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What is Photosystem II?

Photosystem II (PSII), also known as water-plastoquinone oxidoreductase, is a large membrane protein complex located in the thylakoid membrane of organisms ranging from cyanobacteria to higher plants. It is a protein complex that brings about the first reaction of light-dependent reactions during oxygenic photosynthesis.

Photosystem II is the first link that begins the chain of photosynthesis. The system absorbs light to induce a series of light-induced electron transfer reactions that leads to the splitting of water molecules. Read this article to learn in detail about photosystem II.

Structure of PS II

The photosystem II of both cyanobacteria and green algae is composed of 20 subunits with other accessory and light-harvesting proteins.
Each complex has at least 99 cofactors, 35 chlorophyll a, two pheophytin, 12 β-carotene, two heme, two plastoquinone, one bicarbonate, Mn₄CaO₅ cluster, 20 lipids, one non-heme Fe2+ and two Ca2+ ions.
The photosystems are composed of a core complex and a reaction centre.
The core of PS II is a heterodimer that is made up of two homologous proteins, D1 and D2.
The reaction centre is the site where photochemical reactions occur. PS II has a quinone-type reaction centre.
The positive charge on the reaction centre is localised only at one chlorophyll centre, unlike other photosystems, where the charge is equally distributed between chlorophyll molecules.

Functions of Photosystem II

In photosystem II, enzymes absorb the photons of light to energise the electrons. The energised electrons then travel through a series of cofactors and coenzymes to reduce plastoquinone to plastoquinol. The energised electrons also replace the oxidising water to form molecular oxygen and hydrogen ions.

The splitting of water generates electrons that drive the process of photosynthesis. The oxidation of water generates hydrogen ions as well, which create a proton gradient. Thus gradient is used by the ATP synthase complex for the production of ATP. The energised electrons that were transferred to plastoquinol reduce NADP+ to NADPH.

Frequently Asked Questions

What is the difference between photosystem I and photosystem II?

Photosystem I absorbs longer wavelengths of light (~700nm), whereas photosystem II absorbs shorter wavelengths of light (~680nm).

Does photosystem II produce ATP?

Yes, photosystem II generates ATPs.

What is the end product of photosystem II?

The end product of photosystem II is molecular oxygen which is released into the environment.