7. Light-Independent Reactions (Calvin Cycle): Building Sugar

The Calvin cycle, also called the light-independent reactions, uses the ATP and NADPH produced by the light-dependent reactions to build sugar. It does not require light directly, but in most plants it depends on the products of the light reactions and so effectively runs during daylight.

The cycle has three phases. In carbon fixation, an enzyme attaches carbon dioxide from the air to a five-carbon molecule called RuBP. In reduction, ATP and NADPH supply energy and electrons to convert the resulting molecules into a simple three-carbon sugar. In regeneration, most of that sugar is used to rebuild RuBP so the cycle can continue; a small fraction is exported to build glucose and other carbohydrates the plant uses or stores.

Because the Calvin cycle consumes the ATP and NADPH made by the light reactions, the two stages of photosynthesis are tightly linked: neither stage can continue for long without the other.