Studies in irreversible thermodynamics III. models for steady state and active transport across membranes

Abstract

If a lattice model can exchange molecules with two baths, it becomes a membrane model. With this point of departure, we study in this paper the steady state flux properties of 20 membrane models. Although we start with lattice gas models, the type of treatment is more general than this. It applies to any membrane made up of independent units, each of which can exist in a finite number of discrete states. The transition probabilities between states are assumed to be "unimolecular". In biological cases, at least, the states include different macromolecular configurations. In a number of models a "carrier" is introduced. In others, the transport of one or two species across the membrane is coupled to a chemical free energy source such as ATP → ADP + P ("active transport"). Each of the models studied can be represented by a diagram which shows the allowed transitions between states. There is a close relationship between the diagram on the one hand and the steady state force-flux relations on the other. The over-all reciprocal relations for a model are compounded from more fundamental reciprocal ("equal susceptibility") relations for individual cycles (closed paths) in the diagram of the model. © 1966.