From Big Medical Encyclopedia

CELL MEMBRANE (synonym: plasma membrane, plasmolemma) — the lipoproteidny membrane separating cytoplasm of a cell from the environment.

At the person and animal O. to. is the elementary membrane consisting of the double lipidic layer covered with proteinaceous molecules (see. Membranes biological ). At the majority of cells of a cover have width apprx. 6 — 10 nanometers. A proteinaceous component O. to. (makes apprx. 60% of dry weight) it is presented by high-molecular fibrous protein (structural protein). The lipidic component averaging apprx. 40% of dry weight is presented to hl. obr. phospholipids (lecithin, cholesterol). Besides, in O.'s structure to. a number of the enzymes (5 '-nucleotidase, phosphomonoesterase, acid RNK-aza, an alkaline phosphatase and Mg-dependent ATP-ase) playing an important role in implementation of active transport of ions through O. to enters. On a surface of zooblasts various specialized structures are located. A free surface of O. to. it is covered with microvillis, from to-rykh the cuticle (an epithelium of intestines) and an alkaline border (an epithelium of tubules of a kidney) can be formed. Bonds of adjacent surfaces of a cell are carried out by formation of different type of contacts: by means of formation of the folds coming each other (inter-digitation), by merge of periblasts of O. to. (the closing zone, dense contacts — zonula occludens) and intermediate contacts (zonula adhaerens). In the most difficult cases contacts of adjacent surfaces are made by specialized metaplasmatic structures — desmosomes (see). Backboneless animals can have a merge of periblasts of O. to. At the same time true bridges (septirovanny desmosomes) are formed. In epithelial cells on a basal surface there are numerous folds pressing in cytoplasm (a basal labyrinth).

One of the main functions of a cell is connected with a cellular cover — permeability (see), for the account the cut is carried out a metabolism with the environment and maintenance in a cell fiziol, homeostasis (see). Transport of substances through O. to. at the same time it is carried out by passive transfer (diffusion) and transfer against a gradient of concentration — the active transport demanding energy consumptions (see. Transport of ions ). Along with it there are also other mechanisms of absorption by a cell as dense (see. Phagocytosis ), and liquid (see. Pinotsitoz ) substances.

The lake to. possesses special system receptors (see), capable to catch changes of the environment and influence of the nek-ry physiologically active molecules (hormones, mediators, etc.) causing response of a cell. The effect of a number of hormones is carried out by change of activity of the enzyme connected with cellular receptors — adenylatecyclases. Adenylatecyclase catalyzes synthesis cyclic adenosinemonophosphoric to - you are (tsAMF), serving as the direct transmitter of effect of hormone on intracellular processes. The mechanism of action on a cell of neurotransmitters is similar. Ability of cells to a physical activity, formation of pseudopodiums and undulating membranes (the lamellar outgrowths of cytoplasm making oscillating motions), etc. are connected with a cellular cover. Processes of destruction and O.'s synthesis to. — an everyday occurrence at absorption (endocytosis) and release (exocytosis) of alien substances and at fiziol, secretion and excretion.

The cover of a plant cell has a number of the specific features distinguishing it from a zooblast. Cytoplasm of a plant cell is surrounded with the plasma membrane similar to a plasmolemma of a zooblast. However still the system of covers — primary, secondary and tertiary, creating a dense cell wall is outside located. Intercellular substance, connecting covers of the next cells, gives to fabrics high stability. In a growth period the plant cell is surrounded with primary cover which is not interfering increase in its sizes. This cover has small thickness and contains preferential polysaccharides — cellulose, hemicellulose and pectin. In the cells which stopped growth as a result of adjournment of various substances (a lignin, suberene, cutins, various mineral salts) there is a secondary cover. It has considerable thickness and can be exposed to an odereveneniye or suberization. Its inner layer, adjacent to a plasmolemma, sometimes allocate as a tertiary cover. Through a cell wall there pass plazmodesmenny tubules, with the help to-rykh communication of cytoplasm of the next cells, transfer of irritations and the movement of plastic substances between cells is carried out.

See also Cell .

Bibliography: Vasilyev Yu. M. and Malenkov A. G. Cellular surface and reactions of a cell, L., 1968, bibliogr.; JI A. Biokhimiya's enindzher, the lane with English, M., 1976; T r and N to and at with Dzh. From cells to bodies, the lane with English, M., 1972; Troshin A. S. A problem of cellular permeability, M. — JI., 1956, bibliogr.; Finean Dzh., Kolmen R. and Mi persons l of P. Membranes and their functions in a cell, the lane with English, M., 1977; Q u inn P. J. Molecular biology of cell membranes, L., 1976; Robertson J. D. The ultrastructure of cell membranes and their derivatives, in book: The structure and function of subcellular components, ed. byE. M. Crook, p. 3, Cambridge, 1959.

I. A. Alov.