Apparent saturation kinetics of divalent cation uptake in yeast caused by a reduction in the surface potential.

The concentration dependence of the uptake rate of divalent cations in yeast can be described by a simple diffusion process after accounting for the effect of the surface potential upon the divalent cation concentration near the membrane. It is also necessary to correct for the effect of the cell pH upon the rate of translocation. The apparent saturation kinetics is ascribed to the fact that the quotient of the concentration of the divalent cations near the cell membrane and the bulk aqueous phase concentration is reduced on increasing the divalent cation concentration in the medium. The diffusion process regulated by the surface potential even mimics the saturation kinetics of a two-carrier transport system. The selectivity found between Ca2+ and Sr2+ uptake can probably be traced to differences in their affinity for the negative groups on the cell membrane determining the surface potential rather than to differences in their affinity for a transport system. The enhancement of divalent cation uptake by loading the cells with phosphate is probably due to the concomitant increase in the net negative charge of the cell membrane.