Introduction

It is a truism to say that for a heterotrophic organism the rate of nutrient uptake governs the rate of growth. For a fungus, there is a need to supply not only the nutrients from which the polymeric materials of the organism are built but also the inorganic ions that act as cofactors and the nutrients that either directly or indirectly help to generate the internal solute potential. Thus, the rate of functioning of those processes at the plasma membrane will be an important determinant of the rate of growth. At its simplest, the processes at the plasma membrane or indeed any other membrane can be described in terms of two characteristics, namely: (a) the affinity constant, KT, of the transport system (equivalent to the Michaelis constant, Km, but not in a strictly formal manner); and (b) the flux, the number of moles of solute transported per unit surface area of membrane per unit time (mol cm−2 s−1). Assessment of the affinity constant makes assumptions, albeit very rudimentary, about the mechanism of transport, i.e. that there is a transporting molecule to which the solute first binds before crossing the membrane. This is true for many solutes but is not necessarily obligatory. The determination of the flux of a solute, however, makes no presuppositions about the mechanism of transport. The net flux is the difference between influx and efflux, and the flux determined for one of these directions is the sum of those processes acting in that direction.