Question

Describe non-cyclic photophosphorylation with schematic representation. Give its significance.

Answer 1

Non-cyclic photophosphorylation involves the following steps:

1. Photoexcitation of PS-II: PS-II absorbs light and gets excited, which results in ionisation of chlorophyll-a and a high energy electron is expelled from PS-II.
2. Electron Acceptors: The expelled energy rich electron is first accepted by CO-Q (co-enzyme quinone). Electron from CO-Q moves down through various electron carriers and releases energy. From CO-Q, electrons are transferred to plastoquinone (PQ) (it is an iron containing protein). From PQ, electrons are transferred to cytochrome complex (cytochrome-b and cytochrome-f). From cytochrome complex, the electrons are transferred to plastocyanin (PC) (it is a copper containing protein). From PC, the electrons are finally accepted by ionized chlorophyll of PS-I.
3. Synthesis of ATP: One ATP is synthesized when electron passes from cytochrome b6 to cytochrome f.
4. Photoexcitation of PS-I: The light energy absorbed by PS-I is transferred to reaction centre P700. It gets excited and expels energy rich electrons. The electrons are first accepted by unknown electron acceptor called FRS (Ferredoxin Reducing Substance). Electron from FRS moves down through various electron carriers and release energy. From FRS, electrons are transferred to ferredoxin (it is an iron containing protein). The reduced ferredoxin transfers electron to NADP to form NADP^- in the presence of enzyme Fd-NADP-reductase.
5. Photolysis of water: Splitting of water into H+ and OH+ in presence of light and chlorophyll is called photolysis of water. Manganese, calcium and chloride ions present in PS-II play an important role in photolysis of water. Photolysis of water occurs in order to satisfy the electron need of PS-II and proton need of NADP.
6. Assimilatory power: ATP and NADPH2 are together called as assimilating power by Calvin, as it is required for assimilation of CO2.

 

Significance of non-cyclic photophosphorylation:
i. ATP molecules as well as NADPH2 are produced. 
ii. Photolysis of water takes place to release O2.