Introduction

A living cell must do work to maintain the composition of its internal medium different from that of the external. If that cell is growing, work has also to be done to generate the small but necessary osmotic gradient for the inwardly directed influx of water to take place. In a cell with a wall, such as a fungus, the osmotic gradient is much more significant, in order to maintain the necessary internal hydrostatic pressure for turgor (see Chapter 13). Two important loci for the above work are the plasma and the vacuolar membranes. When work is carried out in moving solutes across these two or indeed any other membranes, we can speak of active transport, i.e. the movement of solutes against their electrochemical potential gradient. Of course, there may also be diffusion of solutes across the membrane, albeit almost always, if the solute is polar, aided by the presence of carrier proteins that overcome the activation energy required by such a solute to enter through the lipid portion of the membrane. When diffusion occurs, it must be down the electrochemical potential gradient. However, it needs to be remembered that if the affinity of such a carrier for the solute on one side of the membrane were to be much higher than on the other or the mobility of solute transfer were to be faster in one direction than in the other – both of which changes would depend on the cell doing work – then active transport would occur (Jennings, 1974).