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A2 · Practice questions · The Calvin cycle

Carbon in, sugar out.

Six original Cambridge-style questions on the Calvin cycle, from naming the three stages to predicting what happens to GP and RuBP when conditions change.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

5C in, 3C out.

01
Recall
[3 marks]

Outline the three main stages of the Calvin cycle.

  • Fixation: rubisco catalyses the combination of carbon dioxide with RuBP (5C) to give two molecules of GP (3C). ✓
  • Reduction: GP is reduced to triose phosphate (TP) using reduced NADP and ATP. ✓
  • Regeneration: RuBP is regenerated from TP in reactions that use ATP. ✓
02
Explain
[3 marks]

Explain how the light-independent stage depends on the light-dependent stage.

  • The light-dependent stage supplies ATP and reduced NADP to the stroma. ✓
  • Reduced NADP provides hydrogen / electrons and ATP provides energy to reduce GP to TP. ✓
  • ATP is also needed to regenerate RuBP; without light these run out, and the Calvin cycle stops within seconds or minutes. ✓
03
Data
[4 marks]

A suspension of algae was given radioactive carbon dioxide in bright light. After 5 minutes the light was switched off. The concentration of GP rose to about double its value, while RuBP fell to near zero. Explain these results.

  • In the dark, the light-dependent stage stops, so no ATP or reduced NADP are produced. ✓
  • GP can no longer be reduced to TP, but GP is still formed from RuBP and CO2 by rubisco, so GP accumulates. ✓
  • RuBP is still used in fixation but cannot be regenerated (needs TP and ATP), so its concentration falls. ✓
  • Fixation therefore stops once the RuBP is used up. ✓
04
Calculation
[2 marks]

Calculate the number of turns of the Calvin cycle, and the number of ATP molecules, needed to produce one molecule of glucose.

  • Each turn fixes one CO2; glucose has 6 carbons, so 6 turns are needed (giving 2 TP, which make one hexose). ✓
  • Each turn uses 3 ATP (2 for reduction, 1 for regeneration), so 6 × 3 = 18 ATP (and 12 reduced NADP). ✓
05
Recall
[3 marks]

State how the intermediates GP and TP from the Calvin cycle are used by the plant.

  • GP is used to make some amino acids. ✓
  • TP is used to make carbohydrates, for example glucose, sucrose, starch and cellulose. ✓
  • TP is also used to make lipids (glycerol and fatty acids) and amino acids. ✓
06
Analysis
[2 marks]

Rubisco makes up a large proportion of the protein in a leaf. Suggest why a plant needs such a large amount of this enzyme.

  • Rubisco has a very slow rate of reaction / fixes only a few CO2 molecules per second. ✓
  • A large quantity is needed to fix carbon dioxide fast enough to keep up with the supply of ATP and reduced NADP from the light-dependent stage. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.