Question

Answer the following question.

Describe the light-dependent steps of photosynthesis. How are they linked to the dark reactions?

Answer 1

The light-dependent steps of photosynthesis include cyclic and non-cyclic photophosphorylation.

• Cyclic photophosphorylation:

1. Illumination of photosystem-I causes electrons to move continuously out of the reaction center of photosystem-I and back to it. 
2. The cyclic electron-flow is accompanied by the photophosphorylation of ADP to yield ATP. This is termed as Cyclic photophosphorylation.
3. Since this process involves only pigment system I, photolysis of water and consequent evolution of oxygen does not take place.
   

• Non-cyclic photophosphorylation:

1. It involves both photosystems- PS-I and PS-II.
2. In this case, electron transport chain starts with the release of electrons from PS-II.
3. 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.
4. The reduced NADP⁺ (NADPH) is utilized for the reduction of CO₂ in the dark reaction.
5. Electron-deficient PS-II brings about oxidation of water-molecule. Due to this, protons, electrons and oxygen atom are released.
6. Electrons are taken up by PS-II itself to return to reduced state, protons are accepted by NADP⁺ whereas oxygen is released.
7. 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.
   
   
   where CO-Q: Co-enzyme quinone
   FRS: Ferredoxin Reducing Substance
   PC: Plastocyanin
   PQ: Plastoquinone
   Cyt: Cytochrome 
   Fd: Ferredoxin.

• Link between light-dependent and dark reactions:
  Link between light-dependent and dark reactions: The light reaction gives rise to two important products, a reducing agent NADPH₂ and an energy-rich compound ATP. Both these are utilized in the dark phase of photosynthesis.
  ATP and NADPH₂ molecules function as vehicles for the transfer of energy of sunlight into dark reaction leaving to carbon fixation. In this reaction, CO₂ is reduced to carbohydrates.
  During dark reaction, ATP and NADPH₂ are transformed into ADP, iP and NADP which are transferred to the grana in which light reaction takes place.