Photoelectron transport in the PSII reaction center (see Figure 12-44) resembles that in the purple bacterial reaction center described earlier (see Figure 12-43). Excitation by a photon with a wavelength of <680 nm triggers the loss of an electron from a P₆₈₀ chlorophyll. That electron is transported rapidly to a quinone (Q_A) and then to the primary electron acceptor, Q_B, on the stromal surface of the thylakoid membrane (see Figure 12-44). PSII differs from the bacterial reaction center, however, in that it has an additional three-protein subunit, the oxygen-evolving complex, that faces the thylakoid lumen (in this context, “oxygen-evolving” refers to generating O₂ from water).

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The photochemically oxidized special-pair reaction-center chlorophyll a of PSII, P₆₈₀⁺, is the strongest biological oxidant known. The reduction potential of P₆₈₀⁺ is more positive than that of H₂O, and thus it can oxidize H₂O to generate O₂ and H⁺ ions. The oxidation of H₂O provides the electrons for reduction of P₆₈₀⁺ in PSII, thus replacing the electron released by light absorption. The oxygen-evolving complex is a protein that contains a manganese (Mn), calcium, and oxygen cluster (Mn₄CaO₅) that directly mediates the conversion of H₂O to O₂, as well as a bound Cl⁻ ion that influences the reaction rate (see Figure 12-44). The Mn ions cycle through five different oxidation states during O₂ generation. A nearby tyrosine side chain helps transfer electrons from H₂O to the P₆₈₀⁺. The protons released from H₂O remain in the thylakoid lumen. The oxidation of two molecules of H₂O to form O₂ requires the removal of four electrons, but absorption of each photon by PSII results in the transfer of just one electron. Thus a single PSII must lose an electron and then oxidize the oxygen-evolving complex four times in a row for an O₂ molecule to be formed. Channels in the protein of the oxygen-evolving complex have been proposed to serve as conduits through the complex for the delivery of H₂O to and the removal of O₂ from the active site.

The herbicide atrazine is one of the most commonly used weed killers in US agriculture. Atrazine binds to PSII, blocks the binding of oxidized Q_B, and thus blocks downstream electron transport.