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The Calvin Cycle

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The Calvin cycle is a stage of photosynthesis that occurs during the light-independent reactions. This process allows autotrophs to convert carbon dioxide into sugar (an anabolic process). This sugar is not glucose; it is a three-carbon sugar known as glyceraldehyde 3-phosphate (G3P). Three turns of the cycle produce a net total of one molecule of G3P; therefore, three molecules of CO2 are required. The net synthesis of one G3P also requires nine ATP molecules and six NADPH molecules. The ATP and NADPH are produced during the light-dependent reactions. The Calvin cycle can be divided into three phases: carbon fixation, reduction and regeneration.

Carbon Fixation
A molecule of carbon dioxide is incorporated into a five-carbon sugar known as RuBP (ribulose bisphosphate). This step is catalyzed by the enzyme rubisco, short for RuBP carboxylase-oxygenase. It forms a six-carbon intermediate which is energetically unstable, so it splits in half to form two molecules of 3-phosphoglycerate.

Reduction
3-phosphoglycerate receives a phosphate group from ATP and becomes 1,3-biphosphoglycerate. 1,3-biphosphoglycerate is reduced by NADPH (forming NADP+) and loses a phosphate group, forming G3P.

Regeneration
After 3 turns of the Calvin cycle, six molecules of G3P are produced. Only one molecule leaves the cycle, however; the other five molecules of G3P are rearranged to form three molecules of RuBP.
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