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Practice · Biology · 2025 syllabus · C1.3

IB Biology C1.3 Photosynthesis: practice questions

5 exam-style questions on C1.3 Photosynthesis, SL and HL, 1 marked HL: 3 multiple choice, 1 data-based and 1 short answer. Each one comes with the mark scheme, a model answer and the reasoning behind every mark. Free with your email.

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C1.3-01 · Paper 1A style · multiple choice · 1 mark

Source of oxygen in photosynthesis

What is the source of the oxygen released by plants during photosynthesis? [1]

  1. Carbon dioxide, when carbon is fixed in the Calvin cycle
  2. Glucose, when it is converted to starch for storage
  3. Water, when it is split using energy from light
  4. RuBP, when it is regenerated from triose phosphate
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Answer: C. C is correct. Water molecules are split using energy absorbed from light; the hydrogen (as protons and electrons) is used in photosynthesis and the oxygen is released as a by-product. A is the most common misconception: the oxygen atoms from carbon dioxide end up in the carbohydrate (and water) made, not in the oxygen gas. B and D describe real processes, but neither releases oxygen gas.

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The 5 questions in C1.3

  1. C1.3-01 Source of oxygen in photosynthesisPaper 1A style · multiple choice · 1 mark
  2. C1.3-02 Identifying a limiting factorPaper 1A style · multiple choice · 1 mark
  3. C1.3-03 Triose phosphate in the Calvin cyclePaper 1A style · multiple choice · 1 mark · HL
  4. C1.3-04 Carbon dioxide enrichment of ricePaper 2 style · short answer · 6 marks
  5. C1.3-05 Pigments in sun and shade leavesPaper 1B style · data-based · 9 marks

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Every Biology and Chemistry question here is original, written for practice in the style of the papers set since the 2025 syllabus, with invented data. Developed independently from the International Baccalaureate Organization, which has not endorsed it. Found a mistake? Email vytis@limeadmissions.com with the question id.