Question

Where does cyclic photophosphorylation take place in the leaves? Why is this process called cyclic? Describe cyclic photophosphorylation.

Answer 1

Hint: Electron transport chain is a series of electron carriers over which electrons pass in a downhill journey releasing energy at every step that is used in generating an electrochemical proton gradient which helps in synthesising ATP. Based on the path of the electron, associated photophosphorylation can be identified as non-cyclic and cyclic photophosphorylation.

Complete answer:
The cyclic photophosphorylation usually occurs in the stroma lamellae membrane of the leaves. It is called a cyclic process because electron providers and electron acceptors are the same . The process starts from the same molecule and ends at the same.
Cyclic Photophosphorylation
The process of cyclic photophosphorylation involves only PS I and this process takes place in the stroma lamellae membrane. When only PS I is functional, the electron is circulated within the photosystem and the phosphorylation occurs, due to cyclic flow of electrons.

 The membrane or lamella of the grana have both PS I and PS II, the stroma lamellae membranes lack PS II as well as NADP reductase enzyme. The excited electron does not pass on to NADP+ and is cycled back to the PS i complex through the electron transport chain. Cyclic photophosphorylation also occurs when only light of wavelength beyond 680 nm are available for excitation.

Note: Cyclic photophosphorylation is performed only by photosystem I independently. An external source of electrons is not required in this. It is not connected with the photolysis of water so no oxygen is evolved in this. It synthesises ATP only and operates under low light intensity.