Electron transport from water to NADP an overview

Electron transport from water to NADP an overview 

The photosynthetic apparatus of green plants and cyanobacteria oxidizes water and transfers electrons to NADP, with a net gain in electrochemical potential of 1.13 eV (at pH 7), utilizing the energy of two light quanta per electron. The complete system is contained in the chloroplasts, and is localized within the thylakoid membranes, with the exception of the electron carrier ferredoxin, which is in solution in the stroma, and serves to transfer electrons from the reducing end of photosystem I (PS I) to a membrane-bound flavoprotein which then reduces NADP, and of the copper protein plastocyanin (PC, the electron donor to PS I), which is in solution in the internal phase of thylakoids. 

The two photochemical reactions are performed by two photosystems. Each photosystem consists of a so-called reaction centre, where the primary energy conversion takes place, associated with a few hundred pigment molecules (chlorophylls and carotenoids see Fig. 2) serving as light-harvesting antennas, which transfer the absorbed energy as electronic excitation energy to the reaction centres. 

PS II is responsible for the oxidation of water and the reduction of a stable acceptor at the potential of ca. 0.0 to -0.2 V, while PS I transfers electrons from a donor of = 0.45 V to an acceptor of of ca. -0.65 V. An electron transport chain connects the reducing side of PS II to the oxidizing side of PS I, down the electrochemical gradient. At the reducing side of PS I NADP is reduced, while at the oxidizing side of PS II water is oxidized and Oj is evolved. 

The evolution of O2 from water has been shown to occur every 4th flash, when flashes of saturating intensity, short enough to allow only one turnover of the PS II reaction centres, are fired, separated by a dark period long enough to permit the reoxidation of the electron acceptors on the reducing side of PS II [7]. This observation has been the basis of the S states model. Each flash promotes the transition from the state S to S +, in the sequence [8,9]  

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