Some Properties of Ionic and Nonionic Semipermeable Membranes

Abstract

Some fundamental properties of barriers are reviewed. The barrier of biologic significance is the plasma membrane. The role of the barrier in the transport of solvent and neutral solute species is discussed; 2 fundamental transport processes, namely, diffusion and mass flow are emphasized. The latter is shown to arise as the barrier offers increasing impedance to the movement of a given solute species; the barrier is then referred to as a semipermeable membrane, and the transport of the solvent as osmosis. The osmotic movement, undoubtedly one of the most important transport mechanisms in physiologic systems, is shown to arise from a gradient of hydrostatic pressure in the barrier. The need for the consideration of profiles of thermodynamic quantities is stressed. Finally, the third thermodynamic quantity, electrical potential, is introduced when ionic species are considered in the analysis of the "fixed charge" membrane.