Plastocyanin (Pc) is a soluble copper protein that transfers electrons from cytochromeb6 fto photosystem I (PSI), two protein complexes that are localized in the thylakoid mem-branes in chloroplasts. The surface electrostatic potential distributionof Pc plays akey role in complex formationwith the membrane-bound partners. It is practically identical for Pcs fromplants and green algae, but is quite different for Pc fromferns.