Measurement of the Cytoplasmic and Vacuolar Buffer Capacities in Chara corallina1

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The cytoplasm and the vacuole were isolated from internodal cells of Chara corallina by using the intracellular perfusion technique, and their buffer capacities (βi) were determined from the titration curves. The pH of the isolated vacuolar sap was 5.19 ± 0.029 (mean ± standard error). At this pH, βi was minimal and amounted to 0.933 ± 0.11 millimoles H+/pH unit/liter vacuolar sap. The pH of isolated cytoplasm was 7.22 ± 0.028. βi was minimal in this pH region and amounted to 14.2 ± 0.80 millimoles H+/pH unit/liter cytoplasm. When 1% (volume/volume) Triton X-100 was added to the cytoplasmic solution to permeabilize the subcellular organelles, the cytoplasmic pH increased to 7.32 ± 0.026, where βi was 20.35 ± 2.66 millimoles H+/pH unit/liter cytoplasm. This shows that alkaline subcellular compartments exist in the cytoplasm and also that the cytoplasmic pH before adding Triton X-100 may represent the cytosolic pH. These data indicate that the pH values of the cytoplasm and the vacuole are regulated at the values where the βi values are minimal. This suggests that ATP- and inorganic pyrophosphate-dependent H+ pumps in the plasma membrane and the tonoplast could efficiently regulate the pH of both cytoplasm and vacuole in Chara internodal cells.

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