The distribution of orthophosphate between chloroplasts and the nonchloroplast parts of leaf cells has been investigated. Upon illumination, the percentage of the total orthophosphate located in the chloroplasts decreases rapidly, while darkening results in a slow increase of that percentage to the original dark value. If, in the dark, comparable concentrations of orthophosphate in the chloroplasts and in the cytoplasm are assumed, the concentration of orthophosphate in the vacuole can be calculated from the available data. Values are then obtained, which are considerably lower than the corresponding values in the protoplasm; this indicates that a concentration gradient for orthophosphate is maintained between the cytoplasm and the vacuole.
Leaves of spinach and Elodea, which were fed 32P for 2 to 4 hours and then left to stand for two or three days, showed higher specific activities of orthophosphate in the chloroplasts than in the nonchloroplast parts of the cells. Since there is reasn to assume a fairly rapid exchange of orthophosphate between chloroplasts and cytoplasm, the differences in the specific activities which were observed several days after feeding with 32P, point to a very slow exchange of orthophosphate across the tonoplast membrane.
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