Analog circuit of theAcetabularia membrane

Summary The high membrane potential ofAcetabularia (E_m=–170 mV) is due to an electrogenic pump in parallel with the passive diffusion system (E_d=–80 mV) which could be studied separately in the cold, when the pump is blocked. Electrical measurements under normal conditions show that the pump pathway consists of its electromotive forceE_p with two elementsP₁ andP₂ in series;P₂ is shunted by a large capacitance (C_p=3 mF cm^–2). The nonlinear current-voltage relationship ofP₁ (light- and temperature-sensitive) could be determined separately; it reflects the properties of a carrier-mediated electrogenic pump. The value ofE_p (–190 mV) indicates a stoichiometry of 21 between electrogenically transported charges and ATP. The electrical energy, normally stored inC_p, compares well with the metabolic energy, stored in the ATP pool. The nonlinear current-voltage relationship ofP₂ (attributed to phosphorylating reactions) is also sensitive to light and temperature and is responsible for the region of negative conductance of the overall current-voltage relationship. The power of the pump (1 W cm^–2) amounts to some percent of the total energy turnover. The high Cl^– fluxes (1 nmol cm^–2 sec^–1) and the electrical properties of the plasmalemma are not as closely related as assumed previously. For kinetic reasons, a direct and specific Cl^– pathway between the vacuole and outside is postulated to exist.