Photosystem II complex (PSII) of thylakoid membranes uses light energy to oxidise extremely stable water and produce oxygen (2H2O→O2+4H++4e−). PSII is compared with cytochrome c oxidase that catalyses the opposite reaction coupled to proton translocation. Cytochrome c oxidase has proton and water channels, and a tentative oxygen channel. I propose that functional PSII complexes also need a specific oxygen channel to direct O2 from the water molecules bound to specific Mn atoms of the Mn cluster within PSII out to the membrane surface. The function of this channel will be to prevent oxygen being accessible to the radical pair P680+Pheo−, thereby preventing singlet oxygen generation from the triplet P680 state in functional PSII. The important role of singlet oxygen in structurally perturbed non-functional photosystem II is also discussed.
