IB DP Biology Theme A: Unity and Diversity — Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — the IB holds copyright in its own papers. Use these alongside the official past papers available through your school or the IB store.

Related: Theme A: Unity and Diversity revision notes

---

Section A

1. Explain, in terms of molecular structure, why water is described as a polar molecule. [2]

2. State the four components of a nucleotide. [2]

3. Name two features that distinguish a eukaryotic cell from a prokaryotic cell. [2]

Section B

4. A student compares the incidence of a particular disease across ten countries and finds it correlates strongly with average dietary sugar intake.

(a) Explain why this correlation alone does not prove that dietary sugar intake causes the disease. [2] (b) Suggest one other variable that could account for the pattern observed. [1]

5. Outline the three-step logic connecting variation, natural selection and reproductive isolation to the formation of a new species. [3]

6. A conservation organisation is assessing threats to biodiversity in a coastal wetland ecosystem.

(a) Name two named categories of threat to biodiversity that could apply to this ecosystem. [2] (b) Name two named categories of conservation response that could address these threats. [2]

7. (HL only) A cladogram shows that species X and species Y share a more recent common ancestor with each other than either does with species Z, even though X and Z appear more similar in external body shape. Explain what this shows about the limitations of classifying organisms by surface similarity alone. [3]

8. A virus is described in a textbook as “non-living.” State two features of viruses that support this description, and explain what viruses depend on host cells for. [3]

9. (HL) Outline one hypothesis for how the first living cells could have arisen from non-living chemistry, and state what kind of evidence this hypothesis draws on. [2]

---

Answers

1. The oxygen atom in a water molecule attracts the shared electrons in each O–H bond more strongly than the hydrogen atoms do [1], giving the molecule an uneven charge distribution — a partial negative charge near the oxygen and partial positive charges near the hydrogens [1].

2. A phosphate group, a pentose sugar, and a nitrogenous base [2 — award 2 for all three, 1 for two correct components; the fourth mark point some specifications credit is the way these three link into a repeating polymer unit].

3. Any two of: presence of a membrane-bound nucleus; presence of membrane-bound organelles; circular (prokaryotic) vs linear (eukaryotic) chromosome structure; typical cell size (eukaryotic cells are generally larger) [1 mark each, max 2].

4. (a) A correlation shows that two variables change together but does not on its own establish that one causes the other [1]; other variables that differ between the countries compared (for example genetics, activity levels, or access to healthcare) could also explain the pattern, so causation cannot be confirmed from this data alone [1]. (b) Any plausible confounding variable, e.g. differences in physical activity levels, genetic predisposition, or healthcare access between the countries [1].

5. Variation exists within a population as a starting condition [1]; natural selection acts on that variation because some variants survive and reproduce more successfully in a given environment than others [1]; if a population becomes reproductively isolated (for example geographically), the isolated groups can diverge genetically over time until they become distinct enough to be considered separate species [1].

6. (a) Any two of: habitat loss, invasive species, overexploitation, climate change, pollution [1 mark each, max 2]. (b) Any two of: protected areas, legal frameworks, captive breeding, restoration [1 mark each, max 2].

7. A cladogram groups organisms by shared derived characteristics reflecting evolutionary relationships (most recent common ancestor), rather than by how similar they look on the surface [1]; species X and Y being more closely related to each other than to Z, despite X and Z looking more alike, shows that surface similarity can be misleading — two distantly related species can independently evolve similar features (for example through convergent evolution), while two closely related species can look quite different [1]; this demonstrates why cladistic classification, based on shared derived characteristics, can produce a genuinely different — and more evolutionarily accurate — grouping than one based only on physical resemblance [1].

8. Any two supporting features, e.g. viruses have no independent metabolism of their own, and cannot reproduce independently outside a host cell [1 mark each, max 2]; viruses depend entirely on host-cell machinery (such as ribosomes and enzymes) to replicate their genetic material and construct new virus particles [1].

9. Accept any specification-consistent outline (for example, a hypothesis proposing that simple organic molecules formed under early-Earth conditions and gradually assembled into self-replicating systems enclosed by a membrane) [1]; evidence typically drawn on includes laboratory experiments simulating early-Earth chemical conditions, or comparative biochemistry across simple modern organisms, rather than direct fossil evidence of the very first cells, which have not been preserved [1].

---

Where marks are usually lost

• Stating a correlation “proves” causation, rather than explaining specifically why other variables cannot be ruled out.
• Naming only one component of a nucleotide’s structure instead of all three (phosphate, sugar, base).
• Giving a vague answer on speciation (“evolution happens over time”) instead of the specific three-step variation-selection-isolation sequence the syllabus expects.
• On HL cladogram questions, describing the cladogram’s finding without explaining why it shows surface similarity can be misleading — the explanation, not just the observation, carries the marks.

Official syllabus

International Baccalaureate Organization, Diploma Programme Subject Brief – Sciences: Biology, first assessment 2025, published January 2022 – the same source cited by the Theme A revision notes and the full syllabus guide.