Coupling Ratios H+/e=3 versus H+/e=2 in Chloroplasts and Quantum Requirements of Net Oxygen Exchange during the Reduction of Nitrite, Ferricyanide or Methylviologen

摘要

Using intact and osmotically ruptured chloroplasts, ratios of coupling between deposition of protons in the intrathylakoid space and light-dependent transport of electrons from water to an external acceptor were determined. The data indicate coupling between proton and electron transport at a ratio of H+/e=3 with methylviologen as electron acceptor in thylakoids and with nitrite as electron acceptor in intact chloroplasts. With ferricyanide as electron acceptor in thylakoids, values close to H+/e=2 were observed. Evidence is discussed that H+/e=3 is a fixed value in intact chloroplasts at levels of thylakoid energization sufficient for supporting effective carbon assimilation.In the presence of methylviologen and ascorbate, the minimum quantum requirement of oxygen uptake by thylakoids was about 2.7 quanta of 675 nm light per O2indicating an e/O2ratio of 1.33. In the absence of ascorbate, and with KCN present in addition to methylviologen, e/O2ratios up to 4 were observed. The minimum quantum requirement of oxygen evolution by thylakoids in the presence of ferricyanide and by intact chloroplasts in the presence of nitrite was about 8 quanta/O2.