AMINE UNIPORT AT THE PLASMALEMMA OF CHAROPHYTE CELLS .1. CURRENT-VOLTAGE CURVES, SATURATION KINETICS, AND EFFECTS OF UNSTIRRED LAYERS

In voltage-clamped cells of the algae Chara and Nitella an inward current of positive charge occurs when NH4+ or CH3NH3+ is added to the external medium. There is a simultaneous increase in membrane conductance, agreeing with earlier evidence that the current represents the inward uniport of the amine ion. We have obtained current-voltage curves for this uniport, and show the effect on their shape caused by the unstirred layer of solution adjacent to the cell membrane. The current-voltage curves for CH3NH3+, which are less affected by the unstirred layer, are concave towards the current axis, show no saturation with membrane PD in the range -100 to -300 mV, and show saturation as the concentration is raised. The dependence of current on concentration follows a Michaelis-Menten relation, the parameters having the following values at -200 mV: {Mathematical expression} Both Vm and KM depend on membrane potential, approximately as exp(-Fψ/6 RT) and exp(Fψ/3 RT), respectively. The results suggest a transport channel with a single, selective binding site below the membrane surface and a single potential energy barrier at the center of the membrane. The rate of transport falls as the cell takes up amine, and also varies markedly from culture to culture. The significance of this transport for the biology of the charophyte plant is discussed. © 1979 Springer-Verlag New York Inc.