Educerie · IB Diploma · Biology
Theme A — Topic test
Paper 2 style · 20 marks · 30 minutes
Answer all questions. The number of marks indicates the number of distinct points expected.
1. (a) Outline two properties of water that result from its polarity. [2]
(b) Explain why methane, a molecule of similar size to water, is a gas at room temperature while water is a liquid. [3]
2. (a) State the base that pairs with guanine in DNA, and the number of hydrogen bonds between them. [1]
(b) Distinguish between the structure of a prokaryotic and a eukaryotic cell. [3]
(c) Outline two pieces of evidence that mitochondria evolved from free-living prokaryotes. [2]
3. A biologist sequences a section of the same gene in four primate species and counts the number of base differences from humans.
| Species | Base differences from human |
|---|---|
| Chimpanzee | 4 |
| Gorilla | 9 |
| Orangutan | 21 |
| Gibbon | 34 |
(a) Deduce which species shares the most recent common ancestor with humans, using the data. [2]
(b) Suggest one limitation of using a single gene to determine evolutionary relationships. [2]
4. (a) Define the term species. [1]
(b) Explain how allopatric speciation can occur. [4]
Total: 20 marks
Command terms in this test. Question 1(b) and 4(b) say explain, so a description of what happens will not score — the mechanism or reason is required. Question 2(b) says distinguish, which must be written as a comparison, not two separate accounts. Question 3(a) says deduce, so your answer must refer to the data in the table. Question 3(b) says suggest — the answer is not in the syllabus and you are expected to reason.
Educerie · original questions written against the published IB syllabus structure for Biology Theme A, first assessment 2025. All data are our own. Last reviewed 5 September 2026.