Question

Describe the cyclic photophosphorylation. What is the purpose of proton gradient during the process in thylakoids?

Answer 1

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.

During ATP synthesis in chloroplasts, the proton gradient is formed due to the protons releases after the photolysis of water. This gradient is important because the breakdown of this gradient leads to release of energy. The gradient is broken down due to the movement of protons across the membrane to the stroma through the transmembrane channel of the $F_0$ of the ATPase enzyme. The breakdown of proton gradient provides enough energy to cause a conformation change in $F_1$ part of the ATPase, which makes the enzyme synthesise several molecules of ATP.