Passive Membrane Transport of Nonelectrolytes

Abstract

Transport of nonelectrolytes across cell membranes encompasses any penetration through these membranes by un-ionized substances, whatever the mechanism of the process may be. It will be seen in the following sections that for many, perhaps all, metabolically important substances such as sugars and amino acids efficient mechanisms operate in the cell membranes, transporting such molecules often in a highly specific manner and in most cases against their concentration gradients. However, many of the above substances—as well as a host of other, biochemically little-important, compounds—can enter the cell by paths obeying the laws of simple diffusion. A particular interest for the membrane physiologist is held by un-ionized molecules which seem to penetrate into cells by diffusion across thin lipid layers. This conclusion was reached at the end of the last century by Overton who found that such substances pass across the cell membranes the more rapidly the greater their partition coefficient between oil and water. The correlation between the permeability constant and the partition coefficient was further supported by studies of Collander (see Collander, 1949; Danielli, 1943, 1952; or Stein, 1967; for extensive discussion and references) who compared the rate of permeation of various nonelectrolytes like alcohols, amides, substituted ureas, etc. into the alga Chara ceratophylla with their olive oil: water partition coefficients.