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A-Level Biology May/June 2025 Q9(a): Two of the domains contain only prokaryotic species. With reference to the two prokaryo…
A-Level Biology · Paper 9700/41 · May/June 2025 · Question 9(a) · [3 marks]
Two of the domains contain only prokaryotic species. With reference to the two prokaryotic domains, describe the features used to classify prokaryotic species into two different domains.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
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The two prokaryotic domains are Bacteria and Archaea.
They are classified into separate domains based on several key biochemical differences:
- Cell Wall Composition: The cell walls of Bacteria contain peptidoglycan, whereas the cell walls of Archaea do not contain peptidoglycan.
- Cell Membrane Lipids: In Bacteria, the lipids in the cell membrane have fatty acids joined to glycerol by ester linkages. In Archaea, the lipids have hydrocarbons joined to glycerol by ether linkages.
- Ribosomal RNA (rRNA): Bacteria have one type of RNA polymerase, while Archaea have three types, which are more similar to those found in eukaryotes.
How the marks are awarded
- M1 — Correctly identifying the two prokaryotic domains as Bacteria and Archaea.
- M1 — Describing the difference in cell wall composition, stating that Bacteria have peptidoglycan while Archaea do not.
- M1 — Describing the difference in cell membrane lipid structure, contrasting the ester linkages in Bacteria with the ether linkages in Archaea.
Common mistakes
- Confusing the two domains (Bacteria, Archaea) with the old kingdom classification (Prokaryotae).
- Giving a vague comparison, such as 'they have different cell walls', without specifying the key difference (presence or absence of peptidoglycan).
- Incorrectly assigning features, for example, stating that Archaea have peptidoglycan.
- Only describing the features of one domain without making a direct comparison to the other.
Examiner tip: For classification questions, create summary tables comparing key biochemical features (like cell walls, membrane lipids, and rRNA) across different domains to recall specific, comparative details under exam pressure.
AI-generated model answer, grounded in the official Cambridge mark scheme and reviewed by the MarkScheme team. Mark your own answer to this question →
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