Mechanistic insights into photosynthetic linear electron transport flow: a mathematical modeling approach.

A mathematical model of a photosynthetic electron transport chain part comprising photosystem II (PSII), cytochrome b6 f complex (Cyt b6 f), and the plastoquinone (PQ) pool was developed to investigate the dependence of linear electron flow (LEF) on light intensity in proposition that special regulatory mechanisms are absent. The model predicts a nonmonotonic relationship between LEF rate and light intensity, driven by the imbalance of PSII and Cyt b6 f activity, which causes over-reduction of the PQ pool and further impedes electron transport through Cyt b6 f. These effects arise from the Q-cycle mechanism, where electron transport through the high- and low-potential branches must be balanced. Under PQ pool over-reduction, LEF blockage occurs being still reversible upon lowering light intensity. The mechanism under consideration is an internal property of the system, and its functioning is not associated with the generation of an electrical and proton gradient on the membrane. The model reproduces experimentally observed switching between active and blocked states of the electron transport chain under intense light. These findings demonstrate that rather complex behavior in response to external factors can emerge in a biological system even in the absence of explicit regulation due to the intrinsic properties of complex network of chemical reactions subject to the mass action law.