Models for the active transport of cations ⋯ The steady-state analysis

Abstract

Progress that has been made in the analysis of active transport models is summarized, at the steady-state level. The 2 general classes of models, counter- and co-transport, can be treated by a kinetic analysis which makes no assumptions as to the symmetry or asymmetry of the systems nor as to the presence of any particular rate-limiting steps. Precisely the same formalism is obeyed for primary active transport as for secondary active transport. Both are merely a generalization of facilitated diffusion, in that they follow directly from accepted properties of carrier models. How affinities of such carriers for their substrates affect the efficiency of active transport is discussed. In a number of cases, the affinity changes that the carrier demonstrates arise from inherent properties of the free carrier and not from any "high energy" properties of the chemical reactants. Methods of obtaining the kinetic parameters of the system from experimental data are reviewed, together with methods for testing and characterizing the different transport models.