Correlation between plastoquinone reduction, field formation and proton translocation in photosynthesis.

The electron flow in photosynthesis from H20 to NADP® is driven by two photoactive molecules, chlorophyll-an and chlorophyll-ai. These two chlorophyll molecules are arranged in series within a chain of elec­ tron carriers. The chain is imbedded in the membrane of the thylakoids. The electron carrier plastoquinone is engaged between the two photoactive chlorophyll mole­ cules. During the transfer of one electron from H20 to NADP® (realized by excitation with a short flash of <^0,6 msec) plastoquinone is rapidly reduced (^ 0 ,6 msec) and slowly reoxidized (20 msec) through the cooperation of light reactions II and I. Simultaneously with these events the formation of an electrical field perpendicular to the thylakoid membrane takes place. This membrane potential is indicated by absorp­ tion changes of chlorophyll-b, e. g. at 515 nm (for de­ tails see I .e .1-3). Furthermore a proton transloca­ tion across the membrane into the inner phase of the thylakoids is coupled to the electron flow 4. It has recently been shown that during one electron transfer in each of the two light reactions the mem­ brane potential is raised by the same unit (25 mV) and in each of the two light reactions one proton is taken up 5. Concerning light reaction II the following mechanism has been concluded 5. The formation of the membrane potential occurs through the shift of one electron from H20 at the inner surface of the mem­ brane to plastoquinone at the outer surface. This shift causes a change of the membrane potential of one unit (25 mV). The proton uptake occurs through the ac­ ceptance of a proton by the reduced plastoquinone