Quantum requirements for proton uptake in spinach chloroplasts

Studies of electron and proton transport in chloroplast preparations (Type D) from spinach (Spinacea oleracea L.) yield three basic results. First, in electron transport catalyzed by methyl viologen from water to oxygen at pH 7.6, the quantum requirement for electron transport ($\text{hv}\text{e}{}^{\mathrm{?}}$) was 2.2, while the corresponding requirement for proton transport ($\text{h}\text{v}\text{H}{}^{\mathrm{+}}$) was 1.2. Second, the electron and proton quantum requirements were relatively independent of the individual chloroplast preparation or certain components of the resuspension medium, but did depend upon the reaction medium's initial pH. Third, measurable electron and proton transport did not occur under 715-nm illumination, nor did such activities occur in the presence of DCMU under 645-nm illumination when methyl viologen was used as the electron transport cofactor. These experimental results reconcile the quantum requirement of proton transport with Mitchell's chemiosmotic theory for chloroplast energy transduction and resolve a long standing controversy regarding the quantum requirement in chloroplast thylakoids.