Cyclic And Non Cyclic Photophosphorylation

We all are well aware of the complete process of photosynthesis. Yes, it is the biological process of converting light energy into chemical energy. In this process, the light energy is captured and used for converting carbon dioxide and water into glucose and oxygen gas. The complete process of photosynthesis is carried out into two processes:

Light reaction.

The light reaction takes place in the grana of the chloroplast. Here light energy gets converted to chemical energy as ATP and NADPH. In this very light reaction, the addition of phosphate in the presence of light or the synthesizing of ATP by cells is known as photophosphorylation.

Dark reaction.

While in the dark reaction, the energy produced previously in the light reaction is utilized to fix carbon dioxide into carbohydrates. The location where this happens in the stroma of the chloroplasts.

In this article, we will look at the complete process of photophosphorylation .i.e. the light reaction

Photophosphorylation is the process of synthesizing energy-rich ATP molecules by transferring the phosphate group into ADP molecule in the presence of light. These process may be either a cyclic process or a non-cyclic process

Cyclic Photophosphorylation

The photophosphorylation process which results in the movement of the electrons in a cyclic manner for synthesizing ATP molecules is called cyclic photophosphorylation.

In this process, plant cells just accomplish the ADP to ATP for immediate energy for the cells. This process usually takes place in the thylakoid membrane and uses Photosystem I and the chlorophyll P700.

During cyclic photophosphorylation, the electrons are transferred back to P700 instead of moving into the NADP from the electron acceptor. This downward movement of electrons from an acceptor to P700 results in the formation of ATP molecules.

Non-Cyclic Photophosphorylation

The photophosphorylation process which results in the movement of the electrons in a non-cyclic manner for synthesizing ATP molecules using the energy from excited electrons provided by photosystem II is called as non-cyclic photophosphorylation.

This process is called as non- cyclic phosphorylation, because the lost electrons by P680 of Photosystem II are occupied by P700 of Photosystem I and are not reverted back to P680 . Here the complete movement of the electrons are in a unidirectional or in the non- cyclic manner .

During non-cyclic photophosphorylation, the electrons released by P700 are carried by primary acceptor and are finally passed on to NADP. Here, the electrons combine with the protons – H+ which is produced by splitting up of the water molecule and reduces NADP to NADPH2.

Differences between Cyclic and Non-Cyclic Photophosphorylation

Cyclic Photophosphorylation

Non-Cyclic Photophosphorylation

Only Photosystem I is involved.

Both Photosystem I and II are involved.

P700 is the active reaction center.

P680 is the active reaction center.

Electrons travel in a cyclic manner.

Electrons travel in a non – cyclic manner.

Electrons revert back to Photosystem I

Electrons from Photosystem I are accepted by NADP.

ATP molecules are produced.

Both NADPH and ATP molecules are produced.

Water is not required.

Photolysis of water is present.

NADPH is not synthesized.

NADPH is synthesized.

Oxygen is not evolved as the by-product

Oxygen is evolved as a by-product.

This process is predominant only in bacteria.

This process is predominant in all green plants.

Stay tuned with BYJU’S to learn more about the cyclic and non-cyclic photophosphorylation.


Practise This Question

Which of the following statements about Photosystem II is correct