In the light-dependent reactions, electrons exit the cytochrome b6f complex and must reach the reaction center of photosystem I. Which carrier shuttles them across that span inside the thylakoid lumen?

Core Concept

After electrons leave the cytochrome b6f complex, they must cross the thylakoid lumen to reach oxidized P700 in photosystem I. Plastocyanin, a small soluble protein whose single copper ion alternates between Cu+ and Cu2+, accepts electrons at b6f and donates them to photosystem I. This mobile shuttle links the two photosystems and keeps linear electron flow running.