Chloride Transport inChara corallinaand the Electrochemical Potential Difference for Hydrogen Ions

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 pH_cytoplasm=6·28+0·22 pH_out. 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.