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C1.3.15 (HL)—Carbon fixation by Rubisco

Rubisco catalyses carbon fixation by attaching carbon dioxide to RuBP, forming glycerate phosphate at the start of the Calvin cycle.

Syllabus
First assessment 2025
Objective
C1.3.15
Level
HL

Exam analysis

Chance of appearing9%of analysed past papers
Latest appearanceNovember 2024
Most common paperPaper1
Typical marks1–2

Common command terms

  • State
  • Describe
  • Explain
  • Identify
  • Predict

Recent exam appearances

November 2024Paper2 ["HL"] · TZ05(a)[ 2 ]C1.3.15 (HL)—Carbon fixation by Rubisco
May 2024Paper2 ["HL"] · TZ18(c)[ 7 ]C1.3.15 (HL)—Carbon fixation by Rubisco
May 2024Paper2 ["HL"] · TZ26(c)[ 8 ]C1.3.15 (HL)—Carbon fixation by Rubisco
November 2023Paper1 ["HL"] · TZ227[ 1 ]C1.3.15 (HL)—Carbon fixation by Rubisco
May 2023Paper1 ["HL"] · TZ130[ 1 ]C1.3.15 (HL)—Carbon fixation by Rubisco
Practice this objective

Coverage 2015–2024 · Updated 15 Jul 2026

Rubisco Fixes CO₂ to RuBP

HL only

Rubisco catalyses carbon fixation in the chloroplast stroma by adding CO₂ to the five-carbon acceptor RuBP.

The unstable six-carbon product immediately divides into two molecules of glycerate 3-phosphate (GP). This is carbon's entry into the Calvin cycle.

Substrates: CO₂ and RuBP; enzyme: Rubisco; product: two GP molecules. Rubisco is the most abundant enzyme on Earth, and plants maintain high stromal concentrations because it works relatively slowly and is ineffective at low CO₂ concentrations.

Fixing three CO₂ molecules to three RuBP molecules produces six GP molecules before their reduction to triose phosphate.

Rubisco does not directly produce glucose or triose phosphate. It catalyses the fixation step that produces GP, which is modified in later Calvin-cycle reactions.

Carbon fixation by Rubisco

HL only

Assessment in practice

1–2 marks
How it is assessed

This objective is assessed through structured response, commonly using State / Describe / Explain.

Command terms

State / Describe / Explain / Identify / Predict

What earns marks

Build the answer around this relationship: Rubisco catalyses carboxylation of RuBP.

Representative question

Question 1

[Maximum number: 8]

With reference to Calvin's experiment, explain the fixation of carbon dioxide in photosynthesis.

Trace Photosynthesis Mechanism

HL only

HL photosynthesis links light-dependent reactions to the Calvin cycle. Photosystems are pigment-protein arrays in thylakoid membranes; antenna pigments broaden absorption and pass energy to reaction-centre chlorophyll. PSII uses light to split water, supplying electrons, protons, and oxygen. Electron transport pumps protons into the thylakoid space, ATP synthase makes ATP by photophosphorylation, PSI re-excites electrons, and NADP is reduced. The Calvin cycle uses ATP and reduced NADP: Rubisco fixes CO2 to RuBP to form GP, GP is reduced to triose phosphate, most triose phosphate regenerates RuBP, and some becomes wider biomolecules.

  • Trace energy: pigments -> reaction centres -> ATP and reduced NADP.
  • Trace electrons/protons: water -> PSII -> carriers -> PSI -> NADP; proton gradient -> ATP synthase.
  • Trace carbon: CO2 -> RuBP/GP -> triose phosphate -> RuBP regeneration and biomolecule synthesis.
  • Trace exchange: light-dependent reactions supply ATP/reduced NADP; Calvin cycle returns ADP/NADP.

Concept essentials

  • Rubisco catalyses carboxylation of RuBP.
  • Carbon fixation produces glycerate 3-phosphate early in the Calvin cycle.
  • The Calvin cycle occurs in the chloroplast stroma.
  • Radioactive carbon tracing helped reveal the path of carbon through photosynthesis.