A mathematical model is proposed for potential-dependent proton transport across the algal (Chara corallina) cell membrane. The model is a set of two partial differential equations with proton concentration at the membrane outer side and the transmembrane potential as variables. It describes the experimentally observed inhomogeneous distribution (banding) of the potential and pH along the membrane and their oscillations in time. A mechanism is proposed whereby such spatial and temporal inhomogeneities may arise.