The electric generator in photosynthesis of green plants. I. Vectorial and protolytic properties of the electron transport chain.

Light induces the generation of an electrochemical potential difference across the functional membrane of photosynthesis of green plants. Experimental results on the electrochemical phenomena have been largely interpreted in terms of a vectorial alternating electron hydrogen transport system as originally hypothesized by Mitchell. We asked whether or not the reaction coordinate of the electron transport crosses the membrane, and whether or not the protolytic reactions at either side of the membrane can be understood from the protolytic properties of the redox components involved. For this we studied the flash-light-induced protolytic reactions in the outer and the inner aqueous phase of the chloroplast inner disk membranes. Four sites of protolytic reactions were identified, two at either side of the membrane. One of these sites had to be attributed to the reduction of the terminal electron acceptor at the outer side of the membrane. Evidence is presented for the coupling of the other sites to the oxidation of water at the inner side of the membrane, to the reduction of plastoquinone at the outer side and its oxidation at the inner side, respectively. These results support Mitchell's hypothesis for the generation of an electrochemical potential difference by a vectorial electron transport system.