Translocates protons across the thylakoid membrane and transfers electrons from photosystem II to photosystem I. It receives electrons from the Rieske iron-sulfur protein and passes them to plastocyanin. Binds 1 heme group covalently. Interacts with plastocyanin and Rieske iron-sulfur protein. This polypeptide is nuclear encoded in Euglena gracilis, but is chloroplast encoded in other plant species. Belongs to the cytochrome f family.