Compare prokaryotes with protoctists
Use nucleus organisation and avoid treating all single-celled organisms as one group.
Extended only (Supplement); Biology 0610, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked
Before you begin
- Read the stated observations before choosing a group.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which observation supports a prokaryote?
What you will learn
- Describe prokaryote and protoctist features.
- Classify supplied cell observations in the five-kingdom scheme.
One cell does not imply one kingdom
Prokaryotes include bacteria. Their cells lack a membrane-bound nucleus; genetic material is in the cytoplasm, typically including a circular DNA molecule. They have a membrane, cytoplasm and ribosomes, and commonly a non-cellulose cell wall. Some have additional plasmids.
Protoctists have cells with a nucleus. Many are unicellular, but the group is diverse. Some, such as photosynthetic examples, have chloroplasts; others obtain organic nutrients. Do not require every protoctist to have chloroplasts or one identical feeding method.
Use several observations together
A bacterium and a unicellular protoctist can both be small, but nucleus organisation separates them in this scheme. Absence of a clearly visible nucleus in a low-resolution image is not proof that no nucleus exists; rely on sufficiently clear evidence or an explicit statement.
For the five kingdoms, compare nucleus organisation, wall material, multicellular organisation and nutrition. Animals, plants and fungi all have nuclei; prokaryotes lack the membrane-bound nucleus, while protoctists are nucleated and diverse. The earlier fungal lesson supplies the full five-group comparison.
Pause and explain
Which statement handles protoctist diversity?
Worked example
A supplied unicellular organism has a membrane-bound nucleus and chloroplasts. Is it a prokaryote?
Show the worked solution
- The nucleus contradicts prokaryote organisation.
- Some protoctists have chloroplasts.
- The stated combination supports a photosynthetic protoctist in this exercise.
Answer Protoctist, not prokaryote, on the supplied evidence.
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Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
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 bacterium is described with circular DNA in cytoplasm, ribosomes and no membrane-bound nucleus. Classify it.
Give me a hint
Use cell organisation.
Compare my reasoning
- The DNA is not enclosed in a nucleus.
- The stated structures fit bacteria.
- Bacteria are prokaryotes in this scheme.
Prokaryote.
Look for these in your work
- Use nucleus absence.
- Place bacteria in the correct kingdom.
A single-celled organism has a nucleus but no chloroplasts and takes in organic nutrients. Can it be a protoctist?
Give me a hint
Photosynthesis is not universal in the group.
Compare my reasoning
- A nucleus fits protoctists.
- No chloroplasts is possible.
- The supplied nutrition fits a non-photosynthetic example.
Yes, on the supplied protoctist features.
Look for these in your work
- Allow nutritional diversity.
- Do not equate one cell with a bacterium.
A blurred micrograph does not show a nucleus. Why is prokaryote classification premature?
Give me a hint
Distinguish resolution from true absence.
Compare my reasoning
- A structure may be too small or unclear to resolve.
- Lack of visibility is not established absence.
- Use clearer imaging or other supplied genetic and structural evidence.
The image is insufficient to establish absence of a nucleus.
Look for these in your work
- State the observation limitation.
- Give a specific way to strengthen evidence.
Common mistakes
- Not every unicellular organism is a bacterium.
- An invisible structure may be unresolved rather than absent.
What can you explain now?
Does a protoctist always need chloroplasts?
Compare with the explanation
No.
Protoctists have varied nutrition; some photosynthesise and others do not.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain why nucleus organisation is more informative than single-celled size alone.
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.