What Is the Primary Function of the Calvin Cycle?

The Calvin cycle is the final step of photosynthesis. Here is an explanation of the primary function of this important step:

Converting Carbon Dioxide and Water Into Glucose

In the most general sense, the primary function of the Calvin cycle is to make organic products that plants need using the products from the light reactions of photosynthesis (ATP and NADPH). These organic products include glucose, the sugar made using carbon dioxide and water, plus protein (using nitrogen fixed from the soil) and lipids (e.g., fats and oils).

This is carbon fixation, or fixing inorganic carbon into organic molecules that the plant can use:

3 CO₂ + 6 NADPH + 5 H₂O + 9 ATP → glyceraldehyde-3-phosphate (G3P) + 2 H⁺ + 6 NADP⁺ + 9 ADP + 8 P_i   (P_i = inorganic phosphate)

The key enzyme for the reaction is RuBisCO. Although most texts simply say the cycle makes glucose, the Calvin cycle actually produces 3-carbon molecules, which are eventually converted into the hexose (C6) sugar, glucose.

The Calvin cycle is a set of light-independent chemical reactions, so you might also hear it referred to as the dark reactions. This doesn't mean the Calvin cycle occurs only in the dark; it just doesn't require energy from light for the reactions to occur.

Summary

The primary function of the Calvin cycle is carbon fixation, which is making simple sugars from carbon dioxide and water.