Light-independent reactions and the Calvin cycleEdexcel International A Level Biology: Revision notes
Section 1
Where and why the Calvin cycle happens
The light-independent reactions take place in the stroma of the chloroplast. They use the ATP and reduced NADP made in the light-dependent reactions to turn carbon dioxide into sugars. This is carbon fixation: inorganic carbon in CO₂ is incorporated into organic molecules. The reactions are a cycle, the Calvin cycle, because the carbon dioxide acceptor is regenerated at the end.
They are called light-independent because no step uses light directly, but the cycle stops soon after the light is switched off because it depends on ATP and reduced NADP from the light-dependent reactions.
Section 2
The steps of the cycle
- Fixation: CO₂ combines with the five-carbon acceptor RuBP (ribulose bisphosphate). The reaction is catalysed by RUBISCO and forms an unstable six-carbon compound.
- The six-carbon compound splits immediately into two molecules of GP (glycerate 3-phosphate, three carbons each).
- Reduction: GP is reduced to GALP (glyceraldehyde 3-phosphate, a triose phosphate) using hydrogen from reduced NADP and energy from ATP.
- Regeneration: most GALP is used to make more RuBP, which also needs ATP. The remaining GALP is the net product of the cycle.
Only about one in six GALP molecules leaves the cycle. The rest is recycled so that fixation can continue.
Remember the direction: GP is REDUCED (it gains hydrogen from reduced NADP). Never say GP is oxidised in the Calvin cycle.
Section 3
Energy and reducing power: the numbers
Per CO₂ fixed, two GP are made, so 2 ATP and 2 reduced NADP are used to reduce them. A further 1 ATP is used to regenerate RuBP. So each CO₂ fixed needs 3 ATP and 2 reduced NADP.
To make one hexose (6 carbons) the cycle must turn six times, fixing 6 CO₂ and making 12 GALP. Ten GALP (30 carbons) regenerate six RuBP (30 carbons) and two GALP are left to make one hexose. This needs 18 ATP and 12 reduced NADP.
Do not say the Calvin cycle makes glucose directly. The product that leaves is GALP, which is then converted to glucose and other molecules.
Section 4
Using the products: what GALP becomes
GALP (and GP) are the starting point for every organic molecule the plant makes:
- Carbohydrates: two GALP are joined to make a hexose such as glucose, which is polymerised to starch (storage) or cellulose (cell walls). Sucrose is made for transport.
- Lipids: GALP is converted to glycerol and, via other intermediates, to fatty acids.
- Amino acids and proteins: GP and other intermediates are converted into amino acids, with nitrogen from nitrate, and amino acids are joined into proteins.
- Nucleic acids: sugars provide the pentose sugar (ribose, deoxyribose), and nitrogen and phosphate come from nitrate and phosphate ions in the soil.
- Respiration: some sugar is respired in mitochondria to make ATP for the cell's processes.
Section 5
What limits the Calvin cycle
If light is removed, ATP and reduced NADP run out. GP is still made but cannot be reduced, so GP rises and RuBP falls as RuBP is used up and not regenerated.
If CO₂ is reduced, less GP is made, but the existing GP is still reduced and RuBP is still regenerated, so GP falls and RuBP rises.
Temperature also matters because RUBISCO and the other Calvin cycle enzymes are temperature-sensitive. Interpreting how GP and RuBP change is a common exam question.
Work out what each compound is MADE from and USED for. GP is made from CO₂ + RuBP and used with ATP and reduced NADP; RuBP is made from GALP using ATP and used with CO₂.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Light-independent reactions and the Calvin cycle
- A student isolates intact chloroplasts from spinach leaves and suspends them in a buffer containing hydrogencarbonate ions, which act as a source of carbon dioxide. In the light, the chloroplasts take up carbon dioxide and make GALP.Explain why isolated chloroplasts kept in the dark in this buffer cannot make GALP, even though carbon dioxide is available.2 marks
- A plant biochemist is calculating the energy requirements of the Calvin cycle in a leaf. She assumes that, for every carbon dioxide molecule fixed, the cycle uses 3 molecules of ATP and 2 molecules of reduced NADP.Explain why most of the GALP made in the Calvin cycle is not used to make carbohydrate but is used in another process.2 marks
- Tomato plants grown in a glasshouse are supplied with carbon dioxide containing radioactive ¹⁴C for a short time. Later, radioactivity is found in the starch in the leaves, the cellulose in the cell walls, the proteins in the fruits and the lipids in the seeds.Describe how carbon from carbon dioxide is incorporated into GALP in the Calvin cycle.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).