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Cell

Cell

The cell is the basic unit of a living organism. In multicellular organisms (organisms with more than one cell), a collection of cells that work together to perform similar functions is called a tissue. In the next higher level of organization, various tissues that perform coordinated functions form organs. Finally, organs that work together to perform general processes form body systems.

Types of cells

Multicellular organisms contain a vast array of highly specialized cells. Plants contain root cells, leaf cells, and stem cells. Humans have skin cells, nerve cells, and sex cells. Each kind of cell is structured to perform a highly specialized function. Often, examining a cell's structure reveals much about its function in the organism. For instance, certain cells in the small intestine have developed microvilli (hairs) that promote the absorption of foods. Nerve cells, or neurons, are another kind of specialized cell whose form reflects function. Nerve cells consist of a cell body and long attachments, called axons, that conduct nerve impulses. Dendrites are shorter attachments that receive nerve impulses.

Sensory cells are cells that detect information from the outside environment and transmit that information to the brain. Sensory cells often have unusual shapes and structures that contribute to their function. The rod cells in the retina of the eye, for instance, look like no other cell in the human body. Shaped like a rod, these cells have a light-sensitive region that contains numerous disks. Within each disk is embedded a special light-sensitive pigment that captures light. When the pigment receives light from the outside environment, nerve cells in the eye are triggered to send a nerve impulse in the brain. In this way, humans are able to detect light.

Cells, however, can also exist as single-celled organisms. The organisms called protists, for instance, are single-celled organisms. Examples of protists include the microscopic organism called Paramecium and the single-celled alga called Chlamydomonas.

Prokaryotes and eukaryotes. Two types of cells are recognized in living things: prokaryotes and eukaryotes. The word prokaryote literally means "before the nucleus." As the name suggests, prokaryotes are cells that have no distinct nucleus. Most prokaryotic organisms are single-celled, such as bacteria and algae.

The term eukaryote means "true nucleus." Eukaryotes have a distinct nucleus and distinct organelles. An organelle is a small structure that performs a specific set of functions within the eukaryotic cell. These organelles are held together by membranes. In addition to their lack of a nucleus, prokaryotes also lack these distinct organelles.

The structure and function of cells

The basic structure of all cells, whether prokaryote and eukaryote, is the same. All cells have an outer covering called a plasma membrane. The plasma membrane holds the cell together and permits the passage of substances into and out of the cell. With a few minor exceptions, plasma membranes are the same in prokaryotes and eukaryotes.

The interior of both kinds of cells is called the cytoplasm. Within the cytoplasm of eukaryotes are embedded the cellular organelles. As noted above, the cytoplasm of prokaryotes contains no organelles. Finally, both types of cells contain small structures called ribosomes. Ribosomes are the sites within cells where proteins are produced. (Proteins are large molecules that are essential to the structure and functioning of all living cells.) Ribosomes are not bounded by membranes and are not considered, therefore, to be organelles.

Words to Know

Cell wall: A tough outer covering that overlies the plasma membrane of bacteria and plant cells.

Cilia: Short projections that cover the surface of some cells and provide for movement.

Cytoplasm: The semifluid substance of a cell containing organelles and enclosed by the cell membrane.

Cytoskeleton: The network of filaments that provide structure and movement of a cell.

DNA (deoxyribonucleic acid): The genetic material in the nucleus of cells that contains information for an organism's development.

Endoplasmic reticulum: The network of membranes that extends throughout the cell and is involved in protein synthesis and lipid metabolism.

Enzyme: Any of numerous complex proteins that are produced by living cells and spark specific biochemical reactions.

Eukaryote: A cell that contains a distinct nucleus and organelles.

Flagellum: A whiplike structure that provides for movement in some cells.

Golgi body: Organelle that sorts, modifies, and packages molecules.

Membrane: A thin, flexible layer of plant or animal tissue that covers, lines, separates or holds together, or connects parts of an organism.

Mitochondrion: The power-house of the cell that contains the enzymes necessary for turning food into energy.

Nuclear envelope: The double membrane that surrounds the nucleus.

Nuclear pore: Tiny openings that stud the nuclear envelope.

Nucleolus: The darker region within the nucleolus where ribosomal subunits are manufactured.

Nucleus: The control center of a cell that contains the DNA.

Organelle: A membrane-bounded cellular "organ" that performs a specific set of functions within a eukaryotic cell.

Pili: Short projections that assist bacteria in attaching to tissues.

Plasma membrane: The membrane of a cell.

Plastid: A vesicle-like organelle found in plant cells.

Prokaryote: A cell without a true nucleus.

Protein: Large molecules that are essential to the structure and functioning of all living cells.

Protist: A single-celled eukaryotic organism.

Ribosome: A protein composed of two subunits that functions in protein synthesis.

Vacuole: A space-filling organelle of plant cells.

Vesicle: A membrane-bound sphere that contains a variety of substances in cells.

The structure of prokaryotes. An example of a typical prokaryote is the bacterial cell. Bacterial cells can be shaped like rods, spheres, or corkscrews. Like all cells, prokaryotes are bounded by a plasma membrane. Surrounding this plasma membrane is a cell wall. In addition, in some bacteria, a jelly-like material known as a capsule coats the cell wall. Many disease-causing bacteria have capsules. The capsule provides an extra layer of protection for the bacteria. Pathogenic bacteria with capsules tend to cause much more severe disease than those without capsules.

Within the cytoplasm of prokaryotes is a nucleoid, a region where the cell's genetic material is stored. (Genes determine the characteristics passed on from one generation to the next.) The nucleoid is not a true nucleus because it is not surrounded by a membrane. Also within the cytoplasm are numerous ribosomes.

Attached to the cell wall of some bacteria are flagella, whiplike structures that make it possible for the bacteria to move. Some bacteria also have pili, short, fingerlike projections that help the bacteria to attach to tissues. Bacteria cannot cause disease if they cannot attach to tissues. Bacteria that cause pneumonia, for instance, attach to the tissues of the lung. Bacterial pili greatly facilitate this attachment to tissues. Thus, bacteria with pili, like those with capsules, are often more deadly than those without.

The structure of eukaryotes. The organelles found in eukaryotes include the membrane system, consisting of the plasma membrane, endoplasmic reticulum, Golgi body, and vesicles; the nucleus; cytoskeleton; and mitochondria. In addition, plant cells have special organelles not found in animals cells. These organelles are the chloroplasts, cell wall, and vacuoles. (See the drawing of a plant cell on page 435.)

Plasma membrane. The plasma membrane of the cell is often described as selectively permeable. That term means that some substances are able to pass through the membrane but others are not. For example, the products formed by the breakdown of foods are allowed to pass into a cell, and the waste products formed within the cell are allowed to pass out of the cell. Since the 1960s, scientists have learned a great deal about the way the plasma membrane works. It appears that some materials are able to pass

through tiny holes in the membrane of their own accord. Others are helped to pass through the membrane by molecules located on the surface of and within the membrane itself. The study of the structure and function of the plasma membrane is one of the most fascinating in all of cell biology.

Endoplasmic reticulum. The endoplasmic reticulum (ER) consists of flattened sheets, sacs, and tubes of membrane that cover the entire expanse of a eukaryotic cell's cytoplasm. The ER looks something like a very complex subway or highway system. That analogy is not a bad one, since a major function of ER is to transport materials throughout the cell.

Two kinds of ER can be identified in a cell. One type is called rough ER and the other is called smooth ER. The difference between the two is that rough ER contains ribosomes on its outside surface, giving it a rough or grainy appearance. Rough ER is involved in the process of protein synthesis (production) and transport. Proteins made on the ribosomes attached to rough ER are modified, "packaged," and then shipped to various parts of the cell for use. Some are sent to the plasma membrane, where they are moved out of the cell and into other parts of the organism's body for use.

Smooth ER has many different functions, including the manufacture of lipids (fatlike materials), the transport of proteins, and the transmission of nerve messages.

The Golgi body. The Golgi body is named for its discoverer, the nineteenth century Italian scientist Camillo Golgi (18431926). It is one of the most unusually shaped organelles. Looking somewhat like a stack of pancakes, the Golgi body consists of a pile of membrane-bounded, flattened sacs. Surrounding the Golgi body are numerous small membrane-bounded vesicles (particles). The function of the Golgi body and its vesicles is to sort, modify, and package large molecules that are secreted by the cell or used within the cell for various functions.

The Golgi body can be compared to the shipping and receiving department of a large company. Each Golgi body within a cell has a cis face, which is similar to the receiving division of the department. Here, the Golgi body receives molecules manufactured in the endoplasmic reticulum. The trans face of the Golgi body can be compared to the shipping division of the department. It is the site from which modified and packaged molecules are transported to their destinations.

Vesicles. Vesicles are small, spherical particles that contain various kinds of molecules. Some vesicles, as noted above, are used to transport molecules from the endoplasmic reticulum to the Golgi body and from the Golgi body to various destinations. Special kinds of vesicles perform other functions as well. Lysosomes are vesicles that contain enzymes involved in cellular digestion. Some protists, for instance, engulf other cells for food. In a process called phagocytosis (pronounced FA-go-sy-to-sis), the protist surrounds a food particle and engulfs it within a vesicle. This food-containing vesicle is transported within the protist's cytoplasm until it is brought into contact with a lysosome. The food vesicle and lysosome merge, and the enzymes within the lysosome are released into the food vesicle. The enzymes break down the food into smaller parts for use by the protist.

The nucleus. The nucleus is the control center of the cell. Under a microscope, the nucleus looks like a dark blob, with a darker region, called the nucleolus, centered within it. The nucleolus is the site where parts of ribosomes are manufactured. Surrounding the nucleus is a double membrane called the nuclear envelope. The nuclear envelope is covered with tiny openings called nuclear pores.

The nucleus directs all cellular activities by controlling the synthesis of proteins. Proteins are critical chemical compounds that control almost everything that cells do. In addition, they make up the material from which cells and cell parts themselves are made.

The instructions for making proteins are stored inside the nucleus in a helical molecule called deoxyribonucleic acid, or DNA. DNA molecules differ from each other on the basis of certain chemical units, called nitrogen bases, that they contain. The way nitrogen bases are arranged within any given DNA molecule carries a specific genetic "message." One arrangement of nitrogen bases might carry the instruction "Make protein A," another arrangement of bases might carry the message "Make protein B," yet a third arrangement might code for the message "Make protein C," and so on.

The first step in protein synthesis begins in the nucleus. Within the nucleus, DNA is translated into a molecule called messenger ribonucleic acid (mRNA). MRNA then leaves the nucleus through the nuclear pores. Once in the cytoplasm, mRNA attaches to ribosomes and initiates protein synthesis. The proteins made on ribosomes may be used within the same cell or shipped out of the cell through the plasma membrane for use by other cells.

The cytoskeleton. The cytoskeleton is the skeletal framework of the cell. Instead of bone, however, the cell's skeleton consists of three kinds of protein filaments that form networks. These networks give the cell shape and provide for cellular movement. The three types of cytoskeletal fibers are microtubules, actin filaments, and intermediate filaments.

Microtubules are very thin, long tubes that form a network of "tracks" over which various organelles move within the cell. Microtubules also form small, paired structures called centrioles within animal cells. These structures are not considered organelles because they are not bounded by membranes. Centrioles are involved in the process of cell division (reproduction).

Some eukaryotic cells move about by means of microtubules attached to the exterior of the plasma membrane. These microtubules are called flagella and cilia. Cells with cilia also perform important functions in the human body. The airways of humans and other animals are lined with such cells that sweep debris and bacteria upwards, out of the lungs and into the throat. There, the debris is either coughed from the throat or swallowed into the digestive tract, where digestive enzymes destroy harmful bacteria.

Actin filaments are especially prominent in muscle cells, where they provide for the contraction of muscle tissue. Intermediate filaments are relatively strong and are often used to anchor organelles in place within the cytoplasm.

Mitochondria. The mitochondria are the power plants of cells. Each sausage-shaped mitochondrion is covered by an outer membrane. The inner membrane of a mitochondrion is folded into compartments called cristae (meaning "box"). The matrix, or inner space created by the cristae, contains the enzymes necessary for the many chemical reactions that eventually transform food molecules into energy.

Plant organelles. Plant cells have several organelles not found in animal cells. These include plastids, vacuoles, and a cell wall.

Plastids are vesicle-type organelles that perform a variety of functions in plants. For example, amyloplasts store starch and chromoplasts store pigment molecules that give some plants their vibrant orange and yellow colors. Chloroplasts are plastids that carry out photosynthesis, a process in which water and carbon dioxide are transformed into sugars.

Vacuoles are large vesicles bound by a single membrane. In many plant cells, they occupy about 90 percent of the cellular space. They perform a variety of functions in the cell, including storage of organic compounds, waste products, pigments, and poisonous compounds as well as digestive functions.

All plant cells have a cell wall that surrounds the plasma membrane. The cell wall of plants consists of a tough carbohydrate substance called cellulose laid down in a medium or network of other carbohydrates. (A carbohydrate is a compound consisting of carbon, hydrogen, and oxygen found in plants and used as a food by humans and other animals.) The cell wall provides an additional layer of protection between the contents of the cell and the outside environment. The crunchiness of an apple, for instance, is attributed to the presence of these cell walls.

[See also Chromosome; Enzyme; Neuron; Nucleic acid; Protein; Reproduction; Respiration ]

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Cell

Cell

A cell is the smallest unit of living matter. Cells were first identified in Europe in the seventeenth century by Antoni van Leeuwenhoek and others. They were named by Robert Hooke, an Englishman, who said they reminded him of the rooms or "cells" in a monastery. The cell theory describes some fundamental characteristics of all cells and is one of the unifying concepts in biology. It states that: (1) all organisms are made of cells, a cell is the structural and functional unit of organs, and therefore cells are organisms; and (2) cells are capable of self-reproduction and come only from preexisting cells.

Prokaryotic Cells

Cells come in many shapes and sizes and have different structural features. Bacteria are single-celled organisms approximately 1 to 10 micrometers (.00004 to .0004 inch) in size and can be spherical, rod-shaped, or spiral-shaped. They are known as prokaryotes (from the Greek pro, meaning "before" and karyon, meaning "kernel" or "nucleus") because they contain a nucleoid region rather than a true nucleus where their genetic material is found. All bacteria have cell walls that may be surrounded by a capsule and/or a gelatinous slime layer.

Beneath the cell wall is the plasma membrane responsible for regulating the flow of materials into and out of the cell's cytoplasm within the interior of the cell. The cytoplasm is composed of fluid known as cytosol and solid materials. Within the cytosol are ribosomes , granular bodies that direct the synthesis of all bacterial proteins . Some bacteria have whiplike appendages called flagella that enable them to move. The genetic material of bacteria is deoxyribonucleic acid (DNA), which is contained within a single circular chromosome in the nucleoid region and sometimes also in a smaller ring called a plasmid .

Eukaryotic Cells

Eukaryotic cells (from the Greek eu, meaning "true" and karyon, meaning "kernel" or "nucleus") are more complex than prokaryotic cells and are found in both unicellular organisms like the amoeba and multicellular organisms like sunflowers, mushrooms, and humans. They are generally larger than prokaryotic cells, ranging from about 10 to 100 micrometers (.0004 to .004 inch) in size. In multicellular organisms, there are many different types of cells that perform specialized functions. In animals, for instance, pancreatic cells make and secrete hormones , whereas red blood cells are specialized for transporting oxygen throughout the body. Cells with specialized functions such as these are called "differentiated."

All eukaryotic cells share specific structural characteristics. These include a true nucleus that is bounded by a double-layered membrane known as the nuclear membrane. Within the nucleus is housed the cell's genetic material in the form of linear chromosomes of DNA contained in thread-like structures called chromatin . All eukaryotic cells have a plasma membrane that encloses the cytoplasm. Cells of plants, fungi, and many protists have an additional outer boundary called a cell wall that differs significantly in structure and composition from that of a prokaryotic cell.

Eukaryotic cells have many different kinds of small membrane-bound structures called organelles that, with the exception of ribosomes, are absent from prokaryotic cells. Eukaryotic ribosomes (which are not enclosed by a membrane) float freely in the cytosol or are attached to another organelle known as the endoplasmic reticulum (ER). The ER is a series of membrane-bound, fluid-filled spaces in contact with the nuclear membrane. Its function is to synthesize and/or modify proteins, phospholipids, and cholesterol and to transport substances from the nucleus to the rest of the cell.

When the ER is studded with ribosomes it is called the rough ER. When ribosomes are absent it is called the smooth ER. The Golgi apparatus is a system of membrane-enclosed sacs responsible for transporting newly synthesized proteins and lipids from the ER to other organelles and the plasma membrane. It is also the site of polysaccharide synthesis and modification of proteins and lipids by addition of sugars.

Both animal and plant cells have mitochondria , power houses that convert energy stored in the chemical bonds of nutrients like carbohydrates , proteins, and fats into adenosine triphosphate (ATP), a high-energy chemical compound that is required for many cellular processes. Many plant cells also have chloroplasts, organelles that contain the pigment chlorophyll. Chloroplasts conduct photosynthesis, in which plants use sunlight, water, and carbon dioxide to synthesize the sugar glucose .

Organelles in Eukaryotic Cells
Structure Function
Nucleus Contains genetic material
Ribosomes Protein synthesis
Endoplasmic reticulum Synthesis/modification and transport of proteins and lipids
Golgi apparatus Processing, distribution of proteins, lipids
Lysosomes Digestion of substances in cell
Peroxisomes Digestion and detoxification
Chloroplasts Photosynthesis
Flagella/Cilia Cell movement
Vacuole and vesicle Storage of cellular substances
Centriole Cytoskeletal organization

Lysosomes are membrane-enclosed bodies in plant and animal cells that contain enzymes responsible for digesting substances within the cell. In animal cells, peroxisomes contain enzymes that metabolize lipids and alcohol. In plants, peroxisomes also convert fatty acids into molecules that are precursors of sugars. Both plant and animal cells have vacuoles, membranous sacs that store substances such as water, sugars, and salts. Protozoans, a type of unicellular protist, have specialized contractile vacuoles for removing excess water from the cell.

Most organelles do not flow freely in the cytoplasm but are anchored to a complex intracellular framework known as the cytoskeleton , which is made of three different types of protein fibers: microfilaments, intermediate filaments, and microtubules. The cytoskeleton is involved in maintaining cell shape and participates in cell movement and cell division. The centrosome contains a pair of organelles called centrioles close to the nucleus of animal cells. It is responsible for organizing some of the cytoskeletal components.

Some plant and animal cells have projections from the plasma membrane known as flagella or cilia that are capable of movement. For example, a single flagellum is responsible for the movement of sperm cells.

see also Cell Wall; Chloroplast; Cytoskeleton; DNA; Golgi; History of Biology: Cell Theory and Cell Structure; Mitochondrion; Nucleus; Ribosome; Vacuole

Michele D. Blum

Bibliography

Mader, Sylvia S. Biology, 6th ed. Boston: McGraw-Hill, 1998.

McFadden, Carol, H., and William T. Keeton. Biology: An Exploration of Life. New York: W. W. Norton and Company, Inc., 1995.

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cell

cell The cell is the fundamental unit of all living things. The simplest forms of life are single-celled organisms; these include both ‘prokaryotes’ — bacteria, which have a simple internal structure — and the much more complex ‘eukaryotes’ (pro, before or preceding; eu, good, normal, and karyon, a kernel). Higher organisms, such as man, are sophisticated communities in which groups of eukaryotic cells carry out specialized functions and communicate with each other. Prokaryotes are usually about one thousandth of a millimetre in diameter. Eukaryotic cells are much larger, typically around one to two hundredths of a millimetre. There are about 100 million million cells in the human body. Both prokaryotes and eukaryotes usually multiply by dividing in two, although in multicellular organisms cell division is under strict control.

It was the invention of the microscope, in the seventeenth century, that allowed scientists the first glimpses of individual cells. In particular, the Dutchman, Antoni van Leeuwenhoek described the extraordinary variety of motile single-celled organisms (which he called ‘animalcules’) present in pond water. The word ‘cell’ (from the Latin cella, ‘a small room’) was first coined in 1665, by the English physicist Robert Hooke, to refer to the microscopic structure of cork. Technical improvements in microscopy in the eighteenth and nineteenth centuries allowed more precise observation. It gradually became apparent that cells had a complicated internal structure, and that some features (for example, what we now refer to as the nucleus) were common to most cells, even though the appearance of the cells themselves varied enormously. This in turn hinted that a common basic organization might underlie all living matter.

The first simple distinction had been between nucleus and cytoplasm — the rest of the cellular substance — but by the end of the nineteenth century the principal internal components of cells that we are familiar with today (sub-cellular structures or organelles) had been identified. These included the endoplasmic reticulum (an extensive network of membranes within the cell), mitochondria (cylindrical, membrane-limited structures) and the Golgi complex (a stack of flattened membrane sacs, named after the Italian anatomist who described those and other intracellular structures in 1898, and later shared a Nobel prize with Spaniard Ramón y Cajal). The true complexity of the internal structure of cells, however, only became apparent in the 1950s, when cells were examined with the newly-invented electron microscope, which had much greater resolving power than the conventional light microscope — magnifying 20–30 000 times. It was around this time that the field now known as cell biology began to come to prominence, with the goal of understanding how the various organelles acted together to allow the cell to carry out its many functions. As well as simply observing cell structure, cell biologists now began to take cells apart and purify the different organelles using high-speed centrifugation. It was also shown that the purified organelles could be made to work in isolation, which allowed a detailed study of their functions, and the identification of the mechanisms underlying them.

Our current view of the cell is as an organism-in-miniature. The blueprint is contained in the DNA, packaged into chromosomes in the nucleus. Parts of the DNA sequence are replicated into ‘messenger’ RNA, which exits the nucleus and specifies the sequences of the cell's proteins, which are constructed in the cytoplasm. The power-houses of the cell are the mitochondria, which use nutrients taken up from outside to generate ATP, the energy currency of the cell. (Plant cells have additional organelles, the chloroplasts, which contain chlorophyll, the molecule responsible for capturing the energy of sunlight and initiating the process of photosynthesis. This results in the production of carbon-containing molecules for use by the cell and the generation of oxygen, which is essential for the continuation of life on earth.)

Many cells are responsible for secreting substances which will have external effects. In the pancreas, for instance, some cells secrete enzymes into the gut, where they digest our food, whereas other cells secrete insulin into the bloodstream, which instructs cells in the rest of the body to take up glucose. Both the digestive enzymes and the insulin are packaged into the endoplasmic reticulum and are then transported to the surface of the cell via the Golgi apparatus. Thus although each organelle is a discrete structure, there is extensive communication between organelles. This intracellular trafficking system demands that there be strict controls on the movement of proteins between organelles, and that individual proteins be ‘tagged’ for delivery to particular destinations. Without this control, the organization of the cell would quickly disintegrate.

A single higher organism contains a huge variety of cell types: compare, for example, a neuron with a lymphocyte, or a skeletal muscle cell with a liver cell (hepatocyte). All of these cells were produced from a single fertilized egg, by processes including cell division, migration, differentiation, and death. We are only just beginning to understand how these processes are orchestrated to produce the complete organism. One aspect that is crucial to the development and maintenance of multicellular organisms is communication between cells. Cells are continually signalling to their neighbours through the release of molecules that are detected by specialized receptors on the surface of other cells. In the brain, for example, neurons ‘talk’ to each other by means of small molecules. These molecules, or ‘neurotransmitters’, are packaged in small sacs within the neurons, and are released when an electrical impulse passes to the end of its axon. The neurotransmitter then binds to receptors on the neighbouring neurons and changes the electrical properties of these neurons, making them more or less likely to initiate an electrical impulse themselves. In other parts of the body, neurons communicate in similar fashion with muscle cells, causing them to contract, or with glandular cells, causing them to secrete. Many drugs work by blocking or mimicking the action of these neurotransmitters. Again, some cells release molecules which travel in the blood: messengers which communicate with remotely distant cells that have the appropriate receptors on their surface.

Once tissues and organs have been formed it is essential that cell division be strictly controlled in order to maintain normal function. Many proteins are now known which control cell division, often in response to external stimuli. Mutations in these proteins can result in uncontrolled cell division. This can lead eventually to the formation of tumours, which can be life threatening.

We are now familiar with the idea that cells are produced by the division of progenitor cells. This idea, of course, begs the question as to how the first cell was produced. It has been shown that simple organic molecules can form under conditions believed to be similar to those that existed on earth in its early history. How these molecules became assembled into proteins, and more particularly how the self-replicating ‘blueprint’ molecules such as DNA came about, are fundamental unanswered questions.

Michael Edwardson


See also cell membranes; cell signalling.

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cell (in biology)

cell, in biology, the unit of structure and function of which all plants and animals are composed. The cell is the smallest unit in the living organism that is capable of integrating the essential life processes. There are many unicellular organisms, e.g., bacteria and protozoans, in which the single cell performs all life functions. In higher organisms, a division of labor has evolved in which groups of cells have differentiated into specialized tissues, which in turn are grouped into organs and organ systems.

Cells can be separated into two major groups—prokaryotes, cells whose DNA is not segregated within a well-defined nucleus surrounded by a membranous nuclear envelope, and eukaryotes, those with a membrane-enveloped nucleus. The bacteria (kingdom Monera) are prokaryotes. They are smaller in size and simpler in internal structure than eukaryotes and are believed to have evolved much earlier (see evolution). All organisms other than bacteria consists of one or more eukaryotic cells.

All cells share a number of common properties; they store information in genes made of DNA (see nucleic acid); they use proteins as their main structural material; they synthesize proteins in the cell's ribosomes using the information encoded in the DNA and mobilized by means of RNA; they use adenosine triphosphate as the means of transferring energy for the cell's internal processes; and they are enclosed by a cell membrane, composed of proteins and a double layer of lipid molecules, that controls the flow of materials into and out of the cell.

Cell Structure

In the nucleus the DNA, along with certain proteins, is arranged in long, thin threads called chromatin fibers that coil into bodies called chromosomes during meiosis. The nucleus also contains one or more nucleoli (sing., nucleolus) that participate in the production on the RNA of ribosomes. The portion of the cell outside the nucleus, called the cytoplasm, contains several additional cell structures (often called organelles). Among the important organelles that may be present are the ribosomes; the endoplasmic reticulum, a highly convoluted system of membranes believed to be continuous with the nuclear envelope and responsible for transporting certain newly made proteins; the mitochondria, which extract energy by breaking down the chemical bonds in molecules of complex nutrients during respiration; the chloroplasts, which are present only in green plants and convert energy from sunlight by the process of photosynthesis; lysosomes, which contain digestive enzymes; peroxisomes, which contain a number of specialized enzymes; the centrosomes, which function during cell division; the Golgi apparatus, which functions in the synthesis, storage, and secretion of various cellular products; filaments and microtubules that form a sort of skeletal system known as a cytoskeleton and also participate in movement of cells and organelles; vacuoles containing food in various stages of digestion (see endocytosis); and inert granules and crystals. In plant cells there is, in addition to the cell membrane, a thickened cell wall, usually composed chiefly of cellulose secreted by the cell.

The Study of Cells

Because almost all cells are microscopic, knowledge of the component cell parts increased proportionately to the development of the microscope and other specialized instruments and of allied experimental techniques. Among those who contributed to early knowledge of cells through their use of the microscope were Antony van Leeuwenhoek, Robert Hooke, and Marcello Malpighi. In the 19th cent. Matthias J. Schleiden and Theodor Schwann developed what is now known as the cell theory. The theory was widely promoted after the pronouncement by Rudolf Virchow in 1855 that "omnis cellulae e cellula" [All cells arise from cells]. The study of cell structure came to be called cytology and that of tissues histology. In the 20th cent. appreciation of the biochemistry of the cell has flourished, along with a better understanding of its structure; cell biology now integrates both chemical and structural information.

See also biochemistry.

Bibliography

See L. Thomas, The Lives of a Cell (1974); D. M. Prescott, Cells (1988); B. Alberts et al., Molecular Biology of the Cell (2d ed. 1989); J. M. Lackie and J. A. Dowe, ed., The Dictionary of Cell Biology (1989).

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cell

cell The structural and functional unit of most living organisms (compare coenocyte; syncytium). Cell size varies, but most cells are microscopic (average diameter 0.01–0.1 mm). Cells may exist as independent units of life, as in bacteria and certain protoctists, or they may form colonies or tissues, as in all plants and animals. Each cell consists of a mass of protein material that is differentiated into cytoplasm and a nucleus, which contains DNA. The cell is bounded by a plasma membrane, which in the cells of plants, fungi, algae, and bacteria is surrounded by a cell wall. There are two main types of cell. Prokaryotic cells (bacteria) are the more primitive. The nuclear material is not bounded by a membrane and chemicals involved in cell metabolism are associated with the plasma membrane. Reproduction is generally asexual and involves simple cell cleavage. In eukaryotic cells the nucleus is bounded by a nuclear membrane and the cytoplasm is divided by membranes into a system of interconnected cavities and separate compartments (organelles), e.g. mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, and ribosomes (see illustration). Reproduction can be either asexual (see mitosis) or sexual (see meiosis). Plants and animals consist of eukaryotic cells but plant cells possess chloroplasts and other plastids and bear a rigid cellulose cell wall. See Chronology.

CELL BIOLOGY

1665

English physicist Robert Hooke (1635–1703) coins the word ‘cell’.

1831

Robert Brown discovers the nucleus in plant cells.

1838

German botanist Matthias Schleiden (1804–81) proposes that plants are composed of cells.

1839

Theodor Schwann states that animals are composed of cells and concludes that all living things are made up of cells.

1846

German botanist Hugo von Mohl (1805–72) coins the word ‘protoplasm’ for the living material of cells.

1858

German pathologist Rudolf Virchow (1821–1902) postulates that all cells arise from other cells.

1865

German botanist Julius von Sachs (1832–97) discovers the chlorophyll-containing bodies in plant cells later named chloroplasts.

1876–80

German cytologist Eduard Strasburger (1844–1912) describes cell division in plants and states that new nuclei arise from division of existing nuclei.

1882

German cytologist Walther Flemming (1843–1905) describes the process of cell division in animal cells, for which he coins the term ‘mitosis’. Strasburger coins the words ‘cytoplasm’ and ‘nucleoplasm’.

1886

German biologist August Weismann (1834–1914) proposes his theory of the continuity of the germ plasm.

1887

Belgian cytologist Edouard van Beneden (1846–1910) discovers that the number of chromatin-containing threadlike bodies (subsequently named chromosomes) in the cells of a given species is always the same and that the sex cells contain half this number.

1888

German anatomist Heinrich von Waldeyer (1836–1921) coins the word ‘chromosome’.

1898

Camillo Golgi discovers the Golgi apparatus.

1901

US biologist Clarence McClung (1870–1946) discovers the sex chromosomes.

1911

Thomas Hunt Morgan produces the first chromosome map.

1949

Canadian geneticist Murray Barr (1908–95) discovers Barr bodies.

1955

Belgian biochemist Christian de Duve (1917– ) discovers lysosomes and peroxisomes.

1956

Romanian-born US physiologist George Palade (1912– ) discovers the role of microsomes (later renamed ribosomes).


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"cell." A Dictionary of Biology. 2004. Encyclopedia.com. 29 Aug. 2016 <http://www.encyclopedia.com>.

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cell

cell
1. An address, a location in memory, or a register, usually one capable of holding a binary number. It is sometimes a location capable of holding one bit.

2. The basic unit of a spreadsheet or some other table of text, formed by the intersection of a row and column. It contains a label, value, or formula with attributes such as size, font, and color.

3. The name given to a packet in one version of a packet switching system. Packet switching systems subdivide the data to be transmitted into a number of packets. In contrast to many systems, a cell is short – for instance 53 bytes in the case of an ATM cell – and its internal structure is fixed. Small size and fixed structure allow the cell to be switched using a very simple algorithm; the processing time required for switching is thus reduced, with a corresponding increase in the number of cells switched in a given time.

4. The coverage area provided by a base station to a mobile (wireless) phone. As the user moves geographically, the conversation is “handed over” to another cell at another base station.

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JOHN DAINTITH. "cell." A Dictionary of Computing. 2004. Encyclopedia.com. 29 Aug. 2016 <http://www.encyclopedia.com>.

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cell

cell / sel/ • n. 1. a small room in which a prisoner is locked up or in which a monk or nun sleeps. ∎  a small compartment in a larger structure such as a honeycomb. ∎  hist. a small monastery or nunnery dependent on a larger one. 2. Biol. the smallest structural and functional unit of an organism, typically microscopic and consisting of cytoplasm and a nucleus enclosed in a membrane. ∎  an enclosed cavity in an organism. ∎ fig. a small group forming a nucleus of political activity, typically a secret, subversive one: the weapons may be used to arm terrorist cells. ∎  the local area covered by one of the short-range transmitters in a cellular telephone system. 3. a device containing electrodes immersed in an electrolyte, used for current-generation or electrolysis. ∎  a unit in a device for converting chemical or solar energy into electricity. DERIVATIVES: celled adj. [in comb.] a single-celled organism. cell-like / -ˌlīk/ adj.

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"cell." The Oxford Pocket Dictionary of Current English. 2009. Encyclopedia.com. 29 Aug. 2016 <http://www.encyclopedia.com>.

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cell

cell Basic biological unit of which all plant and animal tissues are composed. The cell is the smallest unit of life that can exist independently, with its own self-regulating chemical system. Most cells consist of a membrane surrounding jelly-like cytoplasm with a central nucleus. The nucleus is the main structure in which DNA is stored in chromosomes. Animal cells vary widely in shape. A red blood cell, for instance, is a biconcave disc, while a nerve cell has a long fibre. The cells of plants and algae are enclosed in a cell wall, which gives them a more rigid shape. Prokaryote cells, as in bacterial, also have a cell wall, but do not have nuclei or chromosomes; instead, they have a loop of DNA floating in the cytoplasm. More advanced cells (those that have nuclei), often have other membrane-bounded structures inside the cell, such as chloroplasts within a plant cell. Cells divide by duplicating the DNA and splitting the nucleus. This takes place by meiosis in sexual reproduction and by mitosis in asexual reproduction. See also eukaryote; mitochondrion

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"cell." World Encyclopedia. 2005. Encyclopedia.com. 29 Aug. 2016 <http://www.encyclopedia.com>.

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cell

cell.
1. Small apartment of any sort, such as a room in a dormitory or inn, but especially a confined study-bedroom allotted to a monk or nun in a monastery. Also a penitential cell in the Middle Ages in which penitents were immured.

2. Secure room with bed or beds in a prison.

3. Any small cavity or room.

4. Cella or naos.

5. Web of a vault framed by the ribs, or one surface of a groin vault.

6. In timber-framed structures, one room or unit. A single-cell plan is one volume, while a two-cell plan may have a cross-entry or cross-passage, and a three-cell plan will have a cross-passage, cross-entry, or lobby-entry.

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JAMES STEVENS CURL. "cell." A Dictionary of Architecture and Landscape Architecture. 2000. Encyclopedia.com. 29 Aug. 2016 <http://www.encyclopedia.com>.

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JAMES STEVENS CURL. "cell." A Dictionary of Architecture and Landscape Architecture. 2000. Retrieved August 29, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O1-cell.html

cell

cell (sel) n. the basic unit of all living organisms, which can reproduce itself exactly (see mitosis). Cells contain cytoplasm, in which are suspended a nucleus and other structures (organelles) specialized to carry out particular activities in the cell. Complex organisms are built up of millions of cells that are specially adapted to carry out particular functions. See illustration overleaf.

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"cell." A Dictionary of Nursing. 2008. Encyclopedia.com. 29 Aug. 2016 <http://www.encyclopedia.com>.

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cell

cell The fundamental autonomous unit of plant and animal bodies, consisting of, at least, a cell membrane containing cytoplasm and nuclear material, but often having a more complex structure. Simple organisms are unicellular, but more complex organisms consist of many co-operating cells. Characteristically, in plants, but not in animals, cells are surrounded by a polysaccharide wall.

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MICHAEL ALLABY. "cell." A Dictionary of Plant Sciences. 1998. Encyclopedia.com. 29 Aug. 2016 <http://www.encyclopedia.com>.

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cell

cell
1. The fundamental autonomous unit of plant and animal bodies, consisting of, at least, a cell membrane containing cytoplasm and nuclear material, but often having a more complex structure. Simple organisms are unicellular, but more complex organisms consist of many co-operating cells. Cells may be eukaryote or prokaryote.

2. See MASS PROVISIONING.

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MICHAEL ALLABY. "cell." A Dictionary of Zoology. 1999. Encyclopedia.com. 29 Aug. 2016 <http://www.encyclopedia.com>.

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cell

cell
1. The fundamental autonomous unit of plant and animal bodies, consisting of, at least, a cell membrane containing cytoplasm and nuclear material, but often having a more complex structure. Simple organisms are unicellular, but more complex organisms consist of many co-operating cells. Cells may be eukaryote or prokaryote.

2. See mass provisioning.

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MICHAEL ALLABY. "cell." A Dictionary of Ecology. 2004. Encyclopedia.com. 29 Aug. 2016 <http://www.encyclopedia.com>.

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cell

cell dependent religious house XII; small dwelling or apartment; cavity in an organism XIV; various subsequent uses. — OF. celle, or its source L. cella store-room, chamber, small apartment, in medL. in the first two senses above, rel. to L. cēlāre CONCEAL.

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T. F. HOAD. "cell." The Concise Oxford Dictionary of English Etymology. 1996. Encyclopedia.com. 29 Aug. 2016 <http://www.encyclopedia.com>.

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Cell

Cell

a small religious group or community connected to a monastery or convent; a unit of persons forming part of a network in a political party; a communist cell; a cell of workers.

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"Cell." Dictionary of Collective Nouns and Group Terms. 1985. Encyclopedia.com. 29 Aug. 2016 <http://www.encyclopedia.com>.

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cell

cell, cell entry See CONTINGENCY TABLE.

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GORDON MARSHALL. "cell." A Dictionary of Sociology. 1998. Encyclopedia.com. 29 Aug. 2016 <http://www.encyclopedia.com>.

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cell (in electricity)

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cell

cellAdele, Aix-la-Chapelle, aquarelle, artel, au naturel, bagatelle, béchamel, befell, bell, belle, boatel, Brunel, Cadell, carousel, cartel, cell, Chanel, chanterelle, clientele, Clonmel, compel, Cornell, crime passionnel, dell, demoiselle, dispel, dwell, el, ell, Estelle, excel, expel, farewell, fell, Fidel, fontanelle, foretell, Gabrielle, gazelle, gel, Giselle, hell, hotel, impel, knell, lapel, mademoiselle, maître d'hôtel, Manuel, marcel, matériel, mesdemoiselles, Michel, Michelle, Miguel, misspell, morel, moschatel, Moselle, motel, muscatel, nacelle, Nell, Nobel, Noel, organelle, outsell, Parnell, pell-mell, personnel, propel, quell, quenelle, rappel, Raquel, Ravel, rebel, repel, Rochelle, Sahel, sardelle, sell, shell, show-and-tell, smell, Snell, spell, spinel, swell, tell, undersell, vielle, villanelle, well, yell •Buñuel • Pachelbel • handbell •barbell • harebell • decibel • doorbell •cowbell • bluebell • Annabel •mirabelle • Christabel • Jezebel •Isabel, Isobel •nutshell • infidel • asphodel •zinfandel • Grenfell • Hillel • parallel •Cozumel • caramel • Fresnel •pimpernel • pipistrelle • Tricel •filoselle

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"cell." Oxford Dictionary of Rhymes. 2007. Encyclopedia.com. 29 Aug. 2016 <http://www.encyclopedia.com>.

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