There is a mitochondrion in almost every cell in your body, and it has its own DNA. Not yours — its own, circular, bacterial-looking DNA. The reason is that mitochondria were once free-living bacteria that got swallowed and never left.
📚 What you need to know
Endosymbiotic theory says mitochondria and chloroplasts were once free-living prokaryotes.
A larger anaerobic cell engulfed a smaller aerobic prokaryote, which was not digested.
Both benefited, so the arrangement lasted — that is what symbiosis means.
The engulfed cell became the mitochondrion. Later, a photosynthetic prokaryote was engulfed and became the chloroplast.
Evidence: a double membrane, their own circular naked DNA, 70S ribosomes, division by binary fission, and a size similar to bacteria.
This explains how the first eukaryotic cells arose.
What is supposed to have happened
Follow it as a sequence. Each step is small and sensible on its own, which is what makes the whole idea believable.
That last point is the neatest part of the theory. The double membrane is not a design feature — it is a leftover from the moment of swallowing.
A large anaerobic prokaryote takes in a smaller aerobic prokaryote by engulfing it, wrapping it in a piece of its own membrane.
The smaller cell is not digested. It carries on respiring aerobically inside the host.
Both gain. The host gets far more ATP from aerobic respiration; the smaller cell gets a steady supply of nutrients and shelter. A relationship where both benefit is mutualism, and living inside the other organism makes it endosymbiosis.
Over many generations the engulfed cell loses the genes it no longer needs and becomes dependent on the host. It is now a mitochondrion.
Later, one of these cells engulfs a photosynthetic prokaryote in the same way. That becomes the chloroplast, which is why only some eukaryotes have them.
Why the order matters. Every eukaryote has mitochondria, but only plants and algae have chloroplasts. That fits the sequence exactly: the mitochondrion came first, into an ancestor shared by everything, and the chloroplast came later into just one branch. The pattern in living organisms today matches the order of events.
The evidence
The theory would be a nice story with nothing behind it if mitochondria and chloroplasts did not still look so much like bacteria. They do, in five specific ways.
The purple loop is the organelle’s own DNA and the brown dots are its 70S ribosomes. Both are prokaryotic features sitting inside a eukaryotic cell.
Evidence
What is observed
Why it supports the theory
Double membrane
Two membranes, and the inner one resembles a prokaryotic membrane
The inner was the bacterium’s own; the outer came from the host engulfing it
Own DNA
A circle of naked DNA, separate from the nuclear DNA
Circular naked DNA is the prokaryotic pattern
70S ribosomes
Ribosomes inside are 70S, not the 80S found in the cytoplasm
70S is the prokaryotic size, so the organelle makes some of its own proteins
Binary fission
They divide in two on their own, independently of the cell
This is how prokaryotes reproduce
Size
Roughly 1 to 5 µm, the same range as bacteria
Consistent with an origin as a free-living prokaryote
🧠
Two, seventy, split
Two membranes, 70S ribosomes, and it splits by binary fission — plus its own circular DNA and a bacterial size. Five points, and most questions here are worth three or four.
A common exam trap is to ask why an organelle contains 70S ribosomes when the rest of the cell has 80S ones. Do not treat it as an oddity to explain away. It is direct evidence: the organelle kept the ribosomes it had when it was a bacterium.
Worked examples
WE 1
Outline the theory
Outline the endosymbiotic theory for the origin of mitochondria. (3 marks)
Point 1: the engulfing
A large anaerobic prokaryote engulfed a smaller aerobic prokaryote.
Point 2: it survived
The engulfed cell was not digested and continued to respire aerobically inside the host.
Point 3: both benefited
The host gained ATP and the engulfed cell gained shelter and nutrients, so the relationship persisted and it became a mitochondrion.
Engulfed, not digested, and both gained — so it stayedthe “not digested” step is often missed, and it is usually a mark
WE 2
Use the evidence
State three pieces of evidence that support the endosymbiotic origin of chloroplasts. (3 marks)
Evidence 1
They contain their own circular naked DNA, like a prokaryote.
Evidence 2
They have 70S ribosomes, the prokaryotic size, rather than the 80S ribosomes of the cytoplasm.
Evidence 3
They are surrounded by a double membrane and divide by binary fission, independently of the cell.
Circular DNA, 70S ribosomes, double membrane and binary fissionsay why each point matters, not just what it is, if the question says “explain”
WE 3
Explain a pattern in living organisms
All eukaryotes have mitochondria but only some have chloroplasts. Suggest how endosymbiotic theory explains this. (3 marks)
Point 1: the order of events
The aerobic prokaryote was engulfed first, in an ancestor of all eukaryotes.
Point 2: the second event
A photosynthetic prokaryote was engulfed later, by only one group of those cells.
Point 3: the consequence
Only the descendants of that group inherited chloroplasts, which is why plants and algae have them and animals do not.
Mitochondria came first and spread to all; chloroplasts came later to one branchthis is a “suggest” question, so build the argument rather than reciting evidence
💡 Exam tips
Use the word engulfed, and add that the cell was not digested.
Say which organism gained what — mutual benefit is the reason it lasted.
Give the ribosome sizes as numbers: 70S inside the organelle, 80S in the cytoplasm.
Explain the double membrane as inner from the bacterium, outer from the host.
Chloroplasts came second, which explains why only some eukaryotes have them.
Link back to earlier pages: circular naked DNA and binary fission are prokaryotic traits you already know.
⚠ Common mistakes
Saying the small cell was eaten and digested. Surviving is the whole point.
Only mentioning the host’s benefit. Symbiosis here means both gained.
Giving 80S ribosomes for mitochondria. They are 70S.
Saying organelle DNA is the same as nuclear DNA. It is separate, circular and naked.
Claiming chloroplasts came first. Mitochondria did, which is why all eukaryotes have them.
Treating the double membrane as coincidence. It is one of the strongest pieces of evidence.
Up next: Cell Differentiation — how cells with identical DNA end up doing completely different jobs.
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