Fig. 4: Comparative schematic of light-induced electron flow between summer (green arrows) and early spring (insets with blue arrows) thylakoid membranes.

Insets show possible alterations in the electron flow in early spring compared to summer as directly measured by our experimental setups. Each step of altered electron flow in early spring is indicated sequentially from A to F. PDB structures used in the model are PSII (Pisum sp) 5XNL⁷⁹; Cyt b₆f (Mastigocladus laminosus) 4H0L⁸⁰; plastocyanin-PSI-ferredoxin (Pisum sp) 6YEZ⁸¹. Note, that the PSII cofactors in contrast to PSI cofactors are not shown in the model as their redox state was not measured experimentally. Electron flow from Cyt b6f to PSI is mediated through plastocyanin that upon binding to Cyt b₆f accepts electrons and diffuses to PSI followed by interaction with PSI and electron donation to P700. However, due to the non-availability of Cyt b6f bound plastocyanin PDB structure plastocyanin is only shown in association with PSI. The position of Flv on PSI does not represent any empirical binding site nor define the exact site of electron transfer from PSI acceptor side to Flv. Bold text in the schematic indicates direct experimental evidence of the phenomena.