The role that plastoquinone plays in photosynthesis, more specifically in the light-dependent reactions of photosynthesis, is that of a mobile electron carrier through the membrane of the thylakoid.
Plastoquinone is reduced when it accepts two electrons from photosystem II and two hydrogen cations (H) from the stroma of the chloroplast, thereby forming plastoquinol (PQH). It transfers the electrons further down the electron transport chain to plastocyanin, a mobile, water-soluble electron carrier, through the cytochrome bf protein complex. The cytochrome bf protein complex catalyzes the electron transfer between plastoquinone and plastocyanin, but also transports the two protons into the lumen of thylakoid discs. This proton transfer forms an electrochemical gradient, which is used by ATP synthase at the end of the light dependent reactions in order to form ATP from ADP and P.