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| The Four - Carbon Pathway |
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| C4 pathway |
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| C4 plants like maize, sugarcane; pearl millet, etc. have evolved a wonderful mechanism to avoid photorespiration, which is considered a wasteful process. |
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| In these plants three or 2 types of photosynthetic cells namely mesophyll cells and bundle sheath cells. The bundle sheath cells are arranged in a wreath like manner, thus calling them as kranz anatomy (kranz : wreath). |
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| One more speciality of C4 plants is that they contain dimorphic chloroplasts, that is chloroplasts in bundle sheath cells are agranal. The presence of two types of cells allows the occurrence of light reactions and carbon reactions separately. |
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| In C4 plants, light reactions occur in the mesophyll cells, whereas CO2 assimilation occurs in the bundle sheath cells. This type of separation does not allow O2 released in mesophyll cells to escape in to the bundle sheath cells. This prevents the oxygenation of RuBP, which is present in the bundle sheath cells. |
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| Transverse section of maize leaf showing the arrangement of mesophyll and bundle-sheath cells. The C4 pathway takes place in the mesophyll cells, and the C3 pathway (Calvin cycle) operates in the bundle-sheath cells. Both types of cells contain chloroplasts. |
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| To further avoid photorespiration, C4 plants have evolved a CO2 concentrating mechanism called the C4 pathway. The main objective of C4 pathway is to build up a high concentration of CO2 in the vicinity of Rubisco, thus favouring carboxytition and suppressing photorespiration. |
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| Many plants like maize and sugar cane, are far more efficient at taking up CO2 than C3 plants. The CO2 acceptor in C4 plants is phosphoenolpyruvate (PEP). PEP reacts with CO2 to form oxaloacetic acid which is reduced by NADPH to form malic acid. The malic acid then reacts with RUBP to form pyruvic acid and PGA. The pyruvic acid is then phosphorylated by ATP to regenerate PEP while PGA is converted to triose phosphate as far as C3 plants. These reactions are called the Hatch - Slack pathway. |
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| These plants, however first fix carbon dioxide in the four - carbon compound oxaloacetic acid. C4 plants have two rings of cells surrounding the vascular bundles. The outer mesophyll cells fix CO2 using PEP and pass malic acid to the inner bundle sheath cells. Here CO2 is released for acceptance by RUBP and the eventual formation of triose phosphate. The photosynthetic efficiency of C4 plants is due to their ability to fix CO2 in environmental conditions where CO2 is the limiting factor. The C4 photosynthetic pathway is more efficient than the C3 pathway due to the absence of photorespiration in C4 plants. |
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| Crassulacean acid metabolism (CAM) |
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| CAM refers to a mechanism of photosynthesis that occurs only in succulents and other plants that grow in dry conditions. |
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| a) In CAM plants, CO2 is taken up by the leaves, which are present on green stems through the stomata. But the stomata remain open only during the night. During the day it remains closed to conserve moisture. |
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| b) The CO2 taken is fixed in the same way as in C4 plants, to from malic acid which is stored in the vacuole. |
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| c) This malic acid formed during the night is used as a source of CO2 during the day for photosynthesis to proceed via the C3 pathway. |
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| Thus CAM is a kind of adaptation in certain plants (grown in dry conditions) to carry out photosynthesis without much loss of water, which is otherwise unavoidable in C3 and C4 photosynthetic mechanisms. |
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| CAM pathway showing carbon dioxide uptake through open stomata during night and its utilisation for the formation of malic acid which is stored in the vacuole. During day, the malic acid is decarboxylated to release carbon dioxide which is re-fixed to produce starch inside choloroplast via C3 calving cycle. |
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