Plastoquinone (PQ) is a quinone molecule involved in the electron transport chain in the light-dependent reactions of photosynthesis. Plastoquinone is reduced when it accepts two electrons from photosystem II and two hydrogen cations (H⁺) from the stromal matrix of the chloroplast), thereby forming plastoquinol. It transports the protons into the lumen of thylakoid discs, while the electrons continue further along the electron transport chain, into the cytochrome b₆f protein complex.