Overview of light-independent reaction

The light-independent reaction occurs in the stroma of chloroplasts.

Reactants:

• Carbon dioxide
• ATP
• Reduced NADP from light-dependent reaction

Products:

• Glucose
• Other organic molecules

Stages of light-independent reaction

There are three main stages of the light-independent reaction: fixation, reduction, and regeneration. As it is a cyclical process, it is sometimes called the Calvin cycle.

Carbon fixation:

1. Carbon dioxide reacts with a 5-carbon (5C) compound ribulose bisphosphate (RuBP) to form an unstable 6C compound.
2. This 6C compound splits into two 3C glycerate-3-phosphate (GP) molecules.
3. This is catalysed by the enzyme ribulose bisphosphate carboxylase (rubisco).

Reduction of GP:

1. GP is reduced into triose phosphate (TP).
2. This uses energy from the hydrolysis of ATP.
3. This also requires protons and electrons from reduced NADP, which itself is oxidised to regenerate NADP.
4. The NADP that is re-formed returns to the light-dependent reaction to be reduced again.

Regeneration of RuBP:

1. Most TP is used to regenerate RuBP using ATP.
2. The rest of the TP can be used to make other organic molecules.

For each turn of the Calvin cycle, five carbons are used to regenerate RuBP (5C) and only one carbon, from CO₂ (1C), is available to make new organic compounds. This means six full turns of the Calvin cycle are needed to make one molecule of glucose (6C).

Uses of triose phosphate

Apart from being used to regenerate RuBP, TP can be used to make other useful molecules.

For example, TP can be used to make:

• Simple sugars (e.g. glucose)
• Larger carbohydrates (e.g. starch, sucrose, and cellulose)
• Amino acids
• Lipids
• Nucleotides