Cytochrome b6f is a is part of the electron transport chain that transfers electrons from photosystem II to photosystem I. Cytochrome b6f is a complex of proteins that includes cytochromes f, b6, and an iron sulfur protein. Cytochrome b6f accepts electrons from plastoquinone QH2 and passes them to plastocyanin (Figure 17.12). In addition to transferring electrons, the cytochrome b6f complex pumps protons into the thylakoid lumen, helping to build the proton gradient, which is used by the CF0-CF1 complex to make ATP.