Note on the Penetration of Electrolytes.

Abstract

When a weak electrolyte penetrates a living cell the distribution at equilibrium 1 between the inside and outside does not tell us whether it enters the cell in ionic or molecular form, for the equilibrium is the same in both cases, being determined by the fact that the ionic activity product (or chemical potential) must be the same inside and outside. Assuming that the dissociation constants are the same inside and outside and that the activity coefficients equal 1 we may write for a weak acid, HA,

where the subscripts o and i denote the external and internal concentrations respectively, and M denotes undissociated molecules. Since the ionic product inside must be equal to that outside we have (H_i) (A_i) = (H_o) (A_o) and therefore M_i = M_o. 2

For convenience we may consider a cylindrical plant cell in contact with a solution at one of its end surfaces, the content of its central vacuole being well stirred (which may sometimes happen as the result of protoplasmic motion): we may assume that the external solution is likewise stirred and has a large volume so that it remains practically constant during penetration. For convenience we shall assume that the pH value is the same inside and outside and remains practically constant during the experiment.

If molecules alone enter and the quantity passing in be called x it is evident that the observed rate will be proportional to the net rate of passage of molecules through the protoplasm, 3 D_M, to the permeability of the protoplasm to molecules, P_M, and to the difference between the external concentration of molecules, M_o, and the internal concentration found in the vacuole, M₁, Hence we may write = D_MP_M(M_o-M₁). This is similar to the formula employed by Northrop for diffusion through collodion membranes in contact with solutions. 4