Figure 3.

Schematic of the electron–proton transport chain of oxygenic photosynthesis in the thylakoid membrane, showing how photosystem I (PSI) and photosystem II (PSII) work together to use absorbed light to oxidize water and reduce NADP⁺, in an alternative representation to the Z-scheme shown in figure 2. The diagram also shows how the vectorial flow of electrons across the membrane generates a proton gradient which is used to power the conversion of ADP to ATP at the ATP synthase complex (CF_oCF₁) which is also embedded in the thylakoid membrane (not shown). In both PSI and PSII, the redox-active cofactors are arranged around a pseudo-twofold axis. In PSII, primary charge separation and subsequent electron flow occurs along one branch of the reaction centre. However, in the case of PSI, electron flow occurs up both branches as shown. Electron flow through the cytochrome b₆ f complex also involves a cyclic process known as the Q cycle. Y_Z = tyrosine; P680 = primary electron donor of PSII composed of chlorophyll (Chl); Pheo = pheophytin; Q_A and Q_B = plastoquinone; Cyt b₆f = cytochrome b₆f complex, consisting of an Fe–S Rieske centre, cytochrome f (Cyt f), cytochrome b low- and high-potential forms (Cyt b_LP and Cyt b_HP), plastoquinone binding sites, Q₁ and Q₀; PC = plastocyanin; P700 = primary electron Chl donor of PSI; A₀ = Chl; A₁[Q] = phylloquinone; F_x, F_A and F_B = Fe–S centres, F_D = ferredoxin; FNR = ferredoxin NADP reductase; NADP⁺ = oxidized nicotinamide adenine dinucleotide phosphate. Y_D = symmetrically related tyrosine to Y_z but not directly involved in water oxidation, and QH₂ = reduced plastoquinone (plastoquinol), which acts as a mobile electron/proton carrier from PSII to the cytochrome b₆f complex. With the exception of the mobile electron carriers Q/QH₂, PC and F_D, the remaining redox-active cofactors are bound to multisubunit protein complexes that span the membrane depicted as coloured boxes.