Dark Reactions

The energy carriers ATP and NADPH generated from light reactions initiate the second stage of photosynthesis, called the Calvin cycle. It was named after the scientist Melvin Calvin, who discovered it.

Because these reactions are light-independent, they're called dark reactions.

This cycle generates glucose and other carbohydrates, which have a backbone of carbon atoms.

But where does the carbon come from? It comes from carbon dioxide, the gas animals exhale with every breath.

Carbon dioxide from the air enters the leaf through tiny pores called stomata. From there, it diffuses to reach the inner region of the chloroplast called the stroma, where the Calvin cycle takes place.

It proceeds in three stages: fixation, reduction, and regeneration.

In the first two stages, carbon dioxide is converted to a 3-carbon sugar with the help of an enzyme called RuBisCO, and the energy carriers ATP and NADPH, which came from the light reaction.

In the third stage, the starting molecule, ribulose bisphosphate or RuBP, is regenerated to continue the cycle.