Effect of pH on the Membrane Potential and Electrical Resistance of Nitella flexilis in the Presence of Calcium

Effects of pH on the membrane potential and electrical resistanceof Nitella were investigated in a bathing medium with or withoutcalcium. The membrane potential became more negative as theexternal pH was raised, at a faster rate in the presence ofcalcium than in its absence. The value then achieved by thepotential could be reversed by restoring the original pH whilstin a Ca-free medium the cell remained ‘hyperpolarized’.Tenfold changes of the external concentration of potassium broughtabout larger modifications of the membrane potential when thepH of the solution was high and calcium concentration low. Theelectrical resistance was lowest in alkaline and calcium-freesolutions. We conclude that calcium prevents the mediation ofsome changes in the membrane structure by lowering the concentrationof external H⁺ ions, and that the permeability of Nitella topotassium increases with rising pH.