Also found in: Dictionary, Medical.
Physiological mechanisms for the maintenance of an optimal and constant level of osmotic activity of the fluid within and around the cells, considered to be most favorable for the initiation and maintenance of vital reactions in the cell and for maximal survival and efficient functioning of the entire organism.
The actions of osmoregulatory mechanisms are, first, to impose constraints upon the passage of water and solute between the organism and its surroundings and, second, to accelerate passage of water and solute between organism and surroundings. The first effect requires a change of architecture of membranes in that they become selectively permeable and achieve their purpose without expenditure of energy. The accelerating effect, apart from requiring a change of architecture of cell membranes, requires expenditure of energy and performance of useful osmotic work. Thus substances may be moved from a region of low to a region of higher chemical activity. Such movement can occur in opposition to the forces of diffusion of an electric field and of a pressure gradient, all of which may act across the cell membrane. It follows that there must be an energy source which is derived from the chemical reactions of cellular metabolism and that part of the free energy so generated must be stored in molecules which are driven across the membrane barrier. Active transport is the modern term for such processes. See Cell membranes