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Learn9700Paper 4 · A Level Structured QuestionsPhotosynthesisCalvin cycle and limiting factors
Photosynthesis · Subtopic 2 of 2

Calvin Cycle and Limiting Factors

The Calvin cycle (stroma) uses ATP and reduced NADP from the light-dependent stage. Fixation: RuBisCO combines CO₂ with ribulose bisphosphate (RuBP, 5C) to form two glycerate 3-phosphate (GP, 3C). Reduction: GP → triose phosphate (TP) using ATP and reduced NADP. Regeneration: most TP (5 of every 6) is used to remake RuBP using ATP. TP is used to make glucose, sucrose, starch, cellulose, lipids and amino acids. Six turns fix six CO₂ to make one hexose. Limiting factors: light intensity, CO₂ concentration and temperature; the factor in shortest supply limits the rate. In the dark, GP accumulates and RuBP falls; with low CO₂, RuBP accumulates and GP falls. C4 plants (maize) fix CO₂ in mesophyll cells and release it to bundle-sheath cells, keeping RuBisCO away from O₂.

Cambridge 9700 Paper 4Edexcel YBI11 WBI14AQA 7402 Paper 2≈ 50 minRevision 1 · updated 2026-11-02
Section 1 of 14

Learning objectives

  • Describe the three stages of the Calvin cycle.
  • Explain changes in GP and RuBP when conditions change.
  • Explain limiting factors and their use in agriculture.
  • Describe C4 adaptations.
Pause and think

If a plant is suddenly put in the dark, why does GP increase?

Section 2 of 14

Core idea

Start hereAir into sugar

Most of a tree's mass comes from carbon dioxide pulled out of the air by the Calvin cycle.

The big idea

RuBisCO fixes CO₂ onto RuBP; ATP and reduced NADP turn the product into triose phosphate, most of which regenerates RuBP.

The Calvin cycle

Three stages in the stroma.

Worked exampleCost of a glucose

ATP and reduced NADP needed per glucose?

Goal
Numbers.
Strategy
Per CO₂: 3 ATP, 2 NADPH; 6 CO₂ per glucose.
  1. 6×3=18 ATP6\times3=18\ \text{ATP}
    Reduction and regeneration.
  2. 6×2=12 reduced NADP6\times2=12\ \text{reduced NADP}
    Reduction of GP.
Answer18 ATP and 12 reduced NADP.
✓ CheckSupplied by light-dependent reactions ✓.

Summary

  • Fixation, reduction, regeneration.
  • Limiting factors.
  • C4 plants.
Key words
RuBisCO
Enzyme that fixes CO₂ to RuBP.
Limiting factor
Factor in shortest supply that limits the rate.
HistoryLollipop apparatus. Melvin Calvin traced radioactive ¹⁴CO₂ through algae in a flat flask nicknamed the "lollipop", sampling every few seconds, and won the 1961 Nobel Prize.
Fun factRuBisCO everywhere. There may be about 5 kg of RuBisCO for every person on Earth.
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