THE INDEPENDENCE PRINCIPLE IN THE PROCESSES OF WATER TRANSPORT

The processes of membrane transport exhibiting permeability coefficien ts depending on the species activities do not obey the ''independence principle'' and are assumed to take place by a mechanism of discrete n ature, analyzable by a kinetic formalism. in this article, we study th e dependence of the osmotice permeability coefficient on the water act ivities, from the steady-state analysis of a kinetic model of single-f ile water transport that simultaneously incorporates the vacancy-media ted and ''knock-on'' mechanisms into the state diagram. In particular, we study the relation between the near-equilibrium osmotic permeabili ty (P-e) and the equilibrium water activity of the compartments (w). T he analysis and numerical calculations performed for a simple case of the model show that, for values of the parameters consistent with expe rimental data, P-e exhibits only a small variation with w within the p hysiological range in the majority of the situations considered here. It is not possible to predict, from the study of these simple models, whether more complicated kinetic diagrams of water transport may be ch aracterized by permeability coefficients with a more evident dependenc e on the water activities. Nevertheless, the results obtained here sug gest that, for the case of physiological water pores, the analysis of the kinetic dependence of the permeability coefficients on the water a ctivities may not yield evidence pointing to a discrete nature for the transport process.