The primary reactions of photosynthesis are catalyzed by three major protein–pigment complexes which are serially connected by the photosynthetic electron transport chain. They include photosystem II (PSII), the cytochrome b6f complex (Cytb6f) and photosystem I (PSI). Both PSII and PSI are associated with their own light-harvesting complexes LHCII and LHCI, respectively, and catalyze the primary photochemical reactions in which the absorbed light energy is used to induce a stable charge separation across the thylakoid membrane. The donor side of PSII creates a strong oxidant capable of extracting electrons from water with their subsequent transfer along the electron
transport chain to plastoquinone, Cytb6f and PSI. The acceptor side of PSI creates in turn a strong reductant capable of reducing ferredoxin and finally, through ferredoxin-NADP reductase (FNR), NADP+ to NADPH. This electron flowis coupled to proton transfer fromthe stroma to the lumen side of the thylakoid membrane and the resulting proton motive force is used by the ATP synthase to generate ATP. Finally, both ATP and NADPH act as energy source and reducing power, respectively,for driving CO2 assimilation by the Calvin–Benson cycle.