The central metabolic pathway of the Calvin cycle, also known as the Calvin-Benson cycle, is a series of biochemical reactions that take place in the stroma of chloroplasts in photosynthetic organisms. Its primary function is to convert carbon dioxide (CO2) from the atmosphere into glucose and other carbohydrates, which serve as energy sources for the organism. The cycle operates in three main stages:
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Carbon Fixation:
- The cycle begins when CO2 is fixed into an organic molecule. This reaction is catalyzed by the enzyme ribulose bisphosphate carboxylase/oxygenase (RuBisCO). CO2 combines with ribulose bisphosphate (RuBP), a 5-carbon sugar, producing a 6-carbon intermediate that quickly splits into two molecules of 3-phosphoglycerate (3-PGA), a 3-carbon compound.
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Reduction Phase:
- The 3-PGA molecules are then phosphorylated by ATP (adenosine triphosphate) and reduced by NADPH (nicotinamide adenine dinucleotide phosphate) to form glyceraldehyde-3-phosphate (G3P), another 3-carbon molecule. Some of these G3P molecules exit the cycle to contribute to the formation of glucose and other carbohydrates, while others are used to regenerate RuBP.
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Regeneration of Ribulose Bisphosphate (RuBP):
- The remaining G3P molecules undergo a series of reactions, utilizing ATP, to regenerate RuBP, allowing the cycle to continue. The regeneration of RuBP is essential to ensure that the cycle can continuously fix CO2.
Overall, the Calvin cycle plays a crucial role in the biosynthesis of carbohydrates and is a key part of the process of photosynthesis. It requires ATP and NADPH, which are produced in the light-dependent reactions of photosynthesis. The cycle typically requires three CO2 molecules to produce one G3P molecule that can be used to synthesize glucose and other carbohydrates.
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