Chloride Transport inChara corallinaand the Electrochemical Potential Difference for Hydrogen Ions

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Abstract
The pH of the cytoplasm of Chara corallina cells has been measured with the weak acid 5,5-dimethyloxazolidine-2,4-dione (DM0). Over an external pH range 4·5–9·5 the results fit the regression equation pHcytoplasm=6·28+0·22 pHout. Using measured values of the electric potential difference across the plasmalemma we have calculated the electrochemical potential difference across this membrane for H+ and Cl−. These data are used to test the hypothesis that the inward transport of Cl− is coupled to the inthix of H+ or, which comes to the same thing, efflux of OH−. One-for-one coupling will not give net Cl− uptake from solutions with pH greater than about 7·2, unless the cytoplasmic Cl− concentration is lower than 10 mM, or the pH just outside the membrane is lower than that in the bulk solution. It is shown that net Cl− uptake proceeds from solutions with pH up to 9. The alternative possibility is that Cl− transport is brought about by co-transport of two H+ for each Cl−; this is not ruled out by the results reported. Such a mechanism might be detectable by its electrogenic effect: although such effects have not been detected, it is shown that they would be small under most conditions. Other possible mechanisms are discussed.

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