Explain how the endosymbiotic origin of chloroplasts is supported by evidence from their structure and molecular biology.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
1. Chloroplasts have a double membrane – the inner membrane comes from the cyanobacterium and the outer from the host cell’s phagocytic membrane, showing engulfment.
2. They contain circular DNA without histones, just like prokaryotic chromosomes, not linear eukaryotic DNA.
3. Chloroplasts have 70S ribosomes, the same size as bacterial ribosomes, rather than the 80S ribosomes of the cytoplasm.
4. They divide by binary fission independently of the host cell’s division, a prokaryotic mode of replication.
2. They contain circular DNA without histones, just like prokaryotic chromosomes, not linear eukaryotic DNA.
3. Chloroplasts have 70S ribosomes, the same size as bacterial ribosomes, rather than the 80S ribosomes of the cytoplasm.
4. They divide by binary fission independently of the host cell’s division, a prokaryotic mode of replication.
Examiner tips
- Use the exact terms – double membrane, circular DNA, 70S ribosomes, binary fission.
- Show each point clearly and link it to the endosymbiotic theory.
- Keep the answer concise – one sentence per point.
- Avoid extra wording such as ‘this shows’ – just state the evidence and its significance.
Common mistakes
- Confusing 80S with 70S ribosomes; students often write 80S. Mistaking chloroplast DNA for linear eukaryotic DNA. Forgetting to mention the double membrane or binary fission.
Mark scheme (4 marks)
- Chloroplasts have a double membrane, consistent with engulfment of a prokaryote by a host cell (inner membrane derived from the cyanobacterium, outer from the host's vacuole/phagocytic membrane).
- Chloroplasts contain their own circular DNA (lacking histones), resembling prokaryotic chromosomes rather than eukaryotic linear chromosomes.
- Chloroplasts possess 70S ribosomes (as in prokaryotes) rather than the 80S ribosomes found in the eukaryotic cytoplasm, supporting a prokaryotic ancestry.
- Chloroplasts reproduce by binary fission independently of cell division, consistent with their descent from free-living prokaryotes.
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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