Calvin cycle

The Calvin cycle is a light independent ("dark-reaction") process of carbon fixation that takes place in the chloroplasts of all plants, whether C3, C4, or CAM plants. The Calvin cycle utilizes light energy stored as ATP and NADPH to convert CO₂ and H₂O to organic compounds.

6 CO₂ + 12 NADPH + 12 H⁺ + 18 ATP → C₆H₁₂O₆ + 6 H₂O + 12 NADP⁺ + 18 ADP + 18 Pi

Enzymes of the Calvin cycle are functionally equivalent to many enzymes involved in other metabolic pathways such as glycolysis and gluconeogenesis, but they are located in the chloroplast stroma rather than in the cytoplasm, thus functionally separating the reactions.

The enzymes are activated by light (hence light independent rather than "dark reaction") and by products of the light-dependent photosynthetic reactions. These regulatory mechanisms prevent the Calvin cycle from operating in reverse to respiration, thus preventing a continuous cycle of CO₂ reduction to carbohydrates from occurring simultaneously with carbohydrate oxidation to CO₂ (respiration). This regulation prevents the waste of energy (as ATP) in simultaneous reverse reactions that would have no net productivity.

Table ~ comparison photosynthesis & respiration : Table ~ comparison plant & bacterial photosynthesis : Table ~ comparison of C-3, C-4, CAM plants :

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