<p>Light-induced redox transformations of P₇₀₀, the reaction center of photosystem I, were studied using the EPR method depending on the illumination conditions of plant leaves (intensity and spectral composition of the active light). Within the framework of a mathematical model, the key stages of electron transfer along the noncyclic electron transport chain containing photosystems I and II and mobile transporters (plastoquinone, plastocyanin, and ferredoxin) and the processes of <i>trans</i>-thylakoid proton transfer associated with ATP synthesis were considered. The mechanisms of pH-dependent regulation of electron transport in chloroplasts at the acceptor and donor sites of photosystem I have been analyzed. The modeling results are in agreement with experimental data on the kinetics of light-induced transformations of P₇₀₀ in chloroplasts of higher plants. The results are discussed in the context of “short-term” pH-dependent mechanisms of electron transport regulation in chloroplasts in situ<i>.</i></p>