Light reaction


LIGHT REACTION



Here we study about the Light-Reaction, involving the process of Photosystem I & II,
and the source of oxygen, hydrogen, and electron




light reaction (chloroplast)
 chloroplast diagram showing Thylakoid and stromal lamellae



The light-reaction is a process through which the plant utilizes the light energy, and transform the inorganic substrates taken through the soil and the atmosphere into organic products.
This process takes place within the thylakoid and stromal lamellae of the chloroplast.
Thylakoid are membranous structure present within the chloroplast, Pile of thylakoid termed as granum  remain in connection with another granum through stromal lamellae.
The light reaction get completed through the two photosynthetic units generally composed of pigment system, proteins which stabilize the pigment system and a number of electron carriers forming the Electron Transport System.
In this process molecules of ATP and NADPH are formed and also, get utilized in some of the in-between processes.
If the light reaction is carried out within the thylakoid the membrane then all the photosynthetic units:- Photosystem I and Photosystem II is functioning, if the light the reaction takes place within the stromal lamellae then only Photosystem I carried out.



LIGHT REACTION IN THYLAKOID

Light-reaction was discovered by Robin Hill in 1937 therefore this reaction is also termed as Hill reaction this reaction is non cyclic therefore some name it as Non-cyclic Photophosphorylation and the path followed is of Z shape therefore also termed as Z reaction.
The term photophosphorylation here means for the production of ATP, and cyclic and non-cyclic represent the path of the electron after getting excited.



Non-cyclic photophosphorylation

In this process when the light of wavelength 680nm fall over the pigment system II then the electron gets excited and reduce the first electron acceptor pheophytin after traveling in the upward direction after that, it transfers the electron in the downward direction to plastoquinone  which further transfers the electron to the cytochrome b6 & f, from here the electron gets transported to the phytochrome and then to the photosystem I and after getting the light of wavelength 700nm it releases the electron to the ferrous sulfate molecule in an upward direction and later moves more up to reduce the ferredoxin molecule, after that finally electron reach to there destiny by moving downward to NADP and reduce it to NADPH +H.
The position of photosystem II and photosystem I is over the lower and upper side of the thylakoid membrane.
Hence the path is:-



Image
Non-cyclic photophosphorylation




Here the electron carrier is:-
·         Phaeophytin
·         Plastoquinone
·         Cytochrome b6 &f
·         Plastocyanin
·         Ferredoxin
·         Ferrous sulfate & ATPase.
In this process when the electron gets excited from the P.S II then the splitting of water takes place in the presence of ions manganese and chlorine within the lumen of thylakoid, in the presence of water splitting complex present at the lower side of the thylakoid membrane. Hence water after splitting produce hydrogen, electron, and oxygen.
Therefore water is the source of byproduct oxygen. Water fulfill the loss of an electron from chlorophyll pigment in P.S II.  Hydrogen get accumulated within the lumen and creates proton gradient this hydrogen at last of Hill reaction transferred from lumen to the stroma and forming NADPH+H in presence of NADP reductase enzyme.
 With the help of enzyme Z-tyrosin electron reaches to P.S II Pigment.
Again at the time when the electrons cross the plastoquinone  two moles of hydrogen ion pass to the lumen and an ATP is produced through ADP and ip.

Cyclic photophosphorylation.


This process of cyclic photophosphorylation takes place within the stroma lamellae of the thylakoid.
Here no splitting of water and reduction of NADPH takes place, in this process only the photosystem I is present therefore when the light of wavelength 700nm fall over the pigment then the electron gets excited and reduce the ferrous sulfate molecule which further transfers the electron to the ferredoxin, from there electron, follow the path of plastoquinone where ATP molecules are produced  then to cytochrome b6 & f complex further to plastocyanin from there it returns back to photosystem I  as this process start and end both at the photosystem I, therefore, this reaction is termed as cyclic. Here the electron transport channel which is involved  are:-
·         Ferrous sulfate
·         Ferredoxin
·         Plastoquinone
·         Cytochrome b6 & f
·         plastocyanin

and the process diagrammatically is:-



Image

cyclic photophosphorylation



Production of ATP in cyclic and noncyclic photophosphorylation is explained by Mitchell in his hypothesis chemiosmotic hypothesis.




Thanks from
Ujjwal Kumar

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