The permeability of the thylakoid membrane for protons

Parallel measurements of light‐induced proton gradients and of electron transport rates were used to characterize proton efflux in lettuce chloroplasts. Proton efflux mediated by the chloroplast H+‐ATPase was resolved from nonspecific leakage, through its dependence on the ratio of internal to external proton concentrations; proton leakage was proportional to the difference between these two concentrations. The proton permeability of the thylakoid membrane was found in these experiments to be 2 × 10−5 cm/s. In spite of this rather high permeability, proton leakage accounted for only ~1% of the total proton flux, due to the low internal proton concentration.