DIFFUSION

From Big Medical Encyclopedia

DIFFUSION (Latin diffusio distribution, spreading) — process of spontaneous interpenetration of the adjoining substances each other due to thermal agitation of particles of substance. Occurs in the direction of falling of concentration of substance and leads to hypodispersion of substances on all volume occupied by them. In biol, objects D. is the basic process providing the directed flow of substance during life activity of an organism (intake of gases, waters, mineral salts etc.). The person generally at the expense of D. has a gas exchange in lungs and fabrics (see. Gas exchange ), water salt metabolism (see), absorption of products of digestion in intestines (see. Absorption ), generation of action potential in nervous and muscle cells (see. Bioelectric potential ) and other processes of life activity. Plays an important role in chemical kinetics and tekhnol. processes (adsorption, cementation, diffusion welding, diffusion metallization, etc.).

Occurs in gases, liquids and solids; can diffuse as the foreign particles (heterodiffusion), and own particles (self-diffusion) which are in them. D.'s speed depends on many factors: density and viscosity of the environment, temperature, the nature of the diffusing particles, any influence of external forces etc. Most quickly D. occurs in gases, more slowly in liquids and even more slowly in solids that is defined by the nature of thermal agitation of particles in these environments.

For D.'s characteristic use the diffusion coefficient of D equal to amount of diffusate through a surface unit for a unit of time at the gradient of concentration equal to unit. Though the particle moves on the broken line and the movement happens in a random way, a mean square of shift (L - 2 ) for a large number of collisions grows in proportion to time (t). Coefficient of proportionality of D in the ratio L - 2 ~ Dt is also D. Sootnosheniye's coefficient was removed by A. Einstein and it is fair for D. in any environments. D.'s coefficient is inversely proportional to pressure of gas and increases with increase of temperature. With increase in molecular weight of the diffusing substance D. decreases.

Diffusion in gases. Each particle of gas moves on a broken trajectory since at collision of a particle change the direction and speed of the movement. Owing to this fact the speed of progress much less the speed of free motion of the molecules (so, rate of propagation of smells is much less than speed of motion of the molecules of odorous substances).

Diffusion in liquids is considered as the movement with friction; apply the second ratio of Einstein to the analysis: D ~ kut, where k — Boltzmann's constant in number equal 1,38•10 - 16 erg/hail, u — mobility of the diffusing particles; at the movement of spherical particles of u = 1/6 πηr where η — a coefficient of viscosity of liquid, r — the radius of particles. D.'s coefficient of liquid increases at temperature increase of liquid. In liquid solutions D. of molecules of solvent through semipermeable membranes leads to emergence osmotic pressure (see) that is used in physical. - chemical methods of division and purification of substances (see. Hemodialysis , Dialysis ).

Diffusion in solids it is carried out due to exchange of places of atoms with unoccupied nodes of a crystal lattice (vacancies), direct exchange of two next atoms, simultaneous cyclic movement of several atoms, the movement of atoms and ions through interstices of a crystal lattice etc. D.'s coefficient in solids to a large extent depends on the defects of crystal structure arising during the heating, deformations, tension and other influences. So, D.'s coefficient of zinc in copper at temperature increase with 20 to 300 ° increases by 1014 times.

In polymers D. occurs due to thermal agitation of polymer molecules or their certain sites. The phenomenon of adhesion (adhesion) of polymers is based on this property. Can diffuse in polymeric materials also particles of foreign substances. So, gas permeability of polymer films is result of consistently proceeding processes: dissolution of gas in a boundary layer of a film, D. of solute through polymer and allocation of molecules of gas on the other side of a film. Gas permeability of polymeric materials depends on flexibility of chain macromolecules, on physical. conditions of polymer, by nature diffusing particles. At crystallization, cross «sewing together» of molecules (curing) with growth of intermolecular forces and package densities gas permeability decreases.

Of low-molecular substances through a polymer film is carried out similarly. Colloid substances poorly diffuse, and colloid solutions almost do not find ability to. At the same time D. of low-molecular substances in colloid solutions of low concentration almost does not differ from D. in pure solvent. With increase in concentration of colloid solution D.'s speed in it low-molecular substances decreases. In gels depends on concentration and the nature of structure-forming substance and by nature and structures of parts of the diffusing substance.

Unit of coefficient of D. in the International System of Units (SI) — m 2 / sec., unit of a flow of D. — 1/m 2 • sec. or kg/m 2 • sec.

Diffusion flow. In practice essential value has not D. of separate particles, and the flow of substance moving towards smaller concentration (a diffusion flow). The diffusion flow (J) is expressed through a difference of the particles crossing unit of area in the direct and opposite direction for a unit of time (Fick's law): j = dm = — DS(C1-C2)/(X1-X2)) dt = — DS(dC/dx)dt, where dm — quantity of the particles of substance diffusing during dt at a gradient of concentration of dC/dx, D — a diffusion coefficient; the minus sign specifies that movement happens in the direction of a decrease of concentration of particles. For biol, systems D.'s coefficient is replaced with a permeability coefficient of P ~ - D/dx representing quantity of penetrating particles through the single platform of a membrane for a unit of time at a difference of concentration on both sides, equal unit (see. Membranes biological , Permeability ).

Diffusion in biological systems

D. plays an important role in biologich. systems, providing intake of gases, waters, mineral substances in fabric of plants and animals. At the expense of D. there are processes of gas exchange in lungs and fabrics, exchange of water and salts in kidneys, absorption of products of digestion from intestines, generation of action potential in nervous and muscle cells, transfer of molecules of a mediator in synapses, movement of substances in a cell etc. Calculation of diffusion flows of solutes through membranes of cells is carried out on Fick's equation, in Krom D.'s coefficient is replaced with a permeability coefficient; on Fick's equation calculate D. of gases and water, replacing at the same time a difference of concentration with values of a difference of pressure of gases or osmotic pressure on both sides of a cell membrane. In most cases D.'s speed through membranes is less than the speed of free D. that is peculiar to the majority of the molecules having the average size or a charge interacting with the loaded membrane (the majority of anions) — limited D. V that case when molecules and ions cannot independently get or poorly get through biol, membranes, and at interaction with nek-ry substances («carriers») their permeability increases, speak about facilitated D. Takaya D. is characteristic of a number of sugars, amino acids and other organic compounds.

Final concentration of molecules or ions in a cell and the environment depends on their D.'s speed through membranes a little. Distribution of ions and molecules between a cell and depends Wednesday on their chemical affinity to substances of cytoplasm, sorption processes, physical. dissolution and other processes proceeding unequally in cytoplasm and pericellular liquid (see. Cell ). Besides, redistribution of ions depends on the processes directed against a concentration (electrochemical) gradient and happening to energy consumption — so-called active transport (see. Transport of ions ). Intake of biopolymers (proteins, nucleinic to - t) can happen at the expense of mechanisms pinocytic (see) and phagocytosis (see).


Bibliography: Bulls G. B. Physical biochemistry, the lane with English, M., 1949, bibliogr.; Girshfelder D., Curtice Ch. and Byrd R. The molecular theory of gases and liquids, the lane with English, M., 1961; Post of X. Physiology of a cell, the lane with English, M., 1975; Nikolsky H. N and Troshin A. S. Transport of sugars through cellular membranes, L., 1973, bibliogr.; F r and N k-K amenetskiyd. A. Diffusion and a heat transfer in chemical kinetics, M., 1967; Hodorov B. I. Problem of excitability, JI., 1969, bibliogr.; Sh yu m about N of the Item. Diffusion in solids, the lane with English, M., 1966.

V. P. Shmelyov.

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