Cells are the basic building blocks of all living things. The human body is composed of trillions of cells. They provide structure for the body, take in nutrients from food, convert those nutrients into energy, and carry out specialized functions. Cells also contain the body’s hereditary material and can make copies of themselves.

Cells have many parts, each with a different function. Some of these parts, called organelles, are specialized structures that perform certain tasks within the cell. Human cells contain the following major parts, listed in alphabetical order:

Cytoplasm

Within cells, the cytoplasm is made up of a jelly-like fluid (called the cytosol) and other structures that surround the nucleus.

Cytoskeleton

The cytoskeleton is a network of long fibers that make up the cell’s structural framework. The cytoskeleton has several critical functions, including determining cell shape, participating in cell division, and allowing cells to move. It also provides a track-like system that directs the movement of organelles and other substances within cells.

Endoplasmic reticulum (ER)

This organelle helps process molecules created by the cell. The endoplasmic reticulum also transports these molecules to their specific destinations either inside or outside the cell.

Golgi apparatus

The Golgi apparatus packages molecules processed by the endoplasmic reticulum to be transported out of the cell.

Lysosomes and peroxisomes

These organelles are the recycling center of the cell. They digest foreign bacteria that invade the cell, rid the cell of toxic substances, and recycle worn-out cell components.

Mitochondria

Mitochondria are complex organelles that convert energy from food into a form that the cell can use. They have their own genetic material, separate from the DNA in the nucleus, and can make copies of themselves.

Nucleus

The nucleus serves as the cell’s command center, sending directions to the cell to grow, mature, divide, or die. It also houses DNA (deoxyribonucleic acid), the cell’s hereditary material. The nucleus is surrounded by a membrane called the nuclear envelope, which protects the DNA and separates the nucleus from the rest of the cell.

Plasma membrane

The plasma membrane is the outer lining of the cell. It separates the cell from its environment and allows materials to enter and leave the cell.

Ribosomes

Ribosomes are organelles that process the cell’s genetic instructions to create proteins. These organelles can float freely in the cytoplasm or be connected to the endoplasmic reticulum (see above).

A cell is the basic building block of living things. All cells can be sorted into one of two groups: eukaryotes and prokaryotes. A eukaryote has a nucleus and membrane-bound organelles, while a prokaryote does not. Plants and animals are made of numerous eukaryotic cells, while many microbes, such as bacteria, consist of single cells. An adult human body is estimated to contain between 10 and 100 trillion cells.

One of the cool things about cells& People always think of them as sort of the building blocks of the body, but I really like to think of them almost individually as their own little kind of nanoparticles, or little nanobots, that have their own individual activities, and together they form things like tissues, organs& And then you have to consider that then there are those cells which act completely independently just on their own, such as bacteria. So you have independently acting cells, but then you also have cells that can act together, such as slime molds, where you have individual cells that come together and form whole entities and have very programmed activities. So the cell is pretty amazing when you think about it, because it can be anything and almost everything to anybody or to any organism. And so again, when I think about cells, I think about these little amazing entities that together form entire bodies. But we also can't dismiss the importance of them individually.

Cytoplasm is the gelatinous liquid that fills the inside of a cell. It is composed of water, salts, and various organic molecules. Some intracellular organelles, such the nucleus and mitochondria, are enclosed by membranes that separate them from the cytoplasm.

Cytoplasm's a funny term. So what does "cyto" mean? "Cyto" means "cell", "plasm" means "stuff", so it's "cell stuff". So think about a cell as a big water balloon, and the water balloon has little pieces of fruit floating around in it. And the cytoplasm is the water in the water balloon, and it's a little bit thicker than water, but it makes up the majority of the inside of most cells. Now, inside the cell, inside that water balloon, there's a nucleus and there's other so-called organelles, like mitochondria, or lysosomes, or the endoplasmic reticulum and other unpronounceable organelles, but the cytoplasm is the ocean in which all of these organelles float.

A Golgi body, also known as a Golgi apparatus, is a cell organelle that helps process and package proteins and lipid molecules, especially proteins destined to be exported from the cell. Named after its discoverer, Camillo Golgi, the Golgi body appears as a series of stacked membranes.

The Golgi body is a portion of the cell that's made up of membranes, and there's different types of membranes. Some of them are tubules, and some of them are vesicles. The Golgi is located right near the nucleus. It's called a perinuclear body, and it's actually right near the endoplasmic reticulum as well. And when proteins come out of the endoplasmic reticulum, they go into the Golgi for further processing. For example, carbohydrates are put on some of the proteins, and then afterwards these glycoproteins--meaning they have carbohydrate as well as protein on them, these glycoproteins move out of the Golgi to the rest of the cell. And they do so inside other vesicles. Those vesicles are actually made from the Golgi network. In fact, one of the functions of the Golgi is to make new vesicles out of the existing membrane of the Golgi and put into those vesicles the glycoproteins and other substances that are made in the Golgi network. And then those vesicles, filled with the Golgi products, move to the rest of the cell, usually through the cell to the plasma membrane, which is their end destination.

Endoplasmic reticulum is a network of membranes inside a cell through which proteins and other molecules move. Proteins are assembled at organelles called ribosomes. When proteins are destined to be part of the cell membrane or exported from the cell, the ribosomes assembling them attach to the endoplasmic reticulum, giving it a rough appearance. Smooth endoplasmic reticulum lacks ribosomes and helps synthesize and concentrate various substances needed by the cell.

The endoplasmic reticulum can either be smooth or rough, and in general its function is to produce proteins for the rest of the cell to function. The rough endoplasmic reticulum has on it ribosomes, which are small, round organelles whose function it is to make those proteins. Sometimes, when those proteins are made improperly, the proteins stay within the endoplasmic reticulum. They're retained and the endoplasmic reticulum becomes engorged because it seems to be constipated, in a way, and the proteins don't get out where they're suppose to go. Then there's the smooth endoplasmic reticulum, which doesn't have those ribosomes on it. And that smooth endoplasmic reticulum produces other substances needed by the cell. So the endoplasmic reticulum is an organelle that's really a workhorse in producing proteins and substances needed by the rest of the cell.

Endoplasmic reticulum is a network of membranes inside a cell through which proteins and other molecules move. Proteins are assembled at organelles called ribosomes. When proteins are destined to be part of the cell membrane or exported from the cell, the ribosomes assembling them attach to the endoplasmic reticulum, giving it a rough appearance.

The endoplasmic reticulum can either be smooth or rough, and in general its function is to produce proteins for the rest of the cell to function. The rough endoplasmic reticulum has on it ribosomes, which are small, round organelles whose function it is to make those proteins. Sometimes, when those proteins are made improperly, the proteins stay within the endoplasmic reticulum. They're retained and the endoplasmic reticulum becomes engorged because it seems to be constipated, in a way, and the proteins don't get out where they're suppose to go. Then there's the smooth endoplasmic reticulum, which doesn't have those ribosomes on it. And that smooth endoplasmic reticulum produces other substances needed by the cell. So the endoplasmic reticulum is an organelle that's really a workhorse in producing proteins and substances needed by the rest of the cell.

A lysosome is a membrane-bound cell organelle that contains digestive enzymes. Lysosomes are involved with various cell processes. They break down excess or worn-out cell parts. They may be used to destroy invading viruses and bacteria. If the cell is damaged beyond repair, lysosomes can help it to self-destruct in a process called programmed cell death, or apoptosis.

Now, the lysosome is a specific type of organelle that's very acidic. So that means that it has to be protected from the rest of the inside of the cell. It's a compartment, then, that has a membrane around it that stores the digestive enzymes that require this acid, low-pH environment. Those enzymes are called hydrolytic enzymes, and they break down large molecules into small molecules. For example, large proteins into amino acids, or large carbohydrates into simple sugars, or large lipids into single fatty acids. And when they do that, they provide for the rest of the cell the nutrients that it needs to... So, for example, if you can't do that, it can't break down large molecules into small molecules. You'll have storage of those large molecules, and this is a disease. There's also another type of lysosome storage disease in which the small molecules that are produced from those large molecules can't get out of the lysosome. They're stored there because the transporters for moving these small molecules out are missing genetically. And finally, one other function of the lysosome is to ingest bacteria so that the bacteria can be destroyed. So the lysosomes also provide a function against infection, and the cell will often engorge a bacterium and put it into its lysosome for destruction. So here's an important organelle that has function against infection and function in a way in nutrition to break down large molecules into small molecules so that they can be reutilized.

Mitochondria are membrane-bound cell organelles (mitochondrion, singular) that generate most of the chemical energy needed to power the cell's biochemical reactions. Chemical energy produced by the mitochondria is stored in a small molecule called adenosine triphosphate (ATP). Mitochondria contain their own small chromosomes. Generally, mitochondria, and therefore mitochondrial DNA, are inherited only from the mother.

Mitochondria are membrane-bound organelles, but they're membrane-bound with two different membranes. And that's quite unusual for an intercellular organelle. Those membranes function in the purpose of mitochondria, which is essentially to produce energy. That energy is produced by having chemicals within the cell go through pathways, in other words, be converted. And the process of that conversion produces energy in the form of ATP, because the phosphate is a high-energy bond and provides energy for other reactions within the cell. So the mitochondria's purpose is to produce that energy. Some different cells have different amounts of mitochondria because they need more energy. So for example, the muscle has a lot of mitochondria, the liver does too, the kidney as well, and to a certain extent, the brain, which lives off of the energy those mitochondria produce. So if you have a defect in the pathways that the mitochondria usually functions with, you're going to have symptoms in the muscle, in the brain, sometimes in the kidneys as well; many different types of symptoms. And we probably don't know all of the different diseases that mitochondrial dysfunction causes.

William Gahl, M.D., Ph.D.

A nucleus is a membrane-bound organelle that contains the cell's chromosomes. Pores in the nuclear membrane allow for the passage of molecules in and out of the nucleus.

The nucleus is one of the most obvious parts of the cell when you look at a picture of the cell. It's in the middle of the cell, and the nucleus contains all of the cell's chromosomes, which encode the genetic material. So this is really an important part of the cell to protect. The nucleus has a membrane around it that keeps all the chromosomes inside and makes the distinction between the chromosomes being inside the nucleus and the other organelles and components of the cell staying outside. Sometimes things like RNA need to traffic between the nucleus and the cytoplasm, and so there are pores in this nuclear membrane that allow molecules to go in and out of the nucleus. It used to be thought that the nuclear membrane only allowed molecules to go out, but now it's realized that there is an active process also for bringing molecules into the nucleus.

The plasma membrane, also called the cell membrane, is the membrane found in all cells that separates the interior of the cell from the outside environment. In bacterial and plant cells, a cell wall is attached to the plasma membrane on its outside surface. The plasma membrane consists of a lipid bilayer that is semipermeable. The plasma membrane regulates the transport of materials entering and exiting the cell.

The plasma membrane, or the cell membrane, provides protection for a cell. It also provides a fixed environment inside the cell. And that membrane has several different functions. One is to transport nutrients into the cell and also to transport toxic substances out of the cell. Another is that the membrane of the cell, which would be the plasma membrane, will have proteins on it which interact with other cells. Those proteins can be glycoprotein, meaning there's a sugar and a protein moiety, or they could be lipid proteins, meaning there's a fat and a protein. And those proteins which stick outside of the plasma membrane will allow for one cell to interact with another cell. The cell membrane also provides some structural support for a cell. And there are different types of plasma membranes in different types of cells, and the plasma membrane has in it in general a lot of cholesterol as its lipid component. That's different from certain other membranes within the cell. Now, there are different plants and different microbes, such as bacteria and algae, which have different protective mechanisms. In fact, they have a cell wall outside of them, and that cell wall is much tougher and is structurally more sound than a plasma membrane is.

A ribosome is an intercellular structure made of both RNA and protein, and it is the site of protein synthesis in the cell. The ribosome reads the messenger RNA (mRNA) sequence and translates that genetic code into a specified string of amino acids, which grow into long chains that fold to form proteins.

Ribosome. A ribosome is the cellular machinery responsible for making proteins. There are many ribosomes in each cell, each made up of two subunits. These two subunits lock around the messenger RNA and then travel along the length of the messenger RNA molecule reading each three-letter codon. The ribosome serves as a docking station for the transfer RNA that matches the sequence of bases on the messenger RNA. Each three-letter codon on the messenger RNA pairs with the matching anticodon on a specific transfer RNA, and that specific RNA allows for the addition of a specific amino acid on the end of the growing protein chain. The ribosome then breaks up after the completion of the protein.

DNA, or deoxyribonucleic acid, is the hereditary material in humans and almost all other organisms. Nearly every cell in a person’s body has the same DNA. Most DNA is located in the cell nucleus (where it is called nuclear DNA), but a small amount of DNA can also be found in the mitochondria (where it is called mitochondrial DNA or mtDNA). Mitochondria are structures within cells that convert the energy from food into a form that cells can use.

The information in DNA is stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Human DNA consists of about 3 billion bases, and more than 99 percent of those bases are the same in all people. The order, or sequence, of these bases determines the information available for building and maintaining an organism, similar to the way in which letters of the alphabet appear in a certain order to form words and sentences.

DNA bases pair up with each other, A with T and C with G, to form units called base pairs. Each base is also attached to a sugar molecule and a phosphate molecule. Together, a base, sugar, and phosphate are called a nucleotide. Nucleotides are arranged in two long strands that form a spiral called a double helix. The structure of the double helix is somewhat like a ladder, with the base pairs forming the ladder’s rungs and the sugar and phosphate molecules forming the vertical sidepieces of the ladder.

An important property of DNA is that it can replicate, or make copies of itself. Each strand of DNA in the double helix can serve as a pattern for duplicating the sequence of bases. This is critical when cells divide because each new cell needs to have an exact copy of the DNA present in the old cell.

A gene is the basic physical and functional unit of heredity. Genes are made up of DNA. Some genes act as instructions to make molecules called proteins. However, many genes do not code for proteins. In humans, genes vary in size from a few hundred DNA bases to more than 2 million bases. An international research effort called the Human Genome Project, which worked to determine the sequence of the human genome and identify the genes that it contains, estimated that humans have between 20,000 and 25,000 genes.

Every person has two copies of each gene, one inherited from each parent. Most genes are the same in all people, but a small number of genes (less than 1 percent of the total) are slightly different between people. Alleles are forms of the same gene with small differences in their sequence of DNA bases. These small differences contribute to each person’s unique physical features.

Scientists keep track of genes by giving them unique names. Because gene names can be long, genes are also assigned symbols, which are short combinations of letters (and sometimes numbers) that represent an abbreviated version of the gene name. For example, a gene on chromosome 7 that has been associated with cystic fibrosis is called the cystic fibrosis transmembrane conductance regulator; its symbol is CFTR.

In the nucleus of each cell, the DNA molecule is packaged into thread-like structures called chromosomes. Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure.

Chromosomes are not visible in the cell’s nucleus—not even under a microscope—when the cell is not dividing. However, the DNA that makes up chromosomes becomes more tightly packed during cell division and is then visible under a microscope. Most of what researchers know about chromosomes was learned by observing chromosomes during cell division.

Each chromosome has a constriction point called the centromere, which divides the chromosome into two sections, or “arms.” The short arm of the chromosome is labeled the “p arm.” The long arm of the chromosome is labeled the “q arm.” The location of the centromere on each chromosome gives the chromosome its characteristic shape, and can be used to help describe the location of specific genes.

In humans, each cell normally contains 23 pairs of chromosomes, for a total of 46. Twenty-two of these pairs, called autosomes, look the same in both males and females. The 23rd pair, the sex chromosomes, differ between males and females. Females have two copies of the X chromosome, while males have one X and one Y chromosome.

Only about 1 percent of DNA is made up of protein-coding genes; the other 99 percent is noncoding. Noncoding DNA does not provide instructions for making proteins. Scientists once thought noncoding DNA was “junk,” with no known purpose. However, it is becoming clear that at least some of it is integral to the function of cells, particularly the control of gene activity. For example, noncoding DNA contains sequences that act as regulatory elements, determining when and where genes are turned on and off. Such elements provide sites for specialized proteins (called transcription factors) to attach (bind) and either activate or repress the process by which the information from genes is turned into proteins (transcription). Noncoding DNA contains many types of regulatory elements:

• Promoters provide binding sites for the protein machinery that carries out transcription. Promoters are typically found just ahead of the gene on the DNA strand.

• Enhancers provide binding sites for proteins that help activate transcription. Enhancers can be found on the DNA strand before or after the gene they control, sometimes far away.

• Silencers provide binding sites for proteins that repress transcription. Like enhancers, silencers can be found before or after the gene they control and can be some distance away on the DNA strand.

• Insulators provide binding sites for proteins that control transcription in a number of ways. Some prevent enhancers from aiding in transcription (enhancer-blocker insulators). Others prevent structural changes in the DNA that repress gene activity (barrier insulators). Some insulators can function as both an enhancer blocker and a barrier.

Other regions of noncoding DNA provide instructions for the formation of certain kinds of RNA molecules. RNA is a chemical cousin of DNA. Examples of specialized RNA molecules produced from noncoding DNA include transfer RNAs (tRNAs) and ribosomal RNAs (rRNAs), which help assemble protein building blocks (amino acids) into a chain that forms a protein; microRNAs (miRNAs), which are short lengths of RNA that block the process of protein production; and long noncoding RNAs (lncRNAs), which are longer lengths of RNA that have diverse roles in regulating gene activity.

Some structural elements of chromosomes are also part of noncoding DNA. For example, repeated noncoding DNA sequences at the ends of chromosomes form telomeres. Telomeres protect the ends of chromosomes from being degraded during the copying of genetic material. Repetitive noncoding DNA sequences also form satellite DNA, which is a part of other structural elements. Satellite DNA is the basis of the centromere, which is the constriction point of the X-shaped chromosome pair. Satellite DNA also forms heterochromatin, which is densely packed DNA that is important for controlling gene activity and maintaining the structure of chromosomes.

Some noncoding DNA regions, called introns, are located within protein-coding genes but are removed before a protein is made. Regulatory elements, such as enhancers, can be located in introns. Other noncoding regions are found between genes and are known as intergenic regions.

The identity of regulatory elements and other functional regions in noncoding DNA is not completely understood. Researchers are working to understand the location and role of these genetic components.

What are cells?

Cells provide structure and function for all living things, from microorganisms to humans. Scientists consider them the smallest form of life. Cells house the biological machinery that makes the proteins, chemicals, and signals responsible for everything that happens inside our bodies.

What do cells look like?

Cells come in different shapes—round, flat, long, star-like, cubed, and even shapeless. Most cells are colorless and see-through. The size of a cell also varies. Some of the smallest are one-celled bacteria, which are too small to see with the naked eye, at 1-millionth of a meter (micrometer) across. Plants have some of the largest cells, 10–100 micrometers across. The human cell with the biggest diameter is the egg. It is about the same diameter as a hair strand (80 micrometers).

How many different types of human cells are there?

The trillions of cells that make up a human are organized into about 200 major types. All of a person’s cells contain the same set of genes. However, each cell type “switches on” a different pattern of genes, and this determines which proteins the cell produces. The unique set of proteins in different cell types allows them to perform specialized tasks. For instance, red blood cells carry oxygen throughout the body. White blood cells kill germ invaders. Intestinal cells release molecules that help digest food. Nerve cells send chemical and electrical messages that produce thoughts and movement. And heart cells contract in unison to pump blood.

What are eukaryotic and prokaryotic cells, and how are they different?

When putting cells into categories, scientists can tell eukaryotic cells apart from prokaryotic cells because they look different. Eukaryotic cells make up animals, plants, fungi, and some single-celled organisms. And they have a number of structures inside them, called organelles. The most prominent organelle is the nucleus, which contains the cell’s genetic material, or DNA (see more on DNA). Prokaryotic cells don’t have a nucleus or other organelles. They are single-celled microorganisms that tend to be smaller than eukaryotic cells. There are two types of prokaryotic cells—bacteria and archaea.

What are some of the major organelles in a human cell?

In addition to the nucleus, the most prominent organelles include the following:

• Mitochondria, the cell’s power plants, convert energy from food into the body’s main energy source, adenosine triphosphate (ATP).
• Ribosomes are molecular factories that make proteins.
• The endoplasmic reticulum (ER), a network of interconnected sacs, processes newly made secreted and membrane proteins and produces fatty substances called lipids.
• The Golgi complex receives proteins and lipids from the ER, packages them, and sends them to their final destinations inside the cell, within the cell membrane, or outside the cell.
• Lysosomes, the cell’s garbage dumps, break down waste materials and dispose of them or recycle them.

How do cells move?

Many types of cells can move. Single-celled organisms move to find food. And even cells inside multicellular organisms may need to get around. For example, immune system cells must move toward invaders. And sperm needs to “swim” to fertilize eggs.

Cells move in several ways. Some simply float through water or other liquids. Some push themselves along using long, thin proteins, called flagella, and cilia that stick outside the cell membrane and wave around. Some “crawl” along, using what’s called amoeboid movements, in which cytoplasm-filled protrusions scoot the cell forward.

Within cells, nutrients and organelles move around to carry out various cellular functions. This kind of internal movement is called cyclosis, or cytoplasmic streaming. The internal structure of cells, which is called the cytoplasm, creates a directional flow that pushes the contents of the cells around.

Scientists study cell movement to better understand how cells work, including how cancer cells move from one tissue to another and how white blood cells move to heal wounds and attack invaders.

How do scientists study cells?

Cell biologists rely on an array of tools to peer into the body and examine cells. Imaging techniques magnify organelles and track cells as they divide, grow, interact, and carry out other vital tasks. Biochemical or genetic tests allow researchers to study how cells respond to environmental stressors, such as rising temperatures or toxins. These tests can also label specific proteins using fluorescent tags and other chemicals that allow scientists to visualize proteins at work inside cells. Sophisticated computational tools then integrate and analyze all the data.

How do our bodies make more cells?

One cell divides into two in a process called mitosis. Mitosis produces two genetically identical “daughter” cells from a single parent cell. Another type of cell division, meiosis, creates four daughter cells that are genetically distinct from one another and from the original parent cell. Only a few special cells can perform meiosis: those that will become eggs in females and sperm in males.

How and why do cells die?

Cells come equipped with what they need to self-destruct. This is called programmed cell death, or apoptosis. And it serves a healthy and protective role in our bodies. For example, it helps shape our fingers and toes before birth, and it kills off diseased cells during our lives. Another kind of cell death, called necrosis, is unplanned and not protective. Necrosis can happen after a sudden traumatic injury, infection, or exposure to a toxic chemical.

What are stem cells?

Stem cells can renew themselves millions of times. Other cells in the body, such as muscle and nerve cells, cannot do this. Embryonic stem cells are undifferentiated, meaning they can turn into any type of cell in the body. Tissue-specific stem cells (sometimes called adult or somatic stem cells) arise later in development. They also can replenish cells. The primary role of tissue-specific stem cells is to maintain and repair the tissue in which they’re found.

How do problems in cells lead to disease?

Changes to the genes inside a cell, called mutations, can alter the cell’s ability to divide, make proteins, remove waste, or perform other tasks. These genetic mutations can lead to birth defects, cancer, and other diseases. Cells that are damaged through physical trauma or infection can, in extreme cases, contribute to harmful inflammation and organ malfunction.

How does studying cells aid our understanding of human health and disease?

Learning about how cells work—and what happens when they don’t work properly—teaches us about the biological processes that keep us healthy. It also uncovers new ways to treat disease. Cellular research has already led to cancer treatments, antibiotics, medicine that lowers cholesterol, and improved methods for delivering drugs. However, much more remains to be discovered. For example, understanding how stem cells and certain other cells regenerate could offer insight on how to repair damaged or lost tissue.

Henrietta Lacks was an African-American woman whose cells were removed during a biopsy in 1951 and used for research without her knowledge or approval. Her cells (commonly known as HeLa cells) have revolutionized the field of medicine and have been used for decades in biomedical research - to study cancer, the effects of radiation, and AIDS among other areas. This timeline includes events from 1920 (when Lacks was born) to 2013 (when NIH announced an agreement with the Lacks family to allow biomedical researchers controlled access to the whole genome data of HeLa cells.

What is the smallest cell?

The smallest cell is probably a bacterial cell. Currently, a type of bacteria called mycoplasma (200-300 nanometers in diameter) are thought to be the smallest cells. But, recent research has identified potentially even smaller cells, like the bacterium Peligiabacter and the archaeon Nanoarchaeum. There is still some controversy over who the winner would be. And as we learn more about life on Earth we might find even smaller cells.

What is the smallest human cell?

Three options:

1. Sperm – diameter of 1-3 micrometers, but its tail is 50 micrometers.
2. Cerebellum granule cell, which play a key role in the sense of smell – diameter of 4-10 micrometers.
3. The red blood cell – diameter of 4-8 micrometers.

What is the largest single cell and how big is it?

By volume, the largest cell is an ostrich egg. But by length, nerve cells win. There are nerve cells that are several feet long, such as nerve cells in our leg, a giraffe neck or the giant axon of the giant squid (imagine that one!).

How many different types of cells can be found inside the human body?

There are about 200 cell types and about 30 trillion cells in the human body. That does not include bacteria, fungus and mites that live on the body.

Which living organism has the most cells?

The winner is bound to be a colony of genetically identical, physically connected clones. One possibility is a grove of aspen trees in Utah known as "Pando," which is projected to weigh about 6 million kg. Another contender is a fungus in eastern Oregon. Nicknamed the "Humongous Fungus," this organism covers almost 9 square kilometers, with most of its bulk underground.

Do any cells have natural color?

Yes! Blueberries are blue, carrots are orange, most plants are green, mustard is yellow, the cells of our retina are black, eggplants are purple, all because of pigments that are present in those cells. Many of the pigments used to dye clothing are isolated from plant and animal sources.

What color is a nucleus?

In a living cell, the nucleus is almost completely transparent and colorless. You can't really see it unless you know what to look for! When scientists take photographs of cells in the microscope, they usually treat the cells first with a dye that makes the nucleus blue or purplish blue. Because of this, the nucleus in most cell images is also blue.

What can stem cells do?

Stem cells can be divided into two major categories—embryonic stem cells and adult stem cells. Embryonic stem cells can differentiate into any cell in the body. Adult stem cells, which are present in tissues throughout the body, have more limited capabilities. They can only differentiate into cell types specific to the tissue in which they are located, whether that is skin, blood, muscle, teeth, brain, liver, heart or other tissue. Their purpose is to supply new cells to replace those lost by disease or injury. Adult stem cells can be extracted for use in biological research and, hopefully in the future, for the development of therapies.

When were cells discovered?

Robert Hooke, a British scientist, coined the term "cell" in 1665 after looking at cork tissue through a microscope. What he saw looked like little boxes with walls around them. They reminded him of the small rooms that monks lived in, which were called...cells.

How do toxins pass through the plasma membrane?

Some toxins produced by bacteria create a channel in the cell's membrane. This channel can either be fatal itself—by allowing ions, water and small molecules to leak out of the cell—or can serve as a portal through which a bacterium injects a toxic protein into the cell. Other toxins can bind to cell surface receptors and hitch a ride into the cell with the receptor. Toxins such as dioxin are small molecules that can pass into the cell membrane because they dissolve in the lipid layer of the membrane.

How do you explain the significance of compartmentalization for eukaryotic cells?

Some biological processes need to be isolated or separated from other reactions so compartmentalization is important. Some reactions require starting material available only in specific intracellular locations. DNA transcription, for example, takes place in the nucleus where DNA is located while translation or protein synthesis takes place in the ribosomes where all the required proteins and nucleic acids are found.

Why does DNA mutate within the cells?

There are lots of mutagenic substances (like cigarette smoke and ultraviolet radiation) that increase the mutation rate. But mutations also happen naturally during the process of DNA replication. Each of your cells has about 6.4 billion base pairs of DNA that need to be copied every time a cell divides. The replication machinery is very accurate but even if it only made one mistake in a billion, the next generation cell would have six to seven mistakes (it is actually a little better than that).

Are there human mutations that are helpful?

Yes, sometimes. For example, there was a mutation that allowed people to digest milk even into adulthood. Because it was helpful for survival, this mutation is now present in about 80 percent of people of European descent. Sometimes a mutation is beneficial, sometimes it is harmful, and sometimes there's a trade-off—maybe it helps in one way but hurts in another.

How fast does it take for a cell to produce two daughter cells?

Bacteria can produce daughter cells very fast when nutrients are available. The doubling time for E. coli bacteria is 20 minutes. Other cells in the human body take hours or days or even years to divide.

How long does it take for one cell to make a whole organism?

If we only think about mammals, we know that the gestation period (how long a mother is pregnant) varies a lot among species. Mice are born after about three weeks, but elephants take over a year.

When do cells stop reproducing?

Some cells stop dividing when they fully differentiate (like nerve cells) and some cells keep reproducing over your entire life (like skin cells).

How long do cells live?

It depends on the type of cell. Cells in your small intestine last only 2-4 days. Skin cells last 10-30 days. Sperm cells last two months. Red blood cells last 70-140 days (about four months). Egg cells, and some cells in the brain and eye last your entire lifetime.

What causes cells to die?

There are two main ways cells can die. Apoptosis, also called programmed cell death, is a normal part of development (it sculpts our fingers and toes and fine-tunes our brains). Necrosis is not normal and results from a sudden traumatic injury, infection or exposure to a toxic chemical.

What are the differences between a virus and a bacterium?

Viruses are really tiny agents that infect cells, including animal cells, plants cells and bacteria. They can replicate only inside a living cell. Bacteria, on the other hand, are single-celled microorganisms that can live and reproduce on their own (they are small but larger than viruses). Not all viruses cause disease. Neither do all bacteria cause disease—in fact, many bacteria are essential to our health! Check out https://biobeat.nigms.nih.gov/2015/04/our-microbial-menagerie/ or do a search on "microbiome" to learn more about this fascinating area of research.

Why do biologists think there are more prokaryotic cells inside the human body than there are eukaryotic cells? Are there really more prokaryotes than eukaryotic cells?

Scientists do not have a complete inventory of the number of cells, prokaryotic or eukaryotic, in our bodies. The numbers they have are estimates. They do know that bacterial cells are many times smaller than eukaryotic cells. So in terms of absolute numbers, there are many more prokaryotes, but they make up less than 3 percent of a person's body mass.

How do viruses attack your cells?

Some viruses attack cells by hijacking normal cell membrane structures and processes. Some may bind to membrane patches, which are rich in membrane fats called cholesterol and sphingolipids. They are then taken up by endocytosis into the cell. The membrane surrounds the virus and engulfs it.

Will they ever find a cure for cancer?

Cancer is a very complicated disease that scientists have been studying for some time. Cancer can involve many different mutations on different genes, and different organs can have their own forms of cancer. The good news is that scientists have made tremendous progress in treating cancer. They have also developed some promising new therapies to treat specific cancers without some of the side effects of chemotherapy.