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Photorespiration - Lesson Summary

Photorespiration is a process that lowers the efficiency of photosynthesis in plants. In this process RuBP reacts with oxygen to release carbon dioxide. This happens during the Calvin cycle due to the catalytic activity of RuBP oxygenase.
Interestingly, though RuBisCo, shows greater affinity towards carbon dioxide than oxygen it can bind with both molecules.  However, this binding is determined by the relative concentration of these gases at the enzyme site of the plant cells. Photorespiration occurs in C3 plants because some oxygen is likely to bind with RuBisCo during  photosynthesis but C4 plants with its special leaf anatomy have evolved to overcome this process. In C3 plants photorespiration is pronounced especially during dry conditions and in high temperatures when the concentration of carbon dioxide is lowered in the leaf. This is due to its utilisation in the Calvin cycle after the closure of stomata to check transpiration. During photorespiration, the active site of RuBisCo combines with oxygen and yields one molecule of phosphoglycerate and one molecule of phosphoglycolate while with carbon dioxide, it yields two molecules of phosphoglycerate. In these plants, oxygen binds with RuBisCo during photosynthesis, which results in reduced carbon dioxide fixation. Additionally, this process does not result in the synthesis of sugars nor of ATP or NADPH. However, it utilises energy in the form of ATP. Photorespiration is thus a wasteful process in C3 plants. However, in C4 plants, photorespiration does not occur due to the special leaf anatomy found in these plants. Moreover, RuBisCo, is absent in the mesophyll cells of c4 plants. However, they have an efficient enzyme in the mesophyll cells called PEP carboxylase, which catalyses the carboxylation reaction. In this reaction, pyruvate accepts the carbon dioxide molecule and forms four carbon acid. The four carbon acid is carried to the bundle sheath cells to release carbon dioxide, where it enters the Calvin cycle. The thick walls of the bundle sheath cells being impervious to gaseous exchange also aid in maximising the concentration of carbon dioxide near RuBisCo. Therefore, the oxygenase activity of RuBisCo is minimised or barely present. As the C4 plants succeed in bypassing the photorespiratory pathway, photosynthesis proves to be more productive, which ultimately increases their yield.


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