Cyclic electron transfer in plant leaf

TLDR

The study proposes that the cyclic electron pathway functions within a supercomplex comprising one PSI, one cytochrome b₆f complex, one plastocyanin, and one ferredoxin. The authors model the system as a supercomplex that dissociates upon illumination, gradually shifting PSI from cyclic to linear electron flow. Fluorescence and membrane potential kinetics reveal that cyclic electron flow operates at near‑maximal rates both when PSII is inhibited and when active, indicating a highly efficient, structurally isolated cyclic chain that supplies ATP for the Calvin cycle while only about half of PSI participates in linear flow.

Abstract

The turnover of linear and cyclic electron flows has been determined in fragments of dark-adapted spinach leaf by measuring the kinetics of fluorescence yield and of the transmembrane electrical potential changes under saturating illumination. When Photosystem (PS) II is inhibited, a cyclic electron flow around PSI operates transiently at a rate close to the maximum turnover of photosynthesis. When PSII is active, the cyclic flow operates with a similar rate during the first seconds of illumination. The high efficiency of the cyclic pathway implies that the cyclic and the linear transfer chains are structurally isolated one from the other. We propose that the cyclic pathway operates within a supercomplex including one PSI, one cytochrome bf complex, one plastocyanin, and one ferredoxin. The cyclic process induces the synthesis of ATP needed for the activation of the Benson–Calvin cycle. A fraction of PSI (∼50%), not included in the supercomplexes, participates in the linear pathway. The illumination would induce a dissociation of the supercomplexes that progressively increases the fraction of PSI involved in the linear pathway.