Educerie · IB Diploma · Biology
Theme A — Topic test · mark scheme
One mark per distinct point, to the maximum shown. Accept any valid alternative wording unless a specific term is required, in which case it is shown in bold. Do not award the same idea twice.
1Water — 5 marks
(a) [2 max] — any two of:
| Cohesion — water molecules attract one another / form hydrogen bonds with each other | 1 |
|---|---|
| Adhesion — water is attracted to other polar surfaces | 1 |
| Solvent — dissolves polar and ionic substances | 1 |
| High specific heat capacity / thermal stability | 1 |
| High latent heat of vaporisation — cooling by evaporation | 1 |
| Surface tension | 1 |
Outline requires only a brief account; a named property with a few words of expansion is sufficient. A bare list of one-word properties may be credited, but "it is a good solvent" without the word polar or ionic is borderline — accept it once.
(b) [3]
| Water is polar / methane is non-polar | 1 |
|---|---|
| Hydrogen bonds form between water molecules, but not between methane molecules | 1 |
| More energy is therefore needed to separate water molecules, so water has a higher boiling point and is liquid at room temperature | 1 |
The third mark requires the consequence to be stated. An answer that ends at "methane has no hydrogen bonds" has described the difference but not explained the observation, and scores 2. Reject "water is heavier" — the two molecules have similar molecular masses, which is the point of the comparison.
2Nucleic acids and cells — 6 marks
(a) [1]
| Cytosine, with three hydrogen bonds | 1 |
|---|
Both parts are required for the single mark.
(b) [3 max] — any three, each written as a comparison:
| Prokaryotes have no nucleus, whereas eukaryotes have a true nucleus with an envelope | 1 |
|---|---|
| Prokaryotic DNA is circular and naked, whereas eukaryotic DNA is linear and associated with histones | 1 |
| Prokaryotes lack membrane-bound organelles, whereas eukaryotes possess them | 1 |
| Prokaryotes have 70S ribosomes, whereas eukaryotes have 80S | 1 |
| Prokaryotes are typically smaller (1–5 µm vs 10–100 µm) | 1 |
A point written as two separate statements does not score. "Prokaryotes have no nucleus." followed later by "Eukaryotes have a nucleus." in a different sentence may be credited if the comparison is unambiguous, but candidates who write two disconnected paragraphs should be capped at 1.
(c) [2 max] — any two of:
| Mitochondria have their own circular DNA | 1 |
|---|---|
| They have 70S ribosomes, as prokaryotes do | 1 |
| They are surrounded by a double membrane | 1 |
| They divide independently of the cell, by binary fission | 1 |
3Molecular evidence — 4 marks
(a) [2]
| Chimpanzee | 1 |
|---|---|
| Because it has the fewest base differences (4), indicating least time since divergence | 1 |
The second mark is for using the data, as deduce requires. Naming the chimpanzee alone scores 1. An answer citing prior knowledge ("chimpanzees are our closest relatives") without reference to the table does not earn the second mark.
(b) [2 max] — any one point developed, or two of:
| Different genes mutate at different rates, so one gene may give a misleading estimate | 1 |
|---|---|
| A single gene is a very small sample of the genome | 1 |
| Some genes are highly conserved and show few differences even between distant species | 1 |
| Mutations can revert or occur repeatedly at the same site, so differences underestimate divergence | 1 |
| Horizontal gene transfer or convergent molecular evolution could mislead | 1 |
4Species and speciation — 5 marks
(a) [1]
| A group of organisms that can interbreed to produce fertile offspring | 1 |
|---|
The word fertile is required. "Can breed together" alone scores 0.
(b) [4 max]
| A population is separated by a geographical barrier | 1 |
|---|---|
| Gene flow between the two populations stops | 1 |
| The two populations experience different selection pressures / different mutations arise | 1 |
| Allele frequencies diverge over many generations | 1 |
| The populations can no longer produce fertile offspring, so are separate species | 1 |
Award to a maximum of 4. The sequence matters: an answer that jumps from separation directly to "they become different species" omits the mechanism and scores 2 at most.
Mark distribution
| Split | Allocation |
|---|---|
| By sub-topic | A1 Molecules 6 · A2 Cells 5 · A3 Organisms 4 · A4 Ecosystems 5 |
| By objective | AO1 knowledge 6 · AO2 application 6 · AO3 analysis and evaluation 8 |
| By command term | Explain 7 · Distinguish 3 · Outline 4 · Deduce 2 · Suggest 2 · State 1 · Define 1 |
Total: 20 marks
Educerie · original mark scheme for original questions, written against the published IB syllabus structure for Biology Theme A, first assessment 2025. Last reviewed 5 September 2026.