| Comments |
P.705 right column: "This is especially likely because ascorbate would be mainly dissociated at the pH of the cytosol, stroma and matrix, because these compartments have pH values in the range pH 7-8, while the pKa of ascorbic acid is 4.17, so all of the ascorbate occurs in vivo in paucivacuolate cells as a salt rather than as a free acid. The ascorbate concentration in such cells is only one to a few mol/m^3 or so (primary sources Aaronson et al. 1977 Shigeoka, Nakano & Kitaoka 1980 Table 2). This presumably constrains the removal of toxic O species by ascorbate, although the possible implications of an increased rate of mutation in low osmolarity cells have not been explored. It is of interest that marine phytoplankton with intracellular osmolarity ≥1000osmol/m^3 have higher ascorbate concentrations than freshwater microalgae or, indeed, seaweeds (Table 2)." |