Fig 5. Proposed mechanism for the involvement of loosely bound plastosemiquinone at the Q_B and Q_C sites in O₂ ^•– formation in PSII.

Superoxide anion radicals are produced via one-electron reduction of molecular oxygen by plastosemiquinones, which are formed via one-electron reduction of plastoquinone at the Q_B sites and one-electron oxidation of plastoquinol at the Q_C site but unlikely at the Q_D site.