Plastocyanin

Plastocyanin is a copper-containing protein that plays a role in the electron transport process associated with photosynthesis. It serves as an electron transfer agent between the cytochrome complex which follows Photosystem II and the entry point to Photosystem I of the non-cyclic electron transfer process.

The sketch below shows the context of plastocyanin's role in the context of the thylakoid membrane in which this process operates.

Plastocyanin also plays a role in the cyclic electron transport employed in some photosynthetic prokaryotes, as indicated below.

In this cyclic process, photosynthesis is used only to provide for the production of ATP for immediate energy needs, and the plastocyanin returns the electrons to Photosystem I.