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| Photophosphorylation (Cyclic and Non-cyclic) |
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| Photosynthetic phosphorylation or photophosphorylation is the process of phosphate group transfer into ADP to synthesize energy rich ATP molecule making use of light as external energy source. According to chemi osmotic hypothesis (Mitchell 1961) the ATP is synthesized on ATPase complexes located on the non appressed portions of thylakoid membranes particularly towards margins. |
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| During photosynthetic electron transport, hydrogen protons (H+) accumulate in the thylakoid space, due to splitting of water and transport between PQH2 to Cytf. Increase in the number of hydrogen protons in the thylakoid space results in increase of proton gradient. Down flow of protons from high to low concentration along H+ conc. gradient through ATPase complex provides the energy that allows an ATP synthase enzyme to produce ATP from ADP + Pi |
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| The electrons lost by P680 (PS-II) are taken up by P700 (PS-I) and do not get back to P680 i.e., unidirectional and hence it is called non- cyclic phosphorylation. The electrons pass through the primary acceptor, plastoquinone (PQ), cytochrome complex, plastocyanin (PC) and finally to P700. |
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| Electron Trasnport flow and Non-cyclic Photophosphorylation |
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| The electrons given out by P700 are taken up by primary acceptor and are ultimately passed on to NADP. The electrons combine with H+ and reduce NADP to NADPH2. The hydrogen ions also called protons are made available by splitting up of water. Non-cyclic photophosphorylation needs a constant supply of water molecules. The net result of non-cyclic phosphorylation is the formation of oxygen, NADPH and ATP molecules. Oxygen is produced as a waste product of photosynthesis. |
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| The electrons released by P700 of PS-I in the presence of light are taken up by the primary acceptor and are then passed on to ferredoxin (Fd), plastoquinone (PQ), cytochrome complex, plastocyanin (PC) and finally back to P700 i.e., electrons come back to the same molecule after cyclic movement. |
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| Cyclic Photophosphorylation |
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| The cyclic photophosphorylation also results in the formation of ATP molecules just like in non - cyclic photo phosphorylation. |
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| As the electrons move downhill in the electron transport chain, they lose potential energy and ATP molecules are formed in the same way as in mitochondria during respiration. |
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| During cyclic photophosphorylation, electrons from photosystem - I are not passed to NADP from the electron acceptor. Instead the electrons are transferred back to P700. This downhill movement of electrons from an electron acceptor to P700 results in the formation of ATP and this is termed as cyclic photophosphorylation. It is very important to note that oxygen and NADPH2 are not formed during cycle photophosphorylation. |
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