Principles Underlying the Exchanges of K and Na Ions Across Cell Membranes

Cells in general are characterized by a high potassium and a low sodium content. This applies both to the animal kind with freely distensible membranes and to the plant variety with comparatively rigid membranes. In the animal organism, whether this is unicellular or multicellular, the K content of the cell is always much greater than that of its environment and the Na content in general is much lower. Why should this be so? One kind of explanation was developed by Macallum (vide his review, 1926) relating the inorganic components of the cell to a very early marine environment presumably much richer in K than in Na and referring also the inorganic composition of the plasma to that of the ocean when an independent circulatory system was developed. The incorrectness of such views has been sufficiently dealt with elsewhere (Conway, 1943). Another type of explanation was advanced in recent years by Ling (1952). The preferential concentrating of K ions as compared with Na is attributed to the smaller size of the hydrated K ion and the existence of a zone of reduced dielectric constant around a univalent ion, the zone radius being 4 angstroms. Anions are regarded as weakly hydrated, especially organic anions so that the charge on a fixed earboxyl group could be regarded as centred in the middle of a naked oxygen atom of 1.4 A radius. Thus at their distance of closest approach between a Na + ion and a carboxyl anion, the average dielectric constant is believed to be considerably higher than for K +, resulting in an electrostatic potential for K +, much greater than for Na +. If this is taken into account, "in constructing a Boltzmann distribution curve for a mixture of K + and Na + in the immediate neighbourhood: of fixed anions, it can be seen that in effect the great majority of anionic sites would be associated with K+. '' The following may be considered a sufficient argument against Ling's views (Conway, 1957b). The total concentration of charges on the proteins that one could consider at all effective in the manner described by Ling is approximately 0.3 M. If around the centre of each charge there is a zone of reduced dielectric of 4 A radius then the total space involved is No x 0.3 x ~ ~r