CELL

in electricity, unit structure used to generate an electrical current by some means other than the motion of a conductor in a magnetic field. A solar cell, for example, consists of a semiconductor junction that converts sunlight directly into electricity. A dry cell is a chemical battery in which no free liquid is present, the electrolyte being soaked up by some absorbent material such as cardboard. A primary, or voltaic, cell produces electricity by means of a chemical reaction but is not rechargeable to any great extent. The conventional dry cell (e.g., flashlight or transistor-radio battery) is a primary cell. A secondary cell, such as a lead-acid storage battery, is rechargeable, as are some primary cells, such as the nickelcadmium cell. A fuel cell produces an electrical current by constantly changing the chemical energy of a fuel and an oxidizing agent, separately stored and supplied to a chamber containing electrodes, to electrical energy. Two or more cells connected together are a battery, although in common usage battery is also used to designate a single cell. in biology, the basic unit of which all living things are composed. The cell is the smallest structural unit of living matter that is capable of functioning independently. All cells are similar in composition, form, and function. A single cell can be a complete organism in itself, as in bacteria and protozoans. Groups of specialized cells are organized into tissues and organs in multicellular organisms such as the higher plants and animals. Cells were first observed in the 17th century, shortly after the discovery of the microscope. Their significance, however, was not understood until the early 19th century, when improvements in microscopy permitted closer observation. Cells are made up of macromolecules (giant molecules) and various smaller molecules. The chief macromolecules are nucleic acids (DNA [deoxyribonucleic acid] and RNA [ribonucleic acid]), proteins, and polysaccharides. DNA comprises the genetic code that carries the essential character of the organism from generation to generation. RNA translates the genetic information into proteins, which carry out vital cell functions. Proteins, for example, recognize and transport specific molecules into and out of the cell and catalyze all chemical reactions within the cell. Polysaccharides function as structural molecules in the rigid cell walls of bacterial and plant cells and as storage molecules in the glycogen granules of vertebrate muscle cells. Important among the smaller molecular components of cells are lipids, ATP (adenosine triphosphate), cyclic AMP (adenosine monophosphate), porphyrins, and water. Lipids are fatty substances that are a major component of cell membranes. ATP is the energy currency of the cell; this energy-rich molecule is formed when the cell needs to store energy and is broken down when the cell requires energy. Cyclic AMP functions as a regulator of cell activities; porphyrins are pigments essential for oxidation and photosynthesis. About 70 to 80 percent of a cell is water, which is vital to the chemistry of life. There are two distinct types of cells: procaryotic cells, found only in blue-green algae and in bacteria, and eucaryotic cells, composing all other life forms. A eucaryotic cell consists of an outer membrane, cytoplasm that contains various membrane-bound structures (organelles), and a membrane-bound nucleus that encloses the gene-bearing chromosomes. Procaryotic cells have a cell membrane and cytoplasm, but they have no nucleus (their genetic material is organized into a single chromosome) and they lack membrane-bound cytoplasmic organelles. The molecular composition and activities of the two types of cells, however, are very similar. A cell is bound by a semipermeable membrane (called the plasma membrane) that enables it to exchange certain materials with its surroundings. The plasma membrane is made up of a double layer of lipids studded with proteins. Some of the proteins extend completely through the lipid layer, others only partially penetrate it, and still others are thought to be completely embedded within the lipid layer. In plants the membrane is enclosed in a rigid cellulose cell wall. The space between cells is filled with the extracellular matrix, a gel of polysaccharides swollen with water molecules in which are suspended protein fibres that hold cells together to form tissues. Within the cytoplasm of both procaryotic and eucaryotic cells are ribosomes, small bodies that are the sites of protein synthesis. In addition, eucaryotic cells have a variety of separate membrane-bound cytoplasmic organelles with special functions. These organelles include the endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria, and plastids. The endoplasmic reticulum is a network of channels that functions in the movement of materials within the cell. Associated with these channels is the Golgi apparatus, which is composed of sacs that bud off from the endoplasmic reticulum. These sacs transport cell products from the endoplasmic reticulum to their appropriate locations either inside or outside the cell. Lysosomes are sacs filled with digestive enzymes; they are capable of digesting worn-out cell parts or extracellular debris, such as dead cells or foreign microorganisms that have been engulfed by the cell. Mitochondria serve as the power plants of the cell; it is within these organelles that ATP is synthesized. Plastids are found in the cells of most plants but are absent from animal cells. Of immense importance are the plastids known as chloroplasts; they contain the machinery for photosynthesis, the process by which the energy of sunlight is captured to produce carbohydrates. The nucleus is the control centre of eucaryotic cells. Within this membrane-bound structure lie the chromosomes, which carry the hereditary material. The DNA of the chromosomes directs protein synthesis in the cell; the DNA instructions are carried from the nucleus to the cytoplasm by messenger RNA (mRNA). Procaryotic cells have no membrane-enclosed nucleus. They do, however, have nuclear matter consisting of a single chromosome. A eucaryotic cell divides, or reproduces, to form two genetically identical daughter cells in a process called mitosis (q.v.). Prior to mitosis, the chromosomes replicate, so that there will be a complete set of hereditary instructions for each daughter cell. During mitosis, the doubled chromosomes are separated, with one copy of each going to each daughter cell. Among sexually reproducing eucaryotes, another type of cell division occurs in the formation of sex cells called gametes (i.e., eggs and sperm). This process is known as meiosis (q.v.). It produces four gametes, each of which contains half the number of chromosomes of the parent cell. When a male gamete and a female gamete unite, they form a new individual in which the full number of chromosomes is restored. Procaryotic cells reproduce in various ways, the most common being binary fission. This process involves replication of the cell's lone chromosome and the subsequent splitting of the parent cell into two daughters. It thus resembles mitosis in eucaryotes, but it lacks the special apparatus involved in true mitotic division. The two main types of cell death are necrotic cell death, or coagulative necrosis, and apoptosis, or programmed cell death. Necrosis occurs in a variety of contexts produced by disease, injury, or accident and is cell death imposed by external factors. A cell undergoing necrosis typically swells in size before its lysosomes rupture and the cell's internal contents spill out into extracellular space. In response to specific intracellular and extracellular signals, cells can also undergo programmed cell death. This apoptosis is a normal cellular process that plays an important role in growth and development. This type of cell death is marked by the shrinking of the cytoplasm and nucleus, degradation of the chromosomes, and the final splitting of the nucleus into a number of membrane-bound fragments. in biology, the basic unit of which all living things are composed. As the smallest units retaining the fundamental properties of life, cells are the atoms of the living world. A single cell is often a complete organism in itself, such as a bacterium or yeast. Other cells, by differentiating in order to acquire specialized functions and cooperating with other specialized cells, become the building blocks of large multicellular organisms as complex as the human being. Although they are much larger than atoms, these building blocks are still very small. The smallest known cells are a group of tiny bacteria called mycoplasmas; some of these single-celled organisms are spheres about 0.3 micrometre in diameter, with a total mass of 10-14 gramequal to that of 8,000,000,000 hydrogen atoms. Human cells typically have a mass 400,000 times larger, but even they are only about 20 micrometres across. It would require a sheet of about 10,000 human cells to cover the head of a pin, and each human being is composed of more than 75,000,000,000,000 cells. This article discusses the cell both as an individual unit and as a contributing part of a larger organism. As an individual unit the cell is capable of digesting its own nutrients, providing its own energy, and replicating itself in order to produce succeeding generations. It can be viewed as an enclosed vessel composed of even smaller units that serve as its skin, skeleton, brain, and digestive tract. Within this vessel innumerable chemical reactions take place simultaneously, all of them controlled so that they contribute to the life and procreation of the cell. In a multicellular organism cells specialize to perform different functions. In order to do this each cell keeps in constant communication with its neighbours. As it receives nutrients from and expels wastes into its surroundings, it adheres to and cooperates with other cells. Cooperative assemblies of similar cells form tissues, and a cooperation between tissues in turn forms organs, the functional units of an organism. Special emphasis is given in this article to animal cells, with some discussion of the energy-synthesizing processes and extracellular components peculiar to plants. For detailed discussion of the biochemistry of plant cells, see photosynthesis. For full-length treatment of the genetic events in the cell nucleus, see heredity. Bruce M. Alberts Additional reading General works Classical microscopy and the excitement about the revelation of the chromosome are discussed in Edmund B. Wilson, The Cell in Development and Heredity, 3rd rev. ed. (1925), a historical work; and Max Verworn, General Physiology: An Outline of the Science of Life (1899; originally published in German, 2nd ed., 1897), recounts the outlook on the cell at the turn of the century. Sir William Maddock Bayliss, Principles of General Physiology, 4th ed. (1924); and Lewis Victor Heilbrunn, An Outline of General Physiology, 3rd ed. (1952), are important historical texts tracing the development of the study of cell function in the 20th century. General works for background reading include Helena Curtis, Biology, 4th ed. (1983); Roger Y. Stanier et al., The Microbial World, 5th ed. (1986); and Benjamin Lewin, Genes, 2nd ed. (1985), which provides modern ideas on the control of gene expression. For details on current research in the field of cell theory the following periodicals are useful: Annual Review of Biochemistry; Annual Review of Genetics; Annual Review of Physiology; Annual Review of Cell Biology; and International Review of Cytology (irregular). Nature and function of cells Comprehensive works touching upon many aspects of the fields of cell biology and physiology include Christian De Duve, A Guided Tour of the Living Cell, 2 vol. (1984); E.J. Ambrose and Dorothy M. Easty, Cell Biology, 2nd ed. (1978); Arthur C. Giese, Cell Physiology, 5th ed. (1979); Don W. Fawcett, The Cell, 2nd ed. (1981); Stephen L. Wolfe, Biology of the Cell, 2nd ed. (1981); Joseph G. Gall, Keith R. Porter, and Philip Siekevits (eds.), Discovery in Cell Biology (1981); Bruce Alberts et al., Molecular Biology of the Cell (1983), a clear and thorough introduction to the subject; Andrew Scott, Pirates of the Cell (1985); Barry King (ed.), Cell Biology (1986); T.A.V. Subramanian (ed.), Cell Metabolism, Growth, and Environment, 2 vol. (1986); J.M. Lackie, Cell Movement and Cell Behaviour (1986); and James Darnell, Harvey Lodish, and David Baltimore, Molecular Cell Biology (1986). Special studies in cell morphology J.B. Finean, R. Coleman, and R.H. Michell, Membranes and Their Cellular Functions, 3rd ed. (1984), a good introduction; Gheorge Benga (ed.), Methodology and Properties of Membranes (1985); Miles D. Houslay and Keith K. Stanley, Dynamics of Biological Membranes: Influence on Synthesis, Structure, and Function (1982), especially good on structure and fluidity; Anthony N. Martonosi (ed.), The Enzymes of Biological Membranes, 2nd ed., 4 vol. (198485), a collection of articles on all aspects of membrane structure and function; Bertil Hille, Ionic Channels of Excitable Membranes (1984); Thomas E. Andreoli et al., Physiology of Membrane Disorders, 2nd ed. (1986); Halvor N. Christensen, Biological Transport, 2nd ed. (1975), an introduction to the problems of the permeability of membranes and active transport; Wilfred D. Stein, Transport and Diffusion Across Cell Membranes (1986), a comprehensive coverage of the aspects of permeability and solute migration; Arthur E. Sowers (ed.), Cell Fusion (1987), a review of mainly plasma membranes; Peter A. Whittaker and Susan M. Danks, Mitochondria: Structure, Function, and Assembly (1978), a comprehensive treatment of the subject; Alexander Tzagoloff, Mitochondria (1982), a well-illustrated, authoritative account; and David G. Nicholls, Bioenergetics: An Introduction to the Chemiosmotic Theory (1982), an exploration of mitochondrial and chloroplastic function.For other studies of the form and structure of cells, see M. Schliwa, The Cytoskeleton: An Introductory Survey (1986); Jerry W. Shay (ed.), Cell and Molecular Biology of the Cytoskeleton (1986); Howard Stebbings and Jeremy S. Hyams, Cell Motility (1979); Eric Holtzman and Alex B. Novikoff, Cells and Organelles, 3rd ed. (1984), a well-illustrated survey of the present knowledge of cell structure, with emphasis on animal cells and tissues; Pierre Dustin, "Microtubules, Scientific American, 243(2):6676 (August 1980), and Microtubules, 2nd rev. ed. (1984), for a fuller exploration; John M. Murray and Annemarie Weber, The Cooperative Action of Muscle Proteins, Scientific American, 230(2):5871 (February 1974); Stephen L. Wolfe, Cell Ultrastructure (1985); J. Richard McIntosh (ed.), Spatial Organization of Eucaryotic Cells (1983); and Gary G. Borisy, Don W. Cleveland, and Douglas B. Murphy (eds.), Molecular Biology of the Cytoskeleton (1984).For plant cells, see Michael McNeil et al., Structure and Function of the Primary Cell Walls of Plants, Annual Review of Biochemistry, 53:625663 (1984), a review of the chemistry of plant cell wall components; Deborah P. Delmer, Cellulose Biosynthesis, Annual Review of Plant Physiology, 38:259290 (1987), an account of current knowledge in the area; Peter Albersheim and Alan G. Darvill, Oligosaccharins, Scientific American, 253(3):5864 (September 1985), an account of how oligosaccharins function in plants; Alan G. Darvill and Peter Albersheim, Phytoalexins and Their Elicitors: A Defense Against Microbial Infections in Plants, Annual Review of Plant Physiology, 35:243276 (1984), a review; Peter H. Raven, Ray F. Evert, and Susan E. Eichhorn, Biology of Plants, 4th ed. (1986), a general work; and Keith Roberts et al. (eds.), The Cell Surface in Plant Growth and Development (1985), a collection of papers, with emphasis on the role of cell walls. Special studies in cell biology On cell growth, see Peter B. Moens (ed.), Meiosis (1987); and Robert A. Schlegel, Margaret S. Halleck, and Potu N. Rao (eds.), Molecular Regulation of Nuclear Events in Mitosis and Meiosis (1987). On cell matrix and cell-to-cell interactions, see M.R. Bernfield, Organization and Remodelling of the Extracellular Matrix in Morphogenesis, in Thomas G. Connelly, Linda L. Brinkley, and Bruce M. Carlson (eds.), Morphogenesis and Pattern Formation (1981); Caroline H. Damsky, K.A. Knudsen, and C.A. Buck, Integral Membrane Glycoproteins in Cell-Cell and CellSubstratum Adhesion, in Raymond J. Ivatt (ed.), The Biology of Glycoproteins (1984); Gerald M. Edelman, Cell Adhesion and the Molecular Processes of Morphogenesis, Annual Review of Biochemistry, 54:135170 (1985); Gerald M. Edelman and Jean-Paul Thiery (eds.), The Cell in Contact: Adhesions and Junctions as Morphogenetic Determinants (1985); Joseph L. Goldstein et al., Receptor-Mediated Endocytosis, Annual Review of Cell Biology, 1:140 (1985); Elizabeth D. Hay, Cell-Matrix Interaction in the Embryo: Cell Shape, Cell Surface, Cell Skeletons and Their Role in Differentiation, in Robert L. Trelstad (ed.), The Role of Extracellular Matrix in Development (1984); J.-P. Revel, B.J. Nicholson, and S.B. Yancey, Chemistry of Gap Junctions, Annual Review of Physiology, 47:263279 (1985); David C. Spray and Michael V.L. Bennett, Physiology and Pharmacology of Gap Junctions, Annual Review of Physiology, 47:281303 (1985); E.L. Hertzberg, Antibody Probes in the Study of Gap Junctional Communication, Annual Review of Physiology, 47:305318 (1985); and Michael V.L. Bennett and David C. Spray (eds.), Gap Junctions (1985). On cell differentiation, see Paul R. Wheater and H. George Burkitt (eds.), Functional Histology, 2nd rev. ed. (1987), a well-illustrated account of the histology of the human body; J.M.W. Slack, From Egg to Embryo: Determinative Events in Early Development (1983), a treatment of the processes leading to cell commitment in embryos; C.S. Potten (ed.), Stem Cells: Their Identification and Characterization (1983); and C.F. Graham and P.F. Wareing (eds.), Developmental Control in Animals and Plants, 2nd ed. (1984). Evolution For insights into chemical evolution on the molecular level, see Manfred Eigen et al., The Origin of Genetic Information, Scientific American, 244(4):88118 (April 1981); Clair Edwin Folsome (ed.), Life: Origin and Evolution: Readings from Scientific American (1979); Bruce M. Alberts, The Function of the Hereditary Materials: Biological Catalyses Reflect the Cell's Evolutionary History, American Zoologist, 26(3):781796 (1986); and D.S. Bendall (ed.), Evolution from Molecules to Men (1983). Bruce M. Alberts Wilfred D. Stein Ronald A. Laskey Merton R. Bernfield L. Andrew Staehelin Jonathan M.W. Slack