Thermodynamics and Transport

The total free energy change of a transport process or a series of processes may have numerous obvious components. Knowledge of these components frequently has practical implications for the interpretation of transport experiments. In addition, knowledge of the free energy changes associated with each component of transport is required in order to fully understand how transport contributes to the work performed by cells. It is proposed in this chapter that the free energy changes associated with the components of primary active transport processes may include much larger changes in the magnitude of enthalpy and entropy than are associated with ATP synthesis or hydrolysis. In addition to the free energy available from solute transport, the cell depends on transport occurring rapidly enough to do work that is useful to it. Transport proteins are the catalysts that permit solutes and the solvent to migrate across biomembranes much more rapidly than they can by permeating phospholipid bilayers. Although ordinary diffusion of solutes and the solvent may be relatively rapid over short distances in intracellular and extracellular aqueous solutions, such diffusion is virtually halted across thin biomembranes.