Figure 1. Energy level diagram depicting the Z scheme of photosynthetic electron transport. Light absorbed by the antennae leads to excitation of the reaction centre chlorophylls of PS II (P₆₈₀) and PS I (P₇₀₀). The primary photochemical event in PS II is the transfer of an electron from the excited state of P₆₈₀^* to the primary acceptor pheophytin (Pheo). Reduced pheophytin transfers electrons to the primary quinone acceptor Q_A, which transfers electrons to the secondary quinone acceptor Q_B, which picks up two protons from the stromal side of the thylakoid membrane and merges into the plastoquinone (PQ) pool. The positively charged hole created by excitation of P₆₈₀ is filled by an electron from the secondary donor, Y_Z, which, in turn, is reduced by electrons supplied by water. The oxygen evolving complex on the donor side of PS II contains a cluster of four manganese (Mn) ions that catalyse the oxidation of water. Excitation of PS I facilitates electron transport beyond plastoquinone. Photochemistry from the excited state of P₇₀₀^* results in the reduction of ferredoxin (Fd) via a series of bound PS I acceptors. Electron donation from the plastoquinone pool, via the cytochrome b/f complex and plastocyanin (PC) restores P₇₀₀. Beyond PS I, the re-oxidation of ferredoxin leads to the reduction of NADP⁺ and, finally, of CO₂ to carbohydrate. Electron transport through the two photosystems generates a transthylakoid proton gradient (pH), which is used to drive the synthesis of ATP from ADP and P_i. The transfer of electrons from water to NADPH requires 109 kJ mol^–1 per electron transferred, which explains why light, with its relatively large amount of energy (Table 3), is needed to drive photosynthesis.

Figure 2. The chemiosmotic mechanism of ATP formation in chloroplasts and mitochondria. In chloroplasts, electron transport results in translocation of protons from the stroma to the thylakoid lumen, while, in mitochondria, protons are translocated from the matrix to the intermembrane space. In both cases, a pH gradient results that is used to drive the formation of ATP by a membrane-bound ATP synthase. cyt, cytochrome.