IB Biology D2.3 Water potential: practice questions

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D2.3-01 · Paper 1A style · multiple choice · 1 mark

Contractile vacuoles

Why does a freshwater unicellular organism such as Paramecium need a contractile vacuole? [1]

1. Water continually leaves the cell by osmosis because the cytoplasm is hypotonic to the pond water.
2. Water continually enters the cell by osmosis because the cytoplasm is hypertonic to the pond water.
3. The vacuole stores water so that the cell can survive if the pond dries out.
4. Water is actively pumped into the cell, so the excess must be removed.

Show the answer and the reasoning

Answer: B. B is correct. The cytoplasm has a higher solute concentration than fresh water, so water moves in by osmosis all the time; without a wall to resist swelling, the cell would burst unless the vacuole pumped the water out. A reverses both the direction and the terms. C gives the vacuole a storage role it does not have. D is wrong because water enters passively by osmosis; it is the removal that uses energy.

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The 5 questions in D2.3

1. D2.3-01 Contractile vacuolesPaper 1A style · multiple choice · 1 mark
2. D2.3-02 Swollen cheek cellsPaper 1A style · multiple choice · 1 mark
3. D2.3-03 Organs awaiting transplantPaper 1A style · multiple choice · 1 mark
4. D2.3-04 Pressure potential at equilibriumPaper 2 style · short answer · 6 marks · HL
5. D2.3-05 Finding the isotonic pointPaper 1B style · data-based · 8 marks

More of D Continuity and change

• D1.1 DNA replication
• D1.2 Protein synthesis
• D1.3 Mutation and gene editing
• D2.1 Cell and nuclear division
• D2.2 Gene expression
• D3.1 Reproduction
• D3.2 Inheritance
• D3.3 Homeostasis
• D4.1 Natural selection
• D4.2 Stability and change
• D4.3 Climate change

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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.