Give long answer on machanism of photosynthesis
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The process of photosynthesis is considered as an oxidation-reduction reaction activated by light absorbed by chlorophyll or certain other substances (e.g., phycocyanin, fucoxanthol), resulting in the evolution of oxygen from water and in the formation of reduced carbon compounds (e.g., carbohydrate) from carbon dioxide. The organisation of the reaction in the cell is not yet understood-the poor photosynthetic activity of light absorbed by chlorophyll in certain red algae may be indicative of a change in this organisation. When chloroplasts are isolated from the cell they are able to catalyse the photochemical reduction of certain hydrogen acceptors with evolution of oxygen from water, but they are unable to reduce carbon dioxide. Analysis of the path of carbon in the green plant cell, both in the light and in the dark, has been made possible by the use of radioactive carbon dioxide. A large number of intermediates in the metabolism of the cell have been identified, but as yet convincing evidence regarding the specific part played by any of these compounds in a particular metabolic process is lacking. The metabolic system of the cell may be analysed in terms of a number of separate processes, but it seems probable that these may have many steps in common. The reactions which must be unique to photosynthesis are those reactions involving at most two light quanta per hydrogen atom transferred which result in the formation of a hydrogen donor capable of reducing carbon dioxide. Comparative studies of the rate of photosynthesis and of the rate of individual steps, as represented for example by the chloroplast reaction and by tracer studies, present the most promising approach to the physiology of photosynthesis.