Plastoquinone is the mobile electron carrier that transfers electrons from Photosystem II toward the cytochrome b6f complex. In its reduced form, plastoquinol, it diffuses within the thylakoid membrane and delivers those electrons to cytochrome b6f.
Inside Photosystem II, plastoquinone occupies two important binding sites called QA and QB. QA accepts one electron at a time and passes electrons to QB. After QB receives two electrons and takes up two protons from the chloroplast stroma, it becomes plastoquinol and leaves Photosystem II for the membrane’s wider plastoquinone pool.
Cytochrome b6f oxidizes plastoquinol, transfers its electrons onward to plastocyanin, and contributes to the proton gradient used to make ATP. The precise chemical distinction explains a frequent terminology issue: plastoquinone is the oxidized carrier, while plastoquinol is its reduced, electron-loaded form. NADP+ and ferredoxin function later near Photosystem I; plastocyanin carries electrons after cytochrome b6f.