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The Calvin cycle

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The Calvin cycle - Lesson Summary

Plants prepare food through Calvin cycle using energy synthesized during light reaction. Oxygen released during photosynthesis is released into the atmosphere through stomata present on the leaves while the ATP and NADPH molecules are used in the synthesis of carbohydrate from carbon dioxide and water. Dark reaction, takes place both in light and dark until the products of light reaction, ATP and NADPH, are exhausted. Melvin Calvin discovered that the first stable product 3-Phosphoglycerate.  However, only some plants showed 3-Phosphoglycerate as the first product of carbon fixation while others showed the formation of a four-carbon organic acid, oxaloacetic acid.  Plants that showed 3-Phosphoglycerate as the first product of carbon fixation were called C 3 plants, and the pathway was called the C 3 pathway. Plants that showed oxaloacetic acid as the first product of carbon fixation were called C 4 plants, and the process of carbon fixation was called the C 4 pathway. The Calvin Cycle has three stages: carboxylation, reduction and regeneration. In carboxylation, a five-carbon molecule, ribulose-1-5 bisphosphate, accepts a carbon dioxide molecule to form an intermediate six-carbon molecule which then splits to form two molecules of 3-Phosphoglycerate. This reaction is catalysed RUBISCO. At first, two molecules of 3-phospho glycerate is phosphorylated to two molecules of 1,3, bisphospho glycerate by utilising two molecules of ATP in the presence of the enzyme phospho glycerate kinase. Second, the enzyme glyceraldehyde-3-phosphate dehydrogenase reduces two molecules of 1,3-biphosphoglycerate to two molecules of glyceraldehyde-3-phosphate. Two molecules of NADPH are used in this reaction. Thus to fix one molecule of carbon dioxide to two molecules of glyceraldehyde-3-phosphate, 2 ATP and 2 NADPH are utilised. Six carbon dioxide molecules are necessary for the formation of one glucose molecule. One carbon dioxide molecule results in two molecules of glyceraldehyde-3-phosphate molecules. Hence, six molecules would yield 12 molecules of glyceraldehyde-3-phosphate.  Of these 12 molecules, two of them are used to make one molecule of glucose.  The remaining ten glyceraldehyde-3-phosphate molecules make up a total of 30 carbon atoms. These are involved in the regeneration of six molecules of Ribulose bisphosphate, a five-carbon compound, thus balancing the reaction.  In the regeneration phase, one ATP molecule is utilised to form 1 RUBP.  Hence during the Calvin Cycle, one carbon dioxide molecule needs 3 ATP molecules and 2 NADPH molecules for fixation.  Therefore, the net utilisation of energy for the formation of one glucose molecule would be 18 ATP and 12 NADPH molecules.  

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