PSIIplastoquinone

PSII plastoquinone refers to the plastoquinone molecule that functions within Photosystem II (PSII) of the photosynthetic apparatus in plants and other photosynthetic organisms. Plastoquinones are small, lipid-soluble molecules that act as mobile electron carriers. In PSII, plastoquinone plays a crucial role in the electron transport chain.

Specifically, the primary plastoquinone molecule in PSII is often denoted as Q A, which is tightly bound to the D1 protein. Q A receives electrons from the reaction center chlorophyll, P680, after it has been excited by light energy. From Q A, electrons are then transferred to a secondary, more mobile plastoquinone molecule, Q B. Q B is also bound to the D1 protein, but it can bind and release plastoquinone molecules.

The interaction with Q B is particularly important because it allows for the sequential reduction of Q B by two electrons. Each reduction of Q B involves the uptake of protons from the stroma. Once Q B has accepted two electrons and two protons, it becomes fully reduced plastoquinol (PQH2). Plastoquinol is then released from PSII and diffuses through the thylakoid membrane to the cytochrome b6f complex, where it donates its electrons, continuing the electron transport chain. This process is essential for generating the proton gradient that drives ATP synthesis. The binding and release cycle of Q B is a rate-limiting step in PSII electron transport and is a common target for herbicides.