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| Photochemical and Biosynthetic Phases |
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| Light Phase consists of photochemical reactions which are carried out by two different photo systems, PS-I and PS-II. In the thylakoids, chlorophyll and other molecules are packed into units called photo systems. |
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| Each photosystem unit contains from 250 to 400 molecules of pigment, which serve as light trapping antennae. Once light energy is absorbed by one of the antenna pigments, it is bounced around among the other pigment molecules of the photosystem until it reaches a special form of chlorophyll-a which is the reaction centre. |
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| In photosystem-I, the reactive chlorophyll-a molecule is known as P700 (P stands for pigments) because one of the peaks of its absorption spectrum is at 700 nm. It is located in the stroma regions of the thylakoid. The reactive chlorophyll a molecule of photosystem-II is P680. They are located in the appressed regions of the grana in the thylakoid. It is believed that these chlorophyll molecules have unusual properties because of their association with special proteins in the membrane. |
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| The main function of these two photosystems is to trap lightenergy and convert it into chemical energy (ATP) which is used by living cells. |
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| The photochemical phase was explained by Arnon and his co-workers in 1958. Whenever a chlorophyll molecule absorbs a photon of light, it is said to move from the ground state to the excited state. The added energy lifts the electrons from the chlorophyll molecule (as given below in the equation) and is ultimately used to reduce NAPH2. |
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| The excited state is unstable and the chlorophyll molecule regains its lost electron from the water molecule through an electron transfer system and gets back to the ground state. |
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| The P680 chlorophyll molecule having lost its electron, is avidly seeking replacement. It finds it in the water molecules, which dissociates into protons and oxygen gas. |
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| The P700 chlorophyll molecule is oxidized and an electron is boosted to a primary electron acceptor from which it goes downhill to NADP. |
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| The electrons removed from P700 molecule is replaced by the electron from photosystem-II. |
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