Alternative pathways of photoinduced electron transport in chloroplasts

The influence of the alternative pathways of electron transport in photosynthetic systems of the oxygen type on the kinetics of the photoinduced redox transitions of P700, ferredoxin, NADP, pH of the intrathylakoid space or lumen, and relative concentration of ATP was studied. The oxygen effect on the kinetics of photooxidation of P700 was analyzed. The retardation of the photooxidation of P700 at low oxygen concentrations can be explained by the “over-reduction” of the acceptor side of PS1 as a result of a decrease in the electron outflow from PS1 to oxygen during hypoxia. The results of numerical experiments are in good agreement with known experimental data that the withdrawal of electrons from PS1 (on the ferredoxin-NADP segment of the chain) can be the limiting stage in the noncyclic electron transport chain. The functioning of the cyclic electron transport chain provides additional synthesis of ATP molecules and weakens the excess reduction of the acceptor segment of PS1. The alternative pathway of electron transport, namely, electron outflow from PS1 to oxygen also favors the optimum conditions for the functioning of the photosynthetic electron transport chain.