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  Cyclic and noncyclic photophosphorylation

Cyclic and noncyclic photophosphorylation:

In chloroplasts,  phosphorylation takes place in two types –

1) Noncyclic photophosphorylation:

While the molecules in the PS I are excited the electrons are released. Thus, an electron deficiency or a hole is prepared in the PS I. Now this electron is transferred to ferredoxin to reduce NADP+. While the molecules in the PS II excited, electrons are released. To fill the hole in PS I they are transferred by cytochrome b6, cytochrome f, plastoquinone and plastocyanin. While the electron is transported among plastoquinone and cytochrome f, ADP is phosphorylated to ATP.

In the PS I the ‘hole’ has been filled through the electron from PS II. After that the electrons are transferred from PS I to NADP+ for reduction. Hence, this electron transport is termed as noncyclic electron transport and the associated phosphorylation like noncyclic photophosphorylation. The noncyclic electron transport occurs in the form of ‘Z’. Therefore, it is also termed as Z-scheme.

1281_non-cyclic phosphyration.jpg


  

                                                  Diagram:  Non-cyclic photophosphorylation

2) Cyclic photophosphorylation:

Within the four conditions of cyclic photophosphorylation occurs. They are as follow:

  • PS I only remains active
  • Photolysis of water does not take place
  • Requirement of ATP is more and
  • Nonavailability of NADP+

While the molecule in the PS I is gets excited, the electrons are released. The electrons are captured through ferredoxin reducing substrate (FRS). Because of non-availability of  NADP+, electrons from ferredoxin  fall back to the molecules of PS I by the electron carriers-cytochrome f, cytochrome b6, and plastocyanin. These electron carriers make easy the downhill transport of electrons to PS I from FRS. Throughout this transport of electrons, two phosphorylations occur-one among the ferredoxin and cytochrome b6 and the other among the cytochrome b6 and cytochrome f. So, two ATP molecules are generated in this cycle.

1132_cyclic phospyration.jpg


  

                                                                            Diagram:  Cyclic photophosphorylation

 

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