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Mechanism ot Photosynthesis Lesson 3 Concept of Light Reaction (Cyclic Photophosphorylation) Dr. Praveen Kumar Agrawal M.Sc., Ph.D., CSIR- NET (JRF), SRF, GATE 20 Years of Pre-medical teaching experience
Light absorption by Photosynthetic pigments Pigments are substances that have an ability to absorb light, at specific wavelengths. In the previous course we have discussed various types of pigments which can absorb light and help in photosynthesis. Chromatographic separation reveals that the leaves of higher plants contain primarily four pigments Chlorophyll a (bright or blue green), chlorophyll b (yellow green), xanthophylls (yellow) and carotenoids (yellow to yellow-orange) .So, the green colour of the leaves is not due to a single pigment but due to a mix of all these four pigments. .The wavelengths at which there is maximum absorption of light by the chlorophyll a molecules, are of blue and red region Minimum absorption occurs in the green region, because this light gets reflected.
_Rate of photosynthesis Absorption Chlorophyll b Carotenoids Chlorophyll o 400 500 600 700 400 500 600 700 Wavelength of light in nanometres (nm) Wavelength of light in nanometres (nm)
Fluorescence in chlorophyll and concept of light reaction Red lig ATP fermati phospho Red li rescence Yuim chlosophyu
Cyclic Photophosphorylation Primary electron acceptor Ferredoxin ATP Only one photosystem, i.e., PS-1 (P 700), participate. No photolysis of water No evolution of oxygen. Cyt-be 2 e 2 e ATP .Reduction of NADP to NADPH H+ does not occur. Assimilatory power is only ATP. Cyt-f Main postulates Only one photosystem 2 e participates PS-1 No photolysis of water " No evolution of oxygen Only assimilatory power PS-I (P 700) S ATP
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