Active Ion Transport through Biomembranes

Abstract

In this chapter we treat active ion transport in biomembranes ranging in complexity from plasma membranes to intact epithelia. Since we deal with studies in the stationary or near-stationary (pseudo-stationary) state, we need to understand the characteristics of stationary states. Accordingly, we may ask what happens if the number of restraints on a system in the steady state is changed. When the maximum number of restraints are applied (all forces being fixed) the stationary state is fully defined, since no more degrees of freedom are left. When no restraints at all are applied (all forces left to float), the system will eventually reach equilibrium. Frequently, however, we impose an intermediate number of restraints. For such situations, PRIGOGINE has shown that in linear systems characterized by ONSAGER symmetry the entropy production assumes the minimal value compatible with the imposed restraints. Thus, if some of the forces are fixed, the remainder will all reach values in the stationary state such that their conjugate fluxes become zero. For example, consider the proton-translocating ATP synthese in mitochondria. If the proton electrochemical potential gradient is clamped, the so-called phosphate potential will reach the steady-state value at which no further phosphorylation occurs (state 4).