Overall survey and conclusions

Available data on the permeability of cell membranes are consistent with the view that the vast majority of substances find the major barrier to their trans-membrane movement to be the tightly packed proximal chains of the phospholipid hydrocarbons, together with the cholesterol moieties and the glycerol backbone. This region has solvent properties well modelled by the less non-polar organic solvents (ether and even w-octanol). The data reveal a steep mass selectivity (three to six power inverse dependence of intramembrane diffusion coefficient on molecular 

7 Stein, W.D. (1967) in The Movement of Molecules Across Cell Membranes, Academic Press, New York. 

10 Crank, J. (1957) The Mathematics of Diffusion, Oxford University Press, London. 

Department of Physiology, University of Connecticut Health Center, Farmington, CT 06032, U.S.A. 

One of the main functions of the plasma membranes of living cells is to control the transport processes into and out of the cells of many substances and thus to regulate the composition of the intracellular fluid. The fluid usually contains solutes at concentrations which are quite different from their corresponding values in the bathing medium. This is achieved by the ability of the membrane to discriminate among various solutes so that some are allowed through, others are kept inside or outside the cell, and still others are carried actively. In addition, important processes such as oxidative metabolism, protein synthesis and several other synthetic processes are intimately connected with and dependent on membrane processes. In fact, continued existence of the cell is critically dependent on its having a functional plasma membrane. 

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