Explain how bacteria, archaea, and eukaryotes are distinguished from one another at the domain level of classification.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Bacteria and archaea are both prokaryotes, lacking a membrane‑bound nucleus, whereas eukaryotes possess a nucleus.
Archaea differ from bacteria in their cell‑membrane lipids: archaea have ether‑linked lipids, while bacteria have ester‑linked lipids.
Bacterial cell walls contain peptidoglycan; archaea lack peptidoglycan.
Molecular data (e.g. rRNA gene sequences) show that archaea are more closely related to eukaryotes than to bacteria, supporting the three‑domain system.
Archaea differ from bacteria in their cell‑membrane lipids: archaea have ether‑linked lipids, while bacteria have ester‑linked lipids.
Bacterial cell walls contain peptidoglycan; archaea lack peptidoglycan.
Molecular data (e.g. rRNA gene sequences) show that archaea are more closely related to eukaryotes than to bacteria, supporting the three‑domain system.
Examiner tips
- Use the exact terms – prokaryote, eukaryote, ether‑linked, ester‑linked, peptidoglycan, rRNA
Common mistakes
- Confusing the presence of a nucleus with prokaryote/eukaryote distinction; mixing up lipid linkages; omitting the molecular evidence point
Mark scheme (4 marks)
- Bacteria and Archaea are both prokaryotes (lacking a membrane-bound nucleus), whereas Eukaryotes possess a membrane-bound nucleus.
- Archaea are distinguished from Bacteria by differences in their cell membrane composition, specifically the presence of ether-linked lipids in Archaea (compared to ester-linked lipids in Bacteria).
- Bacteria have peptidoglycan in their cell walls, whereas Archaea lack peptidoglycan.
- Molecular evidence (such as rRNA / ribosomal gene sequence comparisons) shows that Archaea are more closely related to Eukaryotes than to Bacteria, justifying three separate domains.
Key terms in this question
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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