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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