Photosystem II

Photosystem II is a pigment–protein complex that uses light energy to extract electrons from water, releasing oxygen and supplying electrons to the photosynthetic electron transport chain.

P680: P680 is the chlorophyll a reaction-center pair in photosystem II that becomes strongly oxidizing after absorbing light. Its light-excited state initiates the electron transfer that ultimately removes electrons from water.
Chlorophyll a: Chlorophyll a is a light-absorbing pigment that transfers excitation energy and electrons in oxygenic photosynthesis. Its molecules form the reaction center and contribute to light harvesting in photosystem II.
Cyanobacteria: Cyanobacteria are oxygen-producing photosynthetic bacteria that use chlorophyll a and release oxygen from water. They contain photosystem II and were the source of the photosynthetic machinery inherited by chloroplasts.
Photosystem II inhibitors: Photosystem II inhibitors are compounds that block electron transfer, commonly by binding the QB site of the D1 protein. Their action shows how disrupting this complex can stop photosynthetic electron flow.
Oxygen-evolving complex: The oxygen-evolving complex is a manganese–calcium cluster that catalyzes water oxidation in photosystem II. It replaces electrons lost by P680 by extracting them from water.
Light-harvesting complex II: Light-harvesting complex II is a chlorophyll- and carotenoid-binding antenna that captures light for photosystem II in plants and algae. It transfers absorbed excitation energy to photosystem II's reaction center.
Endosymbiotic theory: Endosymbiotic theory explains how mitochondria and chloroplasts originated from bacteria incorporated into ancestral eukaryotic cells. It accounts for the bacterial ancestry of chloroplast photosystems.
Atrazine: Atrazine is a triazine herbicide that inhibits photosystem II by binding its D1 protein's QB site. It is a widely studied example of a chemical that blocks photosystem II electron transfer.
Plastoquinone: Plastoquinone is a lipid-soluble electron carrier that transfers electrons and protons within the thylakoid membrane. It accepts electrons from photosystem II and carries them toward cytochrome b6f.
Thylakoid membrane: The thylakoid membrane is the internal membrane system in chloroplasts and cyanobacteria where photosynthetic electron transport occurs. Photosystem II is embedded in this membrane, which separates proton-rich compartments.

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