Plastoquinones QH is a molecule in the electron transfer system betweeen photosystem II (PSII) and photosystem I. The sequence of carriers is as follows: Pheophytin -> QA -> QB -> QH -> Cytochrome b6f. Plastoquinone QH2 is the photosynthetic equivalent of coenzyme Q of the electron transport system. It moves freely in the thylakoid membrane, carry electrons between the photosystem II complex and the cytochrome b6f complex (Figure 17.12). Electron transfer involving plastoquinones is shown here. Reduction of a plastoquinone creates a plastoquinol.