Electrical Properties of the Plasmalemma and Tonoplast in Valonia ventricosa

Abstract

Studies were made on the electric potentials of the plasmalemma (Em.) and tonoplast (E,) in small cells (1-3 mm diameter) of Valonia ventricosa.To measure E,,,, 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, except that the reference electrode was placed in the vacuole.Both E,, and E,,, were influenced by light. In the light, E¢o was -70 millivolts and it changed to -60 millivolts in the dark (cytoplasm-negative to outside). For E,,,, the potentials were +86 millivolts in the light and +69 millivolts in the dark (vacuole-positive to cytoplasm). The vacuole potential (E,.) was demonstrated to be the algebraic sum of E". and E,,,> For example, in the light, the sum of the means (±SE) for E>o (= -70 ± 1) and E,,, (= +86± ) is +16 millivolts, which is comparable to the measured E>o of +17 ± 2 millivolts. In the dark, the sum of E,. (= -60 ± 3) and E, (+69 ± 6) is +9 millivolts and the measured value of E"o is +9 ± 4 millivolts.The external K+ concentration had a controlling effect on both E". and the direct current resistance of the plasmalemma, which suggests that E,.. 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.In 1924, Osterhout (25)

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“…1c). Davis (1981) reported relatively stable impalements of the cytoplasm, with PD of )70 mV, whilst the vacuole was 86 mV positive to the cytoplasm, giving the overall positive potential. The cytoplasmic PD depended on external K + , hyperpolarizing by 35 mV when K + decreased from 10 to 1 mM, and depolarizing by 40 mV when K + increased from 10 to 100 mM.…”

“…This PD may be an artifact, or represent transient impalement of a cytoplasmic compartment with a very conductive plasma membrane. Nonetheless, the vacuolar potential shows a near Nernstian dependence on external K + (Davis, 1981;Beilby & Bisson, 1999), and so the potential across the plasmalemma is likely to be due primarily to K + diffusion.…”

“…Aplanospores, however, have a negative, K + -sensitive PD (Damon, 1930;Gutknecht, 1966;Davis, 1981). Aplanospores are small cells that form by segregation of cytoplasmic domains followed by separation from neighboring domains, rounding up as independent cells, and synthesis of cell walls (Kopac, 1933;La Claire, 1982;Nawata et al,.…”

“…It is, however, difficult to distinguish between primary active inward transport of K + and an inward H + pump with tightly coupled K + /H + antiport, as has been postulated for the chloroplast envelope (Song et al, 2004). The vacuolar pH is 5.8 in the light, producing a E H across the tonoplast of )180 mV, assuming cytoplasmic pH is 7.5 (Davis, 1981). Transport into the vacuole is necessarily active, consistent with the decrease in vacuolar pH in the light.…”

“…Davis (1981) placed current-injecting electrodes in the vacuole. Using relatively long single pulses, he monitored the change in membrane potential in the vacuole or cytoplasm, finding a plasma membrane conductance of 7.1 S m )2 and tonoplast conductance of 1.9 S m )2 .…”