Photophosphorylation :
- Formation of ATP in the chloroplasts in presence of light is called photophosphorylation.
- It takes place in the two forms.
- Cyclic photophosphorylation
- Non-cyclic photophosphorylation
i. Cyclic photophosphorylation :
- Illumination of photosystem-I causes electrons to move continuously out of the reaction center of photosystem-I and back to it.
- The cyclic electron-flow is accompanied by the photophosphorylation of ADP to yield ATP. This is termed as Cyclic photophosphorylation.
- Since this process involves only pigment system I, photolysis of water and consequent evolution of oxygen does not takes place.
ii. Non-cyclic photophosphorylation :
- It involves both PS-I and PS-II photosystems.
- In this case, electron transport chain starts with the release of electrons from PS-II.
- In this chain high energy electrons released from PS-II do not return to PS-II
- But after passing through an electron transport chain, reach PS-I, which in turn donates it to reduce NADP+ to NADPH.
- The reduced NADP+ (NADPH) is utilized for the reduction of CO2 in the dark reaction.
- Electron-deficient PS-II brings about oxidation of water-molecule.
- Due to this, protons, electrons and oxygen atom are released.
- Electrons are taken up by PS-II itself to return to reduced state, protons are accepted by NADP+ where as oxygen is released.
- As in this process, high energy electrons released from PS-II do not return to PS-II and it is accompanied with ATP formation, this is called Non-cyclic photophosphorylation.
- Thus, during the photochemical reactions,
- photolysis of water takes place
- O2 is released and
- ATP and NADPH are synthesized.
- ATP and NADPH molecules function as vehicles for transfer of energy of sunlight into dark reaction leaving to carbon fixation.
- In this reaction CO2 is reduced to carbohydrate.
- The light reaction gives rise to two important products :
- A reducing agent NADPH and
- An energy rich compound ATP.
- Both these are utilized in the dark phase of photosynthesis.