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Practice · Biology · 2025 syllabus · B4.1

IB Biology B4.1 Adaptation to environment: practice questions

5 exam-style questions on B4.1 Adaptation to environment, SL and 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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B4.1-03 · Paper 1A style · multiple choice · 1 mark

Sediment and coral reefs

A coral reef grows at a depth of 10 m off a tropical coast. After a nearby forest was cleared, large amounts of soil were washed into the sea, and the reef began to decline.

Which is the most likely explanation for the decline? [1]

  1. The sediment raised the pH of the water, dissolving the coral skeletons.
  2. The sediment increased the salinity beyond the tolerance of the corals.
  3. The sediment made the water turbid, reducing light for photosynthetic algae in the coral tissues.
  4. The sediment lowered the water temperature below the range that corals tolerate.
Show the answer and the reasoning

Answer: C. C is correct. Reef-building corals depend on mutualistic algae (zooxanthellae) living in their tissues, which need light for photosynthesis; this is why reefs form only in shallow, clear water. Suspended soil particles make the water turbid, cutting the light reaching the algae. A is wrong on two counts: soil does not raise pH, and it is a lower pH (acidification) that hinders the deposition of calcium carbonate. B is wrong because run-off from land carries fresh water, which would lower salinity if anything. D is wrong because sediment does not cool the water; the reef is still in tropical surface water.

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The 5 questions in B4.1

  1. B4.1-01 Where coral reefs can formPaper 1A style · multiple choice · 1 mark
  2. B4.1-02 Climate and biomesPaper 1A style · multiple choice · 1 mark
  3. B4.1-03 Sediment and coral reefsPaper 1A style · multiple choice · 1 mark
  4. B4.1-04 Life in hot desertsPaper 2 style · short answer · 6 marks
  5. B4.1-05 Salt marsh transectPaper 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.