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

> IB DP Biology Higher 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 rather than digesting it.
2. The inner membrane of mitochondria is homologous to the plasma membrane of the engulfed prokaryote / mitochondria are surrounded by a double membrane consistent with endocytotic uptake.
3. Mitochondria contain their own circular DNA (and 70S ribosomes) similar to prokaryotes, providing evidence of a formerly free-living bacterial ancestor.
4. The relationship was mutually beneficial: the host provided protection / organic nutrients, while the endosymbiont's aerobic respiration supplied ATP to the host, leading to integration over evolutionary time.

## Key terms

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

## Related

- [Revision notes for IB DP Biology Higher 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-0e2b6c90) · Published by Druglandscape Ltd.