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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

Coverage 2010–2024 · 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.

Oxygen as by-product

[ 1 ]

Which group(s) produce(s) oxygen as a by-product of photosynthesis?

I. Algae
II. Cyanobacteria
III. Fungi

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.