The big idea: Mitochondria and chloroplasts were not always part of eukaryotic cells.
The endosymbiotic theory says they began as free-living prokaryotes (bacteria) that were engulfed by a larger host cell and then survived inside it.
Over many generations the trapped bacterium became a permanent organelle: an aerobic bacterium became the mitochondrion, and a photosynthetic bacterium became the chloroplast.
An ancestral host cell engulfs a free-living aerobic bacterium by endocytosis; the bacterium survives inside and, over many generations, becomes a mitochondrion.
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- Endosymbiotic theory
- The explanation that mitochondria and chloroplasts evolved from free-living prokaryotes that were engulfed by a host cell and survived inside it.
- Endosymbiosis
- One organism living inside the cells of another, where both benefit. 'Endo' = inside, 'symbiosis' = living together.
- Host cell
- The larger ancestral cell that engulfed the bacterium; it became the eukaryotic cell we know today.
- Prokaryote
- A small cell with no nucleus and no membrane-bound organelles (a bacterium). The ancestor of mitochondria and chloroplasts was a prokaryote.
- Organelle
- A membrane-bound structure inside a eukaryotic cell that does a particular job — for example a mitochondrion or a chloroplast.
Read the word: Endo = inside, sym = together, biosis = living.
So endosymbiosis literally means 'living together, inside' — one cell living permanently inside another, with both partners benefiting.
Endosymbiosis happened in a clear sequence. A large host cell took in a smaller bacterium, the bacterium turned out to be useful rather than digested, and so it was kept.
Follow the three steps below — they are exactly what an 'explain the endosymbiotic theory' question is asking you to describe.
The endosymbiotic sequence
- Engulf. A large host cell took in a smaller free-living prokaryote by endocytosis (the membrane folded around it), so the bacterium ended up inside a vesicle.
- Survive, don't digest. The bacterium was not broken down. Instead it survived and reproduced inside the host.
- Mutual benefit. The relationship was beneficial to both: the bacterium gained shelter and nutrients, while the host gained energy (from an aerobic bacterium) or food (from a photosynthetic one).
- Become an organelle. Over many generations the bacterium was retained permanently and became an organelle — a mitochondrion or a chloroplast.
Two organelles, the same story: Both of these organelles arose by endosymbiosis — only the bacterium that was engulfed differs:
an aerobic bacterium became the mitochondrion (it respires to release energy), and a photosynthetic bacterium became the chloroplast (it makes food using light).
This is why mitochondria are found in (almost) all eukaryotes, but chloroplasts only in plants and algae — those cells engulfed the photosynthetic partner.
| Organelle today | Free-living ancestor | Job it does inside the host |
|---|---|---|
| Mitochondrion | An aerobic (oxygen-using) bacterium | Aerobic respiration — releases usable energy (ATP) for the host |
| Chloroplast | A photosynthetic bacterium (a cyanobacterium) | Photosynthesis — makes food (sugar) using light, in plants and algae |
Why the engulfing was kept up: Endocytosis happens all the time, but a one-off meal does not make an organelle.
What made this special is that the relationship was mutually beneficial, so natural selection favoured keeping the bacterium.
That is the part examiners want: the bacterium survived, reproduced, and gave the host an advantage, so it was retained generation after generation.
A memory hook: Engulf -> survive -> benefit both -> keep forever.
If you can say those four words in order, you can answer almost any 'explain the endosymbiotic theory' question.
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How this is tested: On Paper 1 a 1-mark item often asks you to explain how the endosymbiotic theory accounts for the origin of mitochondria — give the core idea: an aerobic bacterium was engulfed by a host cell and survived inside it.
On Paper 2 a 3-mark Explain question widens it to eukaryotic cells and their organelles — here you must mention both mitochondria and chloroplasts, the engulfing, and the survival inside the host.
A favourite Paper-1 data / evidence twist gives you a leaf cell with two ribosome sizes and asks you to explain it using the theory — the 70S ribosomes belong to the chloroplast's bacterial ancestor.
IB-style question — explain the origin of eukaryotic organelles
Explain the endosymbiotic theory for how eukaryotic cells and their organelles originated. [3]
How to score all three marks
- Start with the engulfing. A larger host cell engulfed a smaller free-living prokaryote (bacterium) by endocytosis, so the bacterium ended up inside the host.
- Explain survival and benefit. The bacterium was not digested; it survived and reproduced inside the host, and the relationship was beneficial to both partners, so it was kept.
- Name the organelles formed. Over many generations the engulfed bacterium became an organelle: an aerobic bacterium became the mitochondrion, and a photosynthetic bacterium became the chloroplast. (Award 1 mark for engulfing by the host, 1 mark for survival/mutual benefit inside the host, 1 mark for naming mitochondrion AND chloroplast as the result.)
Final answer
A host cell engulfed free-living prokaryotes that survived inside it; the relationship benefited both, so the bacteria were kept and became organelles — an aerobic bacterium became the mitochondrion and a photosynthetic bacterium became the chloroplast.
✓ Why this scores full marks: A 3-mark Explain needs three distinct, linked points: the engulfing, the survival inside the host, and both organelles named.
Writing only 'a bacterium became a mitochondrion' caps you at 1 mark — the chloroplast and the survival step are the other two marks.
| Organelle today | Free-living ancestor | Job it does inside the host |
|---|---|---|
| Mitochondrion | An aerobic (oxygen-using) bacterium | Aerobic respiration — releases usable energy (ATP) for the host |
| Chloroplast | A photosynthetic bacterium (a cyanobacterium) | Photosynthesis — makes food (sugar) using light, in plants and algae |