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.






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