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cell

Значение термина cell в knolik

cell - cell
cell - (Zool.). Discrete mass of protoplasm, bounded by a plasma membrane. (Bot.). As in Zoology, but including also the surrounding cell wall (usually of cellulose or chitin). Sometimes used for cell-wall only.

Most animals and plants can conveniently be considered as made up of two components, though of course these form an integrated system in the normal organism: (a) the protoplasm, in which are situated the systems of enzymes responsible for controlling the chemical reactions (metabolism) characteristic of living things, and the nucleic acid systems responsible for synthesis of the enzymes and of themselves; (b) a component, made by the protoplasm, devoid of these enzyme systems, largely composed of fibrous materials which provide mechnical support for the protoplasm, e.g. cellulose in plants, collagen in animals.

The protoplasmic component of the larger plants or animals is not usually a continuous mass; it consists of numerous discrete units. Each unit is microscopically small (a man contains something like a million million of them; (each is bounded by an extremely thin membrane (See also: Plasma-membrane) which allows some substances to pass through but not others, and so partially isolates the inside of each unit from its surroundings; and usually each contains one nucleus, sometimes more than one, very rarely none. The shapes of such units are very various, and they may often have some special function highly developed, e.g. power of contraction in muscle; of photosynthesis in green plants. But all of them have the common features of metabolism. Every unit originates from a pre-existing unit, usually by division into two, involving mitosis or some similar process which distributes the nuclear material (DNA) between the two products (but See also: Fertilization).

Each of these units in an animal is called a cell. Each in a plant is usually called a protoplast, every protoplast being consistently surrounded, with peculiar intimacy, by a wall commonly of cellulose (which a zoologist would regard as intercellular material). The protoplast and its cell-wall are together known as the cell in botany. Historically the first use of the word cell was for the plant cell-wall and the space it enclosed: and even when the space contains no protoplast, the word is still applied in this way. Since there is rarely a parallel in animals to the plant cell-wall, divergent usages have unfortunately developed.

Although a cell or protoplast has been characterized above as a discrete unit bounded by a membrane, there are cases where, while the elements are almost entirely separated from each other by their membranes, they are in places continuous (by intercellular bridges or plasmodesmata), in plants constituting a symplasm.

Each almost discrete element is still called a cell (animal) or protoplast (plant). There exist also large continuous masses of protoplasm with many nuclei, which are regarded as units and called syncytia, plasmodia or coenocytes rather than cells or protoplasts, There is no agreed demarcation between a collection of connected cells and a syncytium, etc. Many microscopic organisms (bacteria, algae, fungi, protozoa) consist of a single cell throughout life, in plants and bacteria complete with cell-wall, each with the characteristics (listed above) of a plant or animal protoplasmic element, though naturally without specialization of particular function, since all functions must be fulfilled by the one cell. Every multicellular animal or plant which reproduces sexually, as most do, starts as a single cell (formed by fusion of a male cell with a female cell), which by continuous doubling forms the numerous cells of the adult.

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