Summary
In cells of the freshwater algaHydrodictyon africanum, in solutions where [K+]0=0.1mm and pH0>7.0, the membrane in the light is hyperpolarized. The membrane potential difference {ie179-1} has values from −180 to −275 mV, more negative than any ion diffusion potential difference, and is predominantly a function of pH0, and independent of [K+]0. The hyperpolarization of the membrane appears to arise from an electrogenic efflux of H+, estimated from voltage-clamp data to be about 8 nmol m−2 sec−1 when pH0=8.5. In the light the membrane conductanceg m is about 0.084 S m−2. At light-off, {ie179-2} becomes less negative, with a halftime for change of 15 to 30 sec andg m decreases by about 0.052 S m−2. After dark periods of up to 300 sec, {ie179-3} is largely independent of pH0 for values greater than 6.0 and usually behaves as a combined K+ and Na+ diffusion potential with permeability ratioP Na/P K=0.05 to 0.2. The membrane potassium conductanceg K has either a low value of 2–6×10−2 Sm−2, or a high value of up to 18×10−2 S m−2 depending on [K+]0, the transition from low to high values occurring when {ie179-4} moves over a threshold value that is more negative than {ie179-5}, the electrochemical equilibrium potential for K+. The time for half-change of the transition is about 30 sec. The results are consistent with a model of the membrane in which the pump electromotive force and conductance are in parallel with diffusive electromotive forces and conductances. When the pump is operating its properties determine membrane properties, and when it is inoperative, or running at a diminished rate, the membrane properties are determined more by the diffusive pathways. Changes in both pump rate andg K can account for a variety of characteristic changes in membrane PD and conductance occurring in response to ligh-dark changes, changes in light intensity, pasage of externally applied electric current across the membrane and changes in ionic constituents of the external medium.
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Findlay, G.P. Electrogenic and diffusive components of the membrane ofHydrodictyon africanum . J. Membrain Biol. 68, 179–189 (1982). https://doi.org/10.1007/BF01872263
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DOI: https://doi.org/10.1007/BF01872263