Question

What is ‘photophosphorylation'? Describe non-cyclic photo-phosphorylation with schematic representation. Give its significance.

Answer 1

Hint: Photo-phosphorylation is the process where phosphate group is transferred to the ADP for the synthesis of energy-rich ATP molecules in the presence of light. Non-cyclic photo-phosphorylations transpire as the electrons lost by P680 are taken up by P700. These electrons do not return to P680 hence they are known as non- cyclic phosphorylation.

Complete answer:
In the process of photosynthesis, the phosphorylation of ADP to form the ATP using the energy of sunlight is known as photophosphorylation. The Cyclic photophosphorylation form in both aerobic as well as anaerobic conditions. There are only two sources of energy available to living organisms that are sunlight along with reduction-oxidation reactions.
Non-cyclic photo-phosphorylation consist the following steps:
Photoexcitation of PS-II: PS-II absorbs sunlight along with it gets excited, which results in ionisation of chlorophyll-a as well as a high energy electron is expelled from PS-II.
Photoexcitation of PS-I: The sunlight energy absorbed by PS-I is transferred to reaction centre P700.

This form of photo-phosphorylation is present on the stroma lamellae or fret channels. In cyclic photo-phosphorylation, the high level of energy electrons released from P700 of PS1 and flow down in a cyclic pathway. In there the cyclic electron flow, the electron begins in a pigment complex known as photosystem I, which passes from the primary acceptor to ferredoxin as well as then to plastoquinone, then to cytochrome ${b_6}f$ that is a similar complex to that occur in mitochondria, along with then to plastocyanin before returning to Photosystem-1. This transportation chain creates a proton-motive force, and pumps ${H^ + }$ ions across the membrane. This produces a concentration gradient which can be used to power ATP synthase during chemiosmosis. This pathway is called cyclic photophosphorylation, as well as it produces neither ${O_2}$ nor NADPH. Unlike non-cyclic photophosphorylation.

Significance of non-cyclic photo-phosphorylation-
Non-cyclic phosphorylation outcome in the formation of oxygen, NADPH along with ATP molecules.

Note: The electrons pass through the primary acceptor of non-cyclic photo-phosphorylation are plastoquinone, cytochrome complex, plastocyanin as well as lastly to P700. The electrons released by P700 further combine with $H^+$ which is obtained by splitting up water. Along with ${H^ + }$ it also reduces NADP to NADPH.