plastocyanin

One of the electron transfer components of chloroplasts, mediating transfer of electrons from photosystem II to photosystem I. It is a blue copper-containing protein, and it is the reduction and oxidation of the copper atom that allows plastocyanin to accept, then donate, electrons. The position of plastocyanin in the electron transport chain is thought to be very close to photosystem I, probably between cytochrome f and photosystem I.