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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
Practice this objective

Coverage 2017–2025 · Updated 15 Jul 2026

Photosynthesis equation and oxygen

Simple source-flow diagram for the photosynthesis equation. Show CO2 flowing to the carbon skeleton of glucose/carbohydrate; H2O splitting into hydrogen used to reduce CO2 and oxygen released as O2; light energy shown as the input that drives the process.

The overall photosynthesis equation shows carbon dioxide and water converted to carbohydrate using light energy: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Carbon dioxide supplies the carbon skeleton and is reduced using hydrogen derived from water. Oxygen is released when water is split by photolysis in the light-dependent reactions, not directly from CO₂. Glucose represents a carbohydrate product; the products can be used to make starch, sucrose and other organic compounds.

  • Word equation: carbon dioxide + water → glucose + oxygen, using light energy.
  • Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
  • O₂ comes from photolysis of H₂O.
  • Plants, algae and cyanobacteria perform oxygenic photosynthesis.

CO₂ → glucose

Assessment in practice

1–4 marks
How it is assessed

This objective is assessed through structured response, commonly using State / Outline / Predict.

Command terms

State / Outline / Predict / Suggest

What earns marks

Build the answer around this relationship: Carbon dioxide is the source of carbon for photosynthetic carbohydrates.

Representative question

Question 1

[Maximum number: 4]

Outline how photosynthesis produces glucose.

SL Transfer: Explain Core Photosynthesis

Photosynthesis converts light energy into chemical energy in carbon compounds. Carbon dioxide is reduced to carbohydrate using hydrogen from water, glucose is the main stored product, and released oxygen comes from photolysis of water. Pigment evidence is tested with chromatography and Rf, spectra questions separate absorption from action, and rate/evidence questions use limiting factors, controls, and CO2 enrichment context.

  • Light becomes chemical energy in carbon compounds through chlorophyll-containing photoautotrophs.
  • Carbon in carbohydrate comes from CO2; released oxygen comes from photolysis of water.
  • Chromatography separates pigments using Rf; absorption spectra and action spectra measure different things.
  • Limiting-factor and CO2 enrichment questions require variables, controls, and realistic interpretation.

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.
ConceptIB Biology SL