Abstract
Using dark adapted isolated spinach chloroplasts and sequences of brief saturating flashes the correlation of the uptake and release of protons with electron transport from Photosystem II to Photosystem I were studied. The following observations and conclusions are reported: 1. (1) Flash-induced proton uptake shows a weak, damped binary oscillation, with maxima occurring after the 2nd, 4th, etc. flashes. The damping factor is comparable to that observed in the O 2 flash yield oscillation and therefore explained by misses in Photosystem II. 2. (2) On the average and after a steady state is reached, each flash (i.e. each reduction of Q) induces the uptake of 2H + from outside the chloroplasts. 3. (3) Flash induced proton release inside the chloroplast membrane shows a strong damped binary oscillation with maximum release occurring also after the 2nd, 4th, etc. flashes. 4. (4) This phenomenon is correlated with the earlier reported binary oscillations of electron transport [2] and shows that both electrons and protons are transported in pairs between the photosystems. 5. (5) In two sequential flashes 4H + from the outside of the thylakoid and 2e − from water are accumulated at a binding site B. Subsequently, the two electrons are transferred to non-protonated acceptors in Photosystem I (probably plastocyanin and cytochrome f) and the 4H + are released inside the thylakoid. 6. (6) It is concluded that a primary proton transporting site and/or energy conserving step located between the photosystems is being observed.
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