Question

What is meant by photophosphorylation ? Discuss the cyclic photophosphorylation.

Answer 1

Photophosphorylation is the synthesis of ATP from ADP and inorganic phosphate in the presence of light. The flow of electrons through Electron Transport System (ETS) is linked to photophosphorylation. It is of two main types- cyclic and non-cyclic.

Cyclic photophosphorylation is a process of photophosphorylation in which an electron released by the excited photocentre is returned to it after passing through a series of electron carriers. It takes place in the stroma lamellae membrane of chloroplasts. The process of cyclic photophosphorylation involves only photosystem I. 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 $NAD{P}^{+}$ and is cycled back to the PS I complex through the electron transport chain. Cyclic photophosphorylation is not connected with photolysis of water. Therefore, no oxygen is evolved. It synthesises ATP only. It does not take part in photosynthesis except in certain bacteria.