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• Bal 2012 PLOS ONE  + (The comparison of volumes of cells and sub … The comparison of volumes of cells and subcellular structures with the pH values reported for them leads to a conflict with the definition of the pH scale. The pH scale is based on the ionic product of water, ''K''_w = [H⁺]×[OH⁻]. We used ''K''_w [in a reversed way] to calculate the number of undissociated H₂O molecules required by this equilibrium constant to yield at least one of its daughter ions, H⁺ or OH⁻ at a given pH. In this way we obtained a formula that relates pH to the minimal volume ''V''_pH required to provide a physical meaning to ''K''_w, (where ''N''_A is Avogadro’s number). For example, at pH 7 (neutral at 25 °C) ''V''_pH = 16.6 aL. Any deviation from neutral pH results in a larger ''V''_pH value. Our results indicate that many subcellular structures, including coated vesicles and lysosomes, are too small to contain free H⁺ ions at equilibrium, thus the definition of pH based on ''K''_w is no longer valid. Larger subcellular structures, such as mitochondria, apparently contain only a few free H⁺ ions. These results indicate that pH fails to describe intracellular conditions, and that water appears to be dissociated too weakly to provide free H⁺ ions as a general source for biochemical reactions. Consequences of this finding are discussed. fails to describe intracellular conditions, and that water appears to be dissociated too weakly to provide free H⁺ ions as a general source for biochemical reactions. Consequences of this finding are discussed.)

• Bal 2012 PLOS ONE  +