Bicarbonate-controlled reduction of oxygen by the Q semiquinone in Photosystem II in membranes.

Photosystem II (PSII), the water/plastoquinone photo-oxidoreductase, plays a key energy input role in the biosphere. [Formula: see text], the reduced semiquinone form of the nonexchangeable quinone, is often considered capable of a side reaction with O, forming superoxide, but this reaction has not yet been demonstrated experimentally. Here, using chlorophyll fluorescence in plant PSII membranes, we show that O does oxidize [Formula: see text] at physiological O concentrations with a of 10 s.