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.

• 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.