The Calvin cycle is a metabolic process that uses the carbon from carbon dioxide, along with energy in the form of ATP, to produce sugar. This cycle takes place in the stroma of chloroplasts, which are found in plant cells.
The Calvin cycle occurs in three phases. During the first phase, carbon fixation, carbon dioxide enters the cycle. An enzyme called RuBP carboxylase fixes carbon into ribulose biphosphate, which is a five-carbon sugar abbreviated as RuBP. This reaction produces a six-carbon molecule that splits into two molecules of 3-phosphoglycerate. The 3-phosphoglycerate acts as a metabolic intermediate.
During the second phase of the cycle, called reduction, NADPH2 and ATP are used to convert 3-phosphoglycerate to a carbohydrate precursor to glucose and other types of sugars. This three-carbon precursor is called glyceraldehyde 3-phosphate.
Finally, the cycle begins the regeneration phase. During this third phase, ATP is used to convert some of the glyceraldehyde 3-phosphate to RuBP. Not all of the glyceraldehyde 3-phosphate is converted to RuBP, however. The remaining molecules are used to produce fructose diphosphate, which is used to make starch, sucrose and other sugars. Although the Calvin cycle is a light-independent reaction, it relies indirectly on light reactions because these reactions produce the NADPH2 and ATP needed to complete the cycle.