Figure 1
Model for electron transport chain and cytochrome \({b}_{6}\,f\) complex. (a) The electron transport chain of oxygenic photosynthesis. Solid purple arrows show the ideal linear electron flow (LEF) in which electrons go through the whole chain and are finally transferred to a biological redox agent called NADPH. Dashed purple arrows show the alternative path that makes cyclic electron flow (CEF) possible. (b) Molecular structure of the cytochrome \({b}_{6}\,f\) complex (protein data bank, accession code PDB-1VF5) with a description of the Q-cycle mechanism under CEF conditions. The electron recycling chain (heme \({b}_{L}\) and heme \({b}_{H}\) prosthetic groups) is marked with blue arrows, and biological counterparts for the elements considered in the model are reported. (c) Pictorial representation of the model described in this article. Single electron binding sites are represented as small black circles, and proton ones as small white circles. The PQ purple element represents the plastoquinone/plastoquinol molecule, which is able to commute between N- and P-side (stromal and lumenal, respectively) reaction sites, as indicated by purple arrows. Ferredoxin (Fd) and Plastocyanin (Pc) pools on the stromal and lumenal sides, namely collections of water-soluble single electron carriers, are modeled as fermionic reservoirs. Protons in the bulk aqueous phase on the P- and N-side are described in a similar way. Site \(A\) models the electron re-injection site from the Fd pool to \({\rm{PQ}}\), while \(B\) collectively represents the ISP-cytochrome \(f\) chain that transfers electrons to the Pc pool. We assume that PSI transfers electrons from the Plastocyanin pool to the Ferredoxin pool with the help of external energy coming from light absorption. Sites \(L\) and \(H\), reported in blue consistently with the color code in panel (b), represent the heme \({b}_{L}\) and \({b}_{H}\) groups respectively. The black arrows show the ideal path of the electrons in the system.
