# Explain how the endosymbiotic origin of chloroplasts is supported by evidence from their structure and molecular biology.

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

## Mark scheme (4 marks)

1. Chloroplasts have a double membrane, consistent with engulfment of a prokaryote by a host cell (inner membrane derived from the cyanobacterium, outer from the host's vacuole/phagocytic membrane).
2. Chloroplasts contain their own circular DNA (lacking histones), resembling prokaryotic chromosomes rather than eukaryotic linear chromosomes.
3. Chloroplasts possess 70S ribosomes (as in prokaryotes) rather than the 80S ribosomes found in the eukaryotic cytoplasm, supporting a prokaryotic ancestry.
4. Chloroplasts reproduce by binary fission independently of cell division, consistent with their descent from free-living prokaryotes.

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