plastoquinoni

Plastoquinone is a lipid-soluble, redox-active benzoquinone molecule that serves as a key electron carrier in the photosynthetic electron transport chain of chloroplasts, as well as in cyanobacteria and some algae. It consists of a hydrophobic isoprenoid (polyprenyl) side chain attached to a small, redox-active benzoquinone head. In higher plants and green algae, the common form is plastoquinone-9, named for its nine isoprene units in the tail. The hydrophobic tail anchors plastoquinone in the thylakoid membrane, allowing it to diffuse within the lipid bilayer.

In photosynthesis, plastoquinone accepts electrons from photosystem II and becomes plastoquinol (PQH2). It then diffuses to the cytochrome b6f complex, where it is oxidized back to plastoquinone, releasing protons into the thylakoid lumen. This Q-cycle-like mechanism contributes to the formation of the proton gradient across the membrane, which drives ATP synthesis via the chloroplast ATP synthase. The plastoquinone pool acts as a mobile carrier and reservoir that balances electron flow and helps regulate photosynthetic efficiency.

The plastoquinone pool is distributed within the thylakoid membrane and can be adjusted in response to light conditions and metabolic demand. Different organisms have plastoquinones with varying tail lengths, but the general role of plastoquinone as a membrane-embedded electron carrier and proton-translocating participant in photosynthesis is conserved. Beyond its primary function, plastoquinone and its reduced form can contribute to protective and antioxidant processes in chloroplast membranes.