A Necessary Simplification of the Kinetics of Carrier Transport

A kinetic analysis of facilitated diffusion across cell membranes on the basis of four models is presented. These models are all general in that in no case is symmetry assumed, nor is any particular reaction step considered as rate‐limiting. In one model there is only one form of the free carrier and one form of the bound carrier. In the second model there are two forms of both bound and of free carrier. In the third and fourth models there are two forms of one of these, but only one form of the other. These models fall into two classes: that with one form of the free carrier and that with two forms. Within each class the models give identical predictions for steady‐state fluxes of substrates. It is shown how to distinguish between these two classes using experimental data, and criteria are provided enabling one or other class, or both classes, of model to be rejected. A complete kinetic analysis of a system of either class may be achieved on the basis of a small number of parameters, derived from flux measurements. The significance of these parameters on a molecular level are discussed.