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

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

> Chloroplasts are membrane-bound organelles found in the cells of plants and algae. They contain their own DNA and ribosomes, and are capable of self-replication.

## Mark scheme (4 marks)

1. An ancestral prokaryote (host cell) engulfed a photosynthetic prokaryote by endocytosis / phagocytosis, rather than digesting it.
2. A mutually beneficial (endosymbiotic) relationship developed, with the photosynthetic prokaryote providing organic molecules / sugars to the host and the host providing protection / a stable environment.
3. Evidence for this origin includes the presence of double membranes surrounding chloroplasts, consistent with engulfment by endocytosis forming an outer membrane.
4. Additional evidence includes chloroplasts possessing their own circular DNA and 70S ribosomes, similar to free-living prokaryotes.

## Key terms

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

## 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-4940efb8) · Published by Druglandscape Ltd.