The cytochrome b6f complex (plastoquinol/plastocyanin reductase or plastoquinol/plastocyanin oxidoreductase; EC 7.1.1.6) is an enzyme found in the thylakoid membrane in chloroplasts of plants, cyanobacteria, and green algae, that catalyzes the transfer of electrons from plastoquinol to plastocyanin: The reaction is analogous to the reaction catalyzed by cytochrome bc1 (Complex III) of the mitochondrial electron transport chain.

[5] The inter-monomer space within the core of the cytochrome b6f complex dimer is occupied by lipids,[9] which provides directionality to heme-heme electron transfer through modulation of the intra-protein dielectric environment.

[4] In a separate reaction, the cytochrome b6f complex plays a central role in cyclic photophosphorylation, when NADP+ is not available to accept electrons from reduced ferredoxin.

[20] An integral chlorophyll molecule located within the quinol oxidation site has been suggested to perform a structural, non-photochemical function in enhancing the rate of formation of the reactive oxygen species, possibly to provide a redox-pathway for intra-cellular communication.

[23] The complex contains up to three plastoquinone molecules that form an electron transfer network that are responsible for the operation of the Q cycle and its redox-sensing and catalytic functions in photosynthesis.

The electron from ferredoxin (Fd) is transferred to plastoquinone and then the cytochrome b6f complex to reduce plastocyanin, which is reoxidized by P700 in Photosystem I.