Abstract
Studies were made on the electric potentials of the plasmalemma (Eco) and tonoplast (Evc) in small cells (1-3 mm diameter) of Valonia ventricosa. To measure Eco, 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 Evc except that the reference electrode was placed in the vacuole.
Both Eco and Evc were influenced by light. In the light, Eco was −70 millivolts and it changed to −60 millivolts in the dark (cytoplasm-negative to outside). For Evc, the potentials were +86 millivolts in the light and +69 millivolts in the dark (vacuole-positive to cytoplasm). The vacuole potential (Evo) was demonstrated to be the algebraic sum of Eco and Evc. For example, in the light, the sum of the means (±se) for Eco (= −70 ± 1) and Evc (= +86 ± 5) is +16 millivolts, which is comparable to the measured Evo of +17 ± 2 millivolts. In the dark, the sum of Eco (= −60 ± 3) and Evc (+69 ± 6) is +9 millivolts and the measured value of Evo is +9 ± 4 millivolts.
The external K+ concentration had a controlling effect on both Eco and the direct current resistance of the plasmalemma, which suggests that Eco 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.
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Selected References
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