Analog circuit of theAcetabularia membrane

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Abstract
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 elementsP 1 andP 2 in series;P 2 is shunted by a large capacitance (C p=3 mF cm−2). The nonlinear current-voltage relationship ofP 1 (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 2∶1 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 ofP 2 (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 μW 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.