Besides being the substrate for the carboxylation reaction of photosynthesis, CO 2 (bicarbonate) is required for the activity of Photosystem II (water plastoquinone oxido-reductase). It plays a role on the electron donor side as well as the electron acceptor side. In this contribution, attention will mostly be focused on the history of research into the effects of bicarbonate on electron flow reactions on the acceptor side. Donor side reactions are discussed in this issue by Alan Stemler.

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... iations: Chl -chlorophyll; DCMU (diuron) -3-(3,4-dichlorophenyl)-1,1-dimethylurea; PS II -Photosystem II; PQ -plastoquinone; Q A -primary quinone electron acceptor of PS II; Q B -secondary quinone electron acceptor of PS II