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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
HL

Exam analysis

Chance of appearing2%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper1A
Typical marks1

Common command terms

  • State

Recent exam appearances

May 2025Paper1A ["HL"] · TZ37[ 1 ]C1.3.3—Oxygen as by-product
November 2019Paper2 ["HL"] · TZ04(b)(i)[ 1 ]C1.3.3—Oxygen as by-product
Practice this objective

Coverage 2019–2025 · 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 HL