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| 1. |
4 phases of the light reaction |
| Answer» * Light reaction occurs in the grana of the chloroplasts. Using light energy, water molecules split out to release oxygen.* The chlorophyll pigments trap light energy and excite an electron. This excited electron converts light energy into chemical energy. This chemical energy is stored as ATP (Adenosine Tri Phosphate) and NADPH (Nicotinamide Adenine Dinucleotide Phosphate).2H2O + 2NADP+ + 3ADP + 3Pi --------> O2 + 2NADPH + 3ATPLight reaction occurs in thylakoid membranes of chloroplasts in a leaf.\xa0Light energy is absorbed by photosystems PS-I and PS-II. Cyclic and non-cyclic photophosphorylation occur in the cells. End products are ATP and NADP.H2. Oxygen is released as a by-product of the photolysis if water. Non-cyclic photophosphorylation takes place in the grans of chloroplasts.Non-cyclic photophosphorylation occurs as a result of an interaction between photosystem I and photosystem II. Non-cyclic photophosphorylation helps in the formation of ATP as a result of electron flow from water to NADP. As this is an unidirectional flow, and does not follow any cyclic procedure, is called as non-cyclic photophosphorylation.* Photosystem II gets excited during absorption of light and transfers electrons to electron acceptor molecule. P680 becomes oxidising agent which splits one molecule of water to release oxygen. This light dependent splitting of water molecule is called as photolysis.\xa0* As the water molecule is broken, electrons are generated which are accpeted by P680 molecule to restore its electrons. Now primary electron acceptor is reduced after accepting electrons from P680.* This reduced acceptor donates electrons to the other components of ETS.releasing energy in the form of ATP from ADP.* Photosystem I is excited by absorbing light and gets oxidised. It transfers its electrons to primary electron acceptor which gets reduced. Oxidised P700 obtains electrons from photosystem II, reduced electron acceptor of photosystem I transfer electrons to ferredoxin and ferredoxin-NADP reductase to reduce NADP to NADPH2There is a continuous flow of electrons from water to photosystem II to photosystem I and then to NADP. As these electrons are transferred, there is the formation of ATP from ADP. | |