Photosystem II

The photosystem II ( PSII also ) is a protein complex in which the first step of the light reaction takes place in the context of photosynthesis. The PSII is embedded in the thylakoid membrane of plants, algae, and cyanobacteria and protrudes to both sides in the stroma or lumen. About light absorption gradually plastoquinone is reduced to plastoquinol and thus stored energy in the form of a reduction potential. This reduction potential required for the other in the photosystem I (PSI) running steps of the light reaction.

Structure

The PSII is composed of 20 protein subunits, in which the relevant light absorption light collecting pigments are embedded. The protein structure holds the light collecting pigments in position, so that the energy of the pigment may be transferred as efficiently as possible to pigment. A PSII monomer contains light-harvesting pigments about 35 molecules of chlorophyll a and β -carotene molecules 11. In the center of PSII monomer used is the so-called special pair, two closely spaced chlorophylls, representing the total absorbed energy is transferred. Given its close proximity to each other their energy level is lowered by excitonic interactions, so that they have their absorption maximum at 680 nm. For this reason they are also referred to as P680.

In the immediate spatial vicinity of the special pair are a pheophytin and a plastoquinone molecule over which the energy is further transferred to another plastoquinone molecule. This is reduced and used its reduction potential for the further steps of light reaction. The now positively charged special pair escapes of water molecules through an adjacent manganese complex electron, so that it is again present in the initial state and can absorb energy again.

In nature, the PSII exists not as a monomer, but as a super complex whose structure could be resolved by electron microscopy. Here, a central located dimer of six light -harvesting complex II trimers ( LHCII ) is surrounded, as well as the smaller chlorophyll proteins CP24, CP26 and CP29. Absorb These so-called light-harvesting antennae of the light-harvesting pigments contained energy and forward them to the PSII dimer. The super complex acts like a giant funnel, which focuses on the total absorbed energy to the special pairs.

Expiring light reaction

The overall balance of the proceeding in the PSII reaction is:

For this reaction, a total of four photons to be absorbed.

Manganese complex

The manganese complex, the oxidation of the water takes place. The exact structure of the complex has not been definitively determined, but it is known that it consists of four manganese, four oxygen and a calcium atom. Manganese has the ability to take many different oxidation states. Thus, the complex can be oxidized in four steps ( by donating electrons to the special pair ), and finally two molecules oxidize water to oxygen and release four protons. Through the four protons released to the pH reduced to the thylakoid lumen - side. So overall a proton gradient across the thylakoid membrane is produced.