# Endosymbiosis

The idea that mitochondria and chloroplasts began as free-living bacteria swallowed by an ancestral cell. Evidence: they have their own circular DNA, bacteria-sized ribosomes and double membranes.

Biology (university) · Part 01: Cell structure and membranes · Card 5 of 9 · Last updated 2026-10-19

## Analogy

A lodger who moved in, paid rent in energy and never left.

## In practice

Mitochondria divide on their own inside the cell, much like bacteria do.

## Practice

### Flashcards

- **Give two pieces of evidence for endosymbiosis.** Mitochondria and chloroplasts have their own circular DNA and bacteria-like ribosomes. They also have double membranes.

### Interview questions

**Describe the evidence that mitochondria arose by endosymbiosis.**

- Own circular DNA
- Bacteria-like ribosomes
- Double membrane
- Divide independently by fission-like process

## Questions people ask

### Which organelles came from endosymbiosis?

Mitochondria came from a bacterium that could use oxygen to make energy. Chloroplasts came later, from a photosynthetic cyanobacterium swallowed by a cell that already had mitochondria. That is why plants and algae have both, while animals and fungi have only mitochondria.

### Why do mitochondria have their own DNA?

Because they were once independent bacteria. Over time most of their genes moved to the nucleus, but a small genome remained. In humans it has 37 genes, and it is inherited almost entirely from the mother because sperm mitochondria are generally destroyed after fertilisation.

## Related concepts

- [Nucleus](https://studyvisually.com/learn/science/biology-university/biology-university-part-1/biology-university-part-1-concept-3)
- [Endomembrane system](https://studyvisually.com/learn/science/biology-university/biology-university-part-1/biology-university-part-1-concept-4)

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