Cyclic and Non cyclic photophosphorylation -Light reaction of Photosynthesis

Light harvesting complex is a unit made up of protein and chlorophyll molecules. These are embedded in the thylakoid membrane of plants and cyanobacteria, which transfer light energy .These pigments are organized into two discrete light harvesting complexes (LHC) within photosystem I and photosystem II. Light harvesting complexes are made up of hundreds of pigments .The pigments in photosystem I and photosystem II absorb the lights of different wavelengths and release electrons. In PS I the reaction centre uses light of the wavelength of 700nm, hence called P700. And the PS II reaction centre uses light of wavelength of  680 nm, so called P680.

👌👌Remember -  chlorophyll molecules make the reaction centre in a photosystem.

Non-cyclic photophosphorylation

The photophosphorylation process which leads the movement of the electrons in a non-cyclic manner. In this process, ATP molecules can use the energy from excited electrons given by photosystem II . It is termed as non-cyclic photophosphorylation.

In non-cyclic photophosphorylation, when the photosystem II or P680 receives the light of wavelength 680 by its pigments. It releases an electron and this electron is finally  picked by the  of Photosystem I or P700 and is not reversed back to P680. Here the entire movement of the electrons is given in a unidirectional or a non-cyclic manner.

When light energy is absorbed by the photosystem II it releases an electron that is accepted by the primary electron acceptor plastoquinone , this electron is also transferred by plastoquinone to cytochrome b then cytochrome- f and then plastocyanin.

Now this  electron is transferred by plastocyanin to  Photosystem I to fill the requirement of electron in photosystem I because photosystem I releases the electron after the falling of sunlight and this electron is accepted by ferredoxin ,another electron acceptor.

The passing of electron from plastocyanin to photosystem I is called downhill direction.During downhill direction, release electron reduces the ADP to ATP and NADP+ to NADPH.

When electron is passed from photosystem I to ferrodoxin, it is passed to NADP. Here, the electrons is mixed  with the protons( H+ ).These protons are  released by the photolysis of water. The whole scheme of transfer of electron from photosystem II to ferrodoxin  is called Z-scheme due to its shape.

cyclic photophosphorylation

In cyclic photophosphorylation, when the photosystem I or P700 receives light it releases the electron. This electron is transferred from  photosystem I to  ferredoxin to plastoquinone and returns back again to photosystem I through cytochrome b and cytochrome f in cyclic manner. Hence it is called cyclic photophosphorylation.

Difference between cyclic and non-cyclic photophosphorylation

In cyclic photophosphorylation,

only photosystem I is required independently and an external source of electrons is not required. only ATP is synthesized.It takes place in stromal or intergranal thylakoids.

In  non cyclic photophosphorylation, both

PS I and PS II are required.

The process requires an external electron donor.

It synthesizes ATP and NADH both. It occurs in the granal thylakoids only.