Origins of Cell Compartmentalization
Introduced| English | Meaning |
|---|---|
| endosymbiosis/ˌendəʊˌsɪmbaɪˈəʊsɪs/ | A relationship in which one organism lives inside another; ancestral bacterial endosymbionts gave rise to mitochondria and chloroplasts. |
| endosymbiotic theory/ˌendəʊˌsɪmbaɪˈɒtɪk ˈθɪəri/ | The theory that mitochondria and chloroplasts began as free bacteria living inside another cell. |
Where did the rooms come from?
- Eukaryotic cells are full of membrane-bound organelles.
- But the first cells were simple, like today's bacteria.
- So how did the complex, compartmented cell arise?
- The answer is one of biology's most surprising stories.
Folding the membrane inward
- Some organelles came from the cell's own membrane folding inward.
- These infoldings pinched off to form the nucleus and endomembrane system.
- This built compartments from a structure the cell already had.
- But two organelles have a stranger origin.
Mitochondria under the electron microscope: their double membrane supports the endosymbiotic origin
The nucleus and ER are thought to have formed from…
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The nucleus and endomembrane system likely arose by the plasma membrane folding inward.
Endosymbiosis: a cell within a cell
- Endosymbiosis 内共生 is one cell living permanently inside another.
- Long ago, a large cell engulfed a small bacterium but did not digest it.
- The bacterium made ATP; the host sheltered it — a partnership.
- Over generations, that bacterium became the mitochondrion.
The endosymbiotic theory
Step through how a free-living bacterium became a permanent organelle inside a larger cell.
The endosymbiotic theory explains the origin of…
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Endosymbiosis proposes that mitochondria and chloroplasts were once free-living bacteria engulfed by a host cell.
The evidence
- Mitochondria and chloroplasts keep their own small, circular DNA.
- They are wrapped in a double membrane, a clue to being engulfed.
- They divide on their own, just like bacteria.
- This is why the endosymbiotic theory 内共生学说 is widely accepted.
Which observation supports endosymbiosis?
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Mitochondria keep their own circular DNA, a double membrane, and divide on their own — just like bacteria.
Mitochondria and chloroplasts contain their own DNA.
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They keep a small circular genome, a leftover from their free-living bacterial past.
Select all true statements about the origin of organelles.
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Only mitochondria and chloroplasts were engulfed bacteria — not every organelle. The other three are correct.
Only mitochondria and chloroplasts came from engulfed bacteria. Do not say every organelle was once a bacterium — the nucleus, ER, and Golgi formed by membrane infolding, not endosymbiosis.
A fingerprint from the past:
- Your mitochondria carry a tiny loop of DNA, separate from the DNA in your nucleus.
- That mitochondrial DNA looks bacterial, not like your own.
- It is a molecular fossil of the free-living bacterium your ancestors' cells once swallowed.
Eukaryotic compartments arose two ways. The nucleus and endomembrane system formed by the membrane folding inward. Mitochondria and chloroplasts arose by endosymbiosis — engulfed bacteria that became organelles — supported by their own DNA, double membranes, and self-division (the endosymbiotic theory).
Write from endosymbiosis
- Mitochondria have their own circular DNA and a double membrane.
- Chloroplasts share those features.
Which explanation accounts for those features?
Mitochondria and chloroplasts retain signs of a prokaryotic ancestor.
Where did mitochondria get circular DNA and two membranes?
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An engulfed prokaryote explains both features.