Photo Phosphorylation

  • Photo-phosphorylation is the process of synthesizing ATP from ADP and inorganic phosphate in the presence of light during photosynthesis. 
  • Occurs in chloroplasts during the light-dependent reactions of photosynthesis. 
  • ATP synthesis is driven by light energy absorbed by chlorophyll pigments. 
  • Essential for providing energy for the synthesis of organic compounds during photosynthesis.


Cyclic Photophosphorylation

  • Cyclic photophosphorylation is a type of photo-phosphorylation where only Photosystem I (PS I) is functional, leading to the cyclic flow of electrons. 
  • Electrons circulate within PS I and are cycled back through the electron transport chain. 
  • ATP synthesis occurs, but NADPH + H+ is not produced. 
  • Occurs when only light of wavelengths beyond 680 nm is available for excitation.




Non-cyclic Photophosphorylation

  • Non-cyclic photophosphorylation is a type of photo-phosphorylation where both Photosystem II (PS II) and Photosystem I (PS I) work sequentially, resulting in linear electron flow. 
  • Electrons flow from PS II to PS I through an electron transport chain (the Z scheme). 
  • Both ATP and NADPH + H+ are synthesized through this electron flow. 
  • Oxygen is released as a byproduct of water splitting associated with PS II. 

Chemiosmotic Hypothesis

The chemiosmotic hypothesis explains ATP synthesis in chloroplasts during photosynthesis, similar to ATP synthesis in mitochondria during respiration.

ATP synthesis is linked to the development of a proton gradient across the thylakoid membrane.


  • Proton Gradient Formation:

- Protons (H+) accumulate within the lumen of the thylakoid due to the splitting of water molecules.

- As electrons move through the photosystems, protons are transported across the membrane.

- The primary electron acceptor transfers its electron to an H carrier, removing a proton from the stroma and releasing it into the lumen. 

  • NADP+ Reduction:

- NADP reductase enzyme, located on the stroma side, requires protons for the reduction of NADP+ to NADPH+ H+.

- Protons necessary for NADP+ reduction are also removed from the stroma, resulting in a decrease in proton concentration in the stroma and accumulation in the lumen. 

  • Importance of Proton Gradient:

- The proton gradient across the thylakoid membrane is crucial for ATP synthesis.

- The breakdown of this gradient occurs when protons move across the membrane to the stroma through the transmembrane channel of the ATP synthase enzyme. 

  • ATP Synthesis:

- ATP synthase enzyme consists of two parts: CF0 embedded in the thylakoid membrane and CF1 protruding on the stromal side.

- The breakdown of the gradient provides energy for ATP synthesis as CF1 undergoes a conformational change. 

  • Chemiosmosis Mechanism:

- Chemiosmosis requires a membrane, a proton pump, a proton gradient, and ATP synthase.

- Energy is used to pump protons across the membrane, creating a high concentration of protons within the thylakoid lumen.

- ATP synthase allows diffusion of protons back across the membrane, releasing energy to activate ATP synthesis. 

  • Utilization Of ATP and NADPH:

- ATP and NADPH produced by the movement of electrons are used immediately in biosynthetic reactions in the stroma, such as CO2 fixation and sugar synthesis.