The Energetics of Active Transport

Abstract

This chapter aims to give an overview of the workings of active-transport systems. I shall not go much into the details of any particular transport system—these will be the subjects of later chapters of this treatise—but I shall want to present what seem to me to be the general principles involved in the coupling between two flows of transported substrates or the coupling between the flow of a transported substrate and the progress of the chemical reaction that drives this flow. The standpoint adopted in this chapter is that such coupling, and, hence, active transport itself, arises simply from the properties of the membrane carriers, and, in particular, that such carriers, by virtue of their role as carriers, exist in two conformations facing the two sides of the cell membrane. It is the redistribution of carrier between its two major forms that brings about transport and active transport. The study of active transport is, then, the exploration of the properties of membrane-carrier systems. I shall work step-by-step through the principles of ligand-carrier interactions, transport on the simple carrier, active transport by countertransport, and, finally, active transport by cotransport, and in this way succeed, I hope, in showing that a sound knowledge of carrier kinetics provides the basis for an understanding of active transport. Much has been written on the energetics of active transport, on the interaction between the driving and the driven substrates, on the conformational energetics of the transporters, and on the possible role of high-energy chemical substrates in active transport. But an emphasis on the simple carrier basis of active transport may provide a newer way of looking at these old problems.