Plastocyanin is the copper-containing protein that carries electrons from cytochrome b6f to photosystem I. It is a small, soluble electron carrier located in the thylakoid lumen of chloroplasts and in the corresponding compartment of cyanobacteria and algae.
Cytochrome b6f reduces plastocyanin by transferring an electron to its copper center. In this reduced state, plastocyanin diffuses through the lumen and donates the electron to P700+, the oxidized reaction-center chlorophyll of photosystem I. Plastocyanin is then oxidized and ready to repeat the cycle.
Its intense blue color comes from the type I copper site, known as a blue-copper or cupredoxin center. The copper is coordinated by two histidine residues, a cysteine, and a methionine. This metal center switches between Cu2+ and Cu+ states, enabling rapid reversible electron transfer.
Cytochrome f is part of the cytochrome b6f complex and donates electrons to plastocyanin, but it is not the mobile carrier that completes the link to photosystem I. Plastoquinone operates earlier in the chain, while ferredoxin acts after photosystem I.