5.1.5 The Light-Independent Reaction

Stages of the Calvin Cycle

1. Carbon Dioxide Fixation:

• Carbon dioxide (CO₂) diffuses into the chloroplast through the stomata.
• CO₂ is fixed with ribulose bisphosphate (RuBP) in a process called carboxylation, catalysed by the enzyme Rubisco.
• The reaction forms an unstable 6-carbon compound, which immediately breaks down into two molecules of glycerate-3-phosphate (GP) (a 3-carbon compound).

2. Reduction Phase:

• Each glycerate-3-phosphate (GP) molecule is reduced to triose phosphate (TP).
• ATP provides energy, and reduced NADP donates hydrogen to carry out this reduction.
• At this stage, all reduced NADP from the light-dependent reaction is used, while only some ATP is consumed.

3. Regeneration of RuBP:

• Five molecules of triose phosphate (TP) are used to regenerate three molecules of RuBP.
• ATP from the light-dependent reaction provides energy for this process.

4. Production of Organic Molecules:

• One molecule of triose phosphate (TP) is not recycled and instead combines with another to form hexose sugars, such as glucose.
• This glucose can later be converted into other carbohydrates like starch or used for energy.

Inputs and Outputs

Input	Output
CO₂	Glucose
ATP	ADP + Pi
Reduced NADP	NADP