Compare prokaryotes with protoctists
Use nucleus organisation and avoid treating all single-celled organisms as one group.
Exam scope and assessment notes
Extended only (Supplement) · Biology 0970, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked; human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0970 has the same subject content as IGCSE 0610, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
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.
Try a topic test
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.
Your review choice appears on Today. Sign in to sync it across devices.
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.