RuBP regeneration at Higher Level: An AHL statement that closes the Calvin cycle (3.3.15-3.3.17).
Practise this as you read
- Calculate the numbers of molecules in the cycle.
- Explain why RuBP must be regenerated.
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The big idea: Most of the triose phosphate made in the Calvin cycle is used to regenerate RuBP, so carbon fixation can go on. This needs ATP. Five triose phosphates (15 carbons) make three RuBP (15 carbons); only one TP in six leaves the cycle.
The points to remember
- 5 TP (3C) → 3 RuBP (5C): 15 carbons each side.
- Regeneration uses ATP (but no reduced NADP).
- For one glucose: 6 turns, 6 CO₂, 12 GP, 12 TP.
- 10 TP regenerate the 6 RuBP; 2 TP leave to make one glucose.
- Without regeneration, RuBP would run out and fixation would stop.
Remember it as: Five threes make three fives.
Real example: why the dark stops the cycle. In the dark, ATP runs out, so RuBP cannot be regenerated: RuBP levels fall and glycerate 3-phosphate builds up, which is how the Calvin cycle was first traced with radioactive carbon.
Trap: ATP is used twice in the cycle: to reduce GP and to regenerate RuBP. Reduced NADP is used once.
Counting the carbons for one glucose:
| For one glucose (six turns) | Number | Carbons |
|---|---|---|
| Carbon dioxide fixed | 6 | 6 |
| RuBP used | 6 | 30 |
| Glycerate 3-phosphate made | 12 | 36 |
| Triose phosphate made | 12 | 36 |
| TP back to make RuBP | 10 | 30 |
| TP leaving (one glucose) | 2 | 6 |
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How it is asked: Expect a calculation of the numbers of RuBP, GP or TP for one glucose, or why RuBP must be regenerated.
Calculate how many molecules of triose phosphate are used to regenerate RuBP when 12 molecules of triose phosphate are made.
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