Light-Independent Reactions

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Light-Independent Reactions

These reactions can occur in the light or the dark. They need ATP for energy to drive the reactions, and they need NADPH for reducing power. They occur in the stroma of the chloroplast and are called the Calvin cycle.

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Carbon dioxide combines with a 5-carbon sugar called ribulose bisphosphate (RuBP) to form a 6-carbon sugar. This process is known as carbon fixation and is catalysed by the enzyme ribulose bisphosphate carboxylase (rubisco).

This 6-carbon sugar is unstable and breaks down to form two 3-carbon sugars. These are converted into triose phosphates using the energy from ATP and using the hydrogen from reduced NADP.

Most of this triose phosphate is used to regenerate RuBP, but some is used to produce 6-carbon sugars from which complex carbohydrates, amino acids and other substances are made.

For every twelve 3-carbon sugars (36 carbons):

Two of the 3-carbon sugars form 1 x 6-carbon sugar (6 carbons)

Ten of the 3-carbon sugars form 6 x 5-carbon sugars (RuBP) (30 carbons)