# 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.

> IB DP Biology Higher Level (2023 syllabus) — A2.2 Cell structure · Explain · 4 marks

## Mark scheme (4 marks)

1. 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).
2. 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.
3. 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.
4. 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

- [cilia](https://www.gradenine.co.uk/glossary/cilia)
- [prokaryotic flagella](https://www.gradenine.co.uk/glossary/prokaryotic-flagella)

## Related

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