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7.3.2.1.2.1. Acyclic photophosphorylation

Non-cyclic or acyclic photophosphorylation

In non-cyclic photophosphorylation, photosystems II and I intervene together, and is called the Z-scheme.

A photon of light is captured by photosystem II and chlorophyll P680 emits an electron, and it is oxidized. The photolysis of water produces protons, oxygen and electrons, so the water will be the electron donor for the recovery of chlorophyll P680.

The electron that has been emitted is captured by a primary acceptor, pheophytin, which, in turn, gives it up to an electron transport chain until it ends up in a final acceptor, plastocyanin. This chain of electronic transporters is what communicates photosystem II and photosystem I.

A proton gradient similar to that seen in mitochondria when they produced ATP from ADP is produced in this pathway. This process is called photophosphorylation.

In non-cyclic photophosphorylation, photosystems can work in series, connected by a chain of electrons. The electrons from the photolysis of water will be used to reduce a molecule of NADP and obtain NADPH .

The photons received in photosystem II cause the release of an electron from chlorophyll  P680, which collects pheophytin and which is conducted by the electron transporters of the chain: plastoquinone, cytochrome b6-f and plastocyanin. The electrons emitted by P680 are replaced by electrons from the photolysis of water, which also releases O2.

The cytochrome b6-f complex generates enough energy in electron transport to be able to transport H + into the thylakoid against the electrochemical gradient. The exit of H + into the stroma by the cF 0 F 1 (ATP-synthetase) complex will generate enough energy to form ATP from ADP and Pi, according to Mitchell's chemiosmotic hypothesis for photophosphorylation.

The photosystem I can accept the electron from the plastocyanin, from photosystem II, because another photon had also enabled the release of an electron chlorophyll P 700 . This electron will pass to phylloquinone and ferredoxin, which gives it to the enzyme NADP-reductase that reduces NADP + and NADPH is obtained .

In this way, in the presence of light, there is a continuous flow of electrons from the water to the NADP + , passing through the PS II and PS I. A little more than one ATP molecule is obtained for each pair of electrons that pass from the water to NADP + .

What is the role of light in photosynthesis?

The energy that is contained in the photons of light is used to start the transfer of electrons from the chlorophyll molecule (or carotenoids) of the reaction centers that are located in the thylakoids, from low energy levels to other very high energy levels .