The Relation of the Membrane Potential of Nitella to Photosynthetic Processes and External Conditions

Abstract

In Nitella cells the H+ extrusion pump is believed to be the mechanism for generating the electric potential across the plasmalemma. Raven and Smith (1973) have proposed that the primary role of the H+ pump is to control the internal pH, with the level of the membrane potential (E_m) varying, depending on the H+ pumping rate. The level of the resting potential would also depend upon the magnitude of the return current of H+ into the cell; this in turn is dependent on the external pH. The above hypothesis was tested by imposing external conditions which would lead to changes in internal pH and by observing the resulting changes in E_m. Some of the results were clearly consistent with the hypothesis: (a) Bright light depolarized the membrane as compared to darkness or weak light at pH 8.3; this is ascribed to an increase in the internal pH induced by photosynthesis and a consequent decrease in H+ extrusion. (b) CO_2 addition under the above conditions caused a hyperpolarization even thought the external pH was lowered from 8.3 to 6.5. (c) When cells were conditioned at pH 5.7 a large increase in organic anion content resulted, buffering the cytoplasm and, as predicted, this prevented any significant changes in E_m when bright light or CO_2 were introduced. Studies with monochromatic light indicated that the action spectrum for membrane depolarization is similar to that for photosynthetically-induced changes of the cytoplasmic pH. There is no evidence to indicate or suggest that the level of either ATP or carbon compounds is a rate-limiting factor in the operation of the H+ pump under any of the conditions used in this study.