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C1.3.2—CO₂ → glucose

Photosynthesis uses carbon dioxide as a carbon source to make glucose, starch, oils and other organic compounds in plant cells.

Syllabus
First assessment 2025
Objective
C1.3.2
Level
SL

Exam analysis

Chance of appearing5%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper2
Typical marks1–4

Common command terms

  • State
  • Outline
  • Predict
  • Suggest

Recent exam appearances

May 2025Paper1A ["SL"] · TZ25[ 1 ]C1.3.2—CO₂ → glucose
November 2023Paper2 ["SL"] · TZ26(a)[ 4 ]C1.3.2—CO₂ → glucose
November 2023Paper3 ["SL"] · TZ12(c)[ 2 ]C1.3.2—CO₂ → glucose
May 2023Paper3 ["SL"] · TZ11(d)[ 1 ]C1.3.2—CO₂ → glucose
November 2021Paper1 ["SL"] · TZ08[ 1 ]C1.3.2—CO₂ → glucose

Coverage 2017–2025 · Updated 15 Jul 2026

Carbon Comes from CO₂; Released Oxygen Comes from Water

Overall matter equation

6CO₂ + 6H₂O + light energy →
C₆H₁₂O₆ + 6O₂

Light-dependent reactions use water and release oxygen, while ATP and reduced NADP support carbon dioxide fixation in the stroma.
Atom or product Source and fate
carbon CO₂ is fixed and reduced into carbohydrate
hydrogen water supplies hydrogen carried by reduced NADP
released O₂ produced by photolysis of water, not removed from CO₂

Plants, algae and cyanobacteria carry out oxygenic photosynthesis. Glucose is a useful summary product, but fixed carbon also feeds the synthesis of many other compounds.

SL Checkpoint: Read Photosynthesis as Matter, Energy and Evidence

light energy → excited pigment electrons → ATP and reduced NADP → CO₂ reduced using hydrogen from water → organic carbon compounds; photolysis of water releases O₂

  • A chromatogram separates pigments; Rf compares their movement under matched conditions.
  • An absorption spectrum measures captured wavelengths; an action spectrum measures whole-process rate.
  • A limiting-factor experiment changes one condition, controls the others and measures O₂ production or CO₂ uptake per unit time.

When a rate curve plateaus, identify which factor was relieved and which may now be limiting. When evaluating CO₂ enrichment, separate strong control from ecological realism and do not extrapolate beyond the tested conditions.

CO₂ → glucose

[ 4 ]

Outline how photosynthesis produces glucose.

Concept essentials

  • Carbon dioxide is the source of carbon for photosynthetic carbohydrates.
  • Light supplies the energy needed to convert carbon dioxide into reduced carbon compounds.
  • Glucose can be stored as starch or converted into other organic molecules.
  • Photosynthetic carbon dioxide uptake can change pH in aquatic systems.