The light reactions make ATP and NADPH but no sugar. The Calvin cycle, in the stroma of the chloroplast, spends them to build sugar from CO₂. It is a cycle because the molecule that grabs CO₂ must be rebuilt at the end of each turn.
- Fixation: the enzyme rubisco attaches CO₂ to a 5-carbon sugar, RuBP. The 6-carbon product splits at once into two 3-carbon molecules of 3-PGA.
- Reduction: ATP and NADPH turn each 3-PGA into G3P (glyceraldehyde 3-phosphate), a 3-carbon sugar.
- Regeneration: most G3P is rearranged, using more ATP, back into RuBP. The rest leaves the cycle to build glucose, sucrose, starch and more.
Key idea
Three turns fix 3 CO₂ and make one G3P for the cell, using 9 ATP and 6 NADPH. A glucose (two G3P) takes 6 CO₂, 18 ATP and 12 NADPH.
| Step (for 3 CO₂) | Carbon atoms | Uses |
|---|---|---|
| 3 CO₂ + 3 RuBP (5 C each) | 3 + 15 = 18 | |
| → 6 molecules of 3-PGA (3 C each) | 18 | |
| → 6 G3P | 18 | 6 ATP, 6 NADPH |
| 1 G3P leaves the cycle | 3 | |
| 5 G3P → 3 RuBP | 15 → 15 | 3 ATP |
Why it works
Why does the cycle need both ATP and NADPH? Turning CO₂ into sugar is a reduction: the carbon gains electrons and hydrogen. NADPH supplies them. ATP provides the free energy to make that uphill step go and to rebuild RuBP. Carbon is conserved: the 18 carbons on the table never vanish; only 3 of them, one G3P, are new sugar.
Worked examples
Worked example: Scaling up
How many CO₂, ATP and NADPH are needed to make 4 G3P for the cell?
- Per G3P: 3 CO₂, 9 ATP, 6 NADPH.
- For 4 G3P: $4 \times 3 = 12$ CO₂, $4 \times 9 = 36$ ATP, $4 \times 6 = 24$ NADPH.
Answer: 12 CO₂, 36 ATP and 24 NADPH.
Worked example: Turning off the light
Algae in bright light are suddenly put in the dark. What happens in the next few seconds to the levels of 3-PGA and RuBP?
- Without light, ATP and NADPH run out.
- Rubisco can still fix CO₂ onto the RuBP that is there, making 3-PGA.
- But 3-PGA cannot be reduced, and RuBP cannot be rebuilt.
Answer: 3-PGA builds up and RuBP falls. This kind of experiment helped Melvin Calvin and his team work out the cycle.
Common mistake
Calling it the "dark reactions" and thinking it runs at night. The Calvin cycle needs the ATP and NADPH that the light reactions make, so it runs in the light, and several of its enzymes are switched on by light. "Light-independent" only means that light does not drive it directly.