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The cell is the basic unit of all life. All living things are made up of cells. Some organisms consist of only one cell. Plants and animals are made up of many cells. The human body has more than 10 trillion (10,000,000,000,000) cells.

Most cells are so small they can be seen only with a microscope. It would take about 40,000 of your red blood cells to fill this letter O. It takes millions of cells to make up the skin on the palm of your hand.

Some one-celled organisms lead independent lives. Others live in loosely organized groups. The cells in plants and animals are specialists with particular jobs to do. As you read these words, for example, nerve cells in your eyes are carrying messages of what you are reading to your brain cells. Muscle cells attached to your eyeballs are moving your eyes across the page. Nerve cells, muscle cells, and other specialized cells group together to form tissues, such as nerve tissue or muscle tissue. Different kinds of tissues form organs, such as the eyes, heart, and lungs. All the specialized cells together form you—or some other complete organism.

All cells have some things in common, whether they are specialized cells or one-celled organisms. A cell is alive—as alive as you are. It "breathes," takes in food, and gets rid of wastes. It also grows and reproduces (creates its own kind). And, in time, it dies.

A thin covering called a membrane encloses every cell. The complete contents of a cell are called the protoplasm. Most cells have a structure called the nucleus. The nucleus contains the cell's genetic program, the master plan that controls almost everything the cell does. The part of the protoplasm outside the nucleus is called the cytoplasm.

Just as all living things are made up of cells, every new cell is produced from an existing cell. Cells reproduce by dividing, so that there are two cells where there once was only one cell. When a cell divides, each of the two newly produced cells gets a copy of the genetic program.

The genetic program is "written" in a chemical substance called DNA (deoxyribonucleic acid). All DNA looks much alike and is made up of the same building blocks. But the genetic program carried in DNA makes every living thing different from all others. This program makes a dog different from a fish, a zebra different from a rose, and a willow different from a wasp. It makes you different from other people.

Scientists understand much about a cell's genetic program and the chemical code carried by its DNA. They have used this understanding to alter a cell's genetic program so that an organism develops new characteristics. The new traits of such genetically engineered organisms can have commercially important applications. For example, researchers have developed genetically engineered varieties of tomatoes that stay fresh longer than normal varieties. Scientists hope to eventually control cancer and other diseases by correcting mistakes in a cell's genetic program.

Looking at a cell

One of the most important tools scientists use to study cells is the microscope. An optical microscope can magnify a cell up to 2,000 times. An electron microscope can magnify a cell by 1 million times. An ant magnified 200,000 times would be more than 21/2 miles (4 kilometers) long. But even with such tremendous magnification, the detailed structure of some cell parts still cannot be seen.

Scientists also use dyes to study cells. When various parts of a cell are stained with certain dyes, these parts stand out clearly under a microscope.

Another tool used to study cells is the centrifuge. This instrument separates the various substances in a mixture by whirling the mixture at high speeds. Scientists first grind up the cells. Then they put the mixture containing the cellular parts in a tube. The tube is placed in a centrifuge and whirled rapidly to separate the cellular parts. The heaviest parts move to the bottom of the tube, and the lightest remain at the top. After the parts have been separated, scientists can study their chemical content and activity.

Shapes of cells.

Cells of a plant root tip

Shapes of some types of cells

Sizes of cells.

Inside a living cell

Structures of a cell

Cells differ greatly in size, in shape, and in the special jobs they do. But all cells have certain features, and each cell can be thought of as a tiny chemical factory. It has a control center that tells it what to do and when. It has power plants for generating the energy it needs to function, and it has machinery for making its products or performing its services.

A thin covering called the cell membrane or plasma membrane encloses the cell and regulates substances that pass through it. Membranes consist of a double layer of fatty substance called phospholipid (see Lipid). Outside the membrane, many cells have a special covering that helps protect them or hold them to neighboring cells. In plant cells, this covering is called the cell wall.

Within the cell membrane, all cells except those of bacteria have two main parts: (1) the nucleus and (2) the cytoplasm. Cells with a nucleus are called eukaryotic which means having a true nucleus. All multicellular animals and plants consist of eukaryotic cells, as do the fungi and such unicellular organisms as amebas and diatoms. Bacteria cells lack a nucleus. They are called prokaryotic, which means before the nucleus.

The nucleus is the control center that directs the activities of the cell. A nuclear membrane surrounds the nucleus and separates it from the cytoplasm. The nucleus contains two important types of structures, chromosomes and nucleoli.

Chromosomes are long, threadlike strands of a substance called chromatin. Chromatin consists of DNA and certain proteins. DNA makes up the genes, the basic units of heredity. Genes control the passing on of characteristics from parents to offspring. Each gene consists of part of a DNA molecule. The chemical structure of the DNA that makes up the genes determines that a dog will give birth to a dog instead of a fish or some other organism. This chemical structure determines your blood type, the color of your eyes, the texture of your hair, and thousands of other characteristics.

DNA works its wonders chiefly by directing the production of complicated proteins. The cell's structures are built mostly of proteins. In addition, certain molecules called enzymes speed up chemical reactions in the cell. Without enzymes, these reactions would occur very slowly, and the cell could not function normally (see Enzyme). Thus, the kinds of proteins a cell makes help determine the nature of the cell.

Nucleoli are round bodies that form in certain regions of specific chromosomes. Each nucleus may contain one or more nucleoli, though some cells have none. Nucleoli help in the formation of ribosomes, the cell's centers of protein production. Nucleoli are made up of proteins and RNA (ribonucleic acid). RNA is chemically similar to DNA and plays important roles in making proteins.

The cytoplasm is all the material enclosed by the cell membrane, except for the nucleus. Thus, in prokaryotes, which do not have a nucleus, the cytoplasm includes everything inside the cell membrane. The cytoplasm of all cells contains ribosomes. Proteins manufactured on ribosomes make it possible for the cell to grow, repair itself, and perform the thousands of chemical operations that are required during the cell's lifetime.

The cytoplasm of eukaryotic cells also contains many other small structures called organelles. Each organelle has a particular job to do. The organelles include the mitochondria, endoplasmic reticulum, and Golgi complex. Some cells have other organelles, such as lysosomes, vacuoles, or chloroplasts. All eukaryotic cells also contain a network of proteins known as the cytoskeleton.

Mitochondria are the power plants of the cell. A cell may contain hundreds or even thousands of mitochondria. These structures convert the chemical energy contained in food into a form of energy the cell can use to grow, divide, and do its work.

The endoplasmic reticulum is a complex network of membranes. This network forms a system of pouches that store proteins and help channel substances to various parts of the cell. Some parts of the endoplasmic reticulum have a smooth surface. Other parts of the membrane have many ribosomes attached to their surface. Many of the cell's proteins are made on these ribosomes.

The Golgi complex, also known as the Golgi apparatus, consists of a stack of flat membrane sacs. These sacs process proteins and other substances produced in the cell. Small spheres called vesicles pinch off from the Golgi complex and move some of these substances to the cell membrane. They then may be transported across the membrane to other cells in the body or used to make the cell's covering. Other Golgi vesicles remain inside the cell and fuse with each other to form compartments that store proteins or other substances.

Lysosomes are round bodies containing enzymes that can break down many substances. For example, lysosomes inside white blood cells can destroy harmful bacteria. In plant cells and certain unicellular organisms, large, fluid-filled vacuoles usually perform the same function as lysosomes. In some plant cells, a single vacuole can take up most of the space in the cytoplasm.

Chloroplasts are organelles found in the cells of plants and algae. They contain a green substance called chlorophyll. During a process called photosynthesis, chlorophyll captures the energy of sunlight. Chloroplasts then use this energy to make sugars that are rich in chemical energy (see Photosynthesis). All living things directly or indirectly depend on these sugars for the energy to make all the other chemical substances in cells. For example, animals get energy by eating plants or by eating animals that have eaten plants.

The cytoskeleton consists of several types of protein rods that form a complicated network in the cytoplasm. The position of portions of the network against other portions or the expansion and contraction of parts of the network give a cell its shape, move organelles in the cell, and, in some cells, cause cell movement. Cells that swim do so by means of hairlike structures that extend out from the cell. These structures, called cilia or flagella, contain a bundle of cytoskeleton rods. In many cells, some of the cytoskeleton is found in the centrioles, a pair of short, wide cylinders involved in cell reproduction. Centrioles lie at right angles to each other, usually near the nucleus.

Cell Division

Every living thing is made up of one or more cells, and each of these cells was produced by an already existing cell. New cells are formed by division, so that there are two cells where there once was only one cell. One-celled organisms begin and complete their lives as single cells.

Human beings and other multicellular organisms also develop from a single cell. After the cell grows to a certain size, it divides and forms two cells. These two cells remain attached to each other. They grow and divide, forming four cells. The cells grow and divide over and over again, and during this process they begin to specialize. A dog, a fish, a human being, or some other multicellular organism finally develops from the single cell.

Cell division involves two processes. In the first process, called nuclear division, the nucleus divides. In the second process, called cytokinesis, the cytoplasm divides, and the cell splits in half. There are two types of nuclear division: (1) mitosis and (2) meiosis.

Mitosis. Most eukaryotic cells divide their nucleus by mitosis. In this process, the nucleus divides and forms two identical nuclei. Usually, the cytoplasm divides soon after mitosis, producing two daughter cells with identical nuclei. Most one-celled organisms and most of the cells in multicelled organisms reproduce by mitosis.

Mitosis takes place in four stages: (1) prophase, (2) metaphase, (3) anaphase, and (4) telophase. The period between the completion of one nuclear division and the beginning of the next one is called interphase. During interphase, the cell grows and carries on its normal activities, and its chromosomes are difficult to see with an optical microscope. Each chromosome and centriole makes a copy of itself at a particular time in interphase. The original chromosome and its copy are called sister chromatids. They are joined by a structure called a centromere. After duplication of the centrioles and chromosomes, the cell is ready to undergo mitosis.

The first stage of mitosis is called prophase. At this time, the chromosomes begin to coil up, condensing into visible threads that become progressively shorter and thicker. As the chromosomes condense, part of the cytoskeleton organizes into a network of fibers extending across the cell. This network is called the spindle. The centrioles move apart along the fibers of the spindle until they are at opposite sides of the cell. The centrioles mark the poles of the spindle. Toward the end of prophase, the nuclear membrane breaks apart.

In metaphase, the second stage of mitosis, the sister chromatids move to the spindle's middle, called the equator. They are still joined, but they line up on opposite sides of the equator. Each sister chromatid is attached at its centromere to at least one spindle fiber.

In the third stage, called anaphase, the centromeres divide, and each sister chromatid becomes a new chromosome. The new chromosomes separate and move to opposite poles.

In telophase, the final stage of mitosis, individual chromosomes uncoil and again become hard to see. A new nuclear membrane forms around each new daughter nucleus. Also, the spindle breaks down, and the proteins from spindle fibers form part of the networks of cytoskeleton in the daughter cells.

Usually, division of the cytoplasm also begins during telophase. In animal cells, cytokinesis occurs when the cell membrane pinches between the two daughter nuclei to form two daughter cells. In plant cells and other cells that have a cell wall, a cell wall grows between the daughter nuclei, forming two cells. In either case, each new cell has as many chromosomes as the original cell and contains the same hereditary information.

Cytokinesis does not always create two identical cells. Sometimes, one of the daughter cells receives more of one kind of organelle than does the other cell. Cytokinesis may also result in two different sized cells. In addition, if mitosis occurs more than once in the same cell without cytokinesis, the cell can have more than one nucleus.

Mitosis in plant cells differs somewhat from that in animal cells. Cells in multicellular plants do not have centrioles, but they do form a spindle similar to that formed in animal cells.

Meiosis. Human beings and many other living things reproduce sexually. A new individual can be created only if a male sex cell, called a sperm, unites with a female sex cell, called an egg. Sex cells, also called germ cells, are produced in special reproductive tissues or organs. At first, new sex cells are produced by mitosis. These cells then go through a special kind of cell division called meiosis. To understand why, we must understand something about heredity.

Every species of life has a certain number of chromosomes in each of its somatic (body) cells. These chromosomes exist in pairs. For example, human beings have 23 pairs of chromosomes; frogs, 13 pairs; and pea plants, 7 pairs. The members of each pair are similar in size, shape, and hereditary content. Suppose the egg and sperm cells had the same number of chromosomes as all the other cells in an organism. If they united, the somatic cells in the offspring would have twice the number of chromosomes that they should have.

For example, human beings have 46 chromosomes in their somatic cells. If the father's sperm cells and the mother's egg cells also contained 46 chromosomes, their child's somatic cells would have 92 chromosomes. The next generation would have 184, and so on. To prevent this from happening, the sex cells have half the chromosomes found in the somatic cells. This is accomplished by meiosis.

Meiosis consists of two separate nuclear divisions of sex cells. Each chromosome duplicates before the first division. Then each chromosome, which now consists of two joined sister chromatids, lines up side by side with the other chromosomes of its pair. Each pair of doubled chromosomes moves to the equator. The paired chromosomes then separate. One chromosome, still consisting of two chromatids, goes to one pole. The other chromosome moves to the opposite pole. Cytokinesis occurs, dividing the cytoplasm into two. Each daughter cell thus receives one chromosome, made up of two sister chromatids, from each of the original pairs. These new cells then divide. In this second division, one of each of the sister chromatids goes to each new daughter cell. Thus, the two divisions of meiosis produce a total of four cells. Each cell contains half the number of chromosomes found in all the other cells of the organism.

Human sperm and egg cells have 23 chromosomes each. When a sperm and an egg combine in a process called fertilization, they produce a single cell—the fertilized egg—with 46 chromosomes, or 23 similar pairs. A child develops from this egg. See Heredity (Sex cells and reproduction).

Growth and specialization are the processes by which a single fertilized egg cell develops into a particular organism. The fertilized egg from which you developed contained all the instructions on how you were to grow. The single cell divided by mitosis and cytokinesis. Then, cell after cell divided. After a large mass of cells had formed, the dividing cells began to differentiate (specialize), and became muscle cells, skin cells, nerve cells, and so on. The different cells grouped into tissues. These tissues then formed organs, such as your heart and lungs.

Understanding differentiation is a challenging problem for scientists. Every time a cell divides, it passes on the same heredity material. Scientists think that differentiation occurs when a specific set of genes becomes active in a cell. These genes produce certain proteins, many of which are enzymes, that cause the cell to differentiate. All the cells in an organism have the same genes and the same DNA, so what activates the specific set of genes in one cell type?

Death of a Cell

Like all other living things, cells die. Each day, several billion cells in the body die and are replaced by cell division. Dead skin cells flake off. Dead cells from internal organs pass out of the body with waste products. The life span of cells varies. For example, white blood cells live about 13 days; red blood cells live about 120 days; and liver cells live about 18 months. Nerve cells can live about 100 years.

The Work of a Cell

A cell is intensely active. It carries out life's functions, including growth and reproduction. In addition, cells in multicellular organisms have special jobs. To live and to do its work, a cell must obtain energy. It also must manufacture proteins and other substances needed for the construction of its parts and to speed up the thousands of chemical reactions that occur in the cell.

Producing Energy Most of a body's energy comes from the mitochondria, the power producers of a cell. The mitochondria are like power plants that burn fuel to produce the electric power that runs machines. The food a person eats is the fuel that is "burned" inside the mitochondria. A product of this burning is a compound called adenosine triphosphate (ATP). ATP is the "electric power" that runs a cell's activities. It supplies the energy needed to do work in the cells. For example, ATP supplies the energy to contract a muscle or send a message between nerve cells.

An ATP molecule contains three phosphate groups. Chemical bonds (forces that hold atoms together) link the phosphate groups together like railroad cars. The bonds that attach the second and third phosphate groups are especially rich in energy. When the bonds are broken, energy is released that the cell can use.

The source of energy for most living things—directly or indirectly—is the sun. Plant cells produce ATP during photosynthesis, the process by which green plants capture energy from the sun and use it to make sugars. When sunlight strikes a chlorophyll molecule in a chloroplast, it sets off a series of chemical reactions. The ATP produced provides the energy by which a plant then turns carbon dioxide from air and water from soil into sugars and other substances. Some other organisms, including certain bacteria, also produce ATP by photosynthesis. See Photosynthesis.

Animal cells obtain their energy from food that the animal eats. The animal's digestive system breaks down the food into basic parts. It breaks fats into fatty acids, sugars and starches into simple sugars, and proteins into chemical units called amino acids. The blood carries these substances to cells in the body.