Photophosphorylation (Light reaction)

 

 Photophosphorylation:

Photosynthesis is a process by which chlorophyll-containing cells synthesize carbohydrate from carbon dioxide & water with the help of solar energy & the byproduct is oxygen. It occurs in the Chloroplast. During the light reaction, light energy is converted into chemical energy in the form of ATP & NADPH. This ATP & NADPH is used in the dark reaction to fix CO2in the stroma. Photophosphorylation occurs in the thylakoids (grana) & involves Photosystem I & II. A Photosystem is a group of closely associated pigments molecules that help in converting solar energy into chemical energy. Also referred as Light-harvesting complexes. It is of 2 types, Non- Cyclic & Cyclic.

Noncyclic photophosphorylation:

 

  • Both photosystem I & II are involved.
  • Photosystem I & II are excited simultaneously with solar energy.
  • Electrons are passed through electron transport chain between photosystem I & photosystem II.
  • Electron acceptors are Pheophytin (primary acceptor), Plastoquinone, Cytochrome complex & Plastocyanin.
  • They are arranged in decreasing energy concentration (downhill).
  • Photolysis of water occurs.
  • Oxygen is the by-product.
  • Responsible for synthesis of both ATP & NADPH.

Cyclic photophosphorylation:

  • Occurs in outer thylakoids & in stroma lamellae.
  • They possess Photosystem I.
  • Photosystem II & NAD reductase enzyme is absent.
  • Only photosystem I take part in cyclic photophosphorylation.
  • Only responsible for ATP formation.
  • No splitting of water occurs.

A Chemiosmotic hypothesis for ATP formation:

  • Noncyclic & Cyclic photophosphorylation creates a proton gradient between stroma & thylakoid lumen.
  • Reasons for proton gradient:
In Noncyclic:

1). Photolysis of water at photosystem II releases H+ into the thylakoid lumen. 2). Plastoquinone in the electron transport chain is a hydrogen acceptor, it takes H+ from the stroma & releases them in thylakoid lumen. 3). NADP+ to NADPH by taking H+ from the stroma.

In Cyclic:

4). Releases H+ into the thylakoid lumen.

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  • Thylakoid membrane possesses F0 – F1 particles, where F0 is proton tunnel & F1 is ATP synthetase.  Synthesis of ATP occurs as protons move out of proton tunnel from the thylakoid to stroma.

 

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