Membrane Potential Changes Related to Active Transport of Glycine in Lemna gibba G1

Accumulation of ¹⁴C-labeled glycine and microelectrode techniques were employed to study glycine transport and the effect of glycine on the membrane potential (Δψ) in Lemna gibba G1. Evidence is presented that two processes, a passive uptake by diffusion and a carrier-mediated uptake, are involved in glycine transport into Lemna cells. At the onset of active glycine uptake the component of Δψ which depended on metabolism was decreased. The depolarized membrane repolarized in the presence of glycine. This glycine-induced depolarization followed a saturation curve with increasing glycine concentration which corresponded to carrier-mediated glycine influx kinetics. The transport of glycine was correlated with the metabolically dependent component of Δψ. It is suggested (a) that the transient change in Δψ reflects the operation of an H⁺-glycine cotransport system driven by an electrochemical H⁺ gradient; and (b) that this system is energized by an active H⁺ extrusion. Therefore the maximum depolarization of the membrane consequently depended on both the rate of glycine uptake and the activity of the proton extrusion pump.