From Big Medical Encyclopedia

MEMBRANE EQUILIBRIUM (Latin membrana a thin skin, a cover) — the equilibrium state established in the system formed by solution of polyelectrolyte and the salt divided by a semipermeable membrane. Masculine is observed in living cells, fabrics, biol, liquids and plays an important role in processes of their life activity. Masculine is a special case a so-called Donnan equilibrium (Donnan equilibrium), called so in honor is mute. the scientist F. G. Donnan, to-ry the first quantitatively described this phenomenon. The Donnan equilibrium arises in a colloid system when diffusion of substance on volume for any reason is limited.

At In the masculine establishment a factor of restriction is the semipermeable membrane (see. Membranes biological ). The impossibility of penetration through such membrane of ions of polyelectrolyte (e.g., a squirrel) causes uneven distribution of substances (their ions) on both sides of a membrane, emergence of membrane potential (see. Bioelectric potential ) and gradient osmotic pressure (see). Simple model of system, in a cut Masculine is established, the vessel divided into two parts by a semipermeable membrane can serve. A time of a membrane freely passes monovalent ions (e.g., Na + and Cl - ), but do not pass ions of protein. If on one side of a membrane there is a solution of protein (e.g., a proteinata of sodium), and on other side — solution of sodium chloride, then ions of chlorine will move through a membrane inside, aiming to be distributed evenly on all volume of a vessel. Each ion of chlorine at the same time shall be followed by an ion of sodium as otherwise on outer side of a membrane positive charges would begin to collect. Redistribution of ions on both sides of a membrane results and Masculine, characterized by the following ratio is established:

[Na + ] vnutr. • [Cl - ] vnutr. = [Na + ] vneshn. • [Cl - ] vneshn.

Change of ion concentration of sodium Δ [Na + ] in comparison with initial corresponds:

Δ [Na + ] = ([Na + ] 2 vneshn. ) / ([Na + ] 0vnutr. + 2 [Na + ] 0vneshn. ),

where [Na + ] 0 vneshn. and [Na + ] 0 vnutr. — initial, a [Na + ] vneshn. and [Na + ] vnutr. — equilibrium concentrations of Na + from the external and internal parties of a semipermeable membrane respectively. This equation allows to calculate equilibrium concentrations of ions, knowing their reference values.

This mechanism of uneven distribution of ions plays an important role in regulation and maintenance in cells and living tissues of necessary ion concentrations and their ratios. E.g., at active tissue respiration insignificant decrease in pH in fabrics as a result of formation of CO 2 and H + facilitates release of the oxygen connected with hemoglobin. This process is followed by binding of H+ hemoglobin that increases the buffer capacity of system and recovers an initial pH value of fabric. In this case as system of establishment of Masculine serves the system: an erythrocyte — a blood plasma, divided by a membrane of an erythrocyte; hemoglobin is polyelectrolyte.

At disturbances in fabrics water salt metabolism (see), acid-base equilibrium (see) and nek-ry others patol, states there is In a masculine disturbance, causing the related changes of buffer and osmotic properties of system. Such changes take place, e.g., at acidosis (see) when from accumulation of anions to - the t results excess accumulation in blood of ions of sodium and potassium, falling of pH of a blood plasma, and also at alkalosis (see), to-ry is followed by opposite changes. Small aberrations can be quickly enough liquidated due to activity of systems of an autoregulyation of an organism (see. Biological system ).

At the becoming uneven distribution of ions caused by restriction of mobility of one of components of system is followed by emergence of membrane potential difference. It is considered sometimes as a special case of diffusion potential (see. Gradient ), Nernst described by the equation:

E = (RT/F) of *ln ([Na + ] vnutr. / [Na + ] vneshn. ) = (RT/F) of *ln ([Cl - ] vneshn. / [Cl - ] vnutr. ),

where E — membrane potential; R — a gas characteristic, T — the absolute temperature, F — Faraday's number. Absolute value of this potential of subjects is higher, than concentration and extent of ionization of protein on one side of a membrane are more and than concentration of salt on another is lower. Many cellular membranes have low-permeability not only for protein, but also for ions, in particular cations of Na + and K+. In this case is also established In the masculine and membrane potential is described by Nernst's equation.

Bibliography: Bulls G. B. Physical biochemistry, the lane with English, page 372, M., 1949; Pasynsky A. G. Biophysical chemistry, page 198, M., 1968; Williams V. iuil-I am m with X. Physical chemistry for biologists, the lane with English, page 155, M., 1976.

V. F. Antonov.