Explain how the structure of cilia differs from that of prokaryotic flagella, and suggest why this difference is significant for our understanding of the evolution of eukaryotes.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Cilia are microtubule‑based organelles with a 9+2 arrangement of axonemal microtubules, surrounded by the plasma membrane and powered by ATP‑dependent dynein motors that slide the microtubules to produce bending motion. Prokaryotic flagella are protein filaments made of flagellin, lack microtubules, are not membrane‑bound and rotate like a motor driven by a proton‑motive force. The structural difference shows that cilia did not evolve directly from prokaryotic flagella; instead, eukaryotic motility structures arose independently, supporting theories of endosymbiosis and increased cellular complexity.
Examiner tips
- Use the 9+2 axoneme detail to gain 1 mark; mention membrane‑bound vs extracellular for 1 mark; describe dynein sliding and proton‑motive rotation for 1 mark; link to evolutionary significance for 1 mark.
Common mistakes
- Confusing cilia with eukaryotic flagella; describing prokaryotic flagella as microtubule‑based; omitting the membrane‑bound nature of cilia; failing to connect structure to evolutionary implications.
Mark scheme (4 marks)
- Cilia (and eukaryotic flagella) are composed of microtubules arranged in a 9+2 arrangement, whereas prokaryotic flagella are composed of the protein flagellin (a hollow protein filament with no microtubules).
- Cilia are membrane-bound extensions of the cytoplasm and are enclosed by the plasma membrane, whereas prokaryotic flagella are extracellular protein filaments not enclosed by a membrane.
- Cilia move by bending (using ATP-powered dynein motor proteins sliding microtubules), whereas prokaryotic flagella rotate like a motor driven by a proton gradient / proton-motive force.
- The fundamental difference in structure (microtubule-based vs. flagellin-based) indicates cilia did not evolve directly from prokaryotic flagella, supporting the idea that eukaryotic cell complexity arose through endosymbiosis or other independent evolutionary pathways rather than simple descent from prokaryotic motility structures.
Key terms in this question
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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