Electrical Properties of the Plasmalemma and Tonoplast in Valonia ventricosa

Studies were made on the electric potentials of the plasmalemma (E(co)) and tonoplast (E(vc)) in small cells (1-3 mm diameter) of Valonia ventricosa. To measure E(co), microelectrodes with long tapers were inserted into the vacuole with the path of electrode entry off-center. The microelectrode then was pushed across the vacuole and into the cytoplasm on the opposite side of the cell. A reference electrode was placed in the artificial seawater bathing the cell. A similar method was used to measure E(vc) except that the reference electrode was placed in the vacuole.Both E(co) and E(vc) were influenced by light. In the light, E(co) was -70 millivolts and it changed to -60 millivolts in the dark (cytoplasm-negative to outside). For E(vc), the potentials were +86 millivolts in the light and +69 millivolts in the dark (vacuole-positive to cytoplasm). The vacuole potential (E(vo)) was demonstrated to be the algebraic sum of E(co) and E(vc). For example, in the light, the sum of the means (+/-se) for E(co) (= -70 +/- 1) and E(vc) (= +86 +/- 5) is +16 millivolts, which is comparable to the measured E(vo) of +17 +/- 2 millivolts. In the dark, the sum of E(co) (= -60 +/- 3) and E(vc) (+69 +/- 6) is +9 millivolts and the measured value of E(vo) is +9 +/- 4 millivolts.The external K(+) concentration had a controlling effect on both E(co) and the direct current resistance of the plasmalemma, which suggests that E(co) is largely a K(+) diffusion potential. The tonoplast electrical properties were affected only slightly by external K(+).The data presented are indicative of a K(+) electrogenic influx pump in the tonoplast. It is also considered possible that H(+) might be electrogenically pumped from the cytoplasm both into the vacuole and to the cell exterior.