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| Photorespiration |
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| It has been observed that a high concentration of oxygen, inhibits photosynthesis. This is due to the reason that the CO2 fixing enzyme RUBP carboxylase not only accepts CO2 but can also combine with O2. Since the reaction is an oxygenation reaction, the same enzyme is called RUBP oxygenase. |
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| In the chloroplast, phosphoglycolate is unstable and is converted to glycolate. PGA is used up in the calvin cycle. The peroxisomes present in the cell convert the glycolate into glyoxylate and then into glycine. Glycine is converted to serine and CO2 in the mitochondria. The serine thus formed is converted to glycerate, through a series of reactions which occurs in the peroxisome. |
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| This process is known as photorespiration or photosynthetic carbon oxidation cycle. |
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| Photorespiration is defined as a light dependent uptake of O2 and output of CO2. |
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| Photorespiration involves oxygenation of ribulose-bisphosphate catalysed by the enzyme Rubisco. The reactions lead to the formation of 2-phosphoglycolate, oxidation of which results in release of CO2 . In contrast, carboxylation, also catalysed by Rubisco, results in fixation of CO2 for the production of carbohydrate |
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| Photorespiration is also defined as the respiration initiated in the chloroplasts, in the presence of light only. |
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| The main features are: |
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 The carbon lost is not retrieved. The plant may lose as much as half of the CO2 fixed during photosynthesis by this process. |
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 No energy rich compound is produced and rather is a wasteful loss of ATP and NADPH. |
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 With the increase in O2 concentration, affinity of RUBP carboxylase for O2 increase and for CO2 decreases. Therefore increased oxygen level increases, photorespiration, while increased carbon dioxide level decreases photorespiration. |
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 It involves three organelles - chloroplast, perioxisomes and mitochondria. |
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 Increase in temperature leads to more photorespiration that means more loss of photosynthetically fixed carbon. |
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 It occurs in light and has no relation with the normal respiration. |
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 It decreases the yield of C3 plants. |
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