complex contains cisternae. Cisternae are flat sacs that are stacked in a semicircular, bent formation. Each formation has a membrane to separate it from the cytoplasm of the cell.
  • The Golgi apparatus has several functions, including modification of several products from the endoplasmic reticulum (ER). Examples include phospholipids and proteins. The apparatus can also manufacture its own biological polymers.
  • The Golgi complex is capable of both disassembly and reassembly during mitosis. In the early stages of mitosis, it disassembles while it reassembles in the telophase stage.

Distinguishing Characteristics

A Golgi apparatus is composed of flat sacs known as cisternae. The sacs are stacked in a bent, semicircular shape. Each stacked grouping has a membrane that separates its insides from the cell’s cytoplasm. Golgi membrane protein interactions are responsible for their unique shape. These interactions generate the force that shapes this organelle.

The Golgi apparatus is very polar. Membranes at one end of the stack differ in both composition and in thickness from those at the other end. One end (cis face) acts as the “receiving” department while the other (trans face) acts as the “shipping” department. The cis face is closely associated with the ER.https://8dc61b06b3c25767edb1f21b2f436764.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

Molecule Transport and Modification

Molecules synthesized in the ER exit via special transport vehicles that carry their contents to the Golgi apparatus. The vesicles fuse with Golgi cisternae releasing their contents into the internal portion of the membrane. The molecules are modified as they are transported between cisternae layers.

It is thought that individual sacs are not directly connected, thus the molecules move between cisternae through a sequence of budding, vesicle formation, and fusion with the next Golgi sac. Once the molecules reach the trans face of the Golgi, vesicles are formed to “ship” materials to other sites.

The Golgi apparatus modifies many products from the ER including proteins and phospholipids. The complex also manufactures certain biological polymers of its own.

The Golgi apparatus contains processing enzymes, which alter molecules by adding or removing carbohydrate subunits. Once modifications have been made and molecules have been sorted, they are secreted from the Golgi via transport vesicles to their intended destinations. Substances within the vesicles are secreted by exocytosis.

Some of the molecules are destined for the cell membrane where they aid in membrane repair and intercellular signaling. Other molecules are secreted to areas outside of the cell.

Transport vesicles carrying these molecules fuse with the cell membrane releasing the molecules to the exterior of the cell. Still other vesicles contain enzymes that digest cellular components.

These vesicle form cell structures called lysosomes. Molecules dispatched from the Golgi may also be reprocessed by the Golgi.https://8dc61b06b3c25767edb1f21b2f436764.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

Golgi Apparatus Assembly

Golgi Complex
 The Golgi complex is composed of flat sacs known as cisternae. The sacs are stacked in a bent, semicircular shape. Image credit: Louisa Howard

The Golgi apparatus or Golgi complex is capable of disassembly and reassembly. During the early stages of mitosis, the Golgi disassembles into fragments which further break down into vesicles.

As the cell progresses through the division process, the Golgi vesicles are distributed between the two forming daughter cells by spindle microtubules. The Golgi apparatus reassembles in the telophase stage of mitosis.

The mechanisms by which the Golgi apparatus assembles are not yet understood.

Other Cell Structures

Endoplasmic Reticulum: Structure and Function

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Endoplasmic Reticulum
 The endoplasmic reticulum plays an important role in the biosynthesis, processing, and transport of proteins and lipids. Credit: Encyclopaedia Britannica/UIG/Getty Images


By Regina BaileyUpdated on December 03, 2019

The endoplasmic reticulum (ER) is an important organelle in eukaryotic cells. It plays a major role in the production, processing, and transport of proteins and lipids. The ER produces transmembrane proteins and lipids for its membrane and many other cell components including lysosomes, secretory vesicles, the Golgi appatatus, the cell membrane, and plant cell vacuoles.

Key Takeaways

  • A cell’s endoplasmic reticulum (ER) contains a network of tubules and flattened sacs. The ER performs multiple functions in both plant and animal cells.
  • Endoplasmic reticulum has two major regions: smooth endoplasmic reticulum and rough endoplasmic reticulum. Rough ER contains attached ribosomes while smooth ER does not.
  • Via the attached ribosomes, rough endoplasmic reticulum synthesizes proteins via the translation process. Rough ER also manufactures membranes.
  • Smooth endoplasmic reticulum serves as a transitional area for transport vesicles. It also functions in carbohydrate and lipid synthesis. Cholesterol and phospholipids are examples.
  • Rough and smooth ER are typically connected to one another so that the proteins and membranes made by the rough ER can freely move into the smooth ER for transport to other parts of the cell.

The endoplasmic reticulum is a network of tubules and flattened sacs that serve a variety of functions in plant and animal cells.

The two regions of the ER differ in both structure and function. Rough ER has ribosomes attached to the cytoplasmic side of the membrane. Smooth ER lacks attached ribosomes. Typically, the smooth ER is a tubule network and the rough ER is a series of flattened sacs.

The space inside of the ER is called the lumen. The ER is very extensive extending from the cell membrane through the cytoplasm and forming a continuous connection with the nuclear envelope. Since the ER is connected with the nuclear envelope, the lumen of the ER and the space inside the nuclear envelope are part of the same compartment.

Rough Endoplasmic Reticulum

The rough endoplasmic reticulum manufactures membranes and secretory proteins. The ribosomes attached to the rough ER synthesize proteins by the process of translation. In certain leukocytes (white blood cells), the rough ER produces antibodies. In pancreatic cells, the rough ER produces insulin.

The rough and smooth ER are usually interconnected and the proteins and membranes made by the rough ER move into the smooth ER to be transferred to other locations. Some proteins are sent to the Golgi apparatus by special transport vesicles. After the proteins have been modified in the Golgi, they are transported to their proper destinations within the cell or exported from the cell by exocytosis.https://a59c79358858ab7380089a3f9acb668d.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

Smooth Endoplasmic Reticulum

The smooth ER has a wide range of functions including carbohydrate and lipid synthesis. Lipids such as phospholipids and cholesterol are necessary for the construction of cell membranes. Smooth ER also serves as a transitional area for vesicles that transport ER products to various destinations.

In liver cells the smooth ER produces enzymes that help to detoxify certain compounds. In muscles the smooth ER assists in the contraction of muscle cells, and in brain cells it synthesizes male and female hormones.https://a59c79358858ab7380089a3f9acb668d.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

Eukaryotic Cell Structures

The endoplasmic reticulum is only one component of a cell. The following cell structures can also be found in a typical animal eukaryotic cell:

  • Centrioles: cylindrical groupings of microtubules found in animal cells but not plant cells. They help to organize spindle fibers during cell division.
  • Chromosomes: genetic material consisting of DNA and formed from condensed chromatin.
  • Cilia and flagella: protrusions from a cell that aid in movement and cellular locomotion.
  • Cell membrane: a thin, semi-permeable membrane that surrounds the cytoplasm and encloses the contents of a cell. It protects the integrity of the interior of the cell.
  • Cytoskeleton: a network of fibers throughout the cytoplasm that helps support the cell and aids in organelle movement.
  • Golgi Complex: composed of groupings of flattened sacs known as cisternae, the Golgi generates, processes, stores, and ships cellular products.
  • Lysosomes: membrane-bound sacs of enzymes that digest cellular macromolecules.
  • Mitochondria: organelles that provide energy for the cell by performing cellular respiration.
  • Nucleus: houses chromosomes and controls cell growth and reproduction.
  • Peroxisomes: tiny structures that detoxify alcohol and use oxygen to break down fats.
  • Ribosomes: organelles responsible for protein assembly and production via translation.

Learn About Diffusion

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 This graphic shows the diffusion of water and other molecules from left to right across a semi-permeable membrane. Larger molecules are not able to cross the barrier. Freemesm / Wikimedia Commons / CC BY-SA 3.0


By Regina BaileyUpdated on April 10, 2019

What Is Diffusion?

Diffusion is the tendency of molecules to spread out in order to occupy an available space. Gasses and molecules in a liquid have a tendency to diffuse from a more concentrated environment to a less concentrated environment. Passive transport is the diffusion of substances across a membrane. This is a spontaneous process and cellular energy is not expended. Molecules will move from where a substance is more concentrated to where it is less concentrated. The rate of diffusion for different substances is affected by membrane permeability. For instance, water diffuses freely across cell membranes but other molecules can not. They must be helped across the cell membrane through a process called facilitated diffusion.https://9d92e2ae8ccfee772baabea2280be2b4.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

Key Takeaways: Diffusion

  • Diffusion is the passive movement of molecules from an area of high concentration to an area of low concentration.
  • Passive diffusion is the movement of molecules across a membrane, such as a cell membrane. The movement does not require energy.
  • In facilitated diffusion, a molecule is transported across a membrane with the help of a carrier protein.
  • Osmosis is a type of passive diffusion in which water diffuses across a semi-permeable membrane from an area of low solute concentration to an area of high solute concentration.
  • Respiration and photosynthesis are examples of naturally occurring diffusion processes.
  • Glucose movement into cells is an example of facilitated diffusion.
  • Water absorption in plant roots is an example of osmosis.

What Is Osmosis?

Osmosis is a special case of passive transport. Water diffuses across a semi-permeable membrane which allows some molecules to pass but not others.https://9d92e2ae8ccfee772baabea2280be2b4.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

 Water passing through a semi-permeable membrane by osmosis into a region of higher sugar concentration.  ttsz/iStock/Getty Images Plus

In osmosis, the direction of water flow is determined by solute concentration. Water diffuses from a hypotonic (low solute concentration) solution to a hypertonic (high solute concentration) solution. In the example above, water moves from the left side of the semi-permeable membrane, where the sugar concentration is low, to the right side of the membrane, where the sugar molecule concentration is higher. If the molecule concentration were the same on both sides of the membrane, water would flow equally (isostonic) between both sides of the membrane.

Examples of Diffusion

Gas Exchange in the Lungs
 Both oxygen and carbon dioxide diffuse into the blood and are transported around the body.  ttsz/iStock/Getty Images Plus

A number of naturally occurring processes rely on the diffusion of molecules. Respiration involves the diffusion of gasses (oxygen and carbon dioxide) into and out of the blood. In the lungs, carbon dioxide diffuses from the blood into the air at lung alveoli. Red blood cells then bind the oxygen that diffuses from the air into the blood. Oxygen and other nutrients in the blood are transported to tissues where gasses and nutrients are exchanged. Carbon dioxide and wastes diffuse from tissue cells into the blood, while oxygen, glucose and other nutrients in the blood diffuse into body tissues. This diffusion process occurs at capillary beds.https://9d92e2ae8ccfee772baabea2280be2b4.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

 Diffusion of gasses occurs in photosynthesis in plants.  snapgalleria/iStock/Getty Images Plus

Diffusion also occurs in plant cells. The process of photosynthesis that occurs in plant leaves depends on the diffusion of gasses. In photosynthesis, energy from sunlight, water, and carbon dioxide are used to produce glucose, oxygen, and water. Carbon dioxide diffuses from the air through tiny pores in plant leaves called stomata. Oxygen produced by photosynthesis diffuses from the plant through the stomata into the atmosphere.

Facilitated Diffusion
 This image shows facilitated diffusion of glucose across a cell membrane through a carrier protein.  ttsz/iStock/Getty Images Plus

In facilitated diffusion, larger molecules such as glucose, can not freely diffuse across cell membranes. These molecules must move down their concentration gradient with the help of transport proteins. Protein channels embedded in cell membranes have openings to the outside of a cell that allow certain molecules to fit inside. Only molecules with certain characteristics, such as a certain size and shape are allowed passage from outside of a cell to its intracellular space. Since this process does not require energy, facilitated diffusion is considered passive transport.https://9d92e2ae8ccfee772baabea2280be2b4.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

Osmosis Examples

Wilted Yellow Tulips
 Water moving across plant cell membranes by osmosis helps to restore the plant to an erect position.  berkpixels/Getty Images

Examples of osmosis in the body include the reabsorption of water by nephron tubules in the kidneys and the reabsorption of fluid at tissue capillaries. In plants, osmosis is exhibited in water absorption by plant roots. Osmosis is important to plant stability. Wilted plants are the result of a lack of water in plant vacuoles. Vacuoles help keep plant structures rigid by absorbing water and exerting pressure on plant cell walls. Water moving across plant cell membranes by osmosis helps to restore the plant to an erect position.

How Do Restriction Enzymes Cut DNA Sequences?

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restriction enzymes
 Restriction enzymes are enzymes that cut DNA into fragments based upon recognizing a specific sequence of nucleotides. Restriction enzymes are also known as restriction endonucleases. Callista Images/Cultura/Getty Images


By Regina BaileyPublished on February 27, 2019

In nature, organisms constantly have to protect themselves from foreign invaders, even at the microscopic level. In bacteria, there is a group of bacterial enzymes that work by dismantling foreign DNA. This dismantling process is called restriction and the enzymes that carry out this process are called restriction enzymes.

Restriction enzymes are very important in recombinant DNA technology. Restriction enzymes have been used to help produce vaccines, pharmaceutical products, insect resistant crops, and a host of other products.

Key Takeaways

  • Restriction enzymes dismantle foreign DNA by cutting it into fragments. This disassembling process is called restriction.
  • Recombinant DNA technology relies on restriction enzymes to produce new combinations of genes.
  • The cell protects its own DNA from disassembly by adding methyl groups in a process called modification.
  • DNA ligase is a very important enzyme that helps to join DNA strands together via covalent bonds.

What Is a Restriction Enzyme?

Restriction enzymes are a class of enzymes that cut DNA into fragments based upon recognizing a specific sequence of nucleotides. Restriction enzymes are also known as restriction endonucleases.

While there are hundreds of different restriction enzymes, they all work in essentially the same way. Each enzyme has what is known as a recognition sequence or site. A recognition sequence is typically a specific, short nucleotide sequence in DNA. The enzymes cut at certain points within the recognized sequence. For example, a restriction enzyme may recognize a specific sequence of guanine, adenine, adenine, thymine, thymine, cytosine. When this sequence is present, the enzyme can make staggered cuts in the sugar-phosphate backbone in the sequence.https://a78f4826db2e82f1d99ea62e7d1feb6b.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

But if restriction enzymes cut based on a certain sequence, how do cells like bacteria protect their own DNA from being cut up by restriction enzymes? In a typical cell, methyl groups (CH3) are added to the bases in the sequence to prevent recognition by the restriction enzymes. This process is carried out by complementary enzymes that recognize the same sequence of nucleotide bases as restriction enzymes. The methylation of DNA is known as modification. With the processes of modification and restriction, cells can both cut up foreign DNA that pose a danger to the cell while preserving the important DNA of the cell.

Based on the double-stranded configuration of DNA, recognition sequences are symmetrical on the different stands but run in opposite directions. Recall that DNA has “direction” indicated by the type of carbon at the end of the strand. The 5′ end has a phosphate group attached while the other 3′ end has a hydroxyl group attached. For example:

5′ end – … guanine, adenine, adenine, thymine, thymine, cytosine … – 3′ end

3′ end – … cytosine, thymine, thymine, adenine, adenine, guanine … – 5′ end

If, for example, the restriction enzyme cuts within the sequence between the guanine and adenine, it would do so with both sequences but at opposite ends (since the second sequence runs in the opposite direction). Since the DNA is cut on both strands, there will be complementary ends that can hydrogen bond to one another. These ends are often called “sticky ends.”

What Is DNA Ligase?

The sticky ends of the fragments produced by restriction enzymes are useful in a laboratory setting. They can be used to join DNA fragments from both different sources and different organisms. The fragments are held together by hydrogen bonds. From a chemical perspective, hydrogen bonds are weak attractions and are not permanent. Using another type of enzyme however, the bonds can be made permanent.

DNA ligase is a very important enzyme that functions in both the replication and repair of a cell’s DNA. It functions by helping the joining of DNA strands together. It works by catalyzing a phosphodiester bond. This bond is a covalent bond, much stronger than the aforementioned hydrogen bond and able to hold the different fragments together. When different sources are used, the resulting recombinant DNA that is produced has a new combination of genes.

Restriction Enzyme Types

There are four broad categories of restriction enzymes: Type I enzymes, Type II enzymes, Type III enzymes, and Type IV enzymes. All have the same basic function, but the different types are classified based on their recognition sequence, how they cleave, their composition, and on their substance requirements (the need for and type of cofactors). Generally, Type I enzymes cut DNA at locations distant to the recognition sequence; Type II cut DNA within or close to the recognition sequence; Type III cut DNA near recognition sequences; and Type IV cleave methylated DNA.

A Definition and Explanation of the Steps in Endocytosis

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ttsz/iStock/Getty Images Plus


By Regina BaileyUpdated on June 20, 2019

Endocytosis is the process by which cells internalize substances from their external environment. It is how cells get the nutrients they need to grow and develop. Substances internalized by endocytosis include fluids, electrolytes, proteins, and other macromolecules. Endocytosis is also one of the means by which white blood cells of the immune system capture and destroy potential pathogens including bacteria and protists. The process of endocytosis can be summarized in three basic steps.

The Basic Steps of Endocytosis

  1. The plasma membrane folds inward (invaginates) forming a cavity that fills with extracellular fluid, dissolved molecules, food particles, foreign matter, pathogens, or other substances.
  2. The plasma membrane folds back on itself until the ends of the in-folded membrane meet. This traps the fluid inside the vesicle. In some cells, long channels also form extending from the membrane deep into the cytoplasm.
  3. The vesicle is pinched off from the membrane as the ends of the in-folded membrane fuse together. The internalized vesicle is then processed by the cell.

There are three primary types of endocytosis: phagocytosis, pinocytosis, and receptor-mediated endocytosis. Phagocytosis is also called “cell eating” and involves the intake of solid material or food particles. Pinocytosis, also called “cell drinking”, involves the intake of molecules dissolved in fluid. Receptor-mediated endocytosis involves the intake of molecules based upon their interaction with receptors on a cell’s surface.

The Cell Membrane and Endocytosis

Cell Membrane
Encyclopaedia Britannica/UIG/Getty Images

In order for endocytosis to occur, substances must be enclosed within a vesicle formed from the cell membrane, or plasma membrane. The main components of this membrane are proteins and lipids, which aid in cell membrane flexibility and molecule transport. Phospholipids are responsible for forming a double-layered barrier between the external cellular environment and the cell’s interior. Phospholipids have hydrophilic (attracted to water) heads and hydrophobic (repelled by water) tails. When in contact with liquid, they spontaneously arrange so that their hydrophilic heads face the cytosol and extracellular fluid, while their hydrophobic tails move away from the fluid to the internal region of the lipid bilayer membrane.

The cell membrane is semi-permeable, meaning that only certain molecules are allowed to diffuse across the membrane. Substances that can not diffuse across the cell membrane must be helped across by passive diffusion processes (facilitated diffusion), active transport (requires energy), or by endocytosis. Endocytosis involves the removal of portions of the cell membrane for the formation of vesicles and internalization of substances. In order to maintain cell size, membrane components must be replaced. This is accomplished by the process of exocytosis. Opposite to endocytosis, exocytosis involves the formation, transportation, and fusion of internal vesicles with the cell membrane to expel substances from the cell.https://f9b71365f57a746aa8e619fd6c559cb6.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html


Phagocytosis - White Blood Cell
Juergen Berger/Science Photo Library/Getty Image

Phagocytosis is a form of endocytosis that involves the engulfing of large particles or cells. Phagocytosis allows immune cells, like macrophages, to rid the body of bacteria, cancer cells, virus-infected cells, or other harmful substances. It is also the process by which organisms such as amoebas obtain food from their environment. In phagocytosis, the phagocytic cell or phagocyte must be able to attach to the target cell, internalize it, degrade it, and expel the refuse. This process, as it occurs in immune cells, is described below.

Basic Steps of Phagocytosis

  • Detection: The phagocyte detects the antigen (substance provoking an immune response), such as a bacterium, and moves toward the target cell.
  • Attachment: The phagocyte makes contact with and attaches to the bacterium. This binding initiates the formation of pseudopodia (extensions of the cell) that surround the bacterium.
  • Ingestion: The surrounded bacterium is enclosed within a vesicle formed when pseudopodia membranes fuse. This vesicle with bacterium enclosed, called a phagosome, is internalized by the phagocyte.
  • Fusion: The phagosome fuses with an organelle called a lysosome and becomes known as a phagolysosome. Lysosomes contain enzymes that digest organic material. The release of digestive enzymes within the phagolysosome degrades the bacterium.
  • Elimination: The degraded material is expelled from the cell by exocytosis.

Phagocytosis in protists occurs similarly and more commonly as it is the means by which these organisms obtain food. Phagocytosis in humans is only performed by specialized immune cells.


Endocytosis - Pinocytosis
FancyTapis/iStock/Getty Images Plus

While phagocytosis involves cell eating, pinocytosis involves cell drinking. Fluids and dissolved nutrients are taken into a cell by pinocytosis. The same basic steps of endocytosis are utilized in pinocytosis to internalize vesicles and to transport particles and extracellular fluid inside the cell. Once inside the cell, the vesicle may fuse with a lysosome. The digestive enzymes from the lysosome degrade the vesicle and release its contents into the cytoplasm for use by the cell. In some instances, the vesicle does not fuse with a lysosome but travels across the cell and fuses with the cell membrane on the other side of the cell. This is one means by which a cell can recycle cell membrane proteins and lipids.

Pinocytosis is nonspecific and occurs by two main processes: micropinocytosis and macropinocytosis. As the names suggest, micropinocytosis involves the formation of small vesicles (0.1 micrometers in diameter), while macropinocytosis involves the formation of larger vesicles ( 0.5 to 5 micrometers in diameter). Micropinocytosis occurs in most types of body cells and the tiny vesicles form by budding from the cell membrane. Micropinocytotic vesicles called caveolae were first discovered in blood vessel endothelium. Macropinocytosis is typically observed in white blood cells. This process differs from micropinocytosis in that the vesicles are not formed by budding but by plasma membrane ruffles. Ruffles are extended portions of the membrane that project into the extracellular fluid and then fold back on themselves. In doing so, the cell membrane scoops up the fluid, forms a vesicle, and pulls the vesicle into the cell.

Receptor-mediated Endocytosis

Receptor-mediated Endocytosis
Encyclopaedia Britannica/UIG/Getty Images

Receptor-mediated endocytosis is the process used by cells for the selective internalization of specific molecules. These molecules bind to specific receptors on the cell membrane before they are internalized by endocytosis. Membrane receptors are found in regions of the plasma membrane coated with the protein clatherine known as clatherine-coated pits. Once the specific molecule binds to the receptor, the pit regions are internalized and clatherine-coated vesicles are formed. After fusing with early endosomes (membrane-bound sacs that help sort internalized material), the clatherine coating is removed from the vesicles and the contents are emptied into the cell.

Basic Steps of Receptor-mediated Endocytosis

  • The specified molecule binds to a receptor on the plasma membrane.
  • The molecule-bound receptor migrates along the membrane to a region containing a clatherine-coated pit.
  • After molecule-receptor complexes accumulate in the clatherine-coated pit, the pit region forms an invagination that is internalized by endocytosis.
  • A clatherine-coated vesicle is formed, which encapsulates the ligand-receptor complex and extracellular fluid.
  • The clatherine-coated vesicle fuses with an endosome in the cytoplasm and the clatherine coating is removed.
  • The receptor can be enclosed in a lipid membrane and recycled back to the plasma membrane.
  • If not recycled, the specified molecule remains in the endosome and the endosome fuses with a lysosome.
  • Lysosomal enzymes degrade the specified molecule and deliver the desired contents to the cytoplasm.

Receptor-mediated endocytosis is thought to be more than a hundred times more efficient at taking in selective molecules than pinocytosis.https://f9b71365f57a746aa8e619fd6c559cb6.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

Endocytosis Key Takeaways

  • During endocytosis, cells internalize substances from their external environment and get the nutrients they need to grow and develop.  
  • The three primary types of endocytosis are phagocytosis, pinocytosis, and receptor-mediated Endocytosis.
  • In order for endocytosis to occur, substances must be enclosed within a vesicle formed from the cell (plasma) membrane.
  • Phagocytosis is also known as “cell eating.” It is the process used by immune cells to rid the body of harmful elements and by amoebas to obtain food.
  • In pinocytosis cells “drink” fluids and dissolved nutrients in a process similar to that of phagocytosis.
  • Receptor-mediated endocytosis is a much more efficient process than pinocytosis for internalizing specific molecules. 

What Are Restriction Enzymes?

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Structure of the homodimeric restriction enzyme EcoRI (cyan and green cartoon diagram) bound to double stranded DNA (brown tubes).
Boghog2 / Wikimedia Commons


By Theresa PhillipsUpdated on July 22, 2019

Restriction endonucleases are a class of enzyme that cut DNA molecules. Each enzyme recognizes unique sequences of nucleotides in a DNA strand—usually about four to six base-pairs long. The sequences are palindromic in that the complementary DNA strand has the same sequence in the reverse direction. In other words, both strands of DNA are cut at the same location.

Where These Enzymes Are Found

Restriction enzymes are found in many different strains of bacteria where their biological role is to participate in cell defense. These enzymes restrict foreign (viral) DNA that enters the cells by destroying them. The host cells have a restriction-modification system that methylates their own DNA at sites specific for their respective restriction enzymes, thereby protecting them from cleavage. More than 800 known enzymes have been discovered that recognize more than 100 different nucleotide sequences.https://2b8da4523cb4fba05c242e2acf46fa50.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

Types of Restriction Enzymes

There are five different types of restriction enzymes. Type I cuts DNA at random locations as far as 1,000 or more base-pairs from the recognition site. Type III cuts at approximately 25 base-pairs from the site. Both of these types require ATP and can be large enzymes with multiple subunits. Type II enzymes, which are predominantly used in biotechnology, cut DNA within the recognized sequence without the need for ATP and are smaller and simpler.

Type II restriction enzymes are named according to the bacterial species from which they are isolated. For example, the enzyme EcoRI was isolated from E. coli. Most of the public is familiar with E. coli outbreaks in food.

Type II restriction enzymes can generate two different types of cuts depending on whether they cut both strands at the center of the recognition sequence or each strand closer to one end of the recognition sequence.

The former cut will generate “blunt ends” with no nucleotide overhangs. The latter generates “sticky” or “cohesive” ends because each resulting fragment of DNA has an overhang that complements the other fragments. Both are useful in molecular genetics for making recombinant DNA and proteins. This form of DNA stands out because it is produced by the ligation (bonding together) of two or more different strands that weren’t originally linked together.

Type IV enzymes recognize methylated DNA, and Type V enzymes use RNAs to cut sequences on invading organisms that are not palindromic.

Use in Biotechnology

Restriction enzymes are used in biotechnology to cut DNA into smaller strands in order to study fragment length differences among individuals. This is referred to as restriction fragment length polymorphism (RFLP). They’re also used for gene cloning.

RFLP techniques have been used to determine that individuals or groups of individuals have distinctive differences in gene sequences and restriction cleavage patterns in certain areas of the genome. Knowledge of these unique areas is the basis for DNA fingerprinting. Each of these methods depends on the use of agarose gel electrophoresis for the separation of the DNA fragments. TBE buffer, which is made up of Tris base, boric acid, and EDTA, is commonly used for agarose gel electrophoresis to examine DNA products.

Use in Cloning

Cloning often requires inserting a gene into a plasmid, which is a type of a piece of DNA. Restriction enzymes can assist with the process because of the single-stranded overhangs they leave when they make cuts. DNA ligase, a separate enzyme, can join together two DNA molecules with matching ends.

So, by using restriction enzymes with DNA ligase enzymes, pieces of DNA from different sources can be used to create a single DNA molecule.

Binary Fission vs. Mitosis

Comparing and contrasting cell division in eukaryotes and prokaryotes

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Binary fission is the method of cell division used by prokaryotes or bacteria.
 Binary fission is the method of cell division used by prokaryotes or bacteria. MedicalRF.com / Getty Images


By Anne Marie Helmenstine, Ph.D.Updated on October 04, 2019

Binary fissionmitosis, and meiosis are the main forms of cell division. Binary fission and mitosis are types of asexual reproduction in which the parent cell divides to form two identical daughter cells. Meiosis, on the other hand, is a form of sexual reproduction in which a cell divides its genetic material between the two daughter cells.https://3b6c2568e5945b6945f7b3d3ad077570.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

The Main Difference Between Binary Fission and Mitosis

While both binary fission and mitosis are types of cell division that duplicate cells, fission primarily occurs in prokaryotes (bacteria), while mitosis occurs in eukaryotes (e.g., plant and animal cells).https://3b6c2568e5945b6945f7b3d3ad077570.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

Another way to look at it is that in binary fission cell that divide lack a nucleus, while in mitosis, the cell that divides does possess a nucleus. To get a better understanding of the processes, let’s take a closer look at what’s involved.

Prokaryotic vs. Eukaryotic Cells

Prokaryotes are simple cells that lack a nucleus and organelles. Their DNA consists of one or two circular chromosomes. Eukaryotes, in contrast, are complex cells that have a nucleus, organelles, and multiple linear chromosomes.

In both types of cells, DNA is copied and separated to form new cells in an organized manner. In both types of cells, cytoplasm is divided to form daughter cells via the process of cytokinesis. In both processes, if everything goes as planned, the daughter cells contain an exact copy of the parent cell’s DNA.

In bacterial cells, the process is simpler, making fission faster than mitosis. Because a bacterial cell is a complete organism, fission is a form of reproduction. While there are some single-celled eukaryotic organisms, mitosis is most often used for growth and repair rather than reproduction.https://3b6c2568e5945b6945f7b3d3ad077570.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

While errors in replication in fission are a way to introduce genetic diversity in prokaryotes, errors in mitosis can cause serious problems in eukaryotes (e.g., cancer). Mitosis includes a checkpoint to make certain both copies of DNA are identical. Eukaryotes use meiosis and sexual reproduction to ensure genetic diversity.

Binary Fission Steps

While a bacterial cell lacks a nucleus, its genetic material is found within a special region of the cell called a nucleoid. Copying the round chromosome starts at a site called the origin of replication and moves in both directions, forming two replication sites. As the replication process progresses, the origins move apart and separate the chromosomes. The cell lengthens or elongates.

There are different forms of binary fission: The cell can divide across the transverse (short) axis, the longitudinal (long) axis, at a slant, or in another direction (simple fission). Cytokinesis pulls the cytoplasm toward the chromosomes.

When replication is complete, a dividing line—called a septum—forms, physically separating the cytoplasm of the cells. A cell wall then forms along the septum and the cell pinches in two, forming the daughter cells.

While it’s easy to generalize and say binary fission only occurs in prokaryotes, this isn’t exactly true. Certain organelles in eukaryotic cells, such as mitochondria, also divide by fission. Some eukaryotic cells can divide via fission. For example, algae and Sporozoa may divide via multiple fission in which several copies of a cell are made simultaneously.

Mitosis Steps

Mitosis is part of the cell cycle. The process is much more involved than fission, reflecting the complex nature of eukaryotic cells. There are five phases: prophase, prometaphase, metaphase, anaphase, and telophase.

  • The linear chromosomes replicate and condense early in mitosis, in prophase.
  • In prometaphase, the nuclear membrane and nucleolus disintegrate. Fibers organize to form a structure called the mitotic spindle.
  • Microtubules help align chromosomes on the spindle in metaphase. Molecular machinery checks the DNA to assure replicated chromosomes align toward the proper target cell.
  • In anaphase, the spindle draws the two sets of chromosomes away from each other.
  • In telophase, the spindles and chromosomes move to opposite sides of the cell, a nuclear membrane forms around each set of genetic material, cytokinesis splits the cytoplasm, and cell membrane separates the contents into two cells. The cell enters the non-dividing part of the cell cycle, which is called interphase.

Binary Fission Versus Mitosis

Cell division can be confusing, but similarities and differences between binary fission and mitosis can be summed up in one simple table:

Binary FissionMitosis
Asexual reproduction in which one organism (cell) divides to form two daughter organisms.Asexual reproduction of cells, usually parts of complex organisms.
Occurs in prokaryotes. Some protists and eukaryotic organelles divide via fission.Occurs in eukaryotes.
Primary function is reproduction.Functions include reproduction, repair, and growth.
A simple, rapid process.A complex process that requires more time than binary fission.
No spindle apparatus is formed. DNA attaches to the cell membrane prior to division.A spindle apparatus is formed. DNA attaches to the spindle for division.
DNA replication and separation occur at the same time.DNA replication is completed long before cell division.
Not completely reliable. Daughter cells sometimes get unequal numbers of chromosomes.High fidelity replication in which chromosome number is maintained through a checkpoint at metaphase. Errors occur, but more rarely than in fission.
Uses cytokinesis to divide cytoplasm.Uses cytokinesis to divide cytoplasm.

Binary Fission vs. Mitosis: Key Takeaways

  • Binary fission and mitosis are both forms of asexual reproduction in which a parent cell divides to form two identical daughter cells.
  • Binary fission occurs primarily in prokaryotes (bacteria), while mitosis only occurs in eukaryotes (e.g., plant and animal cells).
  • Binary fission is a simpler and faster process than mitosis.
  • The third main form of cell division is meiosis. Meiosis only occurs in sex cells (gamete formation) and produces daughter cells with half of the chromosomes of the parent cell.

Anabolism and Catabolism Definition and Examples

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Anabolic exercise helps build muscle strength and endurance.
Hans Berggren / Getty Images


By Anne Marie Helmenstine, Ph.D.Updated on February 04, 2020

Anabolism and catabolism are the two broad types of biochemical reactions that make up metabolism. Anabolism builds complex molecules from simpler ones, while catabolism breaks large molecules into smaller ones.https://aca894dcf26063ef0f9a4015c568ca99.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

Most people think of metabolism in the context of weight loss and bodybuilding, but metabolic pathways are important for every cell and tissue in an organism. Metabolism is how a cell gets energy and removes waste. Vitamins, minerals, and cofactors aid the reactions.https://aca894dcf26063ef0f9a4015c568ca99.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

Key Takeaways: Anabolism and Catabolism

  • Anabolism and catabolism are the two broad classes of biochemical reactions that make up metabolism.
  • Anabolism is the synthesis of complex molecules from simpler ones. These chemical reactions require energy.
  • Catabolism is the breakdown of complex molecules into simpler ones. These reactions release energy.
  • Anabolic and catabolic pathways typically work together, with the energy from catabolism providing the energy for anabolism.

Anabolism Definition

Anabolism or biosynthesis is the set of biochemical reactions that construct molecules from smaller components. Anabolic reactions are endergonic, meaning they require an input of energy to progress and are not spontaneous. Typically, anabolic and catabolic reactions are coupled, with catabolism providing the activation energy for anabolism. The hydrolysis of adenosine triphosphate (ATP) powers many anabolic processes. In general, condensation and reduction reactions are the mechanisms behind anabolism.

Anabolism Examples

Anabolic reactions are those that build complex molecules from simple ones. Cells use these processes to make polymers, grow tissue, and repair damage. For example:

  • Glycerol reacts with fatty acids to make lipids:
  • Simple sugars combine to form disaccharides and water:
    C6H12O6 + C6H12O6   →  C12H22O11 + H2O
  • Amino acids join together to form dipeptides:
  • Carbon dioxide and water react to form glucose and oxygen in photosynthesis:
    6CO2 + 6H2O  →  C6H12O6 + 6O2

Anabolic hormones stimulate anabolic processes. Examples of anabolic hormones include insulin, which promotes glucose absorption, and anabolic steroids, which stimulate muscle growth. Anabolic exercise is anaerobic exercise, such as weightlifting, which also builds muscle strength and mass.

Catabolism Definition

Catabolism is the set of biochemical reactions that break down complex molecules into simpler ones. Catabolic processes are thermodynamically favorable and spontaneous, so cells use them to generate energy or to fuel anabolism. Catabolism is exergonic, meaning it releases heat and works via hydrolysis and oxidation.

Cells can store useful raw materials in complex molecules, use catabolism to break them down, and recover the smaller molecules to build new products. For example, catabolism of proteins, lipids, nucleic acids, and polysaccharides generates amino acids, fatty acids, nucleotides, and monosaccharides, respectively. Sometimes waste products are generated, including carbon dioxide, urea, ammonia, acetic acid, and lactic acid.

Catabolism Examples

Catabolic processes are the reverse of anabolic processes. They are used to generate energy for anabolism, release small molecules for other purposes, detoxify chemicals, and regulate metabolic pathways. For example:

  • During cellular respiration, glucose and oxygen react to yield carbon dioxide and water
    C6H12O6 + 6O2  →  6CO2 + 6H2O
  • In cells, hydroxide peroxide decomposes into water and oxygen:
    2H2O2  →  2H2O + O2

Many hormones act as signals to control catabolism. The catabolic hormones include adrenaline, glucagon, cortisol, melatonin, hypocretin, and cytokines. Catabolic exercise is aerobic exercise such as a cardio workout, which burns calories as fat (or muscle) is broken down.

Amphibolic Pathways

A metabolic pathway that can be either catabolic or anabolic depending on energy availability is called an amphibolic pathway. The glyoxylate cycle and the citric acid cycle are examples of amphibolic pathways. These cycles can either produce energy or use it, depending on cellular needs.

Fats, Steroids, and Other Examples of Lipids

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Oil & Water
Credit: Thomas Vogel/E+/Getty Images


By Regina BaileyUpdated on February 04, 2020

Lipids are very diverse in both their respective structures and functions. These diverse compounds that make up the lipid family are so grouped because they are insoluble in water. They are also soluble in other organic solvents such as ether, acetone, and other lipids. Lipids serve a variety of important functions in living organisms. They act as chemical messengers, serve as valuable energy sources, provide insulation, and are the main components of membranes. Major lipid groups include fatsphospholipidssteroids, and waxes.​

Key Takeaways: Lipids

  • Lipids, as a class of compounds, are insoluble in water but are soluble in other organic solvents. Examples of such solvents include acetone and ether.
  • Waxes, steroids, phospholipids, and fats are the most common types of lipid groups.
  • Fats have glycerol in addition to three fatty acids. The structure of the fatty acids determines whether or not the fat is considered saturated or unsaturated.
  • Phospholipids have four major components: fatty acids, a glycerol component, and both a phosphate group and a polar molecule.
  • Human sex hormones, like testosterone and estrogen, are classed as steroids. Steroids most often have a four-fused ring structure.
  • Waxes are composed of alcohol and a fatty acid. Plants often have wax coatings that help them to conserve water.

Lipid Soluble Vitamins

Fat-soluble vitamins are stored in adipose tissue and in the liver. They are eliminated from the body more slowly than water-soluble vitamins. Fat-soluble vitamins include vitamins A, D, E, and K. Vitamin A is important for vision as well as skin, teeth, and bone health. Vitamin D aids in the absorption of other nutrients including calcium and iron. Vitamin E acts as an antioxidant and also aids in immune function. Vitamin K aids in the blood clotting process and maintaining strong bones.

Organic Polymers

  • Biological polymers are vital to the existence of all living organisms. In addition to lipids, other organic molecules include:
  • Carbohydrates: biomolecules that include sugars and sugar derivatives. They not only provide energy but are also important for energy storage.
  • Proteins: composed of amino acids, proteins provide structural support for tissues, act as chemical messengers, move muscles, and much more.
  • Nucleic Acids: biological polymers composed of nucleotides and important for gene inheritance. DNA and RNA are two types of nucleic acids.


Triglyceride Molecule
LAGUNA DESIGN/Science Photo Library/Getty Images

Fats are composed of three fatty acids and glycerol. These so-called triglycerides can be solid or liquid at room temperature. Those that are solid are classified as fats, while those that are liquid are known as oils. Fatty acids consist of a long chain of carbons with a carboxyl group at one end. Depending on their structure, fatty acids can be saturated or unsaturated.

Saturated fats raise LDL (low-density lipoprotein) cholesterol levels in the blood. This increases the chances of developing cardiovascular disease. Unsaturated fats lower LDL levels and reduce the risk of disease. While fats have been denigrated to the point that many believe that fat should be eliminated from the diet, fat serves many useful purposes. Fats are stored for energy in adipose tissue, help to insulate the body, and cushion and protect organs.https://36e5cb259c8452dea9c5c95c35f8e098.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html


Phospholipid Molecule
Stocktrek Images/Getty Images

phospholipid is composed of two fatty acids, a glycerol unit, a phosphate group, and a polar molecule. The phosphate group and polar head region of the molecule are hydrophillic (attracted to water), while the fatty acid tail is hydrophobic (repelled by water). When placed in water, phospholipids will orient themselves into a bilayer in which the nonpolar tail region faces the inner area of the bilayer. The polar head region faces outward and interacts with the water.

Phospholipids are a major component of cell membranes, which enclose and protect the cytoplasm and other contents of a cell. Phospholipids are also a major component of myelin, a fatty substance that is important for insulating nerves and speeding up electrical impulses in the brain. It is the high composition of myelinated nerve fibers that causes white matter in the brain to appear white.https://36e5cb259c8452dea9c5c95c35f8e098.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

Steroids and Waxes

LDL and HDL Particles
JUAN GAERTNER/Science Photo Library/Getty Images

Steroids have a carbon backbone that consists of four fused ring-like structures. Steroids include cholesterol, sex hormones (progesterone, estrogen, and testosterone) produced by gonads and cortisone.

Waxes are composed of an ester of long-chain alcohol and a fatty acid. Many plants have leaves and fruits with wax coatings to help prevent water loss. Some animals also have wax-coated fur or feathers to repel water. Unlike most waxes, ear wax is composed of phospholipids and esters of cholesterol.

Sex Cells Anatomy and Production

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Fertilization of ovum by sperm cell.
Oliver Cleve/Photographer’s Choice/Getty Images


By Regina BaileyUpdated on November 19, 2018

Organisms that reproduce sexually do so via the production of sex cells also called gametes. These cells are very different for the male and female of a species. In humans, male sex cells or spermatozoa (sperm cells), are relatively motile. Female sex cells, called ova or eggs, are non-motile and much larger in comparison to the male gamete.

When these cells fuse in a process called fertilization, the resulting cell (zygote) contains a mix of inherited genes from the father and mother. Human sex cells are produced in the reproductive system organs called gonads. Gonads produce sex hormones needed for the growth and development of primary and secondary reproductive organs and structures.

Key Takeaways: Sex Cells

  • Sexual reproduction occurs through the union of sex cells, or gametes.
  • Gametes differ widely in males versus females for a given organism.
  • For humans, male gametes are called spermatozoa while female gametes are called ova. Spermatozoa are also known as sperm and ova are also known as eggs.

Human Sex Cell Anatomy

Sperm and Ovum
 The male gametes (sperm) are approaching a female gamete (an unfertilized egg) prior to conception. Credit: Science Picture Co/Subjects/Getty Images

Male and female sex cells are dramatically different from one another in size and shape. Male sperm resembles long, motile projectiles. They are small cells that consist of a head region, midpiece region, and tail region. The head region contains a cap-like covering called an acrosome. The acrosome contains enzymes that help the sperm cell penetrate the outer membrane of an ovum. The nucleus is located within the head region of the sperm cell. The DNA within the nucleus is densely packed, and the cell does not contain much cytoplasm. The midpiece region contains several mitochondria which provide the energy for the motile cell. The tail region consists of a long protrusion called a flagellum that aids in cellular locomotion.

Female ova are some of the largest cells in the body and are round in shape. They are produced in the female ovaries and consist of a nucleus, large cytoplasmic region, the zona pellucida, and the corona radiata. The zona pellucida is a membrane covering that surrounds the cell membrane of the ovum. It binds sperm cells and aids in the fertilization of the cell. The corona radiata are outer protective layers of follicular cells that surround the zona pellucida.https://8afb324ff0d37e3f441b7c54fa5c8f74.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

Sex Cell Production

Four Daughter Cells
 Four daughter cells are produced as a result of meiosis. Ed Reschke/Photolibrary/Getty Images

Human sex cells are produced by a two-part cell division process called meiosis. Through a sequence of steps, the replicated genetic material in a parent cell is distributed among four daughter cells. Meiosis produces gametes with one-half the number of chromosomes as the parent cell. Because these cells have one-half the number of chromosomes as the parent cell, they are haploid cells. Human sex cells contain one complete set of 23 chromosomes.

There are two stages of meiosis: meiosis I and meiosis II. Prior to meiosis, the chromosomes replicate and exist as sister chromatids. At the end of meiosis I, two daughter cells are produced. The sister chromatids of each chromosome within the daughter cells are still connected at their centromere. At the end of meiosis II, sister chromatids separate and four daughter cells are produced. Each cell contains one-half the number of chromosomes as the original parent cell.

Meiosis is similar to the cell division process of non-sex cells known as mitosis. Mitosis produces two cells that are genetically identical to and contain the same number of chromosomes as the parent cell. These cells are diploid cells because they contain two sets of chromosomes. Human diploid cells contain two sets of 23 chromosomes for a total of 46 chromosomes. When sex cells unite during fertilization, the haploid cells become a diploid cell.

The production of sperm cells is known as spermatogenesis. This process occurs continuously and takes place within the male testes. Hundreds of millions of sperm must be released in order for fertilization to take place. The vast majority of sperm released never reach the ovum. In oogenesis or ovum development, the daughter cells are divided unequally in meiosis. This asymmetrical cytokinesis results in one large egg cell (oocyte) and smaller cells called polar bodies. The polar bodies degrade and are not fertilized. After meiosis I is complete, the egg cell is called a secondary oocyte. The secondary oocyte will only complete the second meiotic stage if fertilization begins. Once meiosis II is complete, the cell is called an ovum and can fuse with the sperm cell. When fertilization is complete, the united sperm and ovum become a zygote.https://8afb324ff0d37e3f441b7c54fa5c8f74.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

Sex Chromosomes

Human Sex Chromosomes X and Y
 This is a scanning electron micrograph (SEM) of human sex chromosomes X and Y (Pair 23). The X chromosome is much larger than the Y chromosome. Power and Syred/Science Photo Library/Getty Images

Male sperm cells in humans and other mammals are heterogametic and contain one of two types of sex chromosomes. They contain either an X chromosome or a Y chromosome. Female egg cells, however, contain only the X sex chromosome and are therefore homogametic. The sperm cell determines the sex of an individual. If a sperm cell containing an X chromosome fertilizes an egg, the resulting zygote will be XX or female. If the sperm cell contains a Y chromosome, then the resulting zygote will be XY or male.

Spindle Fibers

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Spindle Fibers Mitosis
 This is a fluorescence micrograph of a cell during metaphase of mitosis. During metaphase, chromosomes (green) line up along the center of the cell and spindle fibers (purple) grow from their poles to centromeres (yellow) at the center of each chromosome.DR PAUL ANDREWS, UNIVERSITY OF DUNDEE/Science Photo Library/Getty Images


By Regina BaileyUpdated on November 11, 2019

Spindle fibers are aggregates of microtubules that move chromosomes during cell division. Microtubules are protein filaments that resemble hollow rods. Spindle fibers are found in eukaryotic cells and are a component of the cytoskeleton as well as cilia and flagella.

Spindle fibers are part of a spindle apparatus that moves chromosomes during mitosis and meiosis to ensure even chromosome distribution between daughter cells. The spindle apparatus of a cell is comprised of spindle fibers, motor proteins, chromosomes, and, in some animal cells, microtubule arrays called asters. Spindle fibers are produced in the centrosome from cylindrical microtubules called centrioles.https://439ac5ad179614d6b12d9e461b4dea38.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

Spindle Fibers and Chromosome Movement

Spindle fiber and cell movement occur when microtubules and motor proteins interact. Motor proteins, which are powered by ATP, are specialized proteins that actively move microtubules. Motor proteins such as dyneins and kinesins move along microtubules whose fibers either lengthen or shorten. The disassembly and reassembly of microtubules produces the movement needed for chromosome movement and cell division to occur.

Spindle fibers move chromosomes during cell division by attaching to chromosome arms and centromeres. A centromere is the specific region of a chromosome where duplicates are linked. Identical, joined copies of a single chromosome are known as sister chromatids. The centromere is also where protein complexes called kinetochores are found.

Kinetochores generate fibers that attach sister chromatids to spindle fibers. Kinetochore fibers and spindle polar fibers work together to separate chromosomes during mitosis and meiosis. Spindle fibers that don’t contact chromosomes during cell division extend from one cell pole to the other. These fibers overlap and push cell poles away from one another in preparation for cytokinesis.

Spindle Fibers in Mitosis

Spindle fibers are highly active during mitosis. They migrate throughout the cell and direct chromosomes to go where they need to go. Spindle fibers function similarly in meiosis, where four daughter cells are formed instead of two, by pulling homologous chromosomes apart after they have been duplicated to prepare for division.

Prophase: Spindle fibers form at opposite poles of the cell. In animal cells, a mitotic spindle appears as asters that surround each centriole pair. The cell becomes elongated as spindle fibers stretch from each pole. Sister chromatids attach to spindle fibers at their kinetochores.

Metaphase: Spindle fibers called polar fibers extend from cell poles toward the midpoint of the cell known as the metaphase plate. Chromosomes are held to the metaphase plate by the force of spindle fibers pushing on their centromeres.

Anaphase: Spindle fibers shorten and pull sister chromatids toward spindle poles. Separated sister chromatids move toward opposite cell poles. Spindle fibers not connected to chromatids lengthen and elongate the cell to make room for the cell to separate.

Telophase: Spindle fibers disperse as the chromosomes are separated and become housed within two new nuclei.

Cytokinesis: Two daughter cells are formed, each with the correct number of chromosomes because spindle fibers ensured this. The cytoplasm divides and the distinct daughter cells fully separate.

The Stages of Mitosis and Cell Division

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Illustration depicting the stages of mitosis and cell divison


By Regina BaileyUpdated on July 07, 2019

Mitosis is the phase of the cell cycle where chromosomes in the nucleus are evenly divided between two cells. When the cell division process is complete, two daughter cells with identical genetic material are produced.


Ed Reschke/Photolibrary/Getty Images

Before a dividing cell enters mitosis, it undergoes a period of growth called interphase. About 90 percent of a cell’s time in the normal cell cycle may be spent in interphase.

  • G1 phase: The period prior to the synthesis of DNA. In this phase, the cell increases in mass in preparation for cell division. The G1 phase is the first gap phase.
  • S phase: The period during which DNA is synthesized. In most cells, there is a narrow window of time during which DNA is synthesized. The S stands for synthesis.
  • G2 phase: The period after DNA synthesis has occurred but prior to the start of prophase. The cell synthesizes proteins and continues to increase in size. The G2 phase is the second gap phase.
  • In the latter part of interphase, the cell still has nucleoli present.
  • The nucleus is bounded by a nuclear envelope and the cell’s chromosomes have duplicated but are in the form of chromatin.


Ed Reschke/Photolibrary/Getty Images

In prophase, the chromatin condenses into discrete chromosomes. The nuclear envelope breaks down and spindles form at opposite poles of the cell. Prophase (versus interphase) is the first true step of the mitotic process. During prophase, a number of important changes occur:

  • Chromatin fibers become coiled into chromosomes, with each chromosome having two chromatids joined at a centromere.
  • The mitotic spindle, composed of microtubules and proteins, forms in the cytoplasm.
  • The two pairs of centrioles (formed from the replication of one pair in Interphase) move away from one another toward opposite ends of the cell due to the lengthening of the ​microtubules that form between them.
  • Polar fibers, which are microtubules that make up the spindle fibers, reach from each cell pole to the cell’s equator.
  • Kinetochores, which are specialized regions in the centromeres of chromosomes, attach to a type of microtubule called kinetochore fibers.
  • The kinetochore fibers “interact” with the spindle polar fibers connecting the kinetochores to the polar fibers.
  • The chromosomes begin to migrate toward the cell center.


Ed Reschke/Photolibrary/Getty Images

In metaphase, the spindle reaches maturity and the chromosomes align at the metaphase plate (a plane that is equally distant from the two spindle poles). During this phase, a number of changes occur:

  • The nuclear membrane disappears completely.
  • Polar fibers (microtubules that make up the spindle fibers) continue to extend from the poles to the center of the cell.
  • Chromosomes move randomly until they attach (at their kinetochores) to polar fibers from both sides of their centromeres.
  • Chromosomes align at the metaphase plate at right angles to the spindle poles.
  • Chromosomes are held at the metaphase plate by the equal forces of the polar fibers pushing on the centromeres of the chromosomes.


Ed Reschke/Photolibrary/Getty Images

In anaphase, the paired chromosomes (sister chromatids) separate and begin moving to opposite ends (poles) of the cell. Spindle fibers not connected to chromatids lengthen and elongate the cell. At the end of anaphase, each pole contains a complete compilation of chromosomes. During anaphase, the following key changes occur:

  • The paired centromeres in each distinct chromosome begin to move apart.​
  • Once the paired sister chromatids separate from one another, each is considered a “full” chromosome. They are referred to as daughter chromosomes.​
  • Through the spindle apparatus, the daughter chromosomes move to the poles at opposite ends of the cell.​
  • The daughter chromosomes migrate centromere first and the kinetochore fibers become shorter as the chromosomes near a pole.​
  • In preparation for telophase, the two cell poles also move further apart during the course of anaphase. At the end of anaphase, each pole contains a complete compilation of chromosomes.


Ed Reschke/Photolibrary/Getty Images

In telophase, the chromosomes are cordoned off into distinct new nuclei in the emerging daughter cells. The following changes occur:

  • The polar fibers continue to lengthen.
  • Nuclei begin to form at opposite poles.
  • The nuclear envelopes of these nuclei form from remnant pieces of the parent cell’s nuclear envelope and from pieces of the endomembrane system.
  • Nucleoli also reappear.
  • Chromatin fibers of chromosomes uncoil.
  • After these changes, telophase/mitosis is largely complete. The genetic contents of one cell have been divided equally into two.


Cancer Cell Mitosis
MAURIZIO DE ANGELIS/Science Photo Library/Getty Images

Cytokinesis is the division of the cell’s cytoplasm. It begins prior to the end of mitosis in anaphase and completes shortly after telophase/mitosis. At the end of cytokinesis, two genetically identical daughter cells are produced. These are diploid cells, with each cell containing a full complement of chromosomes.

Cells produced through mitosis are different from those produced through meiosis. In meiosis, four daughter cells are produced. These cells are haploid cells, containing one-half the number of chromosomes as the original cell. Sex cells undergo meiosis. When sex cells unite during fertilization, these haploid cells become a diploid cell.​


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Beginning of Life
Mark Evans/ E+/ Getty Images


By Regina BaileyUpdated on November 06, 2019

Gametes are reproductive cells or sex cells that unite during sexual reproduction to form a new cell called a zygote. Male gametes are called sperm and female gametes are ova (eggs). Sperm are motile and have a long, tail-like projection called a flagellum. Ova are non-motile and relatively large in comparison to the male gamete.

In seed-bearing plants, pollen is a male sperm-producing gametophyte and female sex cells are contained within plant ovules. In animals, gametes are produced in male and female gonads, the site of hormone production. Read to learn more about how gametes divide and reproduce.

Gamete Formation

Gametes are formed through a process of cell division called meiosis. This two-step division process produces four haploid daughter cells. Haploid cells contain only one set of chromosomes. When the haploid male and female gametes unite in a process called fertilization, they form what is called a zygote. The zygote is diploid and contains two sets of chromosomes.

Gametes and Fertilization

Fertilization occurs when male and female gametes fuse. In animal organisms, the union of sperm and egg occurs in the fallopian tubes of the female reproductive tract. Millions of sperm are released during sexual intercourse and these travel from the vagina to the fallopian tubes.


Sperm are specially equipped with burrowing catalysts and mechanisms for fertilizing an egg. The head region contains a cap-like covering called an acrosome that contains enzymes that help the sperm cell penetrate the zona pellucida, the outer covering of an egg cell membrane.

When a sperm reaches the egg cell membrane, its head fuses with the egg. This triggers the release of substances that modify the zona pellucida to prevent any other sperm from fertilizing the egg. This process is crucial as fertilization by multiple sperm cells, or polyspermy, produces a zygote with extra chromosomes. Polyspermy is lethal to a zygote.


Upon fertilization, two haploid gametes become one diploid zygote. A human zygote has 23 pairs of homologous chromosomes and 46 chromosomes total—half from the mother and half from the father. The zygote continues to divide by mitosis until a fully functional individual is formed. The biological sex of this human is decided by the sex chromosomes it inherits.

A sperm cell may either have an X or Y sex chromosome, but an egg cell can only have an X chromosome. A sperm cell with a Y sex chromosome results in a male (XY) and a sperm cell with an X sex chromosome results in a female (XX).

Types of Sexual Reproduction

The type of sexual reproduction of an organism is largely dependent on the size and shape of its gametes. Some male and female gametes are of similar size and shape, while others are vastly different. In some species of algae and fungi, for example, male and female sex cells are almost identical and both are usually motile. The union of similar gametes is known as isogamy.

The process of gametes of dissimilar size and shape joining is called anisogamy or heterogamy. Higher plants, animals, and some species of algae and fungi exhibit a special type of anisogamy called oogamy. In oogamy, the female gamete is non-motile and much larger than the fast-moving male gamete. This is the type of reproduction that occurs in humans.

Types of Cells in the Human Body

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Illustration of the types of cells in the body


By Regina BaileyUpdated on November 13, 2019

Cells in the human body number in the trillions and come in all shapes and sizes. These tiny structures are the basic unit of living organisms. Cells comprise tissues, tissues make up organs, organs form organ systems, and organ systems work together to create an organism and keep it alive.

Each type of cell in the human body is specially equipped for its role. Cells of the digestive system, for instance, are vastly different in structure and function from cells of the skeletal system. Cells of the body depend on each other to keep the body functioning as a unit. There are hundreds of types of cells, but the following are the 11 most common.

Stem Cells

Pluripotent stem cell on a blue background.
 Pluripotent stem cell.Credit: Science Photo Library – STEVE GSCHMEISSNER/Brand X Pictures/Getty Images

Stem cells are unique in that they originate as unspecialized cells and have the ability to develop into specialized cells that can be used to build specific organs or tissues. Stem cells can divide and replicate many times in order to replenish and repair tissue. In the field of stem cell research, scientists take advantage of the renewal properties of these structures by utilizing them to generate cells for tissue repair, organ transplantation, and for the treatment of disease.https://0be02ac3a4d414fc6bb664e3d05edcb0.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

Bone Cells

Osteocyte, or bone cell, close up.
 Colored scanning electron micrograph (SEM) of a freeze-fractured osteocyte (purple) surrounded by bone (gray).Steve Gschmeissner/Science Photo Library/Getty Images

Bones are a type of mineralized connective tissue that comprise a major component of the skeletal system. Bones are made up of a matrix of collagen and calcium phosphate minerals. There are three primary types of bone cells in the body: osteoclasts, osteoblasts, and osteocytes.

Osteoclasts are large cells that decompose bone for resorption and assimilation while they heal. Osteoblasts regulate bone mineralization and produce osteoid, an organic substance of the bone matrix, which mineralizes to form bone. Osteoblasts mature to form osteocytes. Osteocytes aid in the formation of bone and help maintain calcium balance.https://0be02ac3a4d414fc6bb664e3d05edcb0.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

Blood Cells

Blood cells image.
 Red and white blood cells in the bloodstream.Science Photo Library – SCIEPRO/Getty Images

From transporting oxygen throughout the body to fighting infection, blood cell activity is vital to life. Blood cells are produced by bone marrow. The three major types of cells in the blood are red blood cellswhite blood cells, and platelets.

Red blood cells determine blood type and are responsible for transporting oxygen. White blood cells are immune system cells that destroy pathogens and provide immunity. Platelets help clot blood to prevent excessive blood loss due to broken or damaged blood vessels.https://0be02ac3a4d414fc6bb664e3d05edcb0.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

Muscle Cells

Smooth muscle cell image.
 Immunoflourescence of a smooth muscle cell.Beano5/Vetta/Getty Images

Muscle cells form muscle tissue, which enables all bodily movement. The three types of muscle cells are skeletal, cardiac, and smooth. Skeletal muscle tissue attaches to bones to facilitates voluntary movement. These muscle cells are covered by connective tissue, which protects and supports muscle fiber bundles.

Cardiac muscle cells form involuntary muscle, or muscle that doesn’t require conscious effort to operate, found in the heart. These cells aid in heart contraction and are joined to one another by intercalated discs that allow for heartbeat synchronization.

Smooth muscle tissue is not striated like cardiac and skeletal muscle. Smooth muscle is involuntary muscle that lines body cavities and forms the walls of many organs such as kidneys, intestines, blood vessels, and lung airways.

Fat Cells

Close up image of fat cells.
 Adipocytes (fat cells) store energy as an insulating layer of fat and the majority of the cell’s volume is taken up by a large lipid (fat or oil) droplet.Steve Gschmeissner/Science Photo Library/Getty Images

Fat cells, also called adipocytes, are a major cell component of adipose tissue. Adipocytes contain droplets of stored fat (triglycerides) that can be used for energy. When fat is stored, its cells become round and swollen. When fat is used, its cells shrink. Adipose cells also have a critical endocrine function: they produce hormones that influence sex hormone metabolism, blood pressure regulation, insulin sensitivity, fat storage and use, blood clotting, and cell signaling.

Skin Cells

Skin cells close up view.
 This image shows squamous cells from the surface of the skin. These are flat, keratinized, dead cells that are continuously sloughed off and replaced with new cells from below.Science Photo Library/Getty Images

The skin is composed of a layer of epithelial tissue (epidermis) that is supported by a layer of connective tissue (dermis) and an underlying subcutaneous layer. The outermost layer of the skin is composed of flat, squamous epithelial cells that are closely packed together. The skin covers a wide range of roles. It protects internal structures of the body from damage, prevents dehydration, acts as a barrier against germs, stores fat, and produces vitamins and hormones.

Nerve Cells

Nerve cells close up.
Science Picture Co/Collection Mix: Subjects/Getty Images

Nerve cells or neurons are the most basic unit of the nervous system. Nerves send signals between the brainspinal cord, and other body organs via nerve impulses. Structurally, a neuron consists of a cell body and nerve processes. The central cell body contains the neuron’s nucleus, associated cytoplasm, and organelles. Nerve processes are “finger-like” projections (axons and dendrites) that extend from the cell body and transmit signals.https://0be02ac3a4d414fc6bb664e3d05edcb0.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

Endothelial Cells

Endothelial cells close up view.
Dr. Torsten Wittman/Science Photo Library/Getty Images

Endothelial cells form the inner lining of the cardiovascular system and lymphatic system structures. They make up the inner layer of blood vessels, lymphatic vessels, and organs including the brain, lungs, skin, and heart. Endothelial cells are responsible for angiogenesis or the creation of new blood vessels. They also regulate the movement of macromolecules, gases, and fluid between the blood and surrounding tissues as well as help manage blood pressure.

Sex Cells

Human fertilization occurring as sperm cells seek an egg cell.
 This image depicts sperm entering an ovum.Science Picture Co/Collection Mix/Getty Images

Sex cells or gametes are reproductive cells created in male and female gonads that bring new life into existence. Male sex cells or sperm are motile and have long, tail-like projections called flagella. Female sex cells or ova are non-motile and relatively large in comparison to male gametes. In sexual reproduction, sex cells unite during fertilization to form a new individual. While other body cells replicate by mitosis, gametes reproduce by meiosis.

Pancreatic Cells

Pancreas cell close up view.
Steve Gschmeissner/Science Photo Library/Getty Images

The pancreas functions as both an exocrine and endocrine organ, meaning that it discharges hormones both through ducts and directly into other organs. Pancreatic cells are important for regulating blood glucose concentration levels as well as for the digestion of proteins, carbohydrates, and fats.

Exocrine acinar cells, which are produced by the pancreas, secrete digestive enzymes that are transported by ducts to the small intestine. A very small percentage of pancreatic cells have an endocrine function or secrete hormones into cells and tissues. Pancreatic endocrine cells are found in small clusters called islets of Langerhans. Hormones produced by these cells include insulin, glucagon, and gastrin.

Cancer Cells

Cervical cancer cells close up view.
 These cervical cancer cells are dividing.Steve Gschmeissner/Science Photo Library/Getty Images

Unlike all of the other cells listed, cancer cells work to destroy the body. Cancer results from the development of abnormal cell properties that cause cells to divide uncontrollably and spread to other locations. Cancer cell development can originate from mutations stemming from exposure to chemicals, radiation, and ultraviolet light. Cancer can also have genetic origins such as chromosome replication errors and cancer-causing viruses of the DNA.

Cancer cells are allowed to spread rapidly because they develop decreased sensitivity to anti-growth signals and proliferate quickly in the absence of stop commands. They also lose the ability to undergo apoptosis or programmed cell death, making them even more formidable.

How and Why Cells Move

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PRINT Science

By Regina Bailey

Updated on February 25, 2019

Cell movement is a necessary function in organisms. Without the ability to move, cells could not grow and divide or migrate to areas where they are needed. The cytoskeleton is the component of the cell that makes cell movement possible. This network of fibers is spread throughout the cell’s cytoplasm and holds organelles in their proper place. Cytoskeleton fibers also move cells from one location to another in a fashion that resembles crawling.

Why Do Cells Move?

Fibroblast Cell
 This fibroblast cell is important to wound healing. This connective tissue cell migrates to sites of injury to aid in tissue repair. Rolf Ritter/Cultura Science/Getty Images

Cell movement is required for a number of activities to occur within the body. White blood cells, such as neutrophils and macrophages must quickly migrate to sites of infection or injury to fight bacteria and other germs. Cell motility is a fundamental aspect of form generation (morphogenesis) in the construction of tissues, organs and the determination of cell shape. In cases involving wound injury and repair, connective tissue cells must travel to an injury site to repair damaged tissue. Cancer cells also have the ability to metastasize or spread from one location to another by moving through blood vessels and lymphatic vessels. In the cell cycle, movement is required for the cell dividing process of cytokinesis to occur in the formation of two daughter cells.https://daee4fcf90a24f7ff8ac1f1b4b0f032a.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

Steps of Cell Movement

 HeLa cells, fluorescent light micrograph. The cell nuclei contain the genetic material chromatin (red). The proteins making up the cells cytoskeleton have been stained with different colors: actin is blue and microtubules are yellow. DR Torsten Wittmann/Science Photo Library/Getty Image

Cell motility is accomplished through the activity of cytoskeleton fibers. These fibers include microtubules, microfilaments or actin filaments and intermediate filaments. Microtubules are hollow rod-shaped fibers that help support and shape cells. Actin filaments are solid rods that are essential for movement and muscle contraction. Intermediate filaments help stabilize microtubules and microfilaments by keeping them in place. During cell movement, the cytoskeleton disassembles and re-assembles actin filaments and microtubules. The energy required to produce movement comes from adenosine triphosphate (ATP). ATP is a high energy molecule produced in cellular respiration.

Steps of Cell Movement

Cell adhesion molecules on cell surfaces hold cells in place to prevent undirected migration. Adhesion molecules hold cells to other cells, cells to the extracellular matrix (ECM) and the ECM to the cytoskeleton. The extracellular matrix is a network of proteinscarbohydrates and fluids that surround cells. The ECM helps to position cells in tissues, transport communication signals between cells and reposition cells during cell migration. Cell movement is prompted by chemical or physical signals that are detected by proteins found on cell membranes. Once these signals are detected and received, the cell begins to move. There are three phases to cell movement.

  • In the first phase, the cell detaches from the extracellular matrix at its foremost position and extends forward.
  • In the second phase, the detached portion of the cell moves forward and re-attaches at a new forward position. The rear portion of the cell also detaches from the extracellular matrix.
  • In the third phase, the cell is pulled forward to a new position by the motor protein myosin. Myosin utilizes the energy derived from ATP to move along actin filaments, causing cytoskeleton fibers to slide along one another. This action causes the entire cell to move forward.

The cell moves in the direction of the detected signal. If the cell is responding to a chemical signal, it will move in the direction of the highest concentration of signal molecules. This type of movement is known as chemotaxis.https://daee4fcf90a24f7ff8ac1f1b4b0f032a.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

Movement Within Cells

Phagocytosis - White Blood Cell
 This colored scanning electron micrograph (SEM) shows a white blood cell engulfing pathogens (red) by phagocytosis. JUERGEN BERGER/Science Photo Library/Getty Image

Not all cell movement involves the repositioning of a cell from one place to another. Movement also occurs within cells. Vesicle transportation, organelle migration, and chromosome movement during mitosis are examples of types of internal cell movement.

Vesicle transportation involves the movement of molecules and other substances into and out of a cell. These substances are enclosed within vesicles for transportation. Endocytosis, pinocytosis, and exocytosis are examples of vesicle transportation processes. In phagocytosis, a type of endocytosis, foreign substances and unwanted material are engulfed and destroyed by white blood cells. The targeted matter, such as a bacterium, is internalized, enclosed within a vesicle, and degraded by enzymes.

Organelle migration and chromosome movement occur during cell division. This movement ensures that each replicated cell receives the appropriate complement of chromosomes and organelles. Intracellular movement is made possible by motor proteins, which travel along cytoskeleton fibers. As the motor proteins move along microtubules, they carry organelles and vesicles with them.https://daee4fcf90a24f7ff8ac1f1b4b0f032a.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

Cilia and Flagella

Cilia in Trachea
 Colored scanning electron micrograph (SEM) of cilia on the epithelium lining the trachea (windpipe). DR G. MOSCOSO/Science Photo Library/Getty Image

Some cells possess cellular appendage-like protrusions called cilia and flagella. These cell structures are formed from specialized groupings of microtubules that slide against one another allowing them to move and bend. Compared to flagella, cilia are much shorter and more numerous. Cilia move in a wave-like motion. Flagella are longer and have more of a whip-like movement. Cilia and flagella are found in both plant cells and animal cells.

Sperm cells are examples of body cells with a single flagellum. The flagellum propels the sperm cell toward the female oocyte for fertilization. Cilia are found within areas of the body such as the lungs and respiratory system, parts of the digestive tract, as well as in the female reproductive tract. Cilia extend from the epithelium lining the lumen of these body system tracts. These hair-like threads move in a sweeping motion to direct the flow of cells or debris. For example, cilia in the respiratory tract help to propel mucus, pollen, dust, and other substances away from the lungs.

Differences Between Plant and Animal Cells

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Illustration of the differences between plant cells and animal cells
Alison Czinkota / Illustration / ThoughtCo


By Regina BaileyUpdated on May 04, 2019

Animal cells and plant cells are similar in that they are both eukaryotic cells. These cells have a true nucleus, which houses DNA and is separated from other cellular structures by a nuclear membrane. Both of these cell types have similar processes for reproduction, which include mitosis and meiosis. Animal and plant cells obtain the energy they need to grow and maintain normal cellular function through the process of cellular respiration. Both of these cell types also contain cell structures known as organelles, which are specialized to perform functions necessary for normal cellular operation. Animal and plant cells have some of the same cell components in common including a nucleus, Golgi complex, endoplasmic reticulumribosomesmitochondriaperoxisomescytoskeleton, and cell (plasma) membrane. While animal and plant cells have many common characteristics, they are also different.

Differences Between Animal Cells and Plant Cells

Animal Cell Versus Plant Cell
Britannica / UIG / Getty Images


Animal cells are generally smaller than plant cells. Animal cells range from 10 to 30 micrometers in length, while plant cells range from 10 and 100 micrometers in length.


Animal cells come in various sizes and tend to have round or irregular shapes. Plant cells are more similar in size and are typically rectangular or cube shaped.

Energy Storage

Animals cells store energy in the form of the complex carbohydrate glycogen. Plant cells store energy as starch.


Of the 20 amino acids needed to produce proteins, only 10 can be produced naturally in animal cells. The other so-called essential amino acids must be acquired through diet. Plants are capable of synthesizing all 20 amino acids.


In animal cells, only stem cells are capable of converting to other cell types. Most plant cell types are capable of differentiation.


Animal cells increase in size by increasing in cell numbers. Plant cells mainly increase cell size by becoming larger. They grow by absorbing more water into the central vacuole.

Cell Wall

Animal cells do not have a cell wall but have a cell membrane. Plant cells have a cell wall composed of cellulose as well as a cell membrane.


Animal cells contain these cylindrical structures that organize the assembly of microtubules during cell division. Plant cells do not typically contain centrioles.


Cilia are found in animal cells but not usually in plant cells. Cilia are microtubules that aid in cellular locomotion.


Cytokinesis, the division of the cytoplasm during cell division, occurs in animal cells when a cleavage furrow forms that pinches the cell membrane in half. In plant cell cytokinesis, a cell plate is constructed that divides the cell.


These structures are not found in animal cells but are present in plant cells. Glyoxysomes help to degrade lipids, particularly in germinating seeds, for the production of sugar.


Animal cells possess lysosomes which contain enzymes that digest cellular macromolecules. Plant cells rarely contain lysosomes as the plant vacuole handles molecule degradation.


Animal cells do not have plastids. Plant cells contain plastids such as chloroplasts, which are needed for photosynthesis.


Animal cells do not have plasmodesmata. Plant cells have plasmodesmata, which are pores between plant cell walls that allow molecules and communication signals to pass between individual plant cells.


Animal cells may have many small vacuoles. Plant cells have a large central vacuole that can occupy up to 90% of the cell’s volume.

Prokaryotic Cells

E. Coli Bacterium
CNRI / Getty Images 

Animal and plant eukaryotic cells are also different from prokaryotic cells like bacteria. Prokaryotes are usually single-celled organisms, while animal and plant cells are generally multicellular. Eukaryotic cells are more complex and larger than prokaryotic cells. Animal and plant cells contain many organelles not found in prokaryotic cells. Prokaryotes have no true nucleus as the DNA is not contained within a membrane, but is coiled up in a region of the cytoplasm called the nucleoid. While animal and plant cells reproduce by mitosis or meiosis, prokaryotes propagate most commonly by binary fission.https://32e95ceaf6ba4e4b3c4d84bcf818926d.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

Other Eukaryotic Organisms

Haematococcus Algae, Light Micrograph

Plant and animal cells are not the only types of eukaryotic cells. Protists and fungi are two other types of eukaryotic organisms. Examples of protists include algae, euglena, and amoebas. Examples of fungi include mushrooms, yeasts, and molds.

Cell Membrane Function and Structure

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An illustration of cell membrane structure, with important parts labeled
Illustration by Alison Czinkota. ThoughtCo.


By Regina BaileyUpdated on October 07, 2019

The cell membrane (plasma membrane) is a thin semi-permeable membrane that surrounds the cytoplasm of a cell. Its function is to protect the integrity of the interior of the cell by allowing certain substances into the cell while keeping other substances out. It also serves as a base of attachment for the cytoskeleton in some organisms and the cell wall in others. Thus the cell membrane also serves to help support the cell and help maintain its shape.​

Key Takeaways

  • The cell membrane is a multifaceted membrane that envelopes a cell’s cytoplasm. It protects the integrity of the cell along with supporting the cell and helping to maintain the cell’s shape.
  • Proteins and lipids are the major components of the cell membrane. The exact mix or ratio of proteins and lipids can vary depending on the function of a specific cell.
  • Phospholipids are important components of cell membranes. They spontaneously arrange to form a lipid bilayer that is semi-permeable such that only certain substances can diffuse through the membrane to the cell’s interior.
  • Similar to the cell membrane, some cell organelles are surrounded by membranes. The nucleus and mitochondria are two examples.

Another function of the membrane is to regulate cell growth through the balance of endocytosis and ​exocytosis. In endocytosis, lipids and proteins are removed from the cell membrane as substances are internalized. In exocytosis, vesicles containing lipids and proteins fuse with the cell membrane increasing cell size. Animal cellsplant cellsprokaryotic cells, and fungal cells have plasma membranes. Internal organelles are also encased by membranes.

Cell Membrane Structure

A molecular view of the cell membrane highlighting phospholipids, cholesterol, and intrinsic and extrinsic proteins.
Encyclopaedia Britannica / UIG / Getty Images

The cell membrane is primarily composed of a mix of proteins and lipids. Depending on the membrane’s location and role in the body, lipids can make up anywhere from 20 to 80 percent of the membrane, with the remainder being proteins. While lipids help to give membranes their flexibility, proteins monitor and maintain the cell’s chemical climate and assist in the transfer of molecules across the membrane.

Cell Membrane Lipids

Microscopic view of phospholipids.
 Microscopic view of phospholipids.Stocktrek Images / Getty Images

Phospholipids are a major component of cell membranes. Phospholipids form a lipid bilayer in which their hydrophilic (attracted to water) head areas spontaneously arrange to face the aqueous cytosol and the extracellular fluid, while their hydrophobic (repelled by water) tail areas face away from the cytosol and extracellular fluid. The lipid bilayer is semi-permeable, allowing only certain molecules to diffuse across the membrane.

Cholesterol is another lipid component of animal cell membranes. Cholesterol molecules are selectively dispersed between membrane phospholipids. This helps to keep cell membranes from becoming stiff by preventing phospholipids from being too closely packed together. Cholesterol is not found in the membranes of plant cells.

Glycolipids are located on cell membrane surfaces and have a carbohydrate sugar chain attached to them. They help the cell to recognize other cells of the body.

Cell Membrane Proteins

 Lipoproteins and PCSK9 bound to receptors.MAURIZIO DE ANGELIS / SCIENCE PHOTO LIBRARY / Getty Images

The cell membrane contains two types of associated proteins. Peripheral membrane proteins are exterior to and connected to the membrane by interactions with other proteins. Integral membrane proteins are inserted into the membrane and most pass through the membrane. Portions of these transmembrane proteins are exposed on both sides of the membrane. Cell membrane proteins have a number of different functions. 

Structural proteins help to give the cell support and shape.

Cell membrane receptor proteins help cells communicate with their external environment through the use of hormones, neurotransmitters, and other signaling molecules.

Transport proteins, such as globular proteins, transport molecules across cell membranes through facilitated diffusion. 

Glycoproteins have a carbohydrate chain attached to them. They are embedded in the cell membrane and help in cell to cell communications and molecule transport across the membrane.

Organelle Membranes

Rough Endoplasmic Reticulum
 Rough Endoplasmic Reticulum.D Spector / Getty Images

Some cell organelles are also surrounded by protective membranes. The nucleusendoplasmic reticulumvacuoleslysosomes, and Golgi apparatus are examples of membrane-bound organelles. Mitochondria and chloroplasts are bound by a double membrane. The membranes of the different organelles vary in molecular composition and are well suited for the functions they perform. Organelle membranes are important to several vital cell functions including protein synthesis, lipid production, and cellular respiration.

Eukaryotic Cell Structures

Chromosomes, artwork
 Artwork of chromosomes.Science Photo Library – SCIEPRO / Getty Images

The cell membrane is only one component of a cell. The following cell structures can also be found in a typical animal eukaryotic cell:

  • Centrioles—help to organize the assembly of microtubules.
  • Chromosomes—house cellular DNA.
  • Cilia and Flagella—aid in cellular locomotion.
  • Endoplasmic Reticulum—synthesizes carbohydrates and lipids.
  • Golgi Apparatus—manufactures, stores and ships certain cellular products.
  • Lysosomes—digest cellular macromolecules.
  • Mitochondria—provide energy for the cell.
  • Nucleus—controls cell growth and reproduction.
  • Peroxisomes—detoxify alcohol, form bile acid, and use oxygen to break down fats.
  • Ribosomes—responsible for protein production via translation.

Types of White Blood Cells

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White Blood Cells
 Lymphocyte White Blood Cells. Credit: Henrik Jonsson/E+/Getty Images


By Regina BaileyUpdated on November 27, 2019

White blood cells are the defenders of the body. Also called leukocytes, these blood components protect against infectious agents (bacteria and viruses), cancerous cells, and foreign matter. While some white blood cells respond to threats by engulfing and digesting them, others release enzyme-containing granules that destroy the cell membranes of invaders.

White blood cells develop from stem cells in bone marrow. They circulate in blood and lymph fluid and may also be found in body tissues. Leukocytes move from blood capillaries to tissues through a process of cell movement called diapedesis. This ability to migrate throughout the body via the circulatory system allows white blood cells to respond to threats at various locations in the body.


Macrophage and Bacteria
 This is a colored scanning electron micrograph (SEM) of Mycobacterium tuberculosis bacteria (purple) infecting a macrophage. The white blood cell, when activated, will engulf the bacteria and destroy them as part of the body’s immune response. Science Photo Library/Getty Images

Monocytes are the largest of the white blood cells. Macrophages are monocytes that are present in nearly all tissue. They digest cells and pathogens by engulfing them in a process called phagocytosis. Once ingested, lysosomes within the macrophages release hydrolytic enzymes that destroy the pathogen. Macrophages also release chemicals that attract other white blood cells to areas of infection.

Macrophages aid in adaptive immunity by presenting information about foreign antigens to immune cells called lymphocytes. Lymphocytes use this information to quickly mount a defense against these intruders should they infect the body in the future. Macrophages also perform many functions outside of immunity. They assist in sex cell development, steroid hormone production, resorption of bone tissue, and blood vessel network development.

Dendritic Cells

Dendritic Cell
 This is an artistic rendering of the surface of a human dendritic cell illustrating the unexpected discovery of sheet-like processes that fold back onto the membrane surface. National Cancer Institute (NCI)/Sriram Subramaniam/Public Domain

Like macrophages, dendritic cells are monocytes. Dendritic cells have projections that extend from the body of the cell that are similar in appearance to the dendrites of neurons. They are commonly found in tissues in areas that come in contact with the external environment, such as the skin, nose, lungs, and gastrointestinal tract.

Dendritic cells help identify pathogens by presenting information about these antigens to lymphocytes in lymph nodes and lymph organs. They also play an important role in the tolerance of self antigens by removing developing T lymphocytes in the thymus that would harm the body’s own cells.

B Cells

B cell Lymphocyte
 B cells are a type of white blood cell involved in immune response. They account for 10 percent of the body’s lymphocytes. Steve Gschmeissner/Brand X Pictures/Getty Images

B cells are a class of white blood cell known as a lymphocyte. B cells produce specialized proteins called antibodies to counter pathogens. Antibodies help identify pathogens by binding to them and targeting them for destruction by other immune system cells. When an antigen is encountered by B cells that respond to the specific antigen, the B cells rapidly reproduce and develop into plasma cells and memory cells.

Plasma cells produce large quantities of antibodies that are released into circulation to mark any other of these antigens in the body. Once the threat has been identified and neutralized, antibody production is reduced. Memory B cells help protect against future infections from previously encountered germs by retaining information about a germ’s molecular signature. This helps the immune system to quickly identify and respond to a previously encountered antigen and provides long-term immunity against specific pathogens.

T Cells

Cytotoxic T Cell
 This cytotoxic T cell lymphocyte kills cells infected with viruses, or are otherwise damaged or dysfunctional, through release of cytotoxins perforin and granulysin, which cause lysis of the target cell. ScienceFoto.DE ­ Oliver Anlauf/Oxford Scientific/Getty Images

Like B cells, T cells are also lymphocytes. T cells are produced in bone marrow and travel to the thymus where they mature. T cells actively destroy infected cells and signal other immune cells to participate in the immune response. T cell types include:

  • Cytotoxic T cells: actively destroy cells that have become infected
  • Helper T cells: assist in the production of antibodies by B cells and help activate cytotoxic T cells and macrophages
  • Regulatory T cells: suppress B and T cell responses to antigens so an immune response does not last longer than necessary
  • Natural Killer T (NKT) cells: distinguish infected or cancerous cells from normal body cells and attack cells that are not identified as body cells
  • Memory T cells: help to quickly identify previously encountered antigens for a more effective immune response

Reduced numbers of T cells in the body can seriously compromise the ability of the immune system to perform its defensive functions. This is the case with infections such as HIV. In addition, defective T cells may lead to the development of different types of cancer or autoimmune diseases.

Natural Killer Cells

Natural Killer Cell Granule
 This electron micrograph image shows a lytic granule (yellow) within the actin network (blue) at the immune synapse of a natural killer cell. Gregory Rak and Jordan Orange, Children’s Hospital of Philadelphia

Natural killer (NK) cells are lymphocytes that circulate in the blood in search of infected or diseased cells. Natural killer cells contain granules with chemicals inside. When NK cells come across a tumor cell or a cell that is infected with a virus, they surround and destroy the diseased cell by releasing the chemical-containing granules. These chemicals break down the cell membrane of the diseased cell initiating apoptosis and ultimately cause the cell to burst. Natural killer cells should not be confused with certain T cells known as natural Killer T (NKT) cells.


Neutrophil Cell
 This is a stylized image of a neutrophil, one of the white blood cells of the immune system. Science Picture Co/Getty Images

Neutrophils are white blood cells that are classified as granulocytes. They are phagocytic and have chemical-containing granules that destroy pathogens. Neutrophils possess a single nucleus that appears to have multiple lobes. These cells are the most abundant granulocyte in blood circulation. Neutrophils quickly reach sites of infection or injury and are adept at destroying bacteria.


Eosinophil Cell
 This is a stylized image of an eosinophil, one of the white blood cells of the immune system. Science Picture Co/Getty Images

Eosinophils are phagocytic white blood cells that become increasingly active during parasitic infections and allergic reactions. Eosinophils are granulocytes that contain large granules, which release chemicals that destroy pathogens. Eosinophils are often found in connective tissues of the stomach and intestines. The eosinophil nucleus is double-lobed and often appears U-shaped in blood smears.


Basophil Cell
 This is a stylized image of a basophil, one of the white blood cells of the immune system. Science Picture Co/Getty Images

Basophils are granulocytes (granule containing leukocytes) whose granules contain substances such as histamine and heparin. Heparin thins blood and inhibits blood clot formation. Histamine dilates blood vessels and increases blood flow, which helps the flow of white blood cells to infected areas. Basophils are responsible for the body’s allergic response. These cells have a multi-lobed nucleus and are the least numerous of the white blood cells.

Mitochondria: Power Producers

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 CNRI/Science Photo Library/Getty Images


By Regina BaileyUpdated on July 12, 2019

Cells are the basic components of living organisms. The two major types of cells are prokaryotic and eukaryotic cells. Eukaryotic cells have membrane-bound organelles that perform essential cell functions. Mitochondria are considered the “powerhouses” of eukaryotic cells. What does it mean to say that mitochondria are the cell’s power producers? These organelles generate power by converting energy into forms that are usable by the cell. Located in the cytoplasm, mitochondria are the sites of cellular respiration. Cellular respiration is a process that ultimately generates fuel for the cell’s activities from the foods we eat. Mitochondria produce the energy required to perform processes such as cell division, growth, and cell death.

Mitochondria have a distinctive oblong or oval shape and are bounded by a double membrane. The inner membrane is folded creating structures known as cristae. Mitochondria are found in both animal and plant cells. They are found in all body cell types, except for mature red blood cells. The number of mitochondria within a cell varies depending on the type and function of the cell. As mentioned, red blood cells do not contain mitochondria at all. The absence of mitochondria and other organelles in red blood cells leaves room for the millions of hemoglobin molecules needed in order to transport oxygen throughout the body. Muscle cells, on the other hand, may contain thousands of mitochondria needed to provide the energy required for muscle activity. Mitochondria are also abundant in fat cells and liver cells.

Mitochondrial DNA

Mitochondria have their own DNAribosomes and can make their own proteinsMitochondrial DNA (mtDNA) encodes for proteins that are involved in electron transport and oxidative phosphorylation, which occur in cellular respiration. In oxidative phosphorylation, energy in the form of ATP is generated within the mitochondrial matrix. Proteins synthesized from mtDNA also encode for the production of the RNA molecules transfer RNA and ribosomal RNA.

Mitochondrial DNA differs from DNA found in the cell nucleus in that it does not possess the DNA repair mechanisms that help prevent mutations in nuclear DNA. As a result, mtDNA has a much higher mutation rate than nuclear DNA. Exposure to reactive oxygen produced during oxidative phosphorylation also damages mtDNA.

Mitochondrion Anatomy and Reproduction

Animal Mitochondrion
Mariana Ruiz Villarreal

Mitochondrial Membranes

Mitochondria are bounded by a double membrane. Each of these membranes is a phospholipid bilayer with embedded proteins. The outermost membrane is smooth while the inner membrane has many folds. These folds are called cristae. The folds enhance the “productivity” of cellular respiration by increasing the available surface area. Within the inner mitochondrial membrane are a series of protein complexes and electron carrier molecules, which form the electron transport chain (ETC). The ETC represents the third stage of aerobic cellular respiration and the stage where the vast majority of ATP molecules are generated. ATP is the body’s main source of energy and is used by cells to perform important functions, such as muscle contraction and cell division.

Mitochondrial Spaces

The double membranes divide the mitochondrion into two distinct parts: the intermembrane space and the mitochondrial matrix. The intermembrane space is the narrow space between the outer membrane and the inner membrane, while the mitochondrial matrix is the area that is completely enclosed by the innermost membrane. The mitochondrial matrix contains mitochondrial DNA (mtDNA), ribosomes, and enzymes. Several of the steps in cellular respiration, including the Citric Acid Cycle and oxidative phosphorylation occur in the matrix due to its high concentration of enzymes.

Mitochondrial Reproduction

Mitochondria are semi-autonomous in that they are only partially dependent on the cell to replicate and grow. They have their own DNA, ribosomes, make their own proteins, and have some control over their reproduction. Similar to bacteria, mitochondria have circular DNA and replicate by a reproductive process called binary fission. Prior to replication, mitochondria merge together in a process called fusion. Fusion is needed in order to maintain stability as, without it, mitochondria will get smaller as they divide. These smaller mitochondria are not able to produce sufficient amounts of energy needed for proper cell function.urn:uuid:6672fe84-b2ba-acb2-fc13-acb2b2ba6672

Journey Into the Cell

Other important eukaryotic cell organelles include:

  • Nucleus – houses DNA and controls cell growth and reproduction.
  • Ribosomes – aid in the production of proteins.
  • Endoplasmic Reticulum – synthesizes carbohydrates and lipids.
  • Golgi Complex – manufactures, stores, and exports cellular molecules.
  • Lysosomes – digest cellular macromolecules.
  • Peroxisomes – detoxify alcohol, form bile acid, and break down fats.
  • Cytoskeleton – network of fibers that support the cell.
  • Cilia and Flagella – cell appendages that aid in cellular locomotion.

Learn About Plant Cell Types and Organelles

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Ed Reschke / Getty Images

By Regina BaileyUpdated on June 18, 2018

Plant cells are eukaryotic cells or cells with a membrane-bound nucleus. Unlike prokaryotic cells, the DNA in a plant cell is housed within a nucleus that is enveloped by a membrane. In addition to having a nucleus, plant cells also contain other membrane-bound organelles (tiny cellular structures) that carry out specific functions necessary for normal cellular operation. Organelles have a wide range of responsibilities that include everything from producing hormones and enzymes to providing energy for a plant cell.

Plant cells are similar to animal cells in that they are both eukaryotic cells and have similar organelles. However, there are a number of differences between plant and animal cells. Plant cells are generally larger than animal cells. While animal cells come in various sizes and tend to have irregular shapes, plant cells are more similar in size and are typically rectangular or cube shaped. A plant cell also contains structures not found in an animal cell. Some of these include a cell wall, a large vacuole, and plastids. Plastids, such as chloroplasts, assist in storing and harvesting needed substances for the plant. Animal cells also contain structures such as centrioleslysosomes, and cilia and flagella that are not typically found in plant cells.

Plant Cell Organelles

The Cell: Golgi Apparatus Model
 The Golgi Apparatus Model. David Gunn / Getty Images

The following are examples of structures and organelles that can be found in typical plant cells:

  • Cell (Plasma) Membrane: This thin, semi-permeable membrane surrounds the cytoplasm of a cell, enclosing its contents.
  • Cell Wall: This rigid outer covering of the cell protects the plant cell and gives it shape.
  • Chloroplast: Chloroplasts are the sites of photosynthesis in a plant cell. They contain chlorophyll, a green pigment that absorbs energy from sunlight.
  • Cytoplasm: The gel-like substance within the cell membrane is known as cytoplasm. It contains water, enzymes, salts, organelles, and various organic molecules.
  • Cytoskeleton: This network of fibers throughout the cytoplasm helps the cell maintain its shape and gives support to the cell.
  • Endoplasmic Reticulum (ER): The ER is an extensive network of membranes composed of both regions with ribosomes (rough ER) and regions without ribosomes (smooth ER). The ER synthesizes proteins and lipids.
  • Golgi Complex: This organelle is responsible for manufacturing, storing and shipping certain cellular products including proteins.
  • Microtubules: These hollow rods function primarily to help support and shape the cell. They are important for chromosome movement in mitosis and meiosis, as well as cytosol movement within a cell.
  • Mitochondria: Mitochondria generate energy for the cell by converting glucose (produced by photosynthesis) and oxygen to ATP. This process is known as respiration.
  • Nucleus: The nucleus is a membrane-bound structure that contains the cell’s hereditary information (DNA).
    • Nucleolus: This structure within the nucleus helps in the synthesis of ribosomes.
    • Nucleopore: These tiny holes within the nuclear membrane allow nucleic acids and proteins to move into and out of the nucleus.
  • Peroxisomes: Peroxisomes are tiny, single membrane bound structures that contain enzymes which produce hydrogen peroxide as a by-product. These structures are involved in plant processes such as photorespiration.
  • Plasmodesmata: These pores or channels are found between plant cell walls and allow molecules and communication signals to pass between individual plant cells.
  • Ribosomes: Consisting of RNA and proteins, ribosomes are responsible for protein assembly. They can be found either attached to the rough ER or free in the cytoplasm.
  • Vacuole: This plant cell organelle provides support for and participates in a variety of cellular functions including storage, detoxification, protection, and growth. When a plant cell matures, it typically contains one large liquid-filled vacuole.

Plant Cell Types

Plant Tissue Stem
 This is a typical dicotyledon stem (Buttercup). At center is an oval vascular bundle embedded in parenchyma cells (yellow) of the cortex of the stem. Some parenchyma cells contain chloroplasts (green). POWER AND SYRED/SCIENCE PHOTO LIBRARY/Getty Images

As a plant matures, its cells become specialized in order to perform certain functions necessary for survival. Some plant cells synthesize and store organic products, while others help to transport nutrients throughout the plant. Some examples of specialized plant cell types and tissues include: parenchyma cellscollenchyma cellssclerenchyma cells, xylem, and phloem.

Parenchyma Cells

Starch Grains - Carbohydrates
 This image shows starch grains (green) in the parenchyma of a Clematis sp. plant. Starch is synthesized from the carbohydrate sucrose, a sugar produced by the plant during photosynthesis, and used as a source of energy. It is stored as grains in structures called amyloplasts (yellow). STEVE GSCHMEISSNER/Science Photo Library/Getty Images

Parenchyma cells are usually depicted as the typical plant cell because they are not as specialized as other cells. Parenchyma cells have thin walls and are found in dermal, ground, and vascular tissue systems. These cells help to synthesize and store organic products in the plant. The middle tissue layer of leaves (mesophyll) is composed of parenchyma cells, and it is this layer that contains plant chloroplasts.

Chloroplasts are plant organelles that are responsible for photosynthesis and most of the plant’s metabolism takes place in parenchyma cells. Excess nutrients, often in the form of starch grains, are also stored in these cells. Parenchyma cells are not only found in plant leaves, but in the outer and inner layers of stems and roots as well. They are located between xylem and phloem and assist in the exchange of water, minerals, and nutrients. Parenchyma cells are the main components of plant ground tissue and the soft tissue of fruits.

Collenchyma Cells

Collenchyma Cells
 These plant collenchyma cells form supporting tissue. Credit: Ed Reschke/Getty Images

Collenchyma cells have a support function in plants, particularly in young plants. These cells help to support plants, while not restraining growth. Collenchyma cells are elongated in shape and have thick primary cell walls composed of the carbohydrate polymers cellulose and pectin.

Due to their lack of secondary cell walls and the absence of a hardening agent in their primary cell walls, collenchyma cells can provide structural support for tissues while maintaining flexibility. They are able to stretch along with a plant as it grows. Collenchyma cells are found in the cortex (layer between the epidermis and vascular tissue) of stems and along leaf veins.

Sclerenchyma Cells

Sclerenchyma - Plant Vascular Bundle
 This images shows sclerenchyma at vascular bundles of a sunflower stem. Ed Reschke/Photolibrary/Getty Images

Sclerenchyma cells also have a support function in plants, but unlike collenchyma cells, they have a hardening agent in their cell walls and are much more rigid. These cells have thick secondary cell walls and are non-living once matured. There are two types of sclerenchyma cells: sclereids and fibers.

Sclerids have varied sizes and shapes, and most of the volume of these cells is taken up by the cell wall. Sclerids are very hard and form the hard outer shell of nuts and seeds. Fibers are elongated, slender cells that are strand-like in appearance. Fibers are strong and flexible and are found in stems, roots, fruit walls, and leaf vascular bundles.

Conducting Cells – Xylem and Phloem

Xylem and Phloem in Dicotyledon plant
 The center of this stem is filled with large xylem vessels for transporting water and mineral nutrients from the roots to the main body of the plant. Five bundles of phloem tissue (pale green) serve to distribute carbohydrate and plant hormones around the plant. Steve Gschmeissner/Science Photo Library/Getty Images

Water conducting cells of xylem have a support function in plants. Xylem has a hardening agent in the tissue that makes it rigid and capable of functioning in structural support and transportation. The main function of xylem is to transport water throughout the plant. Two types of narrow, elongated cells compose xylem: tracheids and vessel elements. Tracheids have hardened secondary cell walls and function in water conduction. Vessel elements resemble open-ended tubes that are arranged end to end allowing water to flow within the tubes. Gymnosperms and seedless vascular plants contain tracheids, while angiosperms contain both tracheids and vessel members.

Vascular plants also have another type of conducting tissue called phloem. Sieve tube elements are the conducting cells of phloem. They transport organic nutrients, such as glucose, throughout the plant. The cells of sieve tube elements have few organelles allowing for easier passage of nutrients. Since sieve tube elements lack organelles, such as ribosomes and vacuoles, specialized parenchyma cells, called companion cells, must carry out metabolic functions for sieve tube elements. Phloem also contains sclerenchyma cells that provide structural support by increasing rigidity and flexibility.

Amino Acids: Structure, Groups and Function

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Amino Acid
 Ball and stick model of the amino acid glutamate. Callista Images/Image Source/Getty Images

By Regina BaileyUpdated on June 09, 2019

Amino acids are organic molecules that, when linked together with other amino acids, form a protein. Amino acids are essential to life because the proteins they form are involved in virtually all cell functions. Some proteins function as enzymes, some as antibodies, while others provide structural support. Although there are hundreds of amino acids found in nature, proteins are constructed from a set of 20 amino acids.

Key Takeaways

  • Almost all cell functions involve proteins. These proteins are composed of organic molecules called amino acids.
  • While there are many different amino acids in nature, our proteins are formed from twenty amino acids.
  • From a structural perspective, amino acids are typically composed of a carbon atom, a hydrogen atom, a carboxyl group along with an amino group and a variable group.
  • Based on the variable group, amino acids can be classified into four categories: nonpolar, polar, negatively charged, and positively charged.
  • Of the set of twenty amino acids, eleven can be made naturally by the body and are termed nonessential amino acids. Amino acids that can not be naturally made by the body are called essential amino acids.


Amino Acid Structure
 Basic Amino Acid Structure: alpha carbon, hydrogen atom, carboxyl group, amino group, “R” group (side chain). Yassine Mrabet/Wikimedia Commons

Generally, amino acids have the following structural properties:

  • A carbon (the alpha carbon)
  • A hydrogen atom (H)
  • A Carboxyl group (-COOH)
  • An Amino group (-NH2)
  • A “variable” group or “R” group

All amino acids have the alpha carbon bonded to a hydrogen atom, carboxyl group, and amino group. The “R” group varies among amino acids and determines the differences between these protein monomers. The amino acid sequence of a protein is determined by the information found in the cellular genetic code. The genetic code is the sequence of nucleotide bases in nucleic acids (DNA and RNA) that code for amino acids. These gene codes not only determine the order of amino acids in a protein, but they also determine a protein’s structure and function.

Amino Acid Groups

Amino acids can be classified into four general groups based on the properties of the “R” group in each amino acid. Amino acids can be polar, nonpolar, positively charged, or negatively charged. Polar amino acids have “R” groups that are hydrophilic, meaning that they seek contact with aqueous solutions. Nonpolar amino acids are the opposite (hydrophobic) in that they avoid contact with liquid. These interactions play a major role in protein folding and give proteins their 3-D structure. Below is a listing of the 20 amino acids grouped by their “R” group properties. The nonpolar amino acids are hydrophobic, while the remaining groups are hydrophilic.

Nonpolar Amino Acids

  • Ala: Alanine           Gly: Glycine          Ile: Isoleucine           Leu: Leucine
  • Met: Methionine  Trp: Tryptophan    Phe: Phenylalanine    Pro: Proline
  • Val: Valine

Polar Amino Acids

  • Cys: Cysteine         Ser: Serine           Thr: Threonine
  • Tyr: Tyrosine       Asn: Asparagine Gln: Glutamine

Polar Basic Amino Acids (Positively Charged)

  • His: Histidine      Lys: Lysine           Arg: Arginine

Polar Acidic Amino Acids (Negatively Charged)

  • Asp: Aspartate   Glu: Glutamate

While amino acids are necessary for life, not all of them can be produced naturally in the body. Of the 20 amino acids, 11 can be produced naturally. These nonessential amino acids are alanine, arginine, asparagine, aspartate, cysteine, glutamate, glutamine, glycine, proline, serine, and tyrosine. With the exception of tyrosine, nonessential amino acids are synthesized from products or intermediates of crucial metabolic pathways. For example, alanine and aspartate are derived from substances produced during cellular respiration. Alanine is synthesized from pyruvate, a product of glycolysis. Aspartate is synthesized from oxaloacetate, an intermediate of the citric acid cycle. Six of the nonessential amino acids (arginine, cysteine, glutamine, glycine, proline, and tyrosine) are considered conditionally essential as dietary supplementation may be required during the course of an illness or in children. Amino acids that can not be produced naturally are called essential amino acids. They are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Essential amino acids must be acquired through diet. Common food sources for these amino acids include eggs, soy protein, and whitefish. Unlike humans, plants are capable of synthesizing all 20 amino acids.urn:uuid:597e4406-9b3a-5fb2-4a8b-5fb29b3a597e

Amino Acids and Protein Synthesis

Protein Synthesis
 Transmission electron micrograph of DNA (pink). During transcription, mRNA strands (green) are synthesized and translated by ribosomes (blue).DR ELENA KISELEVA/Getty Images

Proteins are produced through the processes of DNA transcription and translation. In protein synthesis, DNA is first transcribed or copied into RNA. The resulting RNA transcript or messenger RNA (mRNA) is then translated to produce amino acids from the transcribed genetic code. Organelles called ribosomes and another RNA molecule called transfer RNA help to translate mRNA. The resulting amino acids are joined together through dehydration synthesis, a process in which a peptide bond is formed between the amino acids. A polypeptide chain is formed when a number of amino acids are linked together by peptide bonds. After several modifications, the polypeptide chain becomes a fully functioning protein. One or more polypeptide chains twisted into a 3-D structure form a protein.

Biological Polymers

While amino acids and proteins play an essential role in the survival of living organisms, there are other biological polymers that are also necessary for normal biological functioning. Along with proteins, carbohydrateslipids, and nucleic acids constitute the four major classes of organic compounds in living cells.

What Are Prokaryotic Cells? Structure, Function, and Definition

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Shigella bacteria, illustration
 Shigella bacteria. KATERYNA KON/SCIENCE PHOTO LIBRARY / Getty Images

By Regina BaileyUpdated on October 30, 2019

Prokaryotes are single-celled organisms that are the earliest and most primitive forms of life on earth. As organized in the Three Domain System, prokaryotes include bacteria and archaeans. Some prokaryotes, such as cyanobacteria, are photosynthetic organisms and are capable of photosynthesis

Many prokaryotes are extremophiles and can live and thrive in various types of extreme environments including hydrothermal vents, hot springs, swamps, wetlands, and the guts of humans and animals (Helicobacter pylori).

Prokaryotic bacteria can be found almost anywhere and are part of the human microbiota. They live on your skin, in your body, and on everyday objects in your environment.

Prokaryotic Cell Structure

Bacterial Cell Structure
 Bacterial Cell Anatomy and Internal Structure. Jack0m/Getty Images

Prokaryotic cells are not as complex as eukaryotic cells. They have no true nucleus as the DNA is not contained within a membrane or separated from the rest of the cell, but is coiled up in a region of the cytoplasm called the nucleoid.

Prokaryotic organisms have varying cell shapes. The most common bacteria shapes are spherical, rod-shaped, and spiral.

Using bacteria as our sample prokaryote, the following structures and organelles can be found in bacterial cells:

  • Capsule: Found in some bacterial cells, this additional outer covering protects the cell when it is engulfed by other organisms, assists in retaining moisture, and helps the cell adhere to surfaces and nutrients.
  • Cell Wall: The cell wall is an outer covering that protects the bacterial cell and gives it shape.
  • Cytoplasm: Cytoplasm is a gel-like substance composed mainly of water that also contains enzymes, salts, cell components, and various organic molecules.
  • Cell Membrane or Plasma Membrane: The cell membrane surrounds the cell’s cytoplasm and regulates the flow of substances in and out of the cell.
  • Pili (Pilus singular): Hair-like structures on the surface of the cell that attach to other bacterial cells. Shorter pili called fimbriae help bacteria attach to surfaces.
  • Flagella: Flagella are long, whip-like protrusions that aid in cellular locomotion.
  • Ribosomes: Ribosomes are cell structures responsible for protein production.
  • Plasmids: Plasmids are gene-carrying, circular DNA structures that are not involved in reproduction.
  • Nucleoid Region: Area of the cytoplasm that contains the single bacterial DNA molecule.

Prokaryotic cells lack organelles found in eukaryoitic cells such as mitochondriaendoplasmic reticuli, and Golgi complexes. According to the Endosymbiotic Theory, eukaryotic organelles are thought to have evolved from prokaryotic cells living in endosymbiotic relationships with one another. 

Like plant cells, bacteria have a cell wall. Some bacteria also have a polysaccharide capsule layer surrounding the cell wall. This is the layer where bacteria produce biofilm, a slimy substance that helps bacterial colonies adhere to surfaces and to each other for protection against antibiotics, chemicals, and other hazardous substances.

Similar to plants and algae, some prokaryotes also have photosynthetic pigments. These light-absorbing pigments enable photosynthetic bacteria to obtain nutrition from light.urn:uuid:df741d42-a1e5-8cba-1461-8cbaa1e5df74

Binary Fission

E. coli Bacterium Binary Fission.
 E. coli bacteria undergoing binary fission. The cell wall is dividing resulting in the formation of two cells. Janice Carr/CDC

Most prokaryotes reproduce asexually through a process called binary fission. During binary fission, the single DNA molecule replicates and the original cell is divided into two identical cells.

Steps of Binary Fission

  • Binary fission begins with DNA replication of the single DNA molecule. Both copies of DNA attach to the cell membrane.
  • Next, the cell membrane begins to grow between the two DNA molecules. Once the bacterium just about doubles its original size, the cell membrane begins to pinch inward.
  • A cell wall then forms between the two DNA molecules dividing the original cell into two identical daughter cells.

Although E.coli and other bacteria most commonly reproduce by binary fission, this mode of reproduction does not produce genetic variation within the organism. 

Prokaryotic Recombination

Bacterial Conjugation
 False-color transmission electron micrograph (TEM) of an Escherichia coli bacterium (bottom right) conjugating with two other E.coli bacteria. The tubes connecting the bacteria are pili, which are used to transfer genetic material between bacteria. DR L. CARO/Science Photo Library/Getty Images

Genetic variation within prokaryotic organisms is accomplished through recombination. In recombination, genes from one prokaryote are incorporated into the genome of another prokaryote.

Recombination is accomplished in bacterial reproduction by the processes of conjugation, transformation, or transduction.

  • In conjugation, bacteria connect through a protein tube structure called a pilus. Genes are transferred between bacteria through the pilus.
  • In transformation, bacteria take up DNA from their surrounding environment. The DNA is transported across the bacterial cell membrane and incorporated into the bacterial cell’s DNA.
  • Transduction involves the exchange of bacterial DNA through viral infection. Bacteriophages, viruses that infect bacteria, transfer bacterial DNA from previously infected bacteria to any additional bacteria that they infect.

What Are Prokaryotic Cells? Structure, Function, and Definition

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Shigella bacteria, illustration
 Shigella bacteria. KATERYNA KON/SCIENCE PHOTO LIBRARY / Getty Images

By Regina BaileyUpdated on October 30, 2019

Prokaryotes are single-celled organisms that are the earliest and most primitive forms of life on earth. As organized in the Three Domain System, prokaryotes include bacteria and archaeans. Some prokaryotes, such as cyanobacteria, are photosynthetic organisms and are capable of photosynthesis

Many prokaryotes are extremophiles and can live and thrive in various types of extreme environments including hydrothermal vents, hot springs, swamps, wetlands, and the guts of humans and animals (Helicobacter pylori).

Prokaryotic bacteria can be found almost anywhere and are part of the human microbiota. They live on your skin, in your body, and on everyday objects in your environment.

Prokaryotic Cell Structure

Bacterial Cell Structure
 Bacterial Cell Anatomy and Internal Structure. Jack0m/Getty Images

Prokaryotic cells are not as complex as eukaryotic cells. They have no true nucleus as the DNA is not contained within a membrane or separated from the rest of the cell, but is coiled up in a region of the cytoplasm called the nucleoid.

Prokaryotic organisms have varying cell shapes. The most common bacteria shapes are spherical, rod-shaped, and spiral.

Using bacteria as our sample prokaryote, the following structures and organelles can be found in bacterial cells:

  • Capsule: Found in some bacterial cells, this additional outer covering protects the cell when it is engulfed by other organisms, assists in retaining moisture, and helps the cell adhere to surfaces and nutrients.
  • Cell Wall: The cell wall is an outer covering that protects the bacterial cell and gives it shape.
  • Cytoplasm: Cytoplasm is a gel-like substance composed mainly of water that also contains enzymes, salts, cell components, and various organic molecules.
  • Cell Membrane or Plasma Membrane: The cell membrane surrounds the cell’s cytoplasm and regulates the flow of substances in and out of the cell.
  • Pili (Pilus singular): Hair-like structures on the surface of the cell that attach to other bacterial cells. Shorter pili called fimbriae help bacteria attach to surfaces.
  • Flagella: Flagella are long, whip-like protrusions that aid in cellular locomotion.
  • Ribosomes: Ribosomes are cell structures responsible for protein production.
  • Plasmids: Plasmids are gene-carrying, circular DNA structures that are not involved in reproduction.
  • Nucleoid Region: Area of the cytoplasm that contains the single bacterial DNA molecule.

Prokaryotic cells lack organelles found in eukaryoitic cells such as mitochondriaendoplasmic reticuli, and Golgi complexes. According to the Endosymbiotic Theory, eukaryotic organelles are thought to have evolved from prokaryotic cells living in endosymbiotic relationships with one another. 

Like plant cells, bacteria have a cell wall. Some bacteria also have a polysaccharide capsule layer surrounding the cell wall. This is the layer where bacteria produce biofilm, a slimy substance that helps bacterial colonies adhere to surfaces and to each other for protection against antibiotics, chemicals, and other hazardous substances.

Similar to plants and algae, some prokaryotes also have photosynthetic pigments. These light-absorbing pigments enable photosynthetic bacteria to obtain nutrition from light.urn:uuid:df741d42-a1e5-8cba-1461-8cbaa1e5df74

Binary Fission

E. coli Bacterium Binary Fission.
 E. coli bacteria undergoing binary fission. The cell wall is dividing resulting in the formation of two cells. Janice Carr/CDC

Most prokaryotes reproduce asexually through a process called binary fission. During binary fission, the single DNA molecule replicates and the original cell is divided into two identical cells.

Steps of Binary Fission

  • Binary fission begins with DNA replication of the single DNA molecule. Both copies of DNA attach to the cell membrane.
  • Next, the cell membrane begins to grow between the two DNA molecules. Once the bacterium just about doubles its original size, the cell membrane begins to pinch inward.
  • A cell wall then forms between the two DNA molecules dividing the original cell into two identical daughter cells.

Although E.coli and other bacteria most commonly reproduce by binary fission, this mode of reproduction does not produce genetic variation within the organism. 

Prokaryotic Recombination

Bacterial Conjugation
 False-color transmission electron micrograph (TEM) of an Escherichia coli bacterium (bottom right) conjugating with two other E.coli bacteria. The tubes connecting the bacteria are pili, which are used to transfer genetic material between bacteria. DR L. CARO/Science Photo Library/Getty Images

Genetic variation within prokaryotic organisms is accomplished through recombination. In recombination, genes from one prokaryote are incorporated into the genome of another prokaryote.

Recombination is accomplished in bacterial reproduction by the processes of conjugation, transformation, or transduction.

  • In conjugation, bacteria connect through a protein tube structure called a pilus. Genes are transferred between bacteria through the pilus.
  • In transformation, bacteria take up DNA from their surrounding environment. The DNA is transported across the bacterial cell membrane and incorporated into the bacterial cell’s DNA.
  • Transduction involves the exchange of bacterial DNA through viral infection. Bacteriophages, viruses that infect bacteria, transfer bacterial DNA from previously infected bacteria to any additional bacteria that they infect.

An Introduction to Hormones

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Hormone replacement therapy
BSIP/UIG/Getty Images 

By Regina BaileyUpdated on September 01, 2019

Hormones regulate various biological activities including growth, development, reproduction, energy use and storage, and water and electrolyte balance. They are molecules that act as chemical messengers in the body’s endocrine system. Hormones are produced by certain organs and glands and are secreted into the blood or other bodily fluids. Most hormones are carried by the circulatory system to different areas, where they influence specific cells and organs. 

Hormone Signaling

Hormones that are circulated in the blood come in contact with a number of cells. However, they influence only target cells, which have receptors for each specific hormone. Target cell receptors can be located on the surface of the cell membrane or inside of the cell. When a hormone binds to a receptor, it causes changes within the cell that influence cellular function. This type of hormone signaling is described as endocrine signaling because the hormones influence target cells over a long distance from where they are secreted. For example, the pituitary gland near the brain secretes growth hormones affecting widespread areas of the body.  

Not only can hormones affect distant cells, but they can also influence neighboring cells. Hormones act on local cells by being secreted into the interstitial fluid that surrounds cells. These hormones then diffuse to nearby target cells. This type of signaling is called paracrine signaling. These travel a much shorter distance between where they’re secreted and where they target.

In autocrine signaling, hormones don’t travel to other cells but cause changes in the very cell that releases them.

Types of Hormones

Thyroid Hormone Activity
BSIP/UIG/Getty Images

Hormones can be classified into two main types: peptide hormones and steroid hormones.

Peptide Hormones

These protein hormones are composed of amino acids. Peptide hormones are water-soluble and are unable to pass through a cell membrane. Cell membranes contain a phospholipid bilayer that prevents fat-insoluble molecules from diffusing into the cell. Peptide hormones must bind to receptors on the cell’s surface, causing changes within the cell by affecting enzymes within the cell’s cytoplasm. This binding by the hormone initiates the production of a second messenger molecule inside the cell, which carries the chemical signal within the cell. Human growth hormone is an example of a peptide hormone.

Steroid Hormones

Steroid hormones are lipid-soluble and able to pass through the cell membrane to enter a cell. Steroid hormones bind to receptor cells in the cytoplasm, and the receptor-bound steroid hormones are transported into the nucleus. Then, the steroid hormone-receptor complex binds to another specific receptor on the chromatin within the nucleus. The complex calls for the production of certain RNA molecules called messenger RNA (mRNA) molecules, which code for the production of proteins.

Steroid hormones cause certain genes to be expressed or suppressed by influencing gene transcription within a cell. Sex hormones (androgens, estrogens, and progesterone), produced by male and female gonads, are examples of steroid hormones.

Hormone Regulation

Thyroid System Hormones
Stocktrek Images/Getty Images

Hormones may be regulated by other hormones, by glands and organs, and by a negative feedback mechanism. Hormones that regulate the release of other hormones are called tropic hormones. The majority of tropic hormones are secreted by the anterior pituitary in the brain. The hypothalamus and thyroid gland also secrete tropic hormones. The hypothalamus produces the tropic hormone thyrotropin-releasing hormone (TRH), which stimulates the pituitary to release thyroid stimulating hormone (TSH). TSH is a tropic hormone that stimulates the thyroid gland to produce and secrete more thyroid hormones.

Organs and glands also aid in hormonal regulation by monitoring blood content. For example, the pancreas monitors glucose concentrations in the blood. If glucose levels are too low, the pancreas will secrete the hormone glucagon to raise glucose levels. If glucose levels are too high, the pancreas secretes insulin to lower glucose levels.

In negative feedback regulation, the initial stimulus is reduced by the response it provokes. The response eliminates the initial stimulus and the pathway is halted. Negative feedback is demonstrated in the regulation of red blood cell production or erythropoiesis. The kidneys monitor oxygen levels in the blood. When oxygen levels are too low, the kidneys produce and release a hormone called erythropoietin (EPO). EPO stimulates red bone marrow to produce red blood cells. As blood oxygen levels return to normal, the kidneys slow the release of EPO, resulting in decreased erythropoiesis.

The Choroid Plexus

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Ependymal Cells
 Colored scanning electron micrograph (SEM) of the lining of the brain, showing ependymal cells (yellow) and ciliary hairs (green).STEVE GSCHMEISSNER/Science Photo Library/Getty Images

By Regina BaileyUpdated on November 13, 2019

The choroid plexus is a network of capillaries and specialized ependymal cells found in the cerebral ventricles of the brain. The choroid plexus serves two roles for the body: it produces cerebrospinal fluid and provides a toxin barrier to the brain and other central nervous system tissue. The choroid plexus and the cerebrospinal fluid that it produces are necessary for proper brain development and central nervous system function.


The choroid plexus is located in the ventricular system. This series of connecting hollow spaces circulates cerebrospinal fluid. Choroid plexus structures are found within both lateral ventricles as well as the third and fourth ventricles of the brain. The choroid plexus resides within the meninges, the membrane linings that cover and protect the central nervous system.

The meninges are composed of three layers known as the dura mater, arachnoid mater, and pia mater. The choroid plexus can be found in the innermost layer of the meninges, the pia mater. The pia mater membrane shelters the cerebral cortex and spinal cord.


The choroid plexus is composed of blood vessels and specialized epithelial tissue called ependyma. Ependymal cells contain hair-like projections called cilia which form a tissue layer that encases the choroid plexus. Ependymal cells also line the cerebral ventricles and spinal cord central canal. These altered epithelial cells are a type of nervous tissue called neuroglia that helps to produce cerebrospinal fluid.


The two important functions of the choroid plexus are to aid in brain development and protection. This is accomplished through cerebrospinal fluid production and brain protection via the blood-cerebrospinal fluid barrier. Read about these below.

Cerebrospinal Fluid Production

Choroid plexus arterial blood and ependymal cells are responsible for producing cerebrospinal fluid. The clear fluid that fills cavities of cerebral ventricles—as well as the central canal of the spinal cord and subarachnoid space of the meninges—is called the cerebrospinal fluid (CSF)Ependyma tissue separates capillaries of the choroid plexus from cerebral ventricles to regulate what enters CSF. It filters water and other substances from blood and transports them across the ependymal layer into brain ventricles.

CSF keeps the brain and spinal cord safe, secure, nourished, and free of waste. As such, it is vital that the choroid plexus function properly and produce the right amount of CSF. The underproduction of CSF can stunt brain growth and overproduction can lead to the accumulation of CSF in brain ventricles, a condition known as hydrocephalus. Hydrocephalus applies excessive pressure to the brain and can cause brain damage.

Blood–Cerebrospinal Fluid Barrier

The choroid plexus also helps prevent blood and other molecules from leaking through—either leaving or entering—perforated blood vessels in the brain. The arachnoid, a largely impenetrable membrane that envelopes the spinal cord, assists the choroid plexus in this task. The protective barrier they form is called the blood-cerebrospinal fluid barrier. Together with the blood-brain barrier, the blood-cerebrospinal fluid barrier serves to block toxic blood-borne substances from entering cerebrospinal fluid and causing damage to the central nervous system.

The choroid plexus also houses and transports other defensive structures that keep the body disease-free. Numerous white blood cells can be found in the choroid plexus—including macrophages, dendritic cells, and lymphocytes—and microglia, or specialized nervous system cells, and other immune cells enter the central nervous system through the choroid plexus. These are important for preventing pathogens from making their way to the brain.

In order for viruses, bacteria, fungi, and other parasites to gain passage to the central nervous system, they must cross the blood-cerebrospinal fluid barrier. This fends off most attacks, but some microbes, such as those that cause meningitis, have developed mechanisms for crossing this barrier.

Nucleic Acids and Their Function

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Illustration of DNA structure
jack0m / DigitalVision Vectors / Getty Images

Table of Contents

By Regina BaileyUpdated on January 24, 2020

Nucleic acids are molecules that allow organisms to transfer genetic information from one generation to the next. These macromolecules store the genetic information that determines traits and makes protein synthesis possible.urn:uuid:9e9038de-3303-4445-3fc5-444533039e90

Key Takeaways: Nucleic Acids

  • Nucleic acids are macromolecules that store genetic information and enable protein production.
  • Nucleic acids include DNA and RNA. These molecules are composed of long strands of nucleotides.
  • Nucleotides are composed of a nitrogenous base, a five-carbon sugar, and a phosphate group.
  • DNA is composed of a phosphate-deoxyribose sugar backbone and the nitrogenous bases adenine (A), guanine (G), cytosine (C), and thymine (T).
  • RNA has ribose sugar and the nitrogenous bases A, G, C, and uracil (U).

Two examples of nucleic acids include deoxyribonucleic acid (better known as DNA) and ribonucleic acid (better known as RNA). These molecules are composed of long strands of nucleotides held together by covalent bonds. Nucleic acids can be found within the nucleus and cytoplasm of our cells.

Nucleic Acid Monomers

 Nucleotides are composed of a nitrogenous base, a five-carbon sugar, and a phosphate group. OpenStax/Wikimedia Commons/CC BY-SA 3.0

Nucleic acids are composed of nucleotide monomers linked together. Nucleotides have three parts:

  • A Nitrogenous Base
  • A Five-Carbon (Pentose) Sugar
  • A Phosphate Group

Nitrogenous bases include purine molecules (adenine and guanine) and pyrimidine molecules (cytosine, thymine, and uracil.) In DNA, the five-carbon sugar is deoxyribose, while ribose is the pentose sugar in RNA. Nucleotides are linked together to form polynucleotide chains.

They are joined to one another by covalent bonds between the phosphate of one and the sugar of another. These linkages are called phosphodiester linkages. Phosphodiester linkages form the sugar-phosphate backbone of both DNA and RNA.

Similar to what happens with protein and carbohydrate monomers, nucleotides are linked together through dehydration synthesis. In nucleic acid dehydration synthesis, nitrogenous bases are joined together and a water molecule is lost in the process.

Interestingly, some nucleotides perform important cellular functions as “individual” molecules, the most common example being adenosine triphosphate or ATP, which provides energy for many cell functions.

DNA Structure

 DNA is composed of a phosphate-deoxyribose sugar backbone and the four nitrogenous bases: adenine (A), guanine (G), cytosine (C), and thymine (T). OpenStax/Wikimedia Commons/CC BY-SA 3.0

DNA is the cellular molecule that contains instructions for the performance of all cell functions. When a cell divides, its DNA is copied and passed from one cell generation to the next.

DNA is organized into chromosomes and found within the nucleus of our cells. It contains the “programmatic instructions” for cellular activities. When organisms produce offspring, these instructions are passed down through DNA.

DNA commonly exists as a double-stranded molecule with a twisted double-helix shape. DNA is composed of a phosphate-deoxyribose sugar backbone and the four nitrogenous bases:

  • adenine (A)
  • guanine (G)
  • cytosine (C)
  • thymine (T)

In double-stranded DNA, adenine pairs with thymine (A-T) and guanine pairs with cytosine (G-C).

RNA Structure

 RNA is composed of a phosphate-ribose sugar backbone and the nitrogenous bases adenine, guanine, cytosine and uracil (U). Sponk/Wikimedia Commons

RNA is essential for the synthesis of proteins. Information contained within the genetic code is typically passed from DNA to RNA to the resulting proteins. There are several types of RNA.

  • Messenger RNA (mRNA) is the RNA transcript or RNA copy of the DNA message produced during DNA transcription. Messenger RNA is translated to form proteins.
  • Transfer RNA (tRNA) has a three-dimensional shape and is necessary for the translation of mRNA in protein synthesis.
  • Ribosomal RNA (rRNA) is a component of ribosomes and is also involved in protein synthesis.
  • MicroRNAs (miRNAs) are small RNAs that help to regulate gene expression.

RNA most commonly exists as a single-stranded molecule composed of a phosphate-ribose sugar backbone and the nitrogenous bases adenine, guanine, cytosine and uracil (U). When DNA is transcribed into an RNA transcript during DNA transcription, guanine pairs with cytosine (G-C) and adenine pairs with uracil (A-U).

DNA and RNA Composition

 This image shows a comparison of a single-stranded RNA molecule and a double-stranded DNA molecule. Sponk/Wikimedia Commons/CC BY-SA 3.0

The nucleic acids DNA and RNA differ in composition and structure. The differences are listed as follows:


  • Nitrogenous Bases: Adenine, Guanine, Cytosine, and Thymine
  • Five-Carbon Sugar: Deoxyribose
  • Structure: Double-stranded

DNA is commonly found in its three-dimensional, double-helix shape. This twisted structure makes it possible for DNA to unwind for DNA replication and protein synthesis.


  • Nitrogenous Bases: Adenine, Guanine, Cytosine, and Uracil
  • Five-Carbon Sugar: Ribose
  • Structure: Single-stranded

While RNA does not take on a double-helix shape like DNA, this molecule is able to form complex three-dimensional shapes. This is possible because RNA bases form complementary pairs with other bases on the same RNA strand. The base pairing causes RNA to fold, forming various shapes.

More Macromolecules

  • Biological Polymers: macromolecules formed from the joining together of small organic molecules.
  • Carbohydrates: include saccharides or sugars and their derivatives.
  • Proteins: macromolecules formed from amino acid monomers.
  • Lipids: organic compounds that include fats, phospholipids, steroids, and waxes.

What Is a Chromatid?

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3D diagram describing the different parts of homologous chromosomes.
Photon Illustration / Stocktrek Images / Getty Images

By Regina BaileyUpdated on November 12, 2019

A chromatid is one half of a replicated chromosome. Prior to cell division, chromosomes are copied and identical chromosome copies join together at their centromeres. Each strand of one of these chromosomes is a chromatid. Joined chromatids are known as sister chromatids. Once linked sister chromatids separate from one another during anaphase of mitosis, each is known as a daughter chromosome.


  • chromatid is one of two strands of a copied chromosome.
  • Chromatids that are joined together at their centromeres are called sister chromatids. These chromatids are genetically identical.
  • Chromatids are formed in both the cellular division processes of mitosis and meiosis.

Chromatid Formation

Chromatids are produced from chromatin fibers during both meiosis and mitosis. Chromatin is composed of DNA and skeletal proteins and is called a nucleosome when wrapped around these proteins in sequence. Even more tightly wound nucleosomes are called chromatin fibers. Chromatin condenses DNA enough to fit within a cell’s nucleus. Condensed chromatin fibers form chromosomes.

Before replication, a chromosome appears as a single-stranded chromatid. After replication, a chromosome appears in an X-shape. Chromosomes are first replicated and their sister chromatids are then separated during cell division to ensure that each daughter cell receives the appropriate number of chromosomes.

Chromatids in Mitosis

When it is time for a cell to replicate, the cell cycle begins. Before the mitosis phase of the cycle, the cell undergoes a period of growth called interphase where it replicates its DNA and organelles to prepare for division. The stages that follow interphase are listed chronologically below.

  • Prophase: Replicated chromatin fibers form chromosomes. Each replicated chromosome consists of two sister chromatids. Chromosome centromeres serve as a place of attachment for spindle fibers during cell division.
  • Metaphase: Chromatin becomes even more condensed and sister chromatids line up along the mid-region of the cell or the metaphase plate.
  • Anaphase: Sister chromatids are separated and pulled toward opposite ends of the cell by spindle fibers.
  • Telophase: Each separated chromatid is known as a daughter chromosome and each daughter chromosome is enveloped in its own nucleus. Two distinct but identical daughter cells are produced from these nuclei following the division of the cytoplasm known as cytokinesis.

Chromatids in Meiosis

Meiosis is a two-part cell division process carried out by sex cells. This process is similar to mitosis in that it consists of prophase, metaphase, anaphase and telophase stages. During meiosis, however, cells go through the stages twice. Because of this, sister chromatids do not separate until anaphase II of meiosis.

After cytokinesis at the end of meiosis II, four haploid daughter cells, containing half the number of chromosomes of the original cell, are produced.

Illustration of sex cells produced during meiosis, showing Interphase, Prophase, Metaphase.
Dorling Kindersley / Getty Images


It is vital that chromosomes separate correctly during cell division. Any failure of homologous chromosomes or chromatids to separate correctly is known as nondisjunction. Nondisjunction occurs during anaphase of mitosis or either stage of meiosis. Half of the resulting daughter cells from nondisjunction have too many chromosomes and the other half have none at all.

The consequences of having either too many or not enough chromosomes are often serious or even fatal. Down syndrome is an example of nondisjunction resulting from an extra chromosome and Turner syndrome is an example of nondisjunction resulting from a missing whole or partial sex chromosome.

Sister Chromatid Exchange

When sister chromatids are in close proximity to one another during cell division, the exchange of genetic material can occur. This process is known as sister-chromatid exchange or SCE. During SCE, DNA material is swapped as portions of chromatids are broken and rebuilt. A low level of material exchange is typically considered safe, but when the exchange reaches excessive levels, it can be hazardous to the individual.

Differences Between Mitosis and Meiosis

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cervical cancer cells
 These cervical cancer cells are dividing. Steve Gschmeissner / Science Photo Library / Getty Images

By Regina BaileyUpdated on August 19, 2019

Organisms grow and reproduce through cell division. In eukaryotic cells, the production of new cells occurs as a result of mitosis and meiosis. These two nuclear division processes are similar but distinct. Both processes involve the division of a diploid cell, or a cell containing two sets of chromosomes (one chromosome donated from each parent).

In mitosis, the genetic material (DNA) in a cell is duplicated and divided equally between two cells. The dividing cell goes through an ordered series of events called the cell cycle. The mitotic cell cycle is initiated by the presence of certain growth factors or other signals that indicate that the production of new cells is needed. Somatic cells of the body replicate by mitosis. Examples of somatic cells include fat cellsblood cells, skin cells, or any body cell that is not a sex cell. Mitosis is necessary to replace dead cells, damaged cells, or cells that have short life spans.

Meiosis is the process by which gametes (sex cells) are generated in organisms that reproduce sexually. Gametes are produced in male and female gonads and contain one-half the number of chromosomes as the original cell. New gene combinations are introduced in a population through the genetic recombination that occurs during meiosis. Thus, unlike the two genetically identical cells produced in mitosis, the meiotic cell cycle produces four cells that are genetically different.

Key Takeaways: Mitosis vs Meiosis

  • Mitosis and meiosis are nuclear division processes that occur during cell division.
  • Mitosis involves the division of body cells, while meiosis involves the division of sex cells.
  • The division of a cell occurs once in mitosis but twice in meiosis.
  • Two daughter cells are produced after mitosis and cytoplasmic division, while four daughter cells are produced after meiosis.
  • Daughter cells resulting from mitosis are diploid, while those resulting from meiosis are haploid.
  • Daughter cells that are the product of mitosis are genetically identical. Daughter cells produced after meiosis are genetically diverse.
  • Tetrad formation occurs in meiosis but not mitosis.

Differences Between Mitosis and Meiosis

Meiosis Telophase II
 Lily Anther Microsporocyte in Telophase II of Meiosis. Ed Reschke/Photolibrary/Getty Images

1. Cell Division

  • Mitosis: A somatic cell divides once. Cytokinesis (the division of the cytoplasm) occurs at the end of telophase.
  • Meiosis: A reproductive cell divides twice. Cytokinesis happens at the end of telophase I and telophase II.

2. Daughter Cell Number

  • Mitosis: Two daughter cells are produced. Each cell is diploid containing the same number of chromosomes.
  • Meiosis: Four daughter cells are produced. Each cell is haploid containing one-half the number of chromosomes as the original cell.

3. Genetic Composition

  • Mitosis: The resulting daughter cells in mitosis are genetic clones (they are genetically identical). No recombination or crossing over occur.
  • Meiosis: The resulting daughter cells contain different combinations of genes. Genetic recombination occurs as a result of the random segregation of homologous chromosomes into different cells and by the process of crossing over (transfer of genes between homologous chromosomes).

4. Length of Prophase

  • Mitosis: During the first mitotic stage, known as prophase, chromatin condenses into discrete chromosomes, the nuclear envelope breaks down, and spindle fibers form at opposite poles of the cell. A cell spends less time in prophase of mitosis than a cell in prophase I of meiosis.
  • Meiosis: Prophase I consists of five stages and lasts longer than prophase of mitosis. The five stages of meiotic prophase I are leptotene, zygotene, pachytene, diplotene, and diakinesis. These five stages do not occur in mitosis. Genetic recombination and crossing over take place during prophase I.

5. Tetrad Formation

  • Mitosis: Tetrad formation does not occur.
  • Meiosis: In prophase I, pairs of homologous chromosomes line up closely together forming what is called a tetrad. A tetrad consists of four chromatids (two sets of sister chromatids).

6. Chromosome Alignment in Metaphase

  • Mitosis: Sister chromatids (duplicated chromosome comprised of two identical chromosomes connected at the centromere region) align at the metaphase plate (a plane that is equally distant from the two cell poles).
  • Meiosis: Tetrads (homologous chromosome pairs) align at the metaphase plate in metaphase I.

7. Chromosome Separation

  • Mitosis: During anaphase, sister chromatids separate and begin migrating centromere first toward opposite poles of the cell. A separated sister chromatid becomes known as daughter chromosome and is considered a full chromosome.
  • Meiosis: Homologous chromosomes migrate toward opposite poles of the cell during anaphase I. Sister chromatids do not separate in anaphase I.

Mitosis and Meiosis Similarities

A plant cell in Interphase
 Plant cell in Interphase. In interphase, the cell is not undergoing cell division. The nucleus and chromatin are evident. Ed Reschke/Getty Images

While the processes of mitosis and meiosis contain a number of differences, they are also similar in many ways. Both processes have a growth period called interphase, in which a cell replicates its genetic material and organelles in preparation for division.

Both mitosis and meiosis involve phases: ProphaseMetaphaseAnaphase and Telophase. Although in meiosis, a cell goes through these cell cycle phases twice. Both processes also involve the lining up of individual duplicated chromosomes, known as sister chromatids, along the metaphase plate. This happens in metaphase of mitosis and metaphase II of meiosis.

In addition, both mitosis and meiosis involve the separation of sister chromatids and the formation of daughter chromosomes. This event occurs in anaphase of mitosis and anaphase II of meiosis. Finally, both processes end with the division of the cytoplasm that produces individual cells.

All About Animal Cells

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Animal Cell
 Animal Cell Components.colematt / iStock / Getty Images Plus 

By Regina BaileyUpdated on October 09, 2019

Animal cells are eukaryotic cells or cells with a membrane-bound nucleus. Unlike prokaryotic cellsDNA in animal cells is housed within the nucleus. In addition to having a nucleus, animal cells also contain other membrane-bound organelles, or tiny cellular structures, that carry out specific functions necessary for normal cellular operation. Organelles have a wide range of responsibilities that include everything from producing hormones and enzymes to providing energy for animal cells.

Key Takeaways

  • Animal cells are eukaryotic cells that have both a membrane-bound nucleus and other membrane-bound organelles. These organelles carry out specific functions that are needed for the normal functioning of the cell.
  • Plant and animal cells are similar in that they are both eukaryotic and have similar types of organelles. Plant cells tend to have more uniform sizes than animal cells.
  • Cell structure and organelle examples include: centrioles, the Golgi complex, microtubules, nucleopores, peroxisomes, and ribosomes.
  • Animals typically contain trillions of cells. Humans, for example, also have hundreds of different cell types. The shape, size and structure of cells go along with their specific function.

Animal Cells vs. Plant Cells

Illustration of an Eukaryotic Animal Cell
 Eukaryotic Animal Cell Illustration.Encyclopaedia Britannica / UIG / Getty Images

Animal cells and plant cells are similar in that they are both eukaryotic cells and have similar organelles. Animal cells are generally smaller than plant cells. While animal cells come in various sizes and tend to have irregular shapes, plant cells are more similar in size and are typically rectangular or cube shaped. A plant cell also contains structures not found in an animal cell. Some of these include a cell wall, a large vacuole, and plastids. Plastids, such as chloroplasts, assist in storing and harvesting needed substances for the plant. Animal cells also contain structures such as centrioles, lysosomes, cilia, and flagella that are not typically found in plant cells.

Organelles and Components of Animal Cells

Illustration of a typical animal cell with labeled organelles
 Animal cell organelles.Mediran / Wikimedia Commons / CC-BY-SA-3.0

The following are examples of structures and organelles that can be found in typical animal cells:

  • Cell (Plasma) Membrane – thin, semi-permeable membrane that surrounds the cytoplasm of a cell, enclosing its contents.
  • Centrioles – cylindrical structures that organize the assembly of microtubules during cell division.
  • Cilia and flagella – specialized groupings of microtubules that protrude from some cells and aid in cellular locomotion.
  • Cytoplasm – gel-like substance within the cell.
  • Cytoskeleton – a network of fibers throughout the cell’s cytoplasm that gives the cell support and helps to maintain its shape.
  • Endoplasmic Reticulum – an extensive network of membranes composed of both regions with ribosomes (rough ER) and regions without ribosomes (smooth ER).
  • Golgi Complex – also called the Golgi apparatus, this structure is responsible for manufacturing, storing and shipping certain cellular products.
  • Lysosomes – sacs of enzymes that digest cellular macromolecules such as nucleic acids.
  • Microtubules – hollow rods that function primarily to help support and shape the cell.
  • Mitochondria – cell components that generate energy for the cell and are the sites of cellular respiration.
  • Nucleus – membrane-bound structure that contains the cell’s hereditary information.
    • Nucleolus – structure within the nucleus that helps in the synthesis of ribosomes.
    • Nucleopore – a tiny hole in the nuclear membrane that allows nucleic acids and proteins to move into and out of the nucleus.
  • Peroxisomes – enzyme containing structures that help to detoxify alcohol, form bile acid, and break down fats.
  • Ribosomes – consisting of RNA and proteins, ribosomes are responsible for protein assembly.

Animal Cell Types

Cilia and mucous cells of oviduct (rat)
 Cilia and mucous cells of rat oviduct.Micro Discovery / Getty Images

In the hierarchical structure of life, cells are the simplest living units. Animal organisms can be composed of trillions of cells. In the human body, there are hundreds of different types of cells. These cells come in all shapes and sizes and their structure suits their function. For example, the body’s nerve cells or neurons have a vastly different shape and function than red blood cells. Nerve cells transport electrical signals throughout the nervous system. They are elongated and thin, with projections that extend out to communicate with other nerve cells in order to conduct and transmit nerve impulses. The major role of red blood cells is to transport oxygen to body cells. Their small, flexible disc shape enables them to maneuver through tiny blood vessels to deliver oxygen to organs and tissues.

The Cell Nucleus

Definition, Structure, and Function

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Human cells, illustration

By Regina BaileyUpdated on November 06, 2019

The cell nucleus​ is a membrane-bound structure that contains a cell’s hereditary information and controls its growth and reproduction. It is the command center of a eukaryotic cell and is usually the most notable cell organelle in both size and function.


The key function of the nucleus is to control cell growth and multiplication. This involves regulating gene expression, initiating cellular reproduction, and storing genetic material necessary for all of these tasks. In order for a nucleus to carry out important reproductive roles and other cell activities, it needs proteins and ribosomes.

Protein and Ribosome Synthesis

The nucleus regulates the synthesis of proteins in the cytoplasm through the use of messenger RNA (mRNA). Messenger RNA is a transcribed DNA segment that serves as a template for protein production. It is produced in the nucleus and travels to the cytoplasm through the nuclear pores of the nuclear envelope, which you’ll read about below. Once in the cytoplasm, ribosomes and another RNA molecule called transfer RNA work together to translate mRNA in order to produce proteins.

Physical Characteristics

The shape of a nucleus varies from cell to cell but is often depicted as spherical. To understand more about the role of the nucleus, read about the structure and function of each of its parts.

Nuclear Envelope and Nuclear Pores

The cell nucleus is bound by a double membrane called the nuclear envelope. This membrane separates the contents of the nucleus from the cytoplasm, the gel-like substance containing all other organelles. The nuclear envelope consists of phospholipids that form a lipid bilayer much like that of the cell membrane. This lipid bilayer has nuclear pores that allow substances to enter and exit the nucleus, or transfer from the cytoplasm to the nucleoplasm.

The nuclear envelope helps to maintain the shape of the nucleus. It is connected to the endoplasmic reticulum (ER) in such a way that the internal chamber of the nuclear envelope is continuous with the lumen, or inside, of the ER. This also allows the transfer of materials as well.


The nucleus houses chromosomes containing DNA. DNA holds heredity information and instructions for cell growth, development, and reproduction. When a cell is “resting”, or not dividing, its chromosomes are organized into long entangled structures called chromatin.


Nucleoplasm is the gelatinous substance within the nuclear envelope. Also called karyoplasm, this semi-aqueous material is similar to cytoplasm in that it is composed mainly of water with dissolved salts, enzymes, and organic molecules suspended within. The nucleolus and chromosomes are surrounded by nucleoplasm, which cushions and protects nuclear contents.

Like the nuclear envelope, the nucleoplasm supports the nucleus to hold its shape. It also provides a medium by which materials, such as enzymes and nucleotides (DNA and RNA subunits), can be transported throughout the nucleus to its various parts.


Contained within the nucleus is a dense, membrane-less structure composed of RNA and proteins called the nucleolus. The nucleolus contains nucleolar organizers, the parts of chromosomes carrying the genes for ribosome synthesis. The nucleolus helps to synthesize ribosomes by transcribing and assembling ribosomal RNA subunits. These subunits join together to form ribosomes during protein synthesis.

The Role of Cytoplasm in a Cell

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Human cells, illustration

By Regina BaileyUpdated on August 21, 2019

Cytoplasm consists of all of the contents outside of the nucleus and enclosed within the cell membrane of a cell. It is clear in color and has a gel-like appearance. Cytoplasm is composed mainly of water but also contains enzymes, salts, organelles, and various organic molecules.urn:uuid:5000e9e8-901c-3216-eb41-3216901c5000

Cytoplasm Functions

  • The cytoplasm functions to support and suspend organelles and cellular molecules.
  • Many cellular processes also occur in the cytoplasm, such as protein synthesis, the first stage of cellular respiration (known as glycolysis), mitosis, and meiosis.
  • The cytoplasm helps to move materials, such as hormones, around the cell and also dissolves cellular waste.


The cytoplasm can be divided into two primary parts: the endoplasm (endo-,-plasm) and ectoplasm (ecto-,-plasm). The endoplasm is the central area of the cytoplasm that contains the organelles. The ectoplasm is the more gel-like peripheral portion of the cytoplasm of a cell.


Prokaryotic cells, such as bacteria and archaeans, do not have a membrane-bound nucleus. In these cells, the cytoplasm consists of all of the contents of the cell inside the plasma membrane. In eukaryotic cells, such as plant and animal cells, the cytoplasm consists of three main components. They are the cytosol, organelles, and various particles and granules called cytoplasmic inclusions.

  • Cytosol: The cytosol is the semi-fluid component or liquid medium of a cell’s cytoplasm. It is located outside of the nucleus and within the cell membrane.
  • Organelles: Organelles are tiny cellular structures that perform specific functions within a cell. Examples of organelles include mitochondriaribosomes, nucleus, lysosomeschloroplastsendoplasmic reticulum, and Golgi apparatus. Also located within the cytoplasm is the cytoskeleton, a network of fibers that help the cell maintain its shape and provide support for organelles.
  • Cytoplasmic Inclusions: Cytoplasmic inclusions are particles that are temporarily suspended in the cytoplasm. Inclusions consist of macromolecules and granules. Three types of inclusions found in the cytoplasm are secretory inclusions, nutritive inclusions, and pigment granules. Examples of secretory inclusions are proteins, enzymes, and acids. Glycogen (glucose storage molecule) and lipids are examples of nutritive inclusions. Melanin found in skin cells is an example of a pigment granule inclusion.

Cytoplasmic Streaming

Cytoplasmic streaming, or cyclosis, is a process by which substances are circulated within a cell. Cytoplasmic streaming occurs in a number of cell types including plant cellsamoeba, protozoa, and fungi. Cytoplasmic movement may be influenced by several factors including the presence of certain chemicals, hormones, or changes in light or temperature.

Plants employ cyclosis to shuttle chloroplasts to areas receiving the most available sunlight. Chloroplasts are the plant organelles responsible for photosynthesis and require light for the process. In protists, such as amoebae and slime molds, cytoplasmic streaming is used for locomotion. Temporary extensions of the cytoplasm known as pseudopodia are generated that are valuable for movement and capturing food. Cytoplasmic streaming is also required for cell division as the cytoplasm must be distributed among daughter cells formed in mitosis and meiosis.

Cell Membrane

The cell membrane or plasma membrane is the structure that keeps cytoplasm from spilling out of a cell. This membrane is composed of phospholipids, which form a lipid bilayer that separates the contents of a cell from the extracellular fluid. The lipid bilayer is semi-permeable, meaning that only certain molecules are able to diffuse across the membrane to enter or exit the cell. Extracellular fluid, proteins, lipids, and other molecules may be added to a cell’s cytoplasm by endocytosis. In this process, molecules and extracellular fluid are internalized as the membrane turns inward forming a vesicle. The vesicle encloses the fluid and molecules and buds off from the cell membrane forming an endosome. The endosome moves within the cell to deliver its contents to their appropriate destinations. Substances are removed from the cytoplasm by exocytosis. In this process, vesicles budding from Golgi bodies fuse with the cell membrane expelling their contents from the cell. The cell membrane also provides structural support for a cell by serving as a stable platform for the attachment of the cytoskeleton and cell wall (in plants).

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Cell Theory: A Core Principle of Biology

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Illustration of cell theory
 Cell Theory. Illustration by Hugo Lin. ThoughtCo. 

By Regina BaileyUpdated on January 24, 2020

Cell Theory is one of the basic principles of biology. Credit for the formulation of this theory is given to German scientists Theodor Schwann (1810–1882), Matthias Schleiden (1804–1881), and Rudolph Virchow (1821–1902).

The Cell Theory states:

  • All living organisms are composed of cells. They may be unicellular or multicellular.
  • The cell is the basic unit of life.
  • Cells arise from pre-existing cells. (They are not derived from spontaneous generation.)

The modern version of the Cell Theory includes the ideas that:

  • Energy flow occurs within cells.
  • Heredity information (DNA) is passed on from cell to cell.
  • All cells have the same basic chemical composition.

In addition to the cell theory, the gene theoryevolutionhomeostasis, and the laws of thermodynamics form the basic principles that are the foundation for the study of life.

What Are Cells?

Cells are the simplest unit of matter that is living. The two primary kinds of cells are eukaryotic cells, which have a true nucleus containing DNA and prokaryotic cells, which have no true nucleus. In prokaryotic cells, the DNA is coiled up in a region called the nucleoid.

Cell Basics

All living organisms in the kingdoms of life are composed of and depend on cells to function normally. Not all cells, however, are alike. There are two primary types of cells: eukaryotic and prokaryotic cells. Examples of eukaryotic cells include animal cellsplant cells, and fungal cellsProkaryotic cells include bacteria and archaeans.

Cells contain organelles, or tiny cellular structures, that carry out specific functions necessary for normal cellular operation. Cells also contain DNA (deoxyribonucleic acid) and RNA (ribonucleic acid), the genetic information necessary for directing cellular activities.

Cell Reproduction

Spirogyra, Green algae. conjugation tubes, zigotes, Active Gametes
Ed Reschke/Getty Images

Eukaryotic cells grow and reproduce through a complex sequence of events called the cell cycle. At the end of the cycle, cells will divide either through the processes of mitosis or meiosis. Somatic cells replicate through mitosis and sex cells reproduce via meiosis. Prokaryotic cells reproduce commonly through a type of asexual reproduction called binary fission. Higher organisms are also capable of asexual reproduction. Plants, algae, and fungi reproduce through the formation of reproductive cells called spores. Animal organisms can reproduce asexually through processes such as budding, fragmentation, regeneration, and parthenogenesis.

Cell Processes: Cellular Respiration and Photosynthesis

Light micrograph of Foveolate stomata of oleander x400
Garry DeLong/Getty Images

Cells perform a number of important processes that are necessary for the survival of an organism. Cells undergo the complex process of cellular respiration in order to obtain energy stored in the nutrients consumed. Photosynthetic organisms including plantsalgae, and cyanobacteria are capable of photosynthesis. In photosynthesis, light energy from the sun is converted to glucose. Glucose is the energy source used by photosynthetic organisms and other organisms that consume photosynthetic organisms.

Cell Processes: Endocytosis and Exocytosis

Volvox colony, light micrograph
Frank Fox/Getty Images

Cells also perform the active transport processes of endocytosis and exocytosis. Endocytosis is the process of internalizing and digesting substances, such as seen with macrophages and bacteria. The digested substances are expelled through exocytosis. These processes also allow for molecule transportation between cells.

Cell Processes: Cell Migration

Plant Mitosis
Ed Reschke/Getty Images

Cell migration is a process that is vital for the development of tissues and organs. Cell movement is also required for mitosis and cytokinesis to occur. Cell migration is made possible by interactions between motor enzymes and cytoskeleton microtubules.

Cell Processes: DNA Replication and Protein Synthesis

The cell process of DNA replication is an important function that is needed for several processes including chromosome synthesis and cell division to occur. DNA transcription and RNA translation make the process of protein synthesis possible.

Ribosomes – The Protein Builders of a Cell

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Ribosome: 3D Model
 This is a 3D computer graphic model of a ribosome. Ribosomes are composed of protein and RNA. They consist of subunits that fit together and work as one to translate mRNA (messenger RNA) into a polypeptide chain during protein synthesis (translation). Credit: Callista Images/Cultura/Getty Images

Table of Contents

By Regina BaileyUpdated on April 04, 2019

There are two major types of cells: prokaryotic and eukaryotic cells. Ribosomes are cell organelles that consist of RNA and proteins. They are responsible for assembling the proteins of the cell. Depending on the protein production level of a particular cell, ribosomes may number in the millions.

Key Takeaways: Ribosomes

  • Ribosomes are cell organelles that function in protein synthesis. Ribosomes in plant and animals cells are larger than those found in bacteria.
  • Ribosomes are composed of RNA and proteins that form ribosome subunits: a large ribosome subunit and small subunit. These two subunits are produced in the nucleus and unite in the cytoplasm during protein synthesis.
  • Free ribosomes are found suspended in the cytosol, while bound ribosomes are attached to the endoplasmic reticulum.
  • Mitochondria and chloroplasts are capable of producing their own ribosomes.

Distinguishing Characteristics

 Structure of a Ribosome. Interaction of a ribosome with mRNA.  ttsz/iStock/Getty Images Plus

Ribosomes are typically composed of two subunits: a large subunit and a small subunit. Eukarotic ribosomes (80S), such as those in plant cells and animal cells, are larger in size than prokaryotic ribosomes (70S), such as those in bacteria. Ribosomal subunits are synthesized in the nucleolus and cross over the nuclear membrane to the cytoplasm through nuclear pores.urn:uuid:6ea7cf1c-4e5e-2580-cfeb-25804e5e6ea7

Both ribosomal subunits join together when the ribosome attaches to messenger RNA (mRNA) during protein synthesis. Ribosomes along with another RNA molecule, transfer RNA (tRNA), help to translate the protein-coding genes in mRNA into proteins. Ribosomes link amino acids together to form polypeptide chains, which are further modified before becoming functional proteins.

Location in the Cell

Anatomy of animal cell
 Ribosomes can be found attached to the endoplasmic reticulum or free within the cytoplasm.  ttsz/iStock/Getty Images Plus

There are two places where ribosomes commonly exist within a eukaryotic cell: suspended in the cytosol and bound to the endoplasmic reticulum. These ribosomes are called free ribosomes and bound ribosomes respectively. In both cases, the ribosomes usually form aggregates called polysomes or polyribosomes during protein synthesis. Polyribosomes are clusters of ribosomes that attach to a mRNA molecule during protein synthesis. This allows for multiple copies of a protein to be synthesized at once from a single mRNA molecule.

Free ribosomes usually make proteins that will function in the cytosol (fluid component of the cytoplasm), while bound ribosomes usually make proteins that are exported from the cell or included in the cell’s membranes. Interestingly enough, free ribosomes and bound ribosomes are interchangeable and the cell can change their numbers according to metabolic needs.

Organelles such as mitochondria and chloroplasts in eukaryotic organisms have their own ribosomes. Ribosomes in these organelles are more like ribosomes found in bacteria with regard to size. The subunits comprising ribosomes in mitochondria and chloroplasts are smaller (30S to 50S) than the subunits of ribosomes found throughout the rest of the cell (40S to 60S).

Ribosomes and Protein Assembly

Ribosome and Protein Synthesis
 Ribosomes interact with mRNA to produce proteins in a process called translation.  ttsz/iStock/Getty Images Plus

Protein synthesis occurs by the processes of transcription and translation. In transcription, the genetic code contained within DNA is transcribed into an RNA version of the code known as messenger RNA (mRNA). The mRNA transcript is transported from the nucleus to the cytoplasm where it undergoes translation. In translation, a growing amino acid chain, also called a polypeptide chain, is produced. Ribosomes help to translate mRNA by binding to the molecule and linking amino acids together to produce a polypeptide chain. The polypeptide chain eventually becomes a fully functioning protein. Proteins are very important biological polymers in our cells as they are involved in virtually all cell functions.

There are some differences between protein synthesis in eukaryotes and prokaryotes. Since eukaryotic ribosomes are larger than those in prokaryotes, they require more protein components. Other differences include different initiator amino acid sequences to start protein synthesis as well as different elongation and termination factors.

Eukaryotic Cell Structures

Animal Cell
 This is a diagram of an animal cell. colematt/iStock/Getty Images Plus 

Ribosomes are only one type of cell organelle. The following cell structures can also be found in a typical animal eukaryotic cell:

What Is an Organelle?

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Animal Cell Organelles
 Animal Cell Organelles.Andrzej Wojcicki/Brand X Pictures/Getty Images 

By Regina BaileyUpdated on June 07, 2019

An organelle is a tiny cellular structure that performs specific functions within a cell. Organelles are embedded within the cytoplasm of eukaryotic and prokaryotic cells. In the more complex eukaryotic cells, organelles are often enclosed by their own membrane. Analogous to the body’s internal organs, organelles are specialized and perform valuable functions necessary for normal cellular operation. Organelles have a wide range of responsibilities that include everything from generating energy for a cell to controlling the cell’s growth and reproduction. 

Key Takeaways

  • Organelles are structures within a cell that perform specific functions like controlling cell growth and producing energy.
  • Plant and animal cells can contain similar types of organelles. However, certain organelles can only be found in plant cells and certain organelles can only be found in animal cells.
  • Examples of organelles found in eukaryotic cells include: the endoplasmic reticulum (smooth and rough ER), the Golgi complex, lysosomes, mitochondria, peroxisomes, and ribosomes.
  • Prokaryotic cells do not have membrane-based organelles. These cells can contain some non-membranous organelles like flagella, ribosomes and circular DNA structures called plasmids.

Eukaryotic Organelles

Illustration of human cell anatomy
 Cellular Organelles in a Human Cell.SCIEPRO/Science Photo Library/Getty Images

Eukaryotic cells are cells with a nucleus. The nucleus is an organelle that is surrounded by a double membrane called the nuclear envelope. The nuclear envelope separates the contents of the nucleus from the rest of the cell. Eukaryotic cells also have a cell membrane (plasma membrane), cytoplasmcytoskeleton, and various cellular organelles. Animals, plants, fungi, and protists are examples of eukaryotic organisms. Animal and plant cells contain many of the same kinds or organelles. There are also certain organelles found in plant cells that are not found in animal cells and vice versa. Examples of organelles found in plant cells and animal cells include:

  • Nucleus – a membrane bound structure that contains the cell’s hereditary (DNA) information and controls the cell’s growth and reproduction. It is commonly the most prominent organelle in the cell.
  • Mitochondria – as the cell’s power producers, mitochondria convert energy into forms that are usable by the cell. They are the sites of cellular respiration which ultimately generates fuel for the cell’s activities. Mitochondria are also involved in other cell processes such as cell division and growth, as well as cell death.​
  • Endoplasmic Reticulum – extensive network of membranes composed of both regions with ribosomes (rough ER) and regions without ribosomes (smooth ER). This organelle manufactures membranes, secretory proteinscarbohydrateslipids, and hormones.​
  • Golgi complex – also called the Golgi apparatus, this structure is responsible for manufacturing, warehousing, and shipping certain cellular products, particularly those from the endoplasmic reticulum (ER).​
  • Ribosomes – these organelles consist of RNA and proteins and are responsible for protein production. Ribosomes are found suspended in the cytosol or bound to the endoplasmic reticulum.​
  • Lysosomes – these membranous sacs of enzymes recycle the cell’s organic material by digesting cellular macromolecules, such as nucleic acids, polysaccharides, fats, and proteins.​
  • Peroxisomes – Like lysosomes, peroxisomes are bound by a membrane and contain enzymes. Peroxisomes help to detoxify alcohol, form bile acid, and break down fats.​
  • Vacuole – these fluid-filled, enclosed structures are found most commonly in plant cells and fungi. Vacuoles are responsible for a wide variety of important functions in a cell including nutrient storage, detoxification, and waste exportation.​
  • Chloroplast – this chlorophyll containing plastid is found in plant cells, but not animal cells. Chloroplasts absorb the sun’s light energy for photosynthesis.​
  • Cell Wall – this rigid outer wall is positioned next to the cell membrane in most plant cells. Not found in animal cells, the cell wall helps to provide support and protection for the cell.​
  • Centrioles – these cylindrical structures are found in animal cells, but not plant cells. Centrioles help to organize the assembly of microtubules during cell division.​
  • Cilia and Flagella – cilia and flagella are protrusions from some cells that aid in cellular locomotion. They are formed from specialized groupings of microtubules called basal bodies.

Prokaryotic Cells

Tongue bacteria
 Prokaryotic cells like these bacteria on the tongue, do not have membrane-based organelles.Steve Gschmeissner/Science Photo Library/Getty Images

Prokaryotic cells have a structure that is less complex than eukaryotic cells since they are the most primitive and earliest forms of life on the planet. They do not have a nucleus or region where the DNA is bound by a membrane. Prokaryotic DNA is coiled up in a region of the cytoplasm called the nucleoid. Like eukaryotic cells, prokaryotic cells contain a plasma membrane, cell wall, and cytoplasm. Unlike eukaryotic cells, prokaryotic cells do not contain membrane-bound organelles. However, they do contain some non-membranous organelles such as ribosomes, flagella, and plasmids (circular DNA structures that are not involved in reproduction). Examples of prokaryotic cells include bacteria and archaeans.

 Different Types of Cells: Prokaryotic and Eukaryotic

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Prokaryotic and Eukaryotic Cells
National Center for Biotechnology Information

By Heather ScovilleUpdated on May 04, 2019

The earth was formed about 4.6 billion years ago. For a very long period of the earth’s history, there was a very hostile and volcanic environment. It is difficult to imagine any life being viable in those types of conditions. It wasn’t until the end of the Precambrian Era of the Geologic Time Scale when life began to form.

There are several theories about how life first came to be on Earth. These theories include the formation of organic molecules within what is known as the “Primordial Soup”, life coming to Earth on asteroids (Panspermia Theory), or the first primitive cells forming in hydrothermal vents.

Prokaryotic Cells

The simplest type of cells were most likely the first type of cells that formed on Earth. These are called prokaryotic cells. All prokaryotic cells have a cell membrane surrounding the cell, cytoplasm where all of the metabolic processes happen, ribosomes that make proteins, and a circular DNA molecule called a nucleoid where the genetic information is held. The majority of prokaryotic cells also have a rigid cell wall that is used for protection. All prokaryotic organisms are unicellular, meaning the entire organism is only one cell.

Prokaryotic organisms are asexual, meaning they do not need a partner to reproduce. Most reproduce through a process called binary fission where basically the cell just splits in half after copying its DNA. This means that without mutations within the DNA, offspring are identical to their parent.

All organisms in the taxonomic domains Archaea and Bacteria are prokaryotic organisms. In fact, many of the species within the Archaea domain are found within hydrothermal vents. It is possible they were the first living organisms on Earth when life was first forming.

Eukaryotic Cells

The other, much more complex, type of cell is called the eukaryotic cell. Like prokaryotic cells, eukaryotic cells have cell membranes, cytoplasm, ribosomes, and DNA. However, there are many more organelles within eukaryotic cells. These include a nucleus to house the DNA, a nucleolus where ribosomes are made, rough endoplasmic reticulum for protein assembly, smooth endoplasmic reticulum for making lipids, Golgi apparatus for sorting and exporting proteins, mitochondria for creating energy, a cytoskeleton for structure and transporting information, and vesicles to move proteins around the cell. Some eukaryotic cells also have lysosomes or peroxisomes to digest waste, vacuoles for storing water or other things, chloroplasts for photosynthesis, and centrioles for splitting the cell during mitosis. Cell walls can also be found surrounding some types of eukaryotic cells.

Most eukaryotic organisms are multicellular. This allows the eukaryotic cells within the organism to become specialized. Through a process called differentiation, these cells take on characteristics and jobs that can work with other types of cells to create an entire organism. There are a few unicellular eukaryotes as well. These sometimes have tiny hair-like projections called cilia to brush away debris and may also have a long thread-like tail called a flagellum for locomotion.

The third taxonomic domain is called the Eukarya Domain. All eukaryotic organisms fall under this domain. This domain includes all animals, plants, protists, and fungi. Eukaryotes may use either asexual or sexual reproduction depending on the organism’s complexity. Sexual reproduction allows more diversity in offspring by mixing the genes of the parents to form a new combination and hopefully a more favorable adaptation for the environment.

The Evolution of Cells

Since prokaryotic cells are simpler than eukaryotic cells, it is thought they came into existence first. The currently accepted theory of cell evolution is called the Endosymbiotic Theory. It asserts that some of the organelles, namely the mitochondria and chloroplast, were originally smaller prokaryotic cells engulfed by larger prokaryotic cells.

White Blood Cells—Granulocytes and Agranulocytes

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White Blood Cells
 This photomicrograph of a blood smear reveals the presence of a few white blood cells.Dr. Candler Ballard / CDC

By Regina BaileyUpdated on October 09, 2019

White blood cells are blood components that protect the body from infectious agents. Also called leukocytes, white blood cells play an important role in the immune system by identifying, destroying, and removing pathogens, damaged cells, cancer cells, and foreign matter from the body.

Leukocytes originate from bone marrow stem cells and circulate in blood and lymph fluid. Leukocytes are able to leave blood vessels to migrate to body tissues.

White blood cells are categorized by the apparent presence or absence of granules (sacs containing digestive enzymes or other chemical substances) in their cytoplasm. If they have granules, they are considered granulocytes. If they do not, they are agranulocytes.

Key Takeaways

  • The primary purpose of white blood cells is to protect the body from infection.
  • White blood cells are produced by bone marrow and their levels of production are regulated by organs such as the spleen, liver, and kidneys.
  • Granulocytes and agranulocytes are the two types of white blood cells or leukocytes.
  • Granulocytes contain granules or sacs in their cytoplasm and agranulocytes do not. Each type of granulocyte and agranulocyte plays a slightly different role in fighting infection and disease.
  • The three types of granulocytes are neutrophils, eosinophils, and basophils.
  • The two types of agranulocytes are lymphocytes and monocytes.

White Blood Cell Production

White blood cells are produced within bones by bone marrow and some then mature in the lymph nodes, spleen, or thymus gland. Blood cell production is often regulated by body structures such as the lymph nodes, spleen, liver, and kidneys. The life span of mature leukocytes can be anywhere from a few hours to several days.

During times of infection or injury, more white blood cells are produced and sent into the blood. A blood test known as a white blood cell count or WBC is used to measure the number of white blood cells present in the blood. There are between 4,300-10,800 white blood cells present per microliter of blood in the average healthy person.

A low WBC count may be due to disease, radiation exposure, or bone marrow deficiency. A high WBC count may indicate the presence of an infectious or inflammatory disease, anemia, leukemia, stress, or tissue damage.


There are three types of granulocytes: neutrophils, eosinophils, and basophils. As seen under a microscope, the granules in these white blood cells are apparent when stained.

  • Neutrophils: These cells have a single nucleus with multiple lobes. Neutrophils are the most abundant white blood cell in circulation. They are chemically drawn to bacteria and migrate through tissue toward infection sites. Neutrophils are phagocytic, meaning that they engulf and destroy target cells. When released, their granules act as lysosomes to digest cellular macromolecules, destroying the neutrophil in the process.
  • Eosinophils: The nucleus of these cells is double-lobed and appears U-shaped in blood smears. Eosinophils are usually found in connective tissues of the stomach and intestines. These are also phagocytic and primarily target antigen-antibody complexes formed when antibodies bind to antigens to signal that they should be destroyed. Eosinophils are most active during parasitic infections and allergic reactions.
  • Basophils: Basophils are the least numerous type of white blood cells. They have a multi-lobed nucleus and their granules contain immune-boosting compounds such as histamine and heparin. Basophils are responsible for the body’s allergic response. Heparin thins the blood and inhibits blood clot formation while histamine dilates blood vessels to increase blood flow and the permeability of capillaries so that leukocytes may be transported to infected areas.


Lymphocytes and monocytes are the two types of agranulocytes or nongranular leukocytes. These white blood cells have no obvious granules. Agranulocytes typically have a larger nucleus due to the lack of noticeable cytoplasmic granules.

  • Lymphocytes: After neutrophils, lymphocytes are the most common type of white blood cell. These cells are spherical in shape with large nuclei and very little cytoplasm. There are three main types of lymphocytes: T cellsB cells, and natural killer cells. T cells and B cells are critical for specific immune responses and natural killer cells provide nonspecific immunity.
  • Monocytes: These cells are the greatest in size of the white blood cells. They have a large, single nucleus that comes in a variety of shapes but is most often kidney-shaped. Monocytes migrate from blood to tissue and develop into either macrophages and dendritic cells. 
    • Macrophages are large cells present in nearly all tissues. They actively perform phagocytic functions. 
    • Dendritic cells reside most often in the tissue of areas that come into contact with external antigens. They are found in the skinlungs, gastrointestinal tract, and inner layers of the nose. Dendritic cells function primarily to present antigenic information to lymphocytes in lymph nodes and lymph organs to aid in the development of antigen immunity. Dendritic cells are so named because they have projections that are similar in appearance to the dendrites of neurons.

Centromere and Chromosome Segregation

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 Chromosomes. Credit: MedicalRF.com/MedicalRF.com/Getty Images

By Regina BaileyUpdated on February 23, 2019

centromere is a region on a chromosome that joins sister chromatids. Sister chromatids are double-stranded, replicated chromosomes that form during cell division. The primary function of the centromere is to serve as a place of attachment for spindle fibers during cell division. The spindle apparatus elongates cells and separates chromosomes to ensure that each new daughter cell has the correct number of chromosomes at the completion of mitosis and meiosis.

The DNA in the centromere region of a chromosome is composed of tightly packed chromatin known as heterochromatin. Heterochromatin is very condensed and is therefore not transcribed. Due to its heterochromatin composition, the centromere region stains more darkly with dyes than the other regions of a chromosome.

Key Takeaways

  • Centromeres are regions on a chromosome that join sister chromatids whose primary function is for the attachment of spindle fibers in cell division.
  • While centromeres are typically located in the central area of a chromosome, they can also be located near the mid-region or at a number of different positions on the chromosome.
  • Specialized zones on centromeres called kinetochores attach the chromosomes to spindle fibers in prophase in mitosis.
  • Kinetochores have protein complexes that generate kinetochore fibers. These fibers help to orient and separate chromosomes during cell division.
  • In meiosis, in metaphase I, the centromeres of homologous chromosomes are oriented toward opposite cell poles while in meiosis II, spindle fibers extending from both cell poles attach to sister chromatids at their centromeres.

Centromere Location

A centromere is not always located in the central area of a chromosome. A chromosome is comprised of a short arm region (p arm) and a long arm region (q arm) that are connected by a centromere region. Centromeres may be located near the mid-region of a chromosome or at a number of positions along the chromosome. ​

  • Metacentric centromeres are located near the chromosome center.
  • Submetacentric centromeres are non-centrally located so that one arm is longer than the other.
  • Acrocentric centromeres are located near the end of a chromosome.
  • Telocentric centromeres are found at the end or telomere region of a chromosome.

The position of the centromere is readily observable in a human karyotype of homologous chromosomes. Chromosome 1 is an example of a metacentric centromere, chromosome 5 is an example of a submetacentric centromere, and chromosome 13 is an example of an acrocentric centromere.

Chromosome Segregation in Mitosis

  • Prior to the start of mitosis, the cell enters a stage known as interphase where it replicates its DNA in preparation for cell division. Sister chromatids are formed that are joined at their centromeres.
  • In prophase of mitosis, specialized regions on centromeres called kinetochores attach chromosomes to spindle polar fibers. Kinetochores are composed of a number of protein complexes that generate kinetochore fibers, which attach to spindle fibers. These fibers help to manipulate and separate chromosomes during cell division.
  • During metaphase, chromosomes are held at the metaphase plate by the equal forces of the polar fibers pushing on the centromeres.
  • During anaphase, paired centromeres in each distinct chromosome begin to move apart as daughter chromosomes are pulled centromere first toward opposite ends of the cell.
  • During telophase, newly formed nuclei enclose separated daughter chromosomes.

After cytokinesis (division of the cytoplasm), two distinct daughter cells are formed.

Chromosome Segregation in Meiosis

In meiosis, a cell goes through two stages of the dividing process. These stages are meiosis I and meiosis II.

  • During metaphase I, the centromeres of homologous chromosomes are oriented toward opposite cell poles. This means that homologous chromosomes will attach at their centromere regions to spindle fibers extending from only one of the two cell poles.
  • When spindle fibers shorten during anaphase I, homologous chromosomes are pulled toward opposite cell poles but sister chromatids remain together.
  • In meiosis II, spindle fibers extending from both cell poles attach to sister chromatids at their centromeres. Sister chromatids are separated in anaphase II when spindle fibers pull them toward opposite poles.

Meiosis results in the division, separation, and distribution of chromosomes among four new daughter cells. Each cell is haploid, containing only half the number of chromosomes as the original cell.

Centromere Anomalies

Centromeres play an important role by participating in the separation process for chromosomes. Their structure however, can make them possible sites for chromosome rearrangements. Keeping the integrity of centromeres intact is thus an important job for the cell. Centromere anomalies have been linked to various diseases like cancer.

A Definition and Explanation of the Steps in Exocytosis

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 In exocytosis, vesicles are carried to the cell membrane, fuse with the membrane, and contents are secreted into the extracellular environment.ttsz / iStock / Getty Images Plus

By Regina BaileyUpdated on February 05, 2020

Exocytosis is the process of moving materials from within a cell to the exterior of the cell. This process requires energy and is therefore a type of active transport. Exocytosis is an important process of plant and animal cells as it performs the opposite function of endocytosis. In endocytosis, substances that are external to a cell are brought into the cell.

In exocytosis, membrane-bound vesicles containing cellular molecules are transported to the cell membrane. The vesicles fuse with the cell membrane and expel their contents to the exterior of the cell. The process of exocytosis can be summarized in a few steps. 

Key Takeaways

  • During exocytosis, cells transport substances from the interior of the cell to the exterior of the cell.
  • This process is important for the removal of waste, for chemical messaging between cells, and for rebuilding the cell membrane.
  • Exocytotic vesicles are formed by the Golgi apparatus, endosomes, and pre-synaptic neurons.
  • Three pathways of exocytosis are constitutive exocytosis, regulated exocytosis, and lysosome mediated exocytosis.
  • Steps of exocytosis include vesicle trafficking, tethering, docking, priming, and fusing.
  • Vesicle fusion with the cell membrane may be complete or temporary.
  • Exocytosis occurs in many cells including pancreatic cells and neurons.

Basic Process of Exocytosis

  1. Vesicles containing molecules are transported from within the cell to the cell membrane.
  2. The vesicle membrane attaches to the cell membrane.
  3. Fusion of the vesicle membrane with the cell membrane releases the vesicle contents outside the cell.

Exocytosis serves several important functions as it allows cells to secrete waste substances and molecules, such as hormones and proteins. Exocytosis is also important for chemical signal messaging and cell to cell communication. In addition, exocytosis is used to rebuild the cell membrane by fusing lipids and proteins removed through endocytosis back into the membrane.

Exocytotic Vesicles

Golgi apparatus and Exocytosis
 The Golgi apparatus transports molecules out of the cell by exocytosis.ttsz / iStock / Getty Images Plus

Exocytotic vesicles containing protein products are typically derived from an organelle called the Golgi apparatus, or Golgi complex. Proteins and lipids synthesized in the endoplasmic reticulum are sent to Golgi complexes for modification and sorting. Once processed, the products are contained within secretory vesicles, which bud from the trans face of the Golgi apparatus.

Other vesicles that fuse with the cell membrane do not come directly from the Golgi apparatus. Some vesicles are formed from early endosomes, which are membrane sacs found in the cytoplasm. Early endosomes fuse with vesicles internalized by endocytosis of the cell membrane. These endosomes sort the internalized material (proteins, lipids, microbes, etc.) and direct the substances to their proper destinations. Transport vesicles bud off from early endosomes sending waste material on to lysosomes for degradation, while returning proteins and lipids to the cell membrane. Vesicles located at synaptic terminals in neurons are also examples of vesicles that are not derived from Golgi complexes.

Types of Exocytosis

 Exocytosis is a process for primary active transport across the cell membrane.Encyclopaedia Britannica / UIG / Getty Images

There are three common pathways of exocytosis. One pathway, constitutive exocytosis, involves the regular secretion of molecules. This action is performed by all cells. Constitutive exocytosis functions to deliver membrane proteins and lipids to the cell’s surface and to expel substances to the cell’s exterior.

Regulated exocytosis relies on the presence of extracellular signals for the expulsion of materials within vesicles. Regulated exocytosis occurs commonly in secretory cells and not in all cell types. Secretory cells store products such as hormones, neurotransmitters, and digestive enzymes that are released only when triggered by extracellular signals. Secretory vesicles are not incorporated into the cell membrane but fuse only long enough to release their contents. Once the delivery has been made, the vesicles reform and return to the cytoplasm.

A third pathway for exocytosis in cells involves the fusion of vesicles with lysosomes. These organelles contain acid hydrolase enzymes that break down waste materials, microbes, and cellular debris. Lysosomes carry their digested material to the cell membrane where they fuse with the membrane and release their contents into the extracellular matrix.

Steps of Exocytosis

Exocytosis Vesicle Transport
 Large molecules are carried across the cell membrane by vesicle transport in exocytosis.FancyTapis / iStock / Getty Images Plus

Exocytosis occurs in four steps in constitutive exocytosis and in five steps in regulated exocytosis. These steps include vesicle trafficking, tethering, docking, priming, and fusing.

  • Trafficking: Vesicles are transported to the cell membrane along microtubules of the cytoskeleton. Movement of the vesicles is powered by the motor proteins kinesins, dyneins, and myosins.
  • Tethering: Upon reaching the cell membrane, the vesicle becomes linked to and pulled into contact with the cell membrane.
  • Docking: Docking involves the attachment of the vesicle membrane with the cell membrane. The phospholipid bilayers of the vesicle membrane and cell membrane begin to merge.
  • Priming: Priming occurs in regulated exocytosis and not in constitutive exocytosis. This step involves specific modifications that must happen in certain cell membrane molecules for exocytosis to occur. These modifications are required for signaling processes that trigger exocytosis to take place.
  • Fusion: There are two types of fusion that can take place in exocytosis. In complete fusion, the vesicle membrane fully fuses with the cell membrane. The energy required to separate and fuse the lipid membranes comes from ATP. The fusion of the membranes creates a fusion pore, which allows the contents of the vesicle to be expelled as the vesicle becomes part of the cell membrane. In kiss-and-run fusion, the vesicle temporarily fuses with the cell membrane long enough to create a fusion pore and release its contents to the exterior of the cell. The vesicle then pulls away from the cell membrane and reforms before returning to the interior of the cell.

Exocytosis in the Pancreas

Exocytosis Pancreas
 The pancreas releases glucagon by exocytosis when blood glucose levels fall too low. Glucagon causes the liver to convert stored glycogen into glucose, which is released into the bloodstream.ttsz / iStock / Getty Images Plus

Exocytosis is used by a number of cells in the body as a means of transporting proteins and for cell to cell communication. In the pancreas, small clusters of cells called islets of Langerhans produce the hormones insulin and glucagon. These hormones are stored in secretory granules and released by exocytosis when signals are received.

When glucose concentration in the blood is too high, insulin is released from islet beta cells causing cells and tissues to take up glucose from the blood. When glucose concentrations are low, glucagon is secreted from islet alpha cells. This causes the liver to convert stored glycogen to glucose. Glucose is then released into the blood causing blood-glucose levels to rise. In addition to hormones, the pancreas also secretes digestive enzymes (proteases, lipases, amylases) by exocytosis.

Exocytosis in Neurons

Neuron Synapse
 Some neurons communicate through the transmission of neurotransmitters. A synaptic vesicle filled with neurotransmitters in the pre-synaptic neuron (above) fuses with the pre-synaptic membrane releasing neurotransmitters into the synaptic cleft (gap between neurons). The neurotransmitters can then bind to receptors on the post-synaptic neuron (below).Stocktrek Images / Getty Images

Synaptic vesicle exocytosis occurs in neurons of the nervous system. Nerve cells communicate by electrical or chemical (neurotransmitters) signals that are passed from one neuron to the next. Neurotransmitters are transmitted by exocytosis. They are chemical messages that are transported from nerve to nerve by synaptic vesicles. Synaptic vesicles are membranous sacs formed by endocytosis of the plasma membrane at pre-synaptic nerve terminals.

Once formed, these vesicles are filled with neurotransmitters and sent toward an area of the plasma membrane called the active zone. The synaptic vesicle awaits a signal, an influx of calcium ions brought on by an action potential, which allows the vesicle to dock at the pre-synaptic membrane. Actual fusion of the vesicle with the pre-synaptic membrane does not occur until a second influx of calcium ions occurs.

After receiving the second signal, the synaptic vesicle fuses with the pre-synaptic membrane creating a fusion pore. This pore expands as the two membranes become one and the neurotransmitters are released into the synaptic cleft (gap between the pre-synaptic and post-synaptic neurons). The neurotransmitters bind to receptors on the post-synaptic neuron. The post-synaptic neuron may either be excited or inhibited by the binding of the neurotransmitters.

Exocytosis versus Endocytosis

While exocytosis is a form of active transport that moves substances and materials from a cell’s interior to the exterior of the cell, endocytosis, is the mirror opposite. In endocytosis, substances and materials that are outside of a cell are transported into the interior of the cell. Like exocytosis, endocytosis requires energy so is also a form of active transport.

Like exocytosis, endocytosis has several different types. The different types are similar in that the basic underlying process involves the plasma membrane forming a pocket or invagination and surrounding the underlying substance that needs to be transported into the cell. There are three major types of endocytosis: phagocytosis, pinocytosis, as well as receptor mediated endocytosis.

Daughter Cells in Mitosis and Meiosis

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Cancer Cell Mitosis
 These cancer cells are undergoing cytokinesis (cell division). Cytokinesis occurs after nuclear division (mitosis), which produces two daughter nuclei. Mitosis produces two identical daughter cells.MAURIZIO DE ANGELIS / Science Photo Library / Getty Images

Table of Contents

By Regina BaileyUpdated on February 10, 2020

Daughter cells are cells that result from the division of a single parent cell. They are produced by the division processes of mitosis and meiosis. Cell division is the reproductive mechanism whereby living organisms grow, develop, and produce offspring.

At the completion of the mitotic cell cycle, a single cell divides forming two daughter cells. A parent cell undergoing meiosis produces four daughter cells. While mitosis occurs in both prokaryotic and eukaryotic organisms, meiosis occurs in eukaryotic animal cellsplant cells, and fungi.

Key Takeaways

  • Daughter cells are cells that are the result of a single dividing parent cell. Two daughter cells are the final result from the mitotic process while four cells are the final result from the meiotic process.
  • For organisms that reproduce via sexual reproduction, daughter cells result from meiosis. It is a two-part cell division process that ultimately produces an organism’s gametes. At the end of this process, the result is four haploid cells.
  • Cells have an error-checking and correcting process that helps to ensure the proper regulation of mitosis. If errors occur, cancerous cells that continue to divide may be the result.

Daughter Cells in Mitosis

daughter cells
 3d illustration depicting cell division, a process whereby a cell divides into two new daughter cells with the same genetic material. somersault18:24 / iStock / Getty Images Plus

Mitosis is the stage of the cell cycle that involves the division of the cell nucleus and the separation of chromosomes. The division process is not complete until after cytokinesis, when the cytoplasm is divided and two distinct daughter cells are formed. Prior to mitosis, the cell prepares for division by replicating its DNA and increasing its mass and organelle numbers. Chromosome movement occurs in the different phases of mitosis:

  • Prophase
  • Metaphase
  • Anaphase
  • Telophase

During these phases, chromosomes are separated, moved to opposite poles of the cell, and contained within newly formed nuclei. At the end of the division process, duplicated chromosomes are divided equally between two cells. These daughter cells are genetically identical diploid cells that have the same chromosome number and chromosome type.

Somatic cells are examples of cells that divide by mitosis. Somatic cells consist of all body cell types, excluding sex cells. The somatic cell chromosome number in humans is 46, while the chromosome number for sex cells is 23.

Daughter Cells in Meiosis

In organisms that are capable of sexual reproduction, daughter cells are produced by meiosis. Meiosis is a two part division process that produces gametes. The dividing cell goes through prophasemetaphaseanaphase, and telophase twice. At the end of meiosis and cytokinesis, four haploid cells are produced from a single diploid cell. These haploid daughter cells have half the number of chromosomes as the parent cell and are not genetically identical to the parent cell.

In sexual reproduction, haploid gametes unite in fertilization and become a diploid zygote. The zygote continues to divide by mitosis and develops into a fully functioning new individual.

Daughter Cells and Chromosome Movement

How do daughter cells end up with the appropriate number of chromosomes after cell division? The answer to this question involves the spindle apparatus. The spindle apparatus consists of microtubules and proteins that manipulate chromosomes during cell division. Spindle fibers attach to replicated chromosomes, moving and separating them when appropriate. The mitotic and meiotic spindles move chromosomes to opposite cell poles, ensuring that each daughter cell gets the correct number of chromosomes. The spindle also determines the location of the metaphase plate. This centrally localized site becomes the plane on which the cell eventually divides.

Daughter Cells and Cytokinesis

The final step in the process of cell division occurs in cytokinesis. This process begins during anaphase and ends after telophase in mitosis. In cytokinesis, the dividing cell is split into two daughter cells with the help of the spindle apparatus.

  • Animal Cells

In animal cells, the spindle apparatus determines the location of an important structure in the cell division process called the contractile ring. The contractile ring is formed from actin microtubule filaments and proteins, including the motor protein myosin. Myosin contracts the ring of actin filaments forming a deep groove called a cleavage furrow. As the contractile ring continues to contract, it divides the cytoplasm and pinches the cell in two along the cleavage furrow.

  • Plant Cells

Plant cells do not contain asters, star-shaped spindle apparatus microtubules, which help determine the site of the cleavage furrow in animal cells. In fact, no cleavage furrow is formed in plant cell cytokinesis. Instead, daughter cells are separated by a cell plate formed by vesicles that are released from Golgi apparatus organelles. The cell plate expands laterally and fuses with the plant cell wall forming a partition between the newly divided daughter cells. As the cell plate matures, it eventually develops into a cell wall.

Daughter Chromosomes

The chromosomes within daughter cells are termed daughter chromosomesDaughter chromosomes result from the separation of sister chromatids occuring in anaphase of mitosis and anaphase II of meiosis. Daughter chromosomes develop from the replication of single-stranded chromosomes during the synthesis phase (S phase) of the cell cycle. Following DNA replication, the single-stranded chromosomes become double-stranded chromosomes held together at a region called the centromere. Double-stranded chromosomes are known as sister chromatids. Sister chromatids are eventually separated during the division process and equally distributed among newly formed daughter cells. Each separated chromatid is known as a daughter chromosome.

Daughter Cells and Cancer

Cancer cell division
 Transmission electron micrograph (TEM) of a section through a cancer cell dividing by mitosis into two new daughter cells. Science Photo Library – STEVE GSCHMEISSNER / Brand X Pictures / Getty Images

Mitotic cell division is strictly regulated by cells to ensure that any errors are corrected and that cells divide properly with the correct number of chromosomes. Should mistakes occur in cell error checking systems, the resulting daughter cells may divide unevenly. While normal cells produce two daughter cells by mitotic division, cancer cells are distinguished for their ability to produce more than two daughter cells.

Three or more daughter cells may develop from dividing cancer cells and these cells are produced at a faster rate than normal cells. Due to the irregular division of cancer cells, daughter cells may also end up with too many or not enough chromosomes. Cancer cells often develop as a result of mutations in genes that control normal cell growth or that function to suppress cancer cell formation. These cells grow uncontrollably, exhausting the nutrients in the surrounding area. Some cancer cells even travel to other locations in the body via the circulatory system or lymphatic system.

Citric Acid Cycle Steps

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Schematic of the Citric Acid Cycle
Evelyn Bailey

By Regina BaileyUpdated on November 04, 2019

 The citric acid cycle, also known as the Krebs cycle or tricarboxylic acid (TCA) cycle, is the second stage of cellular respiration. This cycle is catalyzed by several enzymes and is named in honor of the British scientist Hans Krebs who identified the series of steps involved in the citric acid cycle. The usable energy found in the carbohydratesproteins, and fats we eat is released mainly through the citric acid cycle. Although the citric acid cycle does not use oxygen directly, it works only when oxygen is present.

Key Takeaways

  • The second stage of cellular respiration is called the citric acid cycle. It is also known as the Krebs cycle after Sir Hans Adolf Krebs who discovered its steps.
  • Enzymes play an important role in the citric acid cycle. Each step is catalyzed by a very specific enzyme.
  • In eukaryotes, the Krebs cycle uses a molecule of acetyl CoA to generate 1 ATP, 3 NADH, 1 FADH2, 2 CO2, and 3 H+.
  • Two molecules of acetyl CoA are produced in glycolysis so the total number of molecules produced in the citric acid cycle is doubled (2 ATP, 6 NADH, 2 FADH2, 4 CO2, and 6 H+).
  • Both the NADH and FADH2 molecules made in the Krebs cycle are sent to the electron transport chain, the last stage of cellular respiration.

The first phase of cellular respiration, called glycolysis,​ takes place in the cytosol of the cell’s cytoplasm. The citric acid cycle, however, occurs in the matrix of cell mitochondria. Prior to the beginning of the citric acid cycle, pyruvic acid generated in glycolysis crosses the mitochondrial membrane and is used to form acetyl coenzyme A (acetyl CoA). Acetyl CoA is then used in the first step of the citric acid cycle. Each step in the cycle is catalyzed by a specific enzyme.01of 10

Citric Acid

The two-carbon acetyl group of acetyl CoA is added to the four-carbon oxaloacetate to form the six-carbon citrate. The conjugate acid of citrate is citric acid, hence the name citric acid cycle. Oxaloacetate is regenerated at the end of the cycle so that the cycle may continue. 02of 10


Citrate loses a molecule of water and another is added. In the process, citric acid is converted to its isomer isocitrate. 03of 10

Isocitrate Dehydrogenase

Isocitrate loses a molecule of carbon dioxide (CO2) and is oxidized forming the five-carbon alpha ketoglutarate. Nicotinamide adenine dinucleotide (NAD+) is reduced to NADH + H+ in the process. 04of 10

Alpha Ketoglutarate Dehydrogenase

Alpha ketoglutarate is converted to the 4-carbon succinyl CoA. A molecule of CO2 is removed and NAD+ is reduced to NADH + H+ in the process. 05of 10

Succinyl-CoA Synthetase

CoA is removed from the succinyl CoA molecule and is replaced by a phosphate group. The phosphate group is then removed and attached to guanosine diphosphate (GDP) thereby forming guanosine triphosphate (GTP). Like ATP, GTP is an energy-yielding molecule and is used to generate ATP when it donates a phosphate group to ADP. The final product from the removal of CoA from succinyl CoA is succinate. 06of 10

Succinate Dehydrogenase

Succinate is oxidized and fumarate is formed. Flavin adenine dinucleotide (FAD) is reduced and forms FADH2 in the process. 07of 10


A water molecule is added and bonds between the carbons in fumarate are rearranged forming malate. 08of 10

Malate Dehydrogenase

Malate is oxidized forming oxaloacetate, the beginning substrate in the cycle. NAD+ is reduced to NADH + H+ in the process. 09of 10

Citric Acid Cycle Summary

Sir Hans Adolf Krebs
 Sir Hans Adolf Krebs (1900-1981), British biochemist who discovered the citric acid cycle (Krebs cycle). He won the Nobel Prize for physiology in 1953.Bettmann / Contributor / Bettmann / Getty Images

In eukaryotic cells, the citric acid cycle uses one molecule of acetyl CoA to generate 1 ATP, 3 NADH, 1 FADH2, 2 CO2, and 3 H+. Since two acetyl CoA molecules are generated from the two pyruvic acid molecules produced in glycolysis, the total number of these molecules yielded in the citric acid cycle is doubled to 2 ATP, 6 NADH, 2 FADH2, 4 CO2, and 6 H+. Two additional NADH molecules are also generated in the conversion of pyruvic acid to acetyl CoA prior to the start of the cycle. The NADH and FADH2 molecules produced in the citric acid cycle are passed along to the final phase of cellular respiration called the electron transport chain. Here NADH and FADH2 undergo oxidative phosphorylation to generate more ATP.

Biological Polymers: Proteins, Carbohydrates, Lipids

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Sugar Polymer
David Freund/Stockbyte/Getty Images

By Regina BaileyUpdated on November 27, 2019

Biological polymers are large molecules composed of many similar smaller molecules linked together in a chain-like fashion. The individual smaller molecules are called monomers. When small organic molecules are joined together, they can form giant molecules or polymers. These giant molecules are also called macromolecules. Natural polymers are used to build tissue and other components in living organisms.

Generally speaking, all macromolecules are produced from a small set of about 50 monomers. Different macromolecules vary because of the arrangement of these monomers. By varying the sequence, an incredibly large variety of macromolecules can be produced. While polymers are responsible for the molecular “uniqueness” of an organism, the common monomers are nearly universal.

The variation in the form of macromolecules is largely responsible for molecular diversity. Much of the variation that occurs both within an organism and among organisms can ultimately be traced to differences in macromolecules. Macromolecules can vary from cell to cell in the same organism, as well as from one species to the next. 01of 03


Nucleosome molecule, illustration

There are four basic kinds of biological macromolecules: carbohydrates, lipids, proteins, and nucleic acids. These polymers are composed of different monomers and serve different functions.

  • Carbohydrates: molecules composed of sugar monomers. They are necessary for energy storage. Carbohydrates are also called saccharides and their monomers are called monosaccharides. Glucose is an important monosaccharide that is broken down during cellular respiration to be used as an energy source. Starch is an example of a polysaccharide (many saccharides linked together) and is a form of stored glucose in plants.
  • Lipids: water-insoluble molecules that can be classified as fatsphospholipids, waxes, and steroids. Fatty acids are lipid monomers that consist of a hydrocarbon chain with a carboxyl group attached at the end. Fatty acids form complex polymers such as triglycerides, phospholipids, and waxes. Steroids are not considered true lipid polymers because their molecules do not form a fatty acid chain. Instead, steroids are composed of four fused carbon ring-like structures. Lipids help to store energy, cushion and protect organs, insulate the body, and form cell membranes.
  • Proteins: biomolecules capable of forming complex structures. Proteins are composed of amino acid monomers and have a wide variety of functions including transportation of molecules and muscle movement. Collagen, hemoglobin, antibodies, and enzymes are examples of proteins.
  • Nucleic Acids: molecules consisting of nucleotide monomers linked together to form polynucleotide chains. DNA and RNA are examples of nucleic acids. These molecules contain instructions for protein synthesis and allow organisms to transfer genetic information from one generation to the next.

02of 03

Assembling and Disassembling Polymers

Low-density lipoproteins, illustration

While there is variation among the types of biological polymers found in different organisms, the chemical mechanisms for assembling and disassembling them are largely the same across organisms.

Monomers are generally linked together through a process called dehydration synthesis, while polymers are disassembled through a process called hydrolysis. Both of these chemical reactions involve water.

In dehydration synthesis, bonds are formed linking monomers together while losing water molecules. In hydrolysis, the water interacts with a polymer causing bonds that link monomers to each other to be broken.urn:uuid:94b6cfcd-26c5-4150-c5fa-415026c594b603of 03

Synthetic Polymers

Water Drops on A Pan
MirageC / Getty Images

Unlike natural polymers, which are found in nature, synthetic polymers are made by humans. They are derived from petroleum oil and include products such as nylon, synthetic rubbers, polyester, Teflon, polyethylene, and epoxy.

Synthetic polymers have a number of uses and are widely used in household products. These products include bottles, pipes, plastic containers, insulated wires, clothing, toys, and non-stick pans.

Facts About Cells

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A cell's inability to undergo apoptosis can result in the development of cancer cells, such as this human breast cancer cell
 Human breast cancer cell. Cultura Science / Rolf Ritter / Oxford Scientific / Getty Images

By Regina BaileyUpdated on October 08, 2019

Cells are the fundamental units of life. Whether they be unicellular or multicellular life forms, all living organisms are composed of and depend on cells to function normally. Scientists estimate that our bodies contain anywhere from 75 to 100 trillion cells. In addition, there are hundreds of different types of cells in the body. Cells do everything from providing structure and stability to providing energy and a means of reproduction for an organism. The following 10 facts about cells will provide you with well known and perhaps little known tidbits of information about cells.

Key Takeaways

  • Cells are the basic units of life and are very small in size, ranging from approximately 1 to 100 micrometers. Advanced microscopes allow scientists to be able to see such small entities.
  • There are two major types of cells: eukaryotic and prokaryotic. Eukaryotic cells have a membrane bound nucleus while prokaryotic cells do not have a nucleus that is membrane bound.
  • A cell’s nucleoid region or nucleus contains the cell’s DNA (deoxyribonucleic acid) which contains the cell’s encoded genetic information.
  • Cells reproduce by different methods. Most prokaryotic cells reproduce by binary fission while eukaryotic cells can reproduce asexually or sexually.

Cells are too Small to be Seen Without Magnification

 Biologists are able to obtain detailed observations of cells with microscopes. PeopleImages / E+ / Getty Images

Cells range in size from 1 to 100 micrometers. The study of cells, also called cell biology, would not have been possible without the invention of the microscope. With the advanced microscopes of today, such as the Scanning Electron Microscope and Transmission Electron Microscope, cell biologists are able to obtain detailed images of the smallest of cell structures.

Primary Types of Cells

Eukaryotic and prokaryotic cells are the two main types of cells. Eukaryotic cells are called so because they have a true nucleus that is enclosed within a membrane. Animalsplantsfungi, and protists are examples of organisms that contain eukaryotic cells. Prokaryotic organisms include bacteria and archaeans. The prokaryotic cell nucleus is not enclosed within a membrane.

Prokaryotic Single-Celled Organisms were the Earliest and Most Primitive Forms of Life on Earth

Prokaryotes can live in environments that would be deadly to most other organisms. These extremophiles are able to live and thrive in various extreme habitats. Archaeans for example, live in areas such as hydrothermal vents, hot springs, swamps, wetlands, and even animal intestines.

There are More Bacterial Cells in the Body than Human Cells

Scientists have estimated that about 95% of all the cells in the body are bacteria. The vast majority of these microbes can be found within the digetive tract. Billions of bacteria also live on the skin.

Cells Contain Genetic Material

Cells contain DNA (deoxyribonucleic acid) and RNA (ribonucleic acid), the genetic information necessary for directing cellular activities. DNA and RNA are molecules known as nucleic acids. In prokaryotic cells, the single bacterial DNA molecule is not separated from the rest of the cell but coiled up in a region of the cytoplasm called the nucleoid region. In eukaryotic cells, DNA molecules are located within the cell’s nucleus. DNA and proteins are the major components of chromosomes. Human cells contain 23 pairs of chromosomes (for a total of 46). There are 22 pairs of autosomes (non-sex chromosomes) and one pair of sex chromosomes. The X and Y sex chromosomes determine sex.

Organelles Which Carry Out Specific Functions

Organelles have a wide range of responsibilities within a cell that include everything from providing energy to producing hormones and enzymes. Eukaryotic cells contain several types of organelles, while prokaryotic cells contain a few organelles (ribosomes) and none that are bound by a membrane. There are also differences between the kinds of organelles found within different eukaryotic cell types. Plant cells for example, contain structures such as a cell wall and chloroplasts that are not found in animal cells. Other examples of organelles include:

Reproduce Through Different Methods

Most prokaryotic cells replicate by a process called binary fission. This is a type of cloning process in which two identical cells are derived from a single cell. Eukaryotic organisms are also capable of reproducing asexually through mitosis. In addition, some eukaryotes are capable of sexual reproduction. This involves the fusion of sex cells or gametes. Gametes are produced by a process called meiosis.

Groups of Similar Cells Form Tissues

Tissues are groups of cells with both a shared structure and function. Cells that make up animal tissues are sometimes woven together with extracellular fibers and are occasionally held together by a sticky substance that coats the cells. Different types of tissues can also be arranged together to form organs. Groups of organs can in turn form organ systems.

Varying Life Spans

Cells within the human body have different life spans based on the type and function of the cell. They can live anywhere from a few days to a year. Certain cells of the digestive tract live for only a few days, while some immune system cells can live for up to six weeks. Pancreatic cells can live for as long as a year.

Cells Commit Suicide

cell apoptosis
 Cell Apoptosis. Dr_Microbe / iStock / Getty Images Plus

When a cell becomes damaged or undergoes some type of infection, it will self destruct by a process called apoptosis. Apoptosis works to ensure proper development and to keep the body’s natural process of mitosis in check. A cell’s inability to undergo apoptosis can result in the development of cancer.

Regeneration of Brain Cells

Brave New World of Adult Neurogenesis

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Brain Neural Network
Alfred Pasieka / Science Photo Library / Getty Images

By Regina BaileyUpdated on July 28, 2019

For almost 100 years, it had been a mantra of biology that brain cells or neurons do not regenerate. It was thought that all your significant brain development happened from conception to age 3. Contrary to that widely held popular belief, scientists now know that neurogenesis continuously occurs in specific regions in the adult brain.urn:uuid:c624f0a5-e29d-dbbc-f3fa-dbbce29dc624

In a startling scientific discovery made in the late 1990s, researchers at Princeton University found that new neurons were continually being added to the brains of adult monkeys. The finding was significant because monkeys and humans have similar brain structures.

These findings and several others looking at cell regeneration in other parts of the brain opened up a whole new line of research about “adult neurogenesis,” the process of the birth of neurons from neural stem cells in a mature brain. 

Pivotal Research on Monkeys

Princeton researchers first found cell regeneration in the hippocampus and the subventricular zone of the lateral ventricles in monkeys, which are important structures for memory formation and functions of the central nervous system. 

This was significant but not quite as important as the 1999 finding of neurogenesis in the cerebral cortex section of the monkey brain. The cerebral cortex is the most complex part of the brain and scientists were startled to find neuron formation in this high-function brain area. The lobes of the cerebral cortex are responsible for higher-level decision making and learning.

Adult neurogenesis was discovered in three areas of the cerebral cortex:

  • The prefrontal region, which controls decision-making
  • The inferior temporal region, which plays a role in visual recognition
  • The posterior parietal region, which plays a role in 3D representation

Researchers believed that these results called for a fundamental reassessment of the development of the primate brain. Although the cerebral cortex research had been pivotal for advancing scientific research in this area, the finding remains controversial since it has not yet been proved to occur in the human brain.

Human Research

Since the Princeton primate studies, newer research has shown that human cell regeneration occurs in the olfactory bulb, which is responsible for sensory information for the sense of smell, and the dentate gyrus, a part of the hippocampus responsible for memory formation.

Continued research on adult neurogenesis in humans has found that other areas of the brain may also generate new cells, particularly in the amygdala and the hypothalamus. The amygdala is the part of the brain governing emotions. The hypothalamus helps maintain the autonomic nervous system and the hormone activity of the pituitary, which controls body temperature, thirst, and hunger and is also involved in sleep and emotional activity.

Researchers are optimistic that with further study scientists might one day unlock the key to this process of brain cell growth and use the knowledge to treat a variety of psychiatric disorders and brain diseases, like Parkinson’s and Alzheimer’s.

HeLa Cells and Its Important

The World’s First Immortal Human Cell Line

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HeLa cervical cancer cells were the first immortal cell line.
 HeLa cervical cancer cells were the first immortal cell line. HeitiPaves / Getty Images

By Anne Marie Helmenstine, Ph.D.Updated on November 01, 2018

HeLa cells are the first immortal human cell line. The cell line grew from a sample of cervical cancer cells taken from an African-American woman named Henrietta Lacks on February 8, 1951. The lab assistant responsible for the samples named cultures based on the first two letters of a patient’s first and last name, thus the culture was dubbed HeLa. In 1953, Theodore Puck and Philip Marcus cloned HeLa (the first human cells to be cloned) and freely donated samples to other researchers. The cell line’s initial use was in cancer research, but HeLa cells have led to numerous medical breakthroughs and nearly 11,000 patents.

Key Takeaways: HeLa Cells

  • HeLa cells are the first immortal human cell line.
  • The cells came from a cervical cancer sample obtained from Henrietta Lack in 1951, without her knowledge or permission.
  • HeLa cells have led to many important scientific discoveries, yet there are disadvantages to working with them.
  • HeLa cells have led to the examination of the ethical considerations of working with human cells.

What It Means to Be Immortal

Normally, human cell cultures die within a few days after a set number of cell divisions via a process called senescence. This presents a problem for researchers because experiments using normal cells cannot be repeated on identical cells (clones), nor can the same cells be used for extended study. Cell biologist George Otto Gey took one cell from Henrietta Lack’s sample, allowed that cell to divide, and found the culture survived indefinitely if given nutrients and a suitable environment. The original cells continued to mutate. Now, there are many strains of HeLa, all derived from the same single cell.urn:uuid:22e66d32-c0bd-cafd-679b-cafdc0bd22e6

Researchers believe the reason HeLa cells don’t suffer programmed death is because they maintain a version of the enzyme telomerase that prevents gradual shortening of the telomeres of chromosomes. Telomere shortening is implicated in aging and death.

Notable Achievements Using HeLa Cells

HeLa cells have been used to test the effects of radiation, cosmetics, toxins, and other chemicals on human cells. They have been instrumental in gene mapping and studying human diseases, especially cancer. However, the most significant application of HeLa cells may have been in the development of the first polio vaccine. HeLa cells were used to maintain a culture of polio virus in human cells. In 1952, Jonas Salk tested his polio vaccine on these cells and used them to mass-produce it.

Disadvantages of Using HeLa Cells

While the HeLa cell line has led to amazing scientific breakthroughs, the cells can also cause problems. The most significant issue with HeLa cells is how aggressively they can contaminate other cell cultures in a laboratory. Scientists don’t routinely test the purity of their cell lines, so HeLa had contaminated many in vitro lines (estimated 10 to 20 percent) before the problem was identified. Much of the research conducted on contaminated cell lines had to be thrown out. Some scientists refuse to allow HeLa in their labs in order to control the risk.

Another problem with HeLa is that it doesn’t have a normal human karyotype (the number and appearance of chromosomes in a cell). Henrietta Lacks (and other humans) have 46 chromosomes (diploid or a set of 23 pairs), while the HeLa genome consists of 76 to 80 chromosome (hypertriploid, including 22 to 25 abnormal chromosomes). The extra chromosomes came from the infection by human papilloma virus that led to cancer. While HeLa cells resemble normal human cells in many ways, they are neither normal nor entirely human. Thus, there are limitations to their use.

Issues of Consent and Privacy

The birth of the new field of biotechnology introduced ethical considerations. Some modern laws and policies arose from ongoing issues surrounding HeLa cells.

As was the norm at the time, Henrietta Lacks was not informed her cancer cells were going to be used for research. Years after the HeLa line had become popular, scientists took samples from other members of the Lacks family, but they did not explain the reason for the tests. In the 1970s, the Lacks family was contacted as scientists sought to understand the reason for the aggressive nature of the cells. They finally knew about HeLa. Yet, in 2013, German scientists mapped the entire HeLa genome and made it public, without consulting the Lacks family.

Informing a patient or relatives about the use of samples obtained via medical procedures was not required in 1951, nor is it required today. The 1990 Supreme Court of California case of Moore v. Regents of the University of California ruled a person’s cells are not his or her property and may be commercialized.

Yet, the Lacks family did reach an agreement with the National Institutes of Health (NIH) regarding access to the HeLa genome. Researchers receiving funds from the NIH must apply for access to the data. Other researchers are not restricted, so data about the Lacks’ genetic code is not completely private.

While human tissue samples continue to be stored, specimens are now identified by an anonymous code. Scientists and legislators continue to wrangle with questions of security and privacy, as genetic markers may lead to clues about an involuntary donor’s identity.

Role of Centrioles in Microbiology

Tiny Structures Play Big Part in Cell Division and Mitosis

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Conceptual image of centriole.
Stocktrek Images / Getty Images

By Regina BaileyUpdated on September 01, 2019

In microbiology, centrioles are cylindrical cell structures that are composed of groupings of microtubules, which are tube-shaped molecules or strands of protein. Without centrioles, chromosomes would not be able to move during the formation of new cells. 

Centrioles help to organize the assembly of microtubules during cell division. To put it simply, chromosomes use the centriole’s microtubules as a highway during the cell division process.

Where Centrioles Are Found

Centrioles are found in all animal cells and only a few species of lower plant cells. Two centrioles—a mother centriole and a daughter centriole—are found within the cell in a structure called a centrosome. 


Most centrioles are made up of nine sets of microtubule triplets, with the exception of some species, such as crabs which have nine sets of microtubule doublets. There are a few other species that deviate from the standard centriole structure. Microtubules are composed of a single type of globular protein called tubulin.

Two Main Functions

During mitosis or cell division, the centrosome and centrioles replicate and migrate to opposite ends of the cell. Centrioles help to arrange the microtubules that move chromosomes during cell division to ensure each daughter cell receives the appropriate number of chromosomes. 

Centrioles are also important for the formation of cell structures known as cilia and flagella. Cilia and flagella, found on the outside surface of cells, aid in cellular movement. A centriole combined with several additional protein structures is modified to become a basal body. Basal bodies are the anchoring sites for moving cilia and flagella.

Important Role in Cell Division

Centrioles are located outside of, but near the cell nucleus. In cell division, there are several phases: in order of occurrence they are interphase, prophase, metaphase, anaphase, and telophase. Centrioles have a very important role to play in all phases of cell division. The end goal is in moving replicated chromosomes into a newly created cell.

Interphase and Replication

In the first phase of mitosis, called interphase, centrioles replicate. This is the phase immediately prior to cell division, which marks the start of mitosis and meiosis in the cell cycle.

Prophase and Asters and the Mitotic Spindle

In prophase, each centrosome with centrioles migrates toward opposite ends of the cell. A single pair of centrioles is positioned at each cell pole. The mitotic spindle initially appears as structures called asters which surround each centriole pair. Microtubules form spindle fibers that extend from each centrosome, thereby separating centriole pairs and elongating the cell.

You can think of these fibers as a newly paved highway for the replicated chromosomes to move into the newly formed cell. In this analogy, the replicated chromosomes are a car along the highway.

 Metaphase and Positioning of Polar Fibers

In metaphase, centrioles help to position polar fibers as they extend from the centrosome and position chromosomes along the metaphase plate. In keeping with the highway analogy, this keeps the lane straight.

Anaphase and the Sister Chromatids

In anaphase, polar fibers connected to chromosomes shorten and separate the sister chromatids (replicated chromosomes). The separated chromosomes are pulled toward opposite ends of the cell by polar fibers extending from the centrosome.

At this point in the highway analogy, it is as if one car on the highway has replicated a second copy and the two cars begin moving away from each other, in opposite directions, on the same highway.

Telophase and Two Genetically Identical Daughter Cells

In telophase, the spindle fibers disperse as the chromosomes are cordoned off into distinct new nuclei. After cytokinesis, which is the division of the cell’s cytoplasm, two genetically identical daughter cells are produced each containing one centrosome with one centriole pair.

In this final phase, using the car and highway analogy, the two cars look exactly the same, but are now completely separate and have gone their separate ways.

What Is RNA?

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RNA polymerase
 This illustration shows the process of transcription of deoxyribonucleic acid (DNA, blue) to produce a complementary copy of ribonucleic acid (RNA, green). This is done by the enzyme RNA polymerase (purple). Gunilla Elam / Science Photo Library / Getty Images Plus

By Regina BaileyUpdated on February 05, 2020

RNA molecules are single-stranded nucleic acids composed of nucleotides. RNA plays a major role in protein synthesis as it is involved in the transcription, decoding, and translation of the genetic code to produce proteins. RNA stands for ribonucleic acid and like DNA, RNA nucleotides contain three components:

  • A Nitrogenous Base
  • A Five-Carbon Sugar
  • A Phosphate Group

Key Takeaways

  • RNA is a single-stranded nucleic acid that is composed of three main elements: a nitrogenous base, a five-carbon sugar and a phosphate group.
  • Messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA) are the three major types of RNA.
  • mRNA is involved in the transcription of DNA while tRNA has an important role in the translation component of protein synthesis.
  • As the name implies, ribosomal RNA (rRNA) is found on ribosomes.
  • A less common type of RNA known as small regulatory RNAs possess the ability to regulate the expression of genes. MicroRNAs, a type of regulatory RNA, have also been linked to the development of some types of cancer.

RNA nitrogenous bases include adenine (A)guanine (G)cytosine (C) and uracil (U). The five-carbon (pentose) sugar in RNA is ribose. RNA molecules are polymers of nucleotides joined to one another by covalent bonds between the phosphate of one nucleotide and the sugar of another. These linkages are called phosphodiester linkages.
Although single-stranded, RNA is not always linear. It has the ability to fold into complex three-dimensional shapes and form hairpin loops. When this occurs, the nitrogenous bases bind to one another. Adenine pairs with uracil (A-U) and guanine pairs with cytosine (G-C). Hairpin loops are commonly observed in RNA molecules such as messenger RNA (mRNA) and transfer RNA (tRNA).

Types of RNA

RNA Hairpin Loop
 Although single stranded, RNA is not always linear. It has the ability to fold into complex three dimensional shapes and form hairpin loops. Double-stranded RNA (or dsRNA), as is seen here, can be used to block the expression of specific genes.EQUINOX GRAPHICS / Science Photo Library / Getty Images

RNA molecules are produced in the nucleus of our cells and can also be found in the cytoplasm. The three primary types of RNA molecules are messenger RNA, transfer RNA and ribosomal RNA.

  • Messenger RNA (mRNA) plays an important role in the transcription of DNA. Transcription is the process in protein synthesis that involves copying the genetic information contained within DNA into an RNA message. During transcription, certain proteins called transcription factors unwind the DNA strand and allow the enzyme RNA polymerase to transcribe only a single strand of DNA. DNA contains the four nucleotide bases adenine (A), guanine (G), cytosine (C) and thymine (T) which are paired together (A-T and C-G). When RNA polymerase transcribes the DNA into a mRNA molecule, adenine pairs with uracil and cytosine pairs with guanine (A-U and C-G). At the end of transcription, mRNA is transported to the cytoplasm for the completion of protein synthesis.
  • Transfer RNA (tRNA) plays an important role in the translation portion of protein synthesis. Its job is to translate the message within the nucleotide sequences of mRNA into specific amino acid sequences. The amino acid sequences are joined together to form a protein. Transfer RNA is shaped like a clover leaf with three hairpin loops. It contains an amino acid attachment site on one end and a special section in the middle loop called the anticodon site. The anticodon recognizes a specific area on mRNA called a codon. A codon consists of three continuous nucleotide bases that code for an amino acid or signal the end of translation. Transfer RNA along with ribosomes read the mRNA codons and produce a polypeptide chain. The polypeptide chain undergoes several modifications before becoming a fully functioning protein.
  • Ribosomal RNA (rRNA) is a component of cell organelles called ribosomes. A ribosome consists of ribosomal proteins and rRNA. Ribosomes are typically composed of two subunits: a large subunit and a small subunit. Ribosomal subunits are synthesized in the nucleus by the nucleolus. Ribosomes contain a binding site for mRNA and two binding sites for tRNA located in the large ribosomal subunit. During translation, a small ribosomal subunit attaches to a mRNA molecule. At the same time, an initiator tRNA molecule recognizes and binds to a specific codon sequence on the same mRNA molecule. A large ribosomal subunit then joins the newly formed complex. Both ribosomal subunits travel along the mRNA molecule translating the codons on mRNA into a polypeptide chain as they go. Ribosomal RNA is responsible for creating the peptide bonds between the amino acids in the polypeptide chain. When a termination codon is reached on the mRNA molecule, the translation process ends. The polypeptide chain is released from the tRNA molecule and the ribosome splits back into large and small subunits.



Some RNAs, known as small regulatory RNAs, have the ability to regulate gene expression. MicroRNAs (miRNAs) are a type of regulatory RNA that can inhibit gene expression by halting translation. They do so by binding to a specific location on mRNA, preventing the molecule from being translated. MicroRNAs have also been linked to the development of some types of cancers and a particular chromosome mutation called a translocation.urn:uuid:552925c2-8724-4c79-2fe7-4c7987245529

Transfer RNA

Transfer RNA
 Transfer RNA.Darryl Leja / NHGRI

Transfer RNA (tRNA) is an RNA molecule that assists in protein synthesis. Its unique shape contains an amino acid attachment site on one end of the molecule and an anticodon region on the opposite end of the amino acid attachment site. During translation, the anticodon region of tRNA recognizes a specific area on messenger RNA (mRNA) called a codon. A codon consists of three continuous nucleotide bases that specify a particular amino acid or signal the end of translation. The tRNA molecule forms base pairs with its complementary codon sequence on the mRNA molecule. The attached amino acid on the tRNA molecule is therefore placed in its proper position in the growing protein chain.

How Antibodies Defend Your Body

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Immunoglobulin G Antibody
 Immunoglobulin G is the most abundant immunoglobulin. ALFRED PASIEKA/Science Photo Library/Getty Images

By Regina BaileyUpdated on December 04, 2019

Antibodies (also called immunoglobulins) are specialized proteins that travel through the bloodstream and are found in bodily fluids. They are used by the immune system to identify and defend against foreign intruders to the body.

These foreign intruders, or antigens, include any substance or organism that evokes an immune response.

Examples of antigens that cause immune responses include

Antibodies recognize specific antigens by identifying certain areas on the surface of the antigen known as antigenic determinants. Once the specific antigenic determinant is recognized, the antibody will bind to the determinant. The antigen is tagged as an intruder and labeled for destruction by other immune cells. Antibodies protect against substances prior to cell infection.


Antibodies are produced by a type of white blood cell called a B cell (B lymphocyte). B cells develop from stem cells in bone marrow. When B cells become activated due to the presence of a particular antigen, they develop into plasma cells.

Plasma cells create antibodies specific to a certain antigen. Plasma cells generate the antibodies essential to the branch of the immune system known as the humoral immune system. Humoral immunity relies on the circulation of antibodies in bodily fluids and blood serum to identify and counteract antigens.

When an unfamiliar antigen is detected in the body, it can take up to two weeks before plasma cells can generate enough antibodies to counteract the specific antigen. Once the infection is under control, antibody production decreases and a small sample of antibodies remain in circulation. If this particular antigen should appear again, the antibody response will be much quicker and more forceful.urn:uuid:257612a6-5c13-f10c-1d8d-f10c5c132576


An antibody or immunoglobulin (Ig) is a Y-shaped molecule. It consists of two short polypeptide chains called light chains and two longer polypeptide chains called heavy chains.

The two light chains are identical to each other and the two heavy chains are identical. At the ends of both the heavy and light chains, in the areas that form the arms of the Y-shaped structure, are regions known as antigen-binding sites.

The antigen-binding site is the area of the antibody that recognizes the specific antigenic determinant and binds to the antigen. Since different antibodies recognize different antigens, antigen-binding sites are different for different antibodies. This area of the molecule is known as the variable region. The stem of the Y-shaped molecule is formed by the longer region of the heavy chains. This region is called the constant region.urn:uuid:9ab47553-eb78-587b-7924-587beb789ab4

Classes of Antibodies

Five primary classes of antibodies exist with each class playing a distinct role in the human immune response. These classes are identified as IgG, IgM, IgA, IgD, and IgE. Immunoglobulin classes differ in the structure of the heavy chains in each molecule.

Immunoglobulins (Ig)

  • IgG: These molecules are the most plentiful in circulation. They can cross blood vessels and even the placenta to provide protection to a fetus. The heavy chain type in IgG is a gamma chain.
  • IgM: Of all of the immunoglobulins, these are the most massive. They contain five Y-shaped sections each with two light chains and two heavy chains. Each Y-shaped section is attached to a joining unit called a J chain. IgM molecules play a major role in the primary immune response as the initial respondents to new antigens in the body. The heavy chain type in IgM is a mu chain.
  • IgA: Located mainly in body fluids such as sweat, saliva, and mucus, these antibodies prevent antigens from infecting cells and entering the circulatory system. The heavy chain type in IgA is an alpha chain.
  • IgD: The role of these antibodies in the immune response is currently unknown. IgD molecules are located on the surface membranes of mature B cells. The heavy chain type in IgD is a delta chain.
  • IgE: Found mostly in saliva and mucus, these antibodies are involved in allergic responses to antigens. The heavy chain type in IgE is an epsilon chain.

There are also a few subclasses of immunoglobulins in humans. The differences in subclasses are based on small variations in the heavy chain units of antibodies in the same class. The light chains found in immunoglobulins exist in two major forms. These light chain types are identified as kappa and lambda chains.

What Are Lysosomes and How Are They Formed?

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Lysosome rendering
Stocktrek Images/Getty Images

By Regina BaileyUpdated on August 28, 2018

There are two primary types of cells: prokaryotic and eukaryotic cells. Lysosomes are organelles that are found in most animal cells and act as the digesters of a eukaryotic cell.

What Are Lysosomes?

Lysosomes are spherical membranous sacs of enzymes. These enzymes are acidic hydrolase enzymes that can digest cellular macromolecules. The lysosome membrane helps to keep its internal compartment acidic and separates the digestive enzymes from the rest of the cell. Lysosome enzymes are made by proteins from the endoplasmic reticulum and enclosed within vesicles by the Golgi apparatus. Lysosomes are formed by budding from the Golgi complex.urn:uuid:f82c82bb-b35f-1806-8c74-1806b35ff82c

Lysosome Enzymes

Lysosomes contain various hydrolytic enzymes (around 50 different enzymes) that are capable of digesting nucleic acids, polysaccharides, lipids, and proteins. The inside of a lysosome is kept acidic as the enzymes within work best in an acidic environment. If a lysosome’s integrity is compromised, the enzymes would not be very harmful in the cell’s neutral cytosol.

Lysosome Formation

Lysosomes are formed from the fusion of vesicles from the Golgi complex with endosomes. Endosomes are vesicles that are formed by endocytosis as a section of the plasma membrane pinches off and is internalized by the cell. In this process, extracellular material is taken up by the cell. As endosomes mature, they become known as late endosomes. Late endosomes fuse with transport vesicles from the Golgi that contain acid hydrolases. Once fused, these endosomes eventually develop into lysosomes.

Lysosome Function

Lysosomes act as the “garbage disposal” of a cell. They are active in recycling the cell’s organic material and in the intracellular digestion of macromolecules. Some cells, such as white blood cells, have many more lysosomes than others. These cells destroy bacteria, dead cells, cancerous cells, and foreign matter through cell digestion. Macrophages engulf matter by phagocytosis and enclose it within a vesicle called a phagosome. Lysosomes within the macrophage fuse with the phagosome releasing their enzymes and forming what is known as a phagolysosome. The internalized material is digested within the phagolysosome. Lysosomes are also necessary for the degradation of internal cell components such as organelles. In many organisms, lysosomes are also involved in programmed cell death.

Lysosome Defects

In humans, a variety of inherited conditions can affect lysosomes. These gene mutation defects are called storage diseases and include Pompe’s disease, Hurler Syndrome, and Tay-Sachs disease. People with these disorders are missing one or more of the lysosomal hydrolytic enzymes. This results in the inability of macromolecules to be properly metabolized within the body.

Similar Organelles

Like lysosomes, peroxisomes are membrane-bound organelles that contain enzymes. Peroxisome enzymes produce hydrogen peroxide as a by-product. Peroxisomes are involved in at least 50 different biochemical reactions in the body. They help to detoxify alcohol in the liver, form bile acid, and break down fats.

Eukaryotic Cell Structures

In addition to lysosomes, the following organelles and cell structures can also be found in eukaryotic cells:

  • Cell membrane: Protects the integrity of the interior of the cell.
  • Centrioles: Help to organize the assembly of microtubules.
  • Cilia and Flagella: Aid in cellular locomotion.
  • Chromosomes: Carry heredity information in the form of DNA.
  • Cytoskeleton: A network of fibers that support the cell.
  • Endoplasmic Reticulum: Synthesizes carbohydrates and lipids.
  • Nucleus: Controls cell growth and reproduction.
  • Ribosomes: Involved in protein synthesis.
  • Mitochondria: Provide energy for the cell.

Sister Chromatids: Definition and Example

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Chromosomes, artwork
Science Photo Library – SCIEPRO / Getty Images

By Regina BaileyUpdated on January 23, 2019

Definition: Sister chromatids are two identical copies of a single replicated chromosome that are connected by a centromere. Chromosome replication takes place during interphase of the cell cycleDNA is synthesized during the S phase or synthesis phase of interphase to ensure that each cell ends up with the correct number of chromosomes after cell division. The paired chromatids are held together at the centromere region by a special protein ring and remain joined until a later stage in the cell cycle. Sister chromatids are considered to be a single duplicated chromosome. Genetic recombination or crossing over can occur between sister chromatids or non-sister chromatids (chromatids of homologous chromosomes) during meiosis I. In crossing over, chromosome segments are exchanged between sister chromatids on homologous chromosomes.urn:uuid:1b779750-855d-68c9-9469-68c9855d1b77


Chromosomes are located in the cell nucleus. They exist most of the time as single-stranded structures that are formed from condensed chromatin. Chromatin consists of complexes of small proteins known as histones and DNA. Prior to cell division, single-stranded chromosomes replicate forming double-stranded, X-shaped structures known as sister chromatids. In preparation for cell division, chromatin decondenses forming the less compact euchromatin. This less compact form allows the DNA to unwind so that DNA replication can occur. As the cell progresses through the cell cycle from interphase to either mitosis or meiosis, the chromatin once again becomes tightly packed heterochromatin. The replicated heterochromatin fibers condense further to form sister chromatids. Sister chromatids remain attached until anaphase of mitosis or anaphase II of meiosis. Sister chromatid separation ensures that each daughter cell gets the appropriate number of chromosomes after division. In humans, each mitotic daughter cell would be a diploid cell containing 46 chromosomes. Each meiotic daughter cell would be haploid containing 23 chromosomes.

Sister Chromatids In Mitosis

In prophase of mitosis, sister chromatids begin to move toward the cell center.

In metaphase, sister chromatids align along the metaphase plate at right angles to the cell poles.

In anaphase, sister chromatids separate and begin moving toward opposite ends of the cell. Once the paired sister chromatids separate from one another, each chromatid is considered a single-stranded, full chromosome.

In telophase and cytokinesis, separated sister chromatids are divided into two separate daughter cells. Each separated chromatid is referred to as a daughter chromosome.

Sister Chromatids In Meiosis

Meiosis is a two-part cell division process that is similar to mitosis. In prophase I and metaphase I of meiosis, events are similar with regard to sister chromatid movement as in mitosis. In anaphase I of meiosis, however, sister chromatids remain attached after homologous chromosomes move to opposite poles. Sister chromatids do not separate until anaphase II. Meiosis results in the production of four daughter cells, each with one half the number of chromosomes as the original cell. Sex cells are produced by meiosis.

Related Terms

What Is Bioprinting?

Bioprinted materials can be used to repair damaged organs

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3D printing a heart
 A 3D printer prints out a heart. belekekin / Getty Images.

By Alane LimPublished on May 02, 2018

Bioprinting, a type of 3D printing, uses cells and other biological materials as “inks” to fabricate 3D biological structures. Bioprinted materials have the potential to repair damaged organs, cells, and tissues in the human body. In the future, bioprinting may be used to build entire organs from scratch, a possibility that could transform the field of bioprinting.

Materials That Can Be Bioprinted

Researchers have studied the bioprinting of many different cell types, including stem cells, muscle cells, and endothelial cells. Several factors determine whether or not a material can be bioprinted. First, the biological materials must be biocompatible with the materials in the ink and the printer itself. In addition, the mechanical properties of the printed structure, as well as the time it takes for the organ or tissue to mature, also affect the process. 

Bioinks typically fall into one of two types:

  • Water-based gels, or hydrogels, act as 3D structures in which cells can thrive. Hydrogels containing cells are printed into defined shapes, and the polymers in the hydrogels are joined together or “crosslinked” so that the printed gel becomes stronger. These polymers can be naturally derived or synthetic, but should be compatible with the cells.
  • Aggregates of cells that spontaneously fuse together into tissues after printing.

How Bioprinting Works

The bioprinting process has many similarities with the 3D printing process. Bioprinting is generally divided into the following steps: 

  • Preprocessing: A 3D model based on a digital reconstruction of the organ or tissue to be bioprinted is prepared. This reconstruction can be created based on images captured non-invasively (e.g. with an MRI) or through a more invasive process, such as a series of two-dimensional slices imaged with X-rays.   
  • Processing: The tissue or organ based on the 3D model in the preprocessing stage is printed. Like in other types of 3D printing, layers of material are successively added together in order to print the material.
  • Postprocessing: Necessary procedures are performed to transform the print into a functional organ or tissue. These procedures may include placing the print in a special chamber that helps cells to mature properly and more quickly.

Types of Bioprinters

As with other types of 3D printing, bioinks can be printed several different way. Each method has its own distinct advantages and disadvantages.

  • Inkjet-based bioprinting acts similarly to an office inkjet printer. When a design is printed with an inkjet printer, ink is fired through many tiny nozzles onto the paper. This creates an image made of many droplets that are so small, they are not visible to the eye. Researchers have adapted inkjet printing for bioprinting, including methods that use heat or vibration to push ink through the nozzles. These bioprinters are more affordable than other techniques, but are limited to low-viscosity bioinks, which could in turn constrain the types of materials that can be printed.
  • Laser-assisted bioprinting uses a laser to move cells from a solution onto a surface with high precision. The laser heats up part of the solution, creating an air pocket and displacing cells towards a surface. Because this technique does not require small nozzles like in inkjet-based bioprinting, higher viscosity materials, which cannot flow easily through nozzles, can be used. Laser-assisted bioprinting also allows for very high precision printing. However, the heat from the laser may damage the cells being printed. Furthermore, the technique cannot easily be “scaled up” to quickly print structures in large quantities.
  • Extrusion-based bioprinting uses pressure to force material out of a nozzle to create fixed shapes. This method is relatively versatile: biomaterials with different viscosities can be printed by adjusting the pressure, though care should be taken as higher pressures are more likely to damage the cells. Extrusion-based bioprinting can likely be scaled up for manufacturing, but may not be as precise as other techniques.
  • Electrospray and electrospinning bioprinters make use of electric fields to create droplets or fibers, respectively. These methods can have up to nanometer-level precision. However, they utilize very high voltage, which may be unsafe for cells.

Applications of Bioprinting

Because bioprinting enables the precise construction of biological structures, the technique may find many uses in biomedicine. Researchers have used bioprinting to introduce cells to help repair the heart after a heart attack as well as deposit cells into wounded skin or cartilage. Bioprinting has been used to fabricate heart valves for possible use in patients with heart disease, build muscle and bone tissues, and help repair nerves.

Though more work needs to be done to determine how these results would perform in a clinical setting, the research shows that bioprinting could be used to help regenerate tissues during surgery or after injury. Bioprinters could, in the future, also enable entire organs like livers or hearts to be made from scratch and used in organ transplants.

4D Bioprinting

In addition to 3D bioprinting, some groups have also examined 4D bioprinting, which takes into account the fourth dimension of time. 4D bioprinting is based on the idea that the printed 3D structures may continue to evolve over time, even after they have been printed. The structures may thus change their shape and/or function when exposed to the right stimulus, like heat. 4D bioprinting may find use in biomedical areas, such as making blood vessels by taking advantage of how some biological constructs fold and roll.

The Future

Although bioprinting could help save many lives in the future, a number of challenges have yet to be addressed. For example, the printed structures may be weak and unable to retain their shape after they are transferred to the appropriate location on the body. Furthermore, tissues and organs are complex, containing many different types of cells arranged in very precise ways. Current printing technologies may not be able to replicate such intricate architectures.

Finally, existing techniques are also limited to certain types of materials, a limited range of viscosities, and limited precision. Each technique has the potential to cause damage to the cells and other materials being printed. These issues will be addressed as researchers continue to develop bioprinting to tackle increasingly difficult engineering and medical problems.

Somatic Cells vs. Gametes

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Sperm and Eggs are gametes
 Sperm fertilizing an ovum.Oliver Cleve/Getty Images

By Heather ScovilleUpdated on July 10, 2019

Multicellular eukaryotic organisms have many different types of cells that perform different functions as they combine to form tissues. However, there are two main types of cells within the multicellular organism: somatic cells and gametes, or sex cells.

Somatic cells make up the majority of the body’s cells and account for any regular type of cell in the body that does not perform a function in the sexual reproductive cycle. In humans, these somatic cells contain two full sets of chromosomes (making them diploid cells).

Gametes, on the other hand, are involved directly in the reproductive cycle and are most often haploid cells, meaning they only have one set of chromosomes. This allows each contributing cell to pass on half of the needed complete set of chromosomes for reproduction.

Somatic Cells

Somatic cells are a regular type of body cell that is not involved in any way in sexual reproduction. In humans, such cells are diploid and reproduce using the process of mitosis to create identical diploid copies of themselves when they split.

Other types of species may have haploid somatic cells, and in these individuals, all of the body cells have only one set of chromosomes. This can be found in any sort of species that have haplontic life cycles or follow the alternation of generations’ life cycles.

Humans begin as a single cell when the sperm and the egg fuse during fertilization to form a zygote. From there, the zygote will undergo mitosis to create more identical cells, and eventually, these stem cells will undergo differentiation to create different types of somatic cells. Depending on the time of differentiation and the cells’ exposure to different environments as they develop, the cells will begin down different life paths to create all of the functioning cells of the human body.

Humans have more than three trillion cells as an adult, with somatic cells making up the bulk of that number. The somatic cells that have differentiated can become adult neurons in the nervous system, blood cells in the cardiovascular system, liver cells in the digestive system, or any of the many other types of cells found throughout the body.


Almost all multicellular eukaryotic organisms that undergo sexual reproduction use gametes, or sex cells, to create offspring. Since two parents are necessary to create individuals for the next generation of the species, gametes are typically haploid cells. That way, each parent can contribute half of the total DNA to the offspring. When two haploid gametes fuse during fertilization, they each contribute one set of chromosomes to make a single diploid zygote.

In humans, the gametes are called the sperm (in the male) and the egg (in the female). These are formed by the process of meiosis, which can turn a diploid cell into four haploid gametes. While a human male can continue to make new gametes throughout his life starting at puberty, the human female has a limited number of gametes she can make within a relatively short amount of time.

Mutations and Evolution

Sometimes, during replication, mistakes are made, and these mutations can change the DNA in the cells of the body. However, if there is a mutation in a somatic cell, it most likely will not contribute to the evolution of the species.

Since somatic cells are in no way involved in the process of sexual reproduction, any changes in the DNA of somatic cells will not get passed down to the offspring of the mutated parent. Since the offspring will not receive the changed DNA and any new traits the parent may have will not be passed down, mutations in the DNA of somatic cells will not affect evolution.

If there happens to be a mutation in a gamete, though, that can drive evolution. Mistakes can happen during meiosis that can either change the DNA in the haploid cells or create a chromosome mutation which can add or delete portions of DNA on various chromosomes. If one of the offspring is created from a gamete that has a mutation in it, then that offspring will have different traits that may or may not be favorable for the environment.

Cell Cycle

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The Cell Cycle
By Kelvinsong (Own work) [ CC0], via Wikimedia Commons

By Regina BaileyUpdated on January 17, 2019

The cell cycle is the complex sequence of events by which cells grow and divide. In eukaryotic cells, this process includes a series of four distinct phases. These phases consist of the Mitosis phase (M), Gap 1 phase (G 1), Synthesis phase (S), and Gap 2 phase (G 2). The G 1, S, and G 2 phases of the cell cycle are collectively referred to as interphase. The dividing cell spends most of its time in interphase as it grows in preparation for cell division. The mitosis phase of the cell division process involves the separation of nuclear chromosomes, followed by cytokinesis (division of the cytoplasm forming two distinct cells). At the end of the mitotic cell cycle, two distinct daughter cells are produced. Each cell contains identical genetic material.

The time it takes for a cell to complete one cell cycle varies depending on the type of cell. Some cells, such as blood cells in bone marrowskin cells, and cells lining the stomach and intestines, divide rapidly and constantly. Other cells divide when needed to replaced damaged or dead cells. These cell types include cells of the kidneys, liver, and lungs. Still other cell types, including nerve cells, stop dividing once mature.

Key Takeaways: Cell Cycle

  • Cells grow and divide through the cell cycle.
  • The phases of the cell cycle include Interphase and the Mitotic phase. Interphase consists of the Gap 1 phase (G 1), Synthesis phase (S), and Gap 2 phase (G 2).
  • Dividing cells spend most of their time in interphase, in which they increase in mass and replicate DNA in preparation for cell division.
  • In mitosis, the contents of the dividing cell are equally distributed between two daughter cells.
  • The cell cycle also occurs in the replication of sex cells, or meiosis. Upon completion of the cell cycle in meiosis, four daughter cells are produced.

Phases of the Cell Cycle

Cell Cycle
Darryl Leja, NHGRI

The two main divisions of the cell cycle are interphase and mitosis.


During this segment of the cell cycle, a cell doubles its cytoplasm and synthesizes DNA. It is estimated that a dividing cell spends about 90-95 percent of its time in this phase.

  • G1 phase: The period prior to the synthesis of DNA. In this phase, the cell increases in mass and organelle number in preparation for cell division. Animal cells in this phase are diploid, meaning that they have two sets of chromosomes.
  • S phase: The period during which DNA is synthesized. In most cells, there is a narrow window of time during which DNA replication occurs. The chromosome content is doubled in this phase.
  • G2 phase: The period after DNA synthesis has occurred but prior to the start of mitosis. The cell synthesizes additional proteins and continues to increase in size.

Stages of Mitosis

In mitosis and cytokinesis, the contents of the dividing cell are equally distributed between two daughter cells. Mitosis has four phases: Prophase, Metaphase, Anaphase, and Telophase.

  • Prophase: In this stage, changes occur in both the cytoplasm and nucleus of the dividing cell. The chromatin condenses into discrete chromosomes. The chromosomes begin to migrate toward the cell center. The nuclear envelope breaks down and spindle fibers form at opposite poles of the cell.
  • Metaphase: In this stage, the nuclear membrane disappears completely. The spindle fully develops and the chromosomes align at the metaphase plate (a plane that is equally distant from the two poles).
  • Anaphase: In this stage, paired chromosomes (sister chromatids) separate and begin moving to opposite ends (poles) of the cell. Spindle fibers not connected to chromatids lengthen and elongate the cell.
  • Telophase: In this stage, the chromosomes are cordoned off into distinct new nuclei and the genetic content of the cell is divided equally into two parts. Cytokinesis begins prior to the end of mitosis and completes shortly after telophase.

Once a cell has completed the cell cycle, it goes back into the G 1 phase and repeats the cycle again. Cells in the body can also be placed in a non-dividing state called the Gap 0 phase (G 0) at any point in their life. Cells may remain in this stage for very long periods of time until they are signaled to progress through the cell cycle as initiated by the presence of certain growth factors or other signals. Cells that contain genetic mutations are permanently placed in the G 0 phase to ensure that they are not replicated. When the cell cycle goes wrong, normal cell growth is lost. Cancer cells may develop, which gain control of their own growth signals and continue to multiply unchecked.

Cell Cycle and Meiosis

Meiosis Telophase II
 Lily Anther Microsporocyte in Telophase II of Meiosis. Ed Reschke/Photolibrary/Getty Images

Not all cells divide through the process of mitosis. Organisms that reproduce sexually also undergo a type of cell division called meiosis. Meiosis occurs in sex cells and is similar in process to mitosis. After a complete cell cycle in meiosis, however, four daughter cells are produced. Each cell contains one-half the number of chromosomes as the original parent cell. This means that sex cells are haploid cells. When haploid male and female gametes unite in a process called fertilization, they form one diploid cell called a zygote.

All About Cellular Respiration

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ATP production
 The three processes of ATP production or celluar respiration include glycolysis, the tricarboxylic acid cycle, and oxidative phosphorylation. Encyclopaedia Britannica/UIG/Getty Images

By Regina BaileyUpdated on May 06, 2019

We all need energy to function, and we get that energy from the foods we eat. Extracting those nutrients necessary to keep us going and then converting them into useable energy is the job of our cells. This complex yet efficient metabolic process, called cellular respiration, converts the energy derived from sugars, carbohydrates, fats, and proteins into adenosine triphosphate, or ATP, a high-energy molecule that drives processes like muscle contraction and nerve impulses. Cellular respiration occurs in both eukaryotic and prokaryotic cells, with most reactions taking place in the cytoplasm of prokaryotes and in the mitochondria of eukaryotes. 

There are three main stages of cellular respiration: glycolysis, the citric acid cycle, and electron transport/oxidative phosphorylation.

Sugar Rush

Glycolysis literally means “splitting sugars,” and it is the 10-step process by which sugars are released for energy. Glycolysis occurs when glucose and oxygen are supplied to the cells by the bloodstream, and it takes place in the cell’s cytoplasm. Glycolysis can also occur without oxygen, a process called anaerobic respiration, or fermentation. When glycolysis occurs without oxygen, cells make small amounts of ATP. Fermentation also produces lactic acid, which can build up in muscle tissue, causing soreness and a burning sensation.

Carbs, Proteins, and Fats

The Citric Acid Cycle, also known as the tricarboxylic acid cycle or the Krebs Cycle, begins after the two molecules of the three carbon sugar produced in glycolysis are converted to a slightly different compound (acetyl CoA). It is the process that allows us to use the energy found in carbohydratesproteins, and fats. Although the citric acid cycle does not use oxygen directly, it works only when oxygen is present. This cycle takes place in the matrix of cell mitochondria. Through a series of intermediate steps, several compounds capable of storing “high energy” electrons are produced along with two ATP molecules. These compounds, known as nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD), are reduced in the process. The reduced forms (NADH and FADH2) carry the “high energy” electrons to the next stage.

Aboard the Electron Transport Train

Electron transport and oxidative phosphorylation is the third and final step in aerobic cellular respiration. The electron transport chain is a series of protein complexes and electron carrier molecules found within the mitochondrial membrane in eukaryotic cells. Through a series of reactions, the “high energy” electrons generated in the citric acid cycle are passed to oxygen. In the process, a chemical and electrical gradient is formed across the inner mitochondrial membrane as hydrogen ions are pumped out of the mitochondrial matrix and into the inner membrane space. ATP is ultimately produced by oxidative phosphorylation—the process by which enzymes in the cell oxidize nutrients. The protein ATP synthase uses the energy produced by the electron transport chain for the phosphorylation (adding a phosphate group to a molecule) of ADP to ATP. Most ATP generation occurs during the electron transport chain and oxidative phosphorylation stage of cellular respiration. 

urn:uuid:e2ef61e0-3b10-8a4b-60c5-8a4b3b10e2efabout:blankScience, Tech, Math› Science

Platelets: Cells That Clot Blood

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Activated platelets, artwork
SCIEPRO / Getty Images

By Regina BaileyUpdated on November 18, 2019

Platelets, also called thrombocytes, are the smallest cell type in the blood. Other major blood components include plasma, white blood cells, and red blood cells. The primary function of platelets is to aid in the blood clotting process. When activated, these cells adhere to one another to block the flow of blood from damaged blood vessels. Like red blood cells and white blood cells, platelets are produced from bone marrow stem cells. Platelets are so named because unactivated platelets resemble miniature plates when viewed under a microscope.01of 04

Platelet Production

Platelets are derived from bone marrow cells called megakaryocytes. Megakaryocytes are huge cells that break into fragments to form platelets. These cell fragments have no nucleus but do contain structures called granules. The granules house proteins that are necessary for clotting blood and sealing breaks in blood vessels.

A single megakaryocyte can produce anywhere from 1000 to 3000 platelets. Platelets circulate in the bloodstream for about 9 to 10 days. When they become old or damaged, they are removed from circulation by the spleen. Not only does the spleen filter blood of old cells, but it also stores functional red blood cells, platelets, and white blood cells. In instances where extreme bleeding occurs, platelets, red blood cells, and certain white blood cells (macrophages) are released from the spleen. These cells help to clot blood, compensate for blood loss and fight infectious agents such as bacteria and viruses.urn:uuid:897081ac-9902-0cb2-8603-0cb29902897002of 04

Platelet Function

The role of blood platelets is to clog broken blood vessels to prevent the loss of blood. Under normal conditions, platelets move through blood vessels in an unactivated state. Unactivated platelets have a typical plate-like shape. When there is a break in a blood vessel, platelets become activated by the presence of certain molecules in the blood. These molecules are secreted by blood vessel endothelial cells.

Activated platelets change their shape and become more round with long, finger-like projections extending from the cell. They also become sticky and adhere to one another and to blood vessel surfaces to plug any breaks in the vessel. Activated platelets release chemicals that cause the blood protein fibrinogen to be converted to fibrin. Fibrin is a structural protein that is arranged into long, fibrous chains. As fibrin molecules combine, they form a long, sticky fibrous mesh that traps platelets, red blood cells, and white blood cells. Platelet activation and blood coagulation processes work in conjunction to form a clot. Platelets also release signals that help to summon more platelets to the damaged site, constrict blood vessels, and activate additional clotting factors in blood plasma. urn:uuid:759f6b48-470d-f3e6-62f9-f3e6470d759f03of 04

Platelet Count

Blood counts measure the number of of red blood cells, white blood cells, and platelets in the blood. A normal platelet count is between 150,000 to 450,000 platelets per microliter of blood. A low platelet count may result from a condition called thrombocytopenia. Thrombocytopenia can occur if the bone marrow does not make enough platelets or if the platelets are destroyed. Platelet counts below 20,000 per microliter of blood are dangerous and may result in uncontrollable bleeding. Thrombocytopenia can be caused by a number of conditions, including kidney disease, cancer, pregnancy, and immune system abnormalities. If a person’s bone marrow cells make too many platelets, a condition known as thrombocythemia can develop.

With thrombocythemia, platelet counts may rise above 1,000,000 platelets per microliter of blood for reasons that are unknown. Thrombocythemia is dangerous because the excess platelets may block the blood supply to vital organs such as the heart and brain. When platelet counts are high but not as high as the counts seen with thrombocythemia, another condition called thrombocytosis may develop. Thrombocytosis is not caused by the abnormal bone marrow but by the presence of a disease or another condition, such as cancer, anemia, or an infection. Thrombocytosis is rarely serious and usually improves when the underlying condition subsides.



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