The electric generator in the photosynthesis of green plants. II. Kinetic correlation between protolytic reactions and redox reactions

In a preceding paper (Junge, W. and Ausländer, W. (1974) Biochim. Biophys. Acta 333, 59–70), we attributed the four protolytic reactions at the outer and the inner side of the functional membrane of photosynthesis to the protolytic properties of the redox components, water, plastoquinone and the terminal acceptor. The experimental evidence presented was conclusive except for one argument. The rate of the protolytic reactions as detected by the dye cresol red after a short flash of light was considerably slower than the rate of the corresponding redox reactions.In this communication it is demonstrated that the rate of proton uptake from the outer phase of the functional membrane is slowed down by a diffusion barrier for protons which shields the redox reaction sites at the outer side of the membrane against the outer aqueous phase. This barrier can be lowered by sand grinding the chloroplasts, by digitonin treatment and by uncoupling agents. At the extreme the barrier can be practically eliminated to yield rates of proton uptake matching the rates of the corresponding redox reactions. This gives conclusive evidence that the electrochemical potential difference which light induces across the functional membrane of photosynthesis is generated by a vectorial electron-hydrogen transport system as postulated by Mitchell (e.g. (1966) Biol. Rev. 41, 445–502).