How many molecules of ATP and NADPH are required for every molecule of CO₂ fixed in the Calvin cycle?

The correct answer is 3 molecules of ATP and 2 molecules of NADPH.

The Calvin cycle (the light-independent reactions of photosynthesis) is a metabolic pathway that occurs in the stroma of chloroplasts, converting carbon dioxide into organic compounds. The energy required to fix each molecule of carbon dioxide (${\mathrm{CO}}_2$) is supplied by ATP and NADPH, which are generated during the light-dependent reactions. The process is broken down into three main phases:

1. Carbon Fixation Phase:
One molecule of ${\mathrm{CO}}_2$ is combined with a five-carbon acceptor molecule, ribulose-1,5-bisphosphate (RuBP). This reaction is catalyzed by the enzyme RuBisCO, producing two molecules of a three-carbon compound called 3-phosphoglycerate (3-PGA). No ATP or NADPH is consumed during this initial fixation step.

2. Reduction Phase:
The two molecules of 3-PGA are converted into two molecules of glyceraldehyde-3-phosphate (G3P). This phase requires energy input in two steps:
• Each of the two 3-PGA molecules is phosphorylated by phosphoglycerate kinase, which consumes:

$2\text{molecules of ATP}$

• Each of the resulting molecules is then reduced by G3P dehydrogenase, which consumes:

$2\text{molecules of NADPH}$

3. Regeneration Phase:
To sustain the cycle, RuBP must be regenerated. For every three molecules of ${\mathrm{CO}}_2$ fixed, five molecules of G3P are recycled to regenerate three molecules of RuBP, consuming 3 molecules of ATP in the process. When scaled down to a single molecule of fixed ${\mathrm{CO}}_2$, this regeneration step requires:

$1\text{molecule of ATP}$

Total Requirements Per Fixed ${\mathrm{CO}}_2$ Molecule:
Adding up the energy carriers used in the reduction and regeneration steps:
• Total ATP required:

$2\text{(Reduction)}+1\text{(Regeneration)}=3\text{ATP molecules}$

• Total NADPH required:

$2\text{(Reduction)}=2\text{NADPH molecules}$