Explain why all animals, plants, fungi and protoctists are placed together in the domain Eukarya, whilst prokaryotic organisms are placed in separate domains in the three-domain system.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
All organisms in the domain Eukarya are eukaryotic – their cells contain a true, membrane‑bound nucleus and other membrane‑bound organelles.
Prokaryotic organisms (bacteria and archaea) lack a nucleus and have no membrane‑bound organelles.
Because bacteria and archaea are sufficiently different – for example they have distinct RNA base sequences and archaea are primitive bacteria that thrive in extreme environments – they are placed in separate domains.
The three‑domain system therefore groups organisms by fundamental cell structure and deeper evolutionary relationships, replacing the five‑kingdom split of prokaryotes and eukaryotes with a grouping based on molecular differences.
Prokaryotic organisms (bacteria and archaea) lack a nucleus and have no membrane‑bound organelles.
Because bacteria and archaea are sufficiently different – for example they have distinct RNA base sequences and archaea are primitive bacteria that thrive in extreme environments – they are placed in separate domains.
The three‑domain system therefore groups organisms by fundamental cell structure and deeper evolutionary relationships, replacing the five‑kingdom split of prokaryotes and eukaryotes with a grouping based on molecular differences.
Examiner tips
- Use the word ‘eukaryotic’ and mention the nucleus; mention lack of nucleus for prokaryotes; state the reason for separate domains – different RNA sequences and extreme‑environment adaptation; link to three‑domain system’s basis on cell structure and evolution.
Common mistakes
- Confusing the domains with the five kingdoms; forgetting to mention the nucleus; not explaining why bacteria and archaea are separate domains.
Mark scheme (4 marks)
- All organisms in Eukarya are eukaryotic / have a nucleus (in their cells)
- Prokaryotic organisms (bacteria and archaea) do not have a nucleus / have no membrane-bound nucleus
- Bacteria and Archaea are placed in separate domains because they are sufficiently different from each other / have different RNA base sequences / Archaea are primitive bacteria found in extreme environments
- The three-domain system groups organisms by fundamental cell structure / shared cellular features, which means the five-kingdom system's split of prokaryotes and eukaryotes is replaced by a grouping based on deeper evolutionary / molecular relationships
Key terms in this question
Related
- All Edexcel GCSE Biology (1BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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