Classify organisms into three domains
Place organisms in Archaea, Bacteria or Eukarya and distinguish domains from kingdoms.
Section 18.1 · Classification
A Level; Cambridge International Biology 9700, 2028–2030. Written responses are self-checked; this does not certify supervised fieldwork or practical competence.
Before you begin
- Classification compares shared characteristics; populations occupy environments and contain variation.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which domains contain prokaryotes?
What you will learn
- Name the three domains
- State which domains contain prokaryotes
Build the explanation
The three domains are Archaea, Bacteria and Eukarya. Archaea and Bacteria contain prokaryotic organisms. Eukarya contains organisms whose cells have a nucleus, including animals, plants, fungi and protoctists. Domain is the broad level used here before the classification of groups within Eukarya.
Use evidence within scope
Do not place viruses in one of the three cellular domains: viruses are not cells. Do not use “prokaryote” as a third domain; it describes the cell organisation found in Archaea and Bacteria.
Pause and explain
A learner proposes domains Archaea, Bacteria and “prokaryotes”. Correct the classification.
Worked example
A microscopic organism has no membrane-bound nucleus. Which two domains are consistent with this feature?
Show the worked solution
- No nucleus indicates prokaryotic organisation.
- Both Archaea and Bacteria are prokaryotes.
Answer Archaea or Bacteria; the absence of a nucleus alone does not distinguish between them.
Try a topic test
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A cell has a nucleus and mitochondria. Which domain does this support?
Give me a hint
Identify the required evidence and sequence the reasoning.
Compare my reasoning
- A nucleus is characteristic of eukaryotic cells.
- The domain is Eukarya.
Eukarya, because a nucleus and mitochondria are characteristic of eukaryotic cells.
Look for these in your work
- Relevant evidence selected
- Reasoning linked to conclusion
A prokaryote is found in a sample. Can its domain be identified from “no nucleus” alone?
Give me a hint
Use the supplied example or data to justify a conclusion.
Compare my reasoning
- Both prokaryotic domains lack a nucleus.
- Additional characteristics are needed.
No. It could belong to Archaea or Bacteria; the stated feature is shared by both domains.
Look for these in your work
- Concept applied
- Limit or assumption stated
A learner proposes domains Archaea, Bacteria and “prokaryotes”. Correct the classification.
Give me a hint
Apply the concept in a new context and qualify its limits.
Compare my reasoning
- Prokaryote describes cellular organisation.
- The third domain is Eukarya.
The three domains are Archaea, Bacteria and Eukarya. Archaea and Bacteria are prokaryotes; “prokaryote” is not a separate domain.
Look for these in your work
- New context handled
- Conclusion appropriately bounded
Common mistakes
- Treating Prokaryota as a fourth domain
- Putting viruses in Eukarya because they contain genetic material
What can you explain now?
Which domains contain prokaryotes?
Compare with the explanation
Archaea and Bacteria
Eukarya contains cells with a nucleus.
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Make it stick
What is the difference between a domain and the term prokaryote?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.