FIGURE 12-44 Linear electron flow in plants, which requires both chloroplast photosystems, PSI and PSII. Blue arrows indicate flow of electrons; red arrows indicate proton movement. LHCs are not shown. (Left) In the PSII reaction center, two sequential light-induced excitations of the same special-pair P₆₈₀ chlorophyll result in a two-step reduction of the primary electron acceptor Q_B to QH₂. On the luminal side of PSII, electrons removed from H₂O by the oxygen-evolving complex are transferred to P₆₈₀⁺, restoring the reaction-center chlorophylls to the ground state after each excitation. The oxygen-evolving complex contains a cluster of four manganese ions (Mn, violet), a Ca²⁺ ion (green), and a Cl⁻ ion (teal). These bound ions function in the splitting of H₂O and maintain the environment essential for high rates of O₂ evolution. Four sequential light-induced excitations of the P₆₈₀ (double the number illustrated here) are required to oxidize two water molecules and release four protons and one molecule of molecular oxygen (O₂). A tyrosine on the protein helps conducts electrons from the Mn ions to the oxidized reaction-center chlorophyll (P₆₈₀⁺), reducing it to the ground state (P₆₈₀) after excitation by each photon. (Center) The cytochrome bf complex then accepts electrons from QH₂ and transports two protons into the lumen. Operation of a Q cycle in the cytochrome bf complex translocates additional protons across the membrane to the thylakoid lumen, increasing the proton-motive force. (Right) In the PSI reaction center, each electron released from light-excited P₇₀₀ chlorophylls moves via a series of carriers in the reaction center to the stromal surface, where soluble ferredoxin (an Fe-S protein) transfers the electron to ferredoxin-NADP⁺ reductase (FNR). This enzyme uses the prosthetic group flavin adenine dinucleotide (FAD) and a proton to reduce NADP⁺, forming NADPH. P₇₀₀⁺ is restored to its ground state by addition of an electron carried from PSII via the cytochrome bf complex and plastocyanin, a soluble electron carrier.