A mechanism for multiphasic uptake of solutes in plants

A molecular mechanism is proposed for unitary multiphasic uptake in which the carrier has two binding sites for the substrate. The first site binds n-1 molecules of substrate, and then one additional substrate molecule can become bound at the second binding site. Only this last molecule is transported in the operation of the carrier molecule. In the free state, the carrier can be activated to successive states with increasing affinities for the substrate in the two binding sites. The mechanism is resolved for the steady state conditions, obtaining a simple uptake rate equation, which fits the experimental data. Methods for determining the parameters of the equation are presented. Evidence other than kinetics is discussed for the mechanism. The mechanism also provides a physiological interpretation for multiphasic uptake: the active transport mechanisms (energy-requiring mechanisms) are prevented from operating at high substrate concentrations, thus preventing a waste of energy by the cells.