The generation of latent-ion-transport capacity

Of the various hypotheses designed to explain the movement of ions from the environment into the plant cell, the most attractive suggests that ions traverse cell membranes in combination with an endogenously produced carrier substance.(1,2) The hypothesis stems from the recognition that absorption kinetics are not first-order, but are, rather, best represented by a formulation analogous to that which describes the reversible combination of substrate and enzyme.(3) The carrier hypothesis is at least consistent with the known characteristics of the absorption process. Thus the remarkable specificity of the absorption process may be imputed to specificity on the part of the carriers; the requirement for respiratory energy may be related either to the production of carrier or to the degradation of ion-carrier complex; and the ability of ions to penetrate ion-impermeable cellular membranes may be ascribed to the formation of a permeating ion-carrier complex.