Independent pathways for water and solute movement across the cell membrane

Summary In published studies of the relationship between movement of nonelectrolytes across cell membranes and the lipid solubility of these test molecules, it is generally found that a number of the smaller, more water-soluble molecules deviate significantly from the general pattern relating permeability (or reflection coefficient) to lipid solubility. This is often true of the amides, for example, whose reflection coefficients are considerably lower than expected on the basis of lipid solubility. While this has been interpretep in terms of the movement of these solutes through aqueous channels in the membrane, it now appears that many of these deviant molecules may cross the membrane by means of carrier-mediated diffusion, independent of osmotic water flow. This has important implications for studies in which equivalent pore radius has been estimated from the reflection coefficients of small hydrophilic molecules, and for our present concepts of membrane structure.