CELL, in Biology

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 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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