Thermodynamics of Coupling Mechanisms

Abstract

The energetics of active transport has been the subject of numerous studies from different points of view. Several of these divide the expenditure of metabolic energy into two parts: conversion into electroosmotic work and additional expenditure necessarily associated with a real transport process. The sum of these quantities is defined as work and is compared with estimates of the energy available. In principle, at least, the conversion into electroosmotic work may be evaluated by determining —JΔũ , where J is the net rate of transport (flux) of the test species. On the other hand, the additional energy expenditure is not well defined. USSING [1] and others regarded this quantity, in the case of active sodium transport in frog skin, as the work required to overcome the internal sodium resistance of the skin, evaluating it by means of the flux ratio. According to this interpretation, (RT ln f) represents the internal work done per mole of sodium transported. However, as is readily seen from our earlier discussion, the quantity (RT ln f) does not in fact represent useful conversion of energy, but rather dissipation of energy within the system. On thermodynamic grounds it is clearly inappropriate to include such a term in an expression for the work accomplished by the process. There should be an unambiguous distinction between the rate at which energy is supplied and the rate at which it is recovered.