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C1.3.3—Oxygen as by-product

Oxygen released during photosynthesis is produced when light energy splits water molecules, not when carbon dioxide is broken apart, connecting the mechanism to observable photosynthesis evidence.

Syllabus
First assessment 2025
Objective
C1.3.3
Level
SL

Exam analysis

Chance of appearing4%of analysed past papers
Latest appearanceNovember 2024
Most common paperPaper1
Typical marks1

Common command terms

  • State

Recent exam appearances

November 2024Paper1 ["SL"] · TZ18[ 1 ]C1.3.3—Oxygen as by-product
May 2013Paper1 ["SL"] · TZ113[ 1 ]C1.3.3—Oxygen as by-product
May 2012Paper1 ["SL"] · TZ112[ 1 ]C1.3.3—Oxygen as by-product
May 2011Paper1 ["SL"] · TZ113[ 1 ]C1.3.3—Oxygen as by-product
May 2010Paper2 ["SL"] · TZ22(d)[ 2 ]C1.3.3—Oxygen as by-product
Practice this objective

Coverage 2010–2024 · Updated 15 Jul 2026

Concept essentials

  • Photolysis splits water during the light-dependent reactions.
  • Released oxygen originates from water molecules.
  • Carbon dioxide supplies carbon for organic compounds, not released oxygen.
  • Algae and cyanobacteria can produce oxygen as a photosynthetic by-product.
ConceptIB Biology SL