Properties of the Cell Surface

Summary Measurements of the tension at the cell surface show that the tension lies between 1·0 and 0·1 dyne. The results of measurement of the tension at simple oil‐water interfaces are discussed in relation to this and it is shown that a simple lipoid layer is not sufficient to explain the low tension at the cell surface; the low tension must be brought about by the adsorption of protein upon the lipoid surfaces. This gives rise to a picture of the cell surface which involves, as a minimum, a bimolecular layer of lipoid molecules between two layers of protein molecules. Measurements of the thickness of the plasma membrane, of its permeability and wetting properties, are found to be compatible with this model. The acidic groups of the protein and lipoid can react with cations to give a sodium‐calcium antagonism. The membrane permits preferential penetration of lipoids, and at the same time offers a physical basis for pore and mosaic effects. High temperature coefficients of penetration are found to be explicable by a simple physical mechanism. It is shown that by variation of the p H of the external medium it is possible to distinguish between three possible mechanisms for ionic penetration; in the case of the erythrocyte the mechanism appears to be simple diffusion through a lipoid layer. The wetting properties of the plasma membrane also indicate that it is fatty in character, and that, in the case of Arbacia eggs, the pellicle is liquid. It is concluded that the structure of the plasma membrane must be similar to that suggested as a minimum possible structure.