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Practice · Biology · 2025 syllabus · B3.1

IB Biology B3.1 Gas exchange: practice questions

5 exam-style questions on B3.1 Gas exchange, 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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B3.1-02 · Paper 1A style · multiple choice · 1 mark

Muscles in inspiration

Which events occur during inspiration (breathing in) at rest? [1]

  1. Diaphragm contracts and flattens; external intercostal muscles contract; pressure in the thorax falls
  2. Diaphragm relaxes and domes upwards; external intercostal muscles contract; pressure in the thorax rises
  3. Diaphragm contracts and flattens; external intercostal muscles relax; pressure in the thorax rises
  4. Diaphragm relaxes and domes upwards; external intercostal muscles relax; pressure in the thorax falls
Show the answer and the reasoning

Answer: A. A is correct. In inspiration the diaphragm contracts and flattens and the external intercostal muscles contract, pulling the ribs up and out. The volume of the thorax increases, so the pressure inside falls below atmospheric pressure and air flows in. B and D give the diaphragm's position at rest or during expiration. C has the right diaphragm movement, but if the volume increases the pressure must fall, not rise; air always moves from higher to lower pressure. Note that at rest expiration is mostly passive; the internal intercostal and abdominal muscles contract only in forced expiration.

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The 5 questions in B3.1

  1. B3.1-01 Reading a lung volume tracePaper 1A style · multiple choice · 1 mark
  2. B3.1-02 Muscles in inspirationPaper 1A style · multiple choice · 1 mark
  3. B3.1-03 Carbon dioxide and oxygen releasePaper 1A style · multiple choice · 1 mark · HL
  4. B3.1-04 Alveoli and emphysemaPaper 2 style · short answer · 5 marks
  5. B3.1-05 Stomatal density from leaf castsPaper 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.