# Explain how the endosymbiotic theory accounts for the presence of mitochondria in eukaryotic cells.

> IB DP Biology Standard Level (2023 syllabus) — A2.1 Origins of cells · Explain · 4 marks

## Mark scheme (4 marks)

1. An ancestral prokaryote (host cell) engulfed a smaller aerobic prokaryote by endocytosis / phagocytosis, forming a symbiotic relationship
2. The engulfed prokaryote was not digested but persisted inside the host, providing an advantage such as aerobic respiration / ATP production to the host
3. Evidence: mitochondria have their own circular DNA (similar to prokaryotes) and replicate by binary fission
4. Evidence: mitochondria are surrounded by a double membrane, the inner membrane originating from the prokaryote's plasma membrane and the outer from the host's vesicle / endocytic membrane

## Key terms

- [endosymbiotic theory](https://www.gradenine.co.uk/glossary/endosymbiotic-theory)
- [mitochondria](https://www.gradenine.co.uk/glossary/mitochondria)

## Related

- [Revision notes for IB DP Biology Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-endosymbiotic-theory-accounts-36fc4df5) · Published by Druglandscape Ltd.