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Question : What is 'photophosphorylation' ? Describe non-cyclic photophosphorylation with schematic representation. Give its significance. |
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Answer» Solution :Photophosphortlation : The process in which energy rich ATP molecules are synthesised USING solar energy is called PHOTOPHOSPHORYLATION. Non-cyclic Photophosphorylation : It involves photolysis of water and a continuous unidirectional flow of electrons from water to PS-I, from PS-II to PS-I. Non-cyclic photophosphorylation involves the following steps : (i) Photoexcitation of PS-II : PS-II absorbs light and gets excited, which results in ionisation of CHLOROPHYLLA and a high energy electron is emitted from PS-II. (ii) 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 carries and releases energy. From Co-Q, electrons are transferred to plastoquinone (PQ). From PQ, electrons are transferred to cytochrome complex. From cytochrome complex, the electrons are FINALLY accepted by chlorophyll of PS-I. (iii) Synthesis of ATP : One ATP is synthesized when electron passes from cytochrome `b_(6)` to cytochrome f. (iv) Photoexcitation of PS-I : The light energy absorbed by PS-I is transferred to reaction centre `P_(700)` and get excited. The expelledenergy rich electron is first accepted by unkonwn electron acceptor called FRS (Freedoxin Reducing Substance). Electron from FRS moves down through various electron carries and release energy. From FRS, electrons are transferred to ferredoxin the reduced ferredoxin transfers electrons to NADP to form NADP in the presence of enzyme Fd-NADP reductase. (v) Photolysis of water : Splitting of water into `H^(+)` and `OH^(-)` in presence of light and chlorophyll is called photolysis of water. Manganeses, calcium and chloride ions present in PS-II and proton needs of NADP. (vi) ATP and `"NADPH"_(2)` together called as assimilatory powers by Calvin, as it is required for assimilation of `CO_(2)`. Significance of Non-cyclic photophosphorylation : 1. It yields ATP 2. It yields `"NADPH"_(2)` 3. It yields metabolic `"H"_(2)"O"` |
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