Determination and Analysis of the Buffer Capacity of Isolated Chloroplasts in the Light and in the Dark

The ability of isolated chloroplasts to bind protons has been measured by means of titration with acid or alkali over the range of pH 3.7 to 10.8. It was found that the total amount of protons bound over the whole range titrated was equal for chloroplasts in the light and in the dark (around 6 μmol H±/mg chlorophyll), whereas the buffer capacity, i.e. the pH‐dependence of the binding, drastically changed. Values for the buffer capacity at different pH‐values in the dark and in the light were calculated from titration curves and estimated directly with similar results. An analysis of these changes shows that most, if not all of the light‐minus‐dark differences can be accounted for by the previously demonstrated large change in the pH gradient across the thylakoids in the light versus that in the dark.