Figure 1. The ‘Z scheme’. This shows how the two primary photoacts of oxygenic photosynthesis are linked by thermochemical electron transfer through the cytochrome b₆f complex coupled to ATP synthesis. The energetics of electron transfer are illustrated by reference to the scale of midpoint redox potentials (E_m) drawn underneath the ‘Z’. P680, the special pair of chlorophyll a molecules acting as the primary electron donor of photosystem II; hv, a quantum of light absorbed by antenna chlorophylls; Q_A/B, the primary and secondary plastoquinone acceptor molecules associated with photosystem II; PQ, the pool of plastoquinone molecules in the thylakoid membrane; cyt, cytochrome; FeS, the ‘Rieske’ iron–sulfur protein; PC, plastocyanin; P700, the primary donor chlorophylls a of photosystem I; F_B, one of two iron–sulfur centres associated with subunit PsaC of photosystem I; Fd, ferredoxin.

Figure 2. The vectorial ‘Z scheme’. This represents the Z scheme in terms of the arrangement of the components within the thylakoid membrane according to the chemiosmotic theory, and illustrates the accumulation of H⁺ in the thylakoid lumen (‘in’) coupled to electron transfer. hem b_L, hem b_H, low- and high-potential haems of cytochrome b₆; other abbreviations as in Figure 1. The stoichiometry of H⁺ translocation is not fully represented.

Figure 3. Diagrammatic arrangement of protein complexes and diffusible components in relation to a thylakoid. PSII, photosystem II; D1/D2, the two main subunits of the reaction centre of photosystem II; O, subunit PsbO, which protects the manganese cluster of the OEC; b, f, cytochromes b₆ and f, respectively, of the cytochrome b₆f complex; IV, subunit IV of the cytochrome b₆f complex; PC, plastocyanin; c₆, cytochrome c₆; PSI, photosystem I; A/B, the two main subunits of photosystem I (PsaA, PsaB); C, D, E, F, subunits of photosysem I; Fd, ferredoxin; FNR, ferredoxin–NADP oxidoreductase; F₀, F₁, intrinsic and extrinsic components of ATP synthase; III, multiple copies of subunit III involved in proton translocation; , , subunits of F₁.

Figure 4. Cofactors of the reaction centres of photosystems II and I, drawn from the X-ray crystal structures. The plane of the thylakoid membrane is horizontal and at right angles to the plane of view. Light-induced electron transfer takes place from bottom to top of the figure. (a) Photosystem II from Thermosynechococcus elongatus, pdb entry 1S5L. Oxygen evolving centre (OEC); Tyr_Z, side-chain of Tyr-161 of D1 which acts as an intermediate in electron transfer between OEC and P680; Tyr_D, side-chain of Tyr-160 of D2, which can donate electrons to P680, but is inactive in water oxidation; P680, the special pair of chlorophyll a molecules acting as the primary electron donor of photosystem II; Chl_D1, Chl_D2 chlorophyll a molecules bound to D1 and D2, respectively, only the former normally being active in electron transfer; Pheo_D1, Pheo_D2, phaeophytin molecules associated with D1 and D2, respectively; Q_A, tightly bound molecule of plastoquinone acting as primary acceptor; Fe, ferrous ion important for electron transfer from Q_A to Q_B; Q_B, loosely bound molecule of plastoquinone, the secondary acceptor. (b) Photosystem I from Synechococcus elongatus, pdb entry 1JB0; P700(eC1), pair of chlorophyll a molecules associated with PsaA and PsaB respectively; Q_K, a pair of vitamin K (naphthoquinone) molecules; F_x, iron–sulfur centre bound to interface between PsaA and PsaB; F_A, F_B, iron–sulfur centres bound to the subunit PsaC.

Figure 5. Cofactors of the cytochrome b₆f complex of Chlamydomonas reinhardtii, drawn from the X-ray crystal structure, pdb entry 1Q90. The orientation is as in Figure 4. Hem b_L, Hem b_H, low-potential and high-potential haems, respectively, of cytochrome b₆; Hem f, a c-type haem bound to cytochrome f; Fes, iron–sulfur centre of the Rieske iron–sulfur protein subunit; Hem c_i, a c-type haem associated with the cytochrome b polypeptide and of uncertain function; Chl, a molecule of chlorophyll a associated with the cytochrome b polypeptide and of unknown function.

Figure 6. Model of thylakoid membrane structure in higher-plant chloroplasts, illustrating the distribution of the major protein complexes between grana and stroma lamellae, and between appressed membranes, grana margins and end membranes within the grana stacks.