Figure 3. Charge recombination pathways in PSII.

The energy gaps are not drawn to scale. Safe routes for charge recombination between P⁺ and Q_A⁻ are indicated in blue, the damaging route producing ¹O₂ in red and radiative pathway in green. Stabilization of the P⁺PheoQ_A⁻ state helps prevent reverse electron flow to form P⁺Pheo⁻Q_A and subsequent charge recombination to form PPheoQ_A. For clarity, the details of the additional electron transfer steps, including the oxidation of water and the reduction of plastoquinone to plastoquinol by PSII, collectively termed as photosynthesis, are omitted. Abbreviations: P, primary electron donor of PSII; Pheo, pheophytin electron acceptor; Q_A, primary plastoquinone electron acceptor; ¹O₂, singlet oxygen; ³O₂, triplet oxygen; ³P, triplet excited state of P.