The Chara plasmalemma at high pH. Electrical measurements show rapid specific passive uniport of H⁺ or OH^-

Above a critical external pH (about 10.5), the Chara membrane acquires new propertes. In this state the membrane potential is close to the equilibrium potentials for H⁺ and OH^-, hyperpolarizing as external pH increases with a slope of -59 mV/pH unit. The membrane conductance increases by an average factor of 2.4 above the critical pH. These changes are explained by an increase in permeability to OH^- (or H⁺). The establishment of a OH^- (or H⁺ permeable membrane at high pH suggests that the large fluxes of OH^- (or H⁺ which occur in the alkaline band in photosynthesizing cells are passive.