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 bacteria [pl. of bacterium], microscopic unicellular prokaryotic organisms characterized by the lack of a membrane-bound nucleus and membrane-bound organelles. Once considered a part of the plant kingdom, bacteria were eventually placed in a separate kingdom, Monera .
..... Click the link for more information. and protozoans protozoan (prō'təzō`ən), informal term for the unicellular heterotrophs of the kingdom Protista .
..... Click the link for more information. , 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 tissue, in biology, aggregation of cells that are similar in form and function and the intercellular substances produced by them. The fundamental tissues in animals are epithelial, nerve, connective, and muscle tissue; blood and lymph are commonly classed separately
..... Click the link for more information. , 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 eukaryote (y
..... Click the link for more information. , those with a membrane-enveloped nucleus. The bacteria (kingdom Monera Monera, taxonomic kingdom that comprises the prokaryotes ( bacteria and cyanobacteria ). Prokaryotes are single-celled organisms that lack a membrane-bound nucleus and usually lack membrane-bound organelles (mitochondria, chloroplasts; see cell , in biology).
..... Click the link for more information. ) are prokaryotes. They are smaller in size and simpler in internal structure than eukaryotes and are believed to have evolved much earlier (see evolution evolution, concept that embodies the belief that existing animals and plants developed by a process of gradual, continuous change from previously existing forms. This theory, also known as descent with modification, constitutes organic evolution.
..... Click the link for more information. ). 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 gene, the structural unit of inheritance in living organisms. A gene is, in essence, a segment of DNA that has a particular purpose, i.e., that codes for (contains the chemical information necessary for the creation of) a specific enzyme or other protein.
..... Click the link for more information. made of DNA (see nucleic acid nucleic acid, any of a group of organic substances found in the chromosomes of living cells and viruses that play a central role in the storage and replication of hereditary information and in the expression of this information through protein synthesis.
..... Click the link for more information. ); they use proteins protein, any of the group of highly complex organic compounds found in all living cells and comprising the most abundant class of all biological molecules. Protein comprises approximately 50% of cellular dry weight.
..... Click the link for more information. 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 adenosine triphosphate (ATP) (ədĕn`əsēn trī'fŏs`fāt)
..... Click the link for more information. 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 lipids, a broad class of organic products found in living systems. Most are insoluble in water but soluble in nonpolar solvents. The definition excludes the mineral oils and other petroleum products obtained from fossil material.
..... Click the link for more information. 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 chromosome (krō`məsōm')
..... Click the link for more information. during meiosis meiosis (mīŏ`sĭs)
..... Click the link for more information. . 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 photosynthesis (fō'tōsĭn`thəsĭs)
..... Click the link for more information. ; 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 endocytosis (ĕn'dōsītō`səs), in biology, process by which substances are taken into the cell .
..... Click the link for more information. ); 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 cellulose, chief constituent of the cell walls of plants. Chemically, it is a carbohydrate that is a high molecular weight polysaccharide. Raw cotton is composed of 91% pure cellulose; other important natural sources are flax, hemp, jute, straw, and wood.
..... Click the link for more information. 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 microscope, optical instrument used to increase the apparent size of an object.

Simple Microscopes

A magnifying glass, an ordinary double convex lens having a short focal length, is a simple microscope.
..... Click the link for more information. 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 Leeuwenhoek, Antony van (än`tōnē vän lā`vənh
..... Click the link for more information. , Robert Hooke Hooke, Robert (h
..... Click the link for more information. , and Marcello Malpighi Malpighi, Marcello (märchĕl`lō mälpē`gē), 1628–94, Italian anatomist.
..... Click the link for more information. . In the 19th cent. Matthias J. Schleiden Schleiden, Matthias Jakob (mätē`äs yä`kôp shlī`dən), 1804–81, German botanist.
..... Click the link for more information. and Theodor Schwann Schwann, Theodor (tā`ōdōr shvän), 1810–82, German physiologist and histologist. He was a student of J. P.
..... Click the link for more information. 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 biochemistry, science concerned chiefly with the chemistry of biological processes; it attempts to utilize the tools and concepts of chemistry, particularly organic and physical chemistry, for elucidation of the living system.
..... Click the link for more information. .

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).

cell, in electricity

cell: see battery, electric storage battery is generally of the wet-cell type; i.e., it uses a liquid electrolyte and can be recharged many times. The storage battery consists of several cells connected in series.
..... Click the link for more information. .

cell

Principal structures of an animal cell

In biology, the basic unit of which all living things are composed; the smallest structural unit of living matter that is able to function independently. 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 higher plants and animals. There are two distinct types of cells: prokaryotic cells and eukaryotic cells. Though the structures of prokaryotic and eukaryotic cells differ (see prokaryote, eukaryote), their molecular compositions and activities are very similar. The chief molecules in cells are nucleic acids, proteins, and polysaccharides. A cell is bounded by a membrane that enables it to exchange certain materials with its surroundings. In plant cells, a rigid cell wall encloses this membrane.

(1) A geographic area in a cellular phone system. See cellphone.

(2) In a spreadsheet, the intersection of a row and column.

(3) Short for "cellphone."

(4) An elementary unit of storage for data (bit) or power (battery).

(5) See Cell chip.

1.	(spreadsheet)	cell - In a spreadsheet, the intersection of a row a column and a sheet, the smallest addressable unit of data. A cell contains either a constant value or a formula that is used to calculate a value. The cell has a format that determines how to display the value. A cell can be part of a range. A cell is usually referred to by its column (labelled by one or more letters from the sequence A, B, ..., Z, AA, AB, ..., AZ, BA, BB, ..., BZ, ... ) and its row number counting up from one, e.g. cell B3 is in the second column across and the third row down. A cell also belongs to a particular sheet, e.g. "Sheet 1".
2.	(networking)	cell - ATM's term for a packet.