IB Biology C4.1 Populations and communities: practice questions

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C4.1-01 · Paper 1A style · multiple choice · 1 mark

Benefits in root nodule mutualism

Bacteria of the genus Rhizobium live inside nodules on the roots of pea plants, which belong to the legume family (Fabaceae).

Which row shows what each partner gains from this relationship? [1]

1. Pea plant gains: carbon dioxide; bacteria gain: oxygen
2. Pea plant gains: sugars; bacteria gain: ammonium ions
3. Pea plant gains: ammonium ions made from nitrogen gas; bacteria gain: sugars and other carbon compounds
4. Pea plant gains: protection from herbivores; bacteria gain: nitrate absorbed from the soil

Show the answer and the reasoning

Answer: C. C is correct. The bacteria fix nitrogen gas from the air in the soil, converting it to ammonium, which the plant uses to make amino acids and other nitrogen compounds. In return the plant supplies the bacteria with sugars and other carbon compounds made in photosynthesis, and a protected place to live inside the nodule. Both partners benefit, so this is mutualism. B reverses the direction of the exchange. A names gases that are not what the partners trade: the plant takes in its carbon dioxide from the air through its leaves. D is wrong because the nodules do not protect the plant from herbivores, and the bacteria's gain is a supply of carbon compounds, not soil nitrate.

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The 5 questions in C4.1

1. C4.1-01 Benefits in root nodule mutualismPaper 1A style · multiple choice · 1 mark
2. C4.1-02 Top-down control of grazersPaper 1A style · multiple choice · 1 mark
3. C4.1-03 Lost marks in a recapture studyPaper 1A style · multiple choice · 1 mark
4. C4.1-04 An invasive plant on riverbanksPaper 2 style · short answer · 7 marks
5. C4.1-05 Estimating a beetle populationPaper 1B style · data-based · 9 marks

More of C Interaction and interdependence

• C1.1 Enzymes and metabolism
• C1.2 Cell respiration
• C1.3 Photosynthesis
• C2.1 Chemical signalling
• C2.2 Neural signalling
• C3.1 Integration of body systems
• C3.2 Defence against disease
• C4.2 Transfers of energy and matter

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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.