In the stroma, carbon dioxide is captured by a five-carbon acceptor and built up into sugars. The cycle runs on the ATP and reduced NADP from the thylakoids, and it has to rebuild its own acceptor to keep turning.
The Calvin cycle (light-independent stage) takes place in the stroma and uses the ATP and reduced NADP from the light-dependent stage to make complex organic molecules. It has three main stages. Fixation: the enzyme rubisco combines carbon dioxide with ribulose bisphosphate (RuBP, 5C), giving two molecules of glycerate 3-phosphate (GP, 3C). Reduction: GP is reduced to triose phosphate (TP, 3C) in reactions using reduced NADP and ATP. Regeneration: most of the TP is used to regenerate RuBP, in reactions that use ATP. The remaining TP is used to make carbohydrates, lipids and amino acids, and some GP is used to make amino acids.
Click each stage chip in turn: carbon fixation, reduction, regeneration and products. The lab follows three turns of the cycle: 3 RuBP and 3 CO2 give 6 GP, then 6 ATP and 6 reduced NADP (labelled NADPH) make 6 TP, then 3 ATP rebuild 3 RuBP from 5 of the TP, and 1 TP leaves.
| Stage | What happens | For 3 CO2 (three turns) |
|---|---|---|
| 1 fixation | rubisco catalyses the combination of CO2 with RuBP (5C); the unstable 6C product splits at once into two GP (3C) | 3 RuBP + 3 CO2 → 6 GP |
| 2 reduction | GP is reduced to TP; reduced NADP supplies the hydrogen (reducing power) and ATP supplies energy | 6 GP + 6 ATP + 6 reduced NADP → 6 TP |
| 3 regeneration | TP molecules are rearranged into RuBP, using ATP | 5 TP (15 C) + 3 ATP → 3 RuBP (15 C) |
| net gain | one TP is free to leave the cycle | 1 TP (3 C), the 3 C fixed |
Rubisco fixes carbon dioxide; it does not use ATP. ATP and reduced NADP are used in the reduction of GP to TP, and ATP again in regenerating RuBP. Also: RuBP is 5C, GP and TP are both 3C.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
In the carbon fixation stage of the simulator, what does rubisco join together?
Which stage in the simulator uses the 6 ATP and 6 reduced NADP tokens?
In the lab's three turns, why are 5 of the 6 TP molecules kept in the cycle?
How many carbon atoms are in RuBP, GP and TP?
Click products in the simulator. One TP in six leaves the cycle; the rest must stay to rebuild RuBP, or the cycle would stop.
| Intermediate | Used to make |
|---|---|
| triose phosphate (TP) | carbohydrates: two TP make a hexose such as glucose, then sucrose, starch and cellulose lipids: TP gives glycerol and is used to make fatty acids amino acids |
| glycerate 3-phosphate (GP) | some amino acids |
In data questions about switching the light off, the first change is in the reduction step: GP rises because it is still being made from RuBP but can no longer be reduced, and RuBP falls because it is not regenerated.
Unlocks once the four checks above are done. Worth more XP, written to A Level Paper 4 standard.
An alga is photosynthesising in bright light. The light is switched off. What happens to the concentrations of GP and RuBP in the next minute?
The carbon dioxide supply to a plant in constant light is cut off. What happens to GP and RuBP?
How many molecules of ATP and reduced NADP are used to fix 6 molecules of carbon dioxide?
Which molecules can be made from triose phosphate leaving the Calvin cycle?
This skill is now recorded on your profile. Keep the chain going.