photosynthesis. (a) Linear electron flow begins with excitation of the photosynthetic
reaction centers, PSII and PSI, and with splitting of water by the oxygen-evolving
complex of PSII. Electrons are passed from PSII to the plastoquinone (PQ) pool and
then to PSI, where NADPH is formed. The cytochrome complex (cyt b 6 f) pumps in
protons that drive ATP production. (b) Under conditions of high light and low
nutrients, plastoquinol terminal oxidase (PTOX) is activated and serves as an electron
relief valve that uses excess electrons to reduce O 2 to water, so that less NADPH is
formed than during linear electron transport.
(^) (After Zehr & Kudela 2009.)
The asterisks indicate that the oxygen produced is from the water reactant, not from