Serine transport and membrane depolarization in the liverwort Riccia fluitans

The plasmalemma of thallus cells of the aquatic liverwort, Riccia fluitans, is reversibly depolarized by L- and D-serine. At 0.1 mM K+ in the medium, the depolarization saturates at 50 mV; half-maximal depolarization occurs at 13 microM L-serine and 30 microM D-serine, respectively. Uptake of 14C-labelled L-serine depends upon the K+ concentration and is sensitive to the membrane potential as indicated by its reduction through 1 mM sodium cyanide. We propose that serine binds to and is transported by an electrogenic carrier. However, an interaction of serine with K+ channels of the membrane seems also possible.