Explain how oxygen is produced as a by-product during the light-dependent reactions of photosynthesis.

IB DP Biology Higher Level (2023 syllabus) — C1.3 Photosynthesis · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Water is split (photolysed) in the thylakoid lumen at Photosystem II by light energy absorbed by chlorophyll.
The reaction 2H₂O → 4H⁺ + 4e⁻ + O₂ releases electrons and protons that replace those lost from oxidised P680.
The electrons travel through the electron‑transport chain, while the protons contribute to the proton gradient.
The oxygen produced is released as a waste product of the photolysis step.

Examiner tips

  • Show the photolysis reaction explicitly; link light absorption to electron loss at P680.
  • Mention the thylakoid lumen and Photosystem II to demonstrate location.
  • Explain that O₂ is a waste product, not used in the chain.
  • Use correct chemical equation and terms (photolysis, oxidised P680).

Common mistakes

  • Forgetting to specify Photosystem II or the thylakoid lumen.
  • Confusing the source of electrons (water) with the source of protons (ATP synthase).
  • Writing the reaction incorrectly (e.g., 2H₂O → 4H⁺ + 4e⁻ + 2O₂).

Mark scheme (4 marks)

  1. Water molecules are split (photolysed) in the light-dependent reactions
  2. Photolysis occurs in the thylakoid / at Photosystem II, using light energy absorbed by chlorophyll
  3. Electrons (and protons/H⁺) released from water replace those lost by oxidised P680 / Photosystem II
  4. Oxygen is released as a waste product when the water molecule is split: 2H₂O → 4H⁺ + 4e⁻ + O₂

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