A new concept of electrochemical membrane equilibrium. Coupled transport and membrane potential

A generalized concept of membrane equilibrium is proposed which is based on the assumption that ion transport across the membrane ceases. The Nernst-Donnan equilibrium does not contradict this concept, but is considered as a particular case where the transport of ions across the membrane ceases when their electrochemical potentials in the aqueous phases are equal. The Nernst-Donnan equilibrium occurs provided the ions penetrate across the membrane independently; in this case both conditions of equilibrium - the cessation of transport and the equality of the ion electrochemical potentials in the solution - are equivalent. In more complicated types of transport, when flows of several different species are coupled, the transport may cease even though the corresponding electrochemical potentials in the aqueous phases are not equal. In this case membrane equilibrium occurs in the absence of the Donnan equilibrium, and an anomalous potential, which does not obey the Nernst relation, is established across the membrane. If there exists a concentration gradient for one type of ion, the sign of the potential may be opposite to that of the equilibrium potential of the membrane when it is selective for this type of ion. and the absolute value of the potential may be higher than the corresponding equilibrium potential. The proposal of anomalous potentials may reveal new aspects of the potential control mechanisms in biological membranes where coupled transport is observed quite frequently.