Studies on the physical state of water in living cells and model systems: IX. Theoretical significance of a straight line relationship between intracellular concentration of a partially excluded solute and its concentration in the bathing medium.

The experimentally observed steady-level distribution of Na+ (25 degrees) and of D-glucose (0 degree c) in frog muscle were chosen as examples of solute distribution patterns observed in living cells, for comparison with those predicted by two theoretical models: one derived from the membrane-pump theory and the other from the association-induction (AI) hypothesis. Neither the distribution of Na+ nor that of D-glucose follows the pattern predicted by the membrane-pump models for solutes maintained at lower level than in the external medium, in which the plot of intracellular solute concentration as ordinate against different external concentrations as abscissa bends upward with increasing external solute concentration. Instead, both Na+ and D-glucose exhibit either straight line distribution with unchanging (below unity) slopes, or that of a hyperbola superimposed on such a straight line, both in agreement with the AI hypothesis.