Question

Where does non-cyclic photophosphorylation take place? Describe the process. Why is the process referred to as non-cyclic?

Answer 1

Hint:Here several photosystems take place for carrying out the procedure. The process where a phosphate group is attached to the molecule and helps in ATP synthesis by using the gradient energy, it is known as phosphorylation.

Complete answer:
Non-cyclic phosphorylation takes place in the granal thylakoid region of chloroplast. Two photosystems i.e. Photosystem-I and Photosystem-II is involved in the process of non-cyclic phosphorylation. At first, Photosystem-II and then Photosystem-I comes and thus works like a series. The process of non-cyclic phosphorylation is described below:

During the light absorption, Photosystem-II becomes excited and electrons are transferred to the acceptor molecule. One molecule of water is splitted for releasing oxygen by the help of P680, an oxidizing agent molecule. This process is called photolysis of water.
After breaking the water molecule, the electrons are again generated and they are restored. Once the electrons are accepted from P680, the acceptor molecule becomes reduced.
Then the electrons are donated to the electron transport system by the acceptor molecule and thus energy is released and it forms ATP from ADP.
In this state, Photosystem-I becomes activated, gets excited and takes part in the oxidation process. The electrons are transferred from it and become reduced. After some times the electrons are transferred to ferredoxin and the enzyme ferredoxin mediated NADP reductase that can reduce NADP to NADPH.
The process to electron flow to Photosystem-II from the water molecule is a continuous process. And thus ATP is formed.

The non-cyclic phosphorylation process is also called a dynamic process that means the electrons produced do not go back to Photosystem-I, so they are moved to NADP.

Note: In contrast to non-cyclic photophosphorylation, in cyclic phosphorylation the electrons are transferred back to P700 instead of moving into the NADP from the electron acceptor. In this process, water is not required and no NADPH is not synthesized.