Plastocyanin is an electron carrier in the system that operates between photosystem II and I (Figure 17.12). Plastocyanin accepts electrons from cytochrome f of the cytochrome b6f complex and donates them to photosystem I. Plastocyanin contains a copper atom, which is reduced to Cu⁺ upon accepting an electron and Cu²⁺ upon losing the electron.

See also: Photosystem II, Photosystem I, Cyclic Electron Flow, Cytochrome b6f