The Calvin cycle
The Calvin cycle
- The light-independent stage builds sugars in the stroma.
- It uses the ATP and reduced NADP made by the light stage.
- It runs as a cycle, in three main steps.
Step 1: Fixation
- The enzyme rubisco joins carbon dioxide to a 5-carbon molecule, RuBP (ribulose bisphosphate).
- This makes two molecules of a 3-carbon compound, GP (glycerate 3-phosphate).
Practice
In fixation, the enzyme rubisco:
Rubisco catalyses carbon fixation: CO₂ + RuBP → 2 GP.
Steps 2 & 3: Reduction and regeneration
- Reduction: GP is reduced to TP (triose phosphate), using reduced NADP and ATP.
- Regeneration: most TP is used (with more ATP) to regenerate RuBP, so the cycle keeps running.
Practice
GP is reduced to TP using:
The reduction step uses the reduced NADP and ATP made in the light-dependent stage.
Practice
Why must some TP regenerate RuBP?
RuBP is the CO₂ acceptor; regenerating it (using ATP) lets the cycle continue.
Practice
Put the steps of the Calvin cycle in order.
CO₂ is fixed onto RuBP (fixation) → GP reduced to TP (reduction) → RuBP regenerated (regeneration).
Making useful molecules
- Some TP leaves the cycle to make carbohydrates, lipids and amino acids.
- GP can also be used to make some amino acids.
Practice
What happens to the TP that leaves the cycle?
TP that leaves the cycle builds useful molecules: carbohydrates, lipids and amino acids (GP can make amino acids too).
You've got it
Key idea
- fixation: rubisco + CO₂ + RuBP (5C) → 2 GP (3C)
- reduction: GP → TP using reduced NADP + ATP
- regeneration: TP → RuBP (more ATP) to keep the cycle going
- some TP leaves → carbohydrates, lipids, amino acids