The Molecular Basis of Simple Diffusion within Biological Membranes

The primary function of the cell membrane is to separate the interior of the cell from its external environment. The rapid progress made in the past decade in the analysis of the diffusional process in synthetic polymers has made it possible to understand, in some detail, the corresponding diffusional process within biological membranes. By considering the membrane as a hydrophobic but polymeric phase, most of the observed properties of this barrier can be accounted for, as far as nonelectrolytes are concerned. Diffusion coefficients in both biological membranes and polymers show very similar dependencies upon diffusant size, diffusant shape, temperature, and the presence of low-molecular-weight additives. The overall process of permeation across both biological and synthetic membranes is the resultant of two processes-partitioning into and out of the membrane and diffusing within the membrane. The overall process is, therefore, characterized by two parameters––the equilibrium distribution coefficient “K” between membrane and external phases and the diffusion coefficient “D_mem” for movement inside the membrane.