From Wikepedia:
In photosynthesis, plastocyanin functions as an electron transfer agent
between cytochrome f of the cytochrome b6f complex from photosystem II and
P700+ from photosystem I. Cytochrome b6f complex and P700+ are both
membrane-bound proteins with exposed residues on the lumen-side of the
thylakoid membrane of chloroplasts. Cytochrome f acts as an electron donor
while P700+ accepts electrons from reduced plastocyanin.