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Year : 2011  |  Volume : 4  |  Issue : 10  |  Page : 61-64  

The Endosymbiotic Theory

The King's School, Canterbury, United Kingdom

Date of Web Publication24-Jan-2012

Correspondence Address:
Cleodie Swire
The King's School, Canterbury
United Kingdom
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DOI: 10.4103/0974-6102.92200

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   Definitions Top

Prokaryote - Organism with cells without a true nucleus or other membrane-bound organelles

Eukaryote - Organism whose cell(s) contain(s) a distinct, membrane-bound nucleus

Autotroph - An organism that can make its own food

Heterotroph - An organism that must obtain ready-made food

Endocytosis - A process in which a cell takes in materials by engulfing them and fusing them with its membrane, as shown in [Figure 1]
Figure 1: A diagram showing endocytosis [available from Average_prokaryote_cell-_en.svg]

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Aerobic - Organism that requires oxygen for survival

Anaerobic - Organism that can function without oxygen

Symbiosis - Two different organisms benefit from living and working together

Endosymbiosis - One organism lives inside another

Mitochondrion - Organelle where aerobic respiration occurs within the cell

Carbohydrate + Oxygen → Carbon dioxide + Water + Energy

Chloroplast - Organelle where photosynthesis occurs in plant cells

Carbon dioxide + Water (with sunlight and chlorophyll) → Carbohydrate + Oxygen

[Figure 2] shows that mitochondria and chloroplasts are very similar to prokaryotic cells; these observations lead to The Endosymbiotic Theory.
Figure 2: A table comparing various characteristics of different cells and cell types

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   Theory Top

Researchers comparing the structures of prokaryotes and cell organelles, as shown in [Figure 2], came to the conclusion that organelles such as mitochondria and chloroplasts had originally been bacteria that were taken into larger bacteria by endocytosis and not digested. The cells would have had a mutually beneficial (symbiotic) relationship. The ingested cells developed into organelles, such as mitochondria and chloroplasts, which now cannot live outside the host cell.

   Mitochondria Top

Aerobic bacteria were taken in by anaerobic bacteria. The enveloped bacteria would have used the oxygen from the air (which was useless to its host) to provide far more adenosine triphosphate (ATP) (useful energy) than the host could produce on its own, while the host cell would provide materials to respire, protection, and a steady environment.

   Chloroplasts Top

Autotrophic photosynthetic bacteria cells were taken in by the heterotrophic prokaryote cells. The ingested cell would continue to provide glucose and oxygen (which could be used by the mitochondria as endocytosis of the photosynthetic prokaryote occurred after the endocytosis of aerobic cells) by photosynthesis. The host cell would provide carbon dioxide and nitrogen for the engulfed cell, as well as protecting it.

Over time, both cells lost their ability to survive without each other.

   Proof Top

Similarities to bacteria

[Figure 2] shows that mitochondria and chloroplasts have many similarities to prokaryotic bacteria. They are of a similar size and have 70S ribosomes, as opposed to the 80S ribosomes found in eukaryotic cells.

These cells all divide by binary fission, as shown in [Figure 3].
Figure 3: A diagram showing binary fission of a prokaryotic cell [available from Binary_fission.svg]

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The organelles have their own DNA, separate to the DNA found in the nucleus of the cell, which they use to produce enzymes and proteins to aid their function. This was predicted by the researchers, and was later proved to be true for mitochondria and chloroplasts.

All of these likenesses suggest that mitochondria and chloroplasts developed from prokaryotes.

Double outer membranes

Mitochondrion and chloroplasts have double outer membranes - the inner layer came from the engulfed cell and the outer membrane from the host cell during endocytosis.


Mitochondrion and chloroplasts can only arise from pre-existing organelles - the DNA that codes for them is not found in the nucleus of the cell, but in naked loops of DNA within the organelles themselves. This suggests that these organelles were originally separate cells that needed to replicate themselves.

Fossil record [Figure 4]

Fossil evidence shows that bacteria were present 3.8 billion years ago, when there was no oxygen in the atmosphere and all organisms were anaerobic.

Photosynthetic bacteria appeared about 3.2 billion years ago, producing oxygen. As the oxygen levels increased, the anaerobic organisms began to die out as oxygen is toxic to most cells (even ours!).
Figure 4: Timeline of events affecting The Endosymbiotic Theory

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Organisms that could respire aerobically developed about 2.5 billion years ago.

This evidence suggests that the 'ancestors' to the mitochondria and chloroplasts developed outside the cell, and later merged with other larger prokaryotes, leading to the development of eukaryotes.


The discoveries regarding the origins of the mitochondria and chloroplasts have led to several scientific applications.

   History of Evolution Top

The DNA found in mitochondria (mtDNA) is passed directly from mother to child, and changes much more slowly than other types of DNA, providing information about evolutionary history. This can be used to determine how closely related two species are to one another and migration patterns as shown in [Figure 5].
Figure 5: A graph showing human migration patterns, created by studying mtDNA. The letters denote the different groups of mtDNA [available from Migration_map4.png]

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   Astrobiology Top

Organisms called archaebacteria, which live in the most extreme habitats on Earth, have been studied as they are the organism believed to be most like the bacteria that inhabited the Earth billions of years ago.

They now inhabit salt ponds and boiling hot springs. As they live in places previously assumed to be unsuitable for life, they are being studied as they may provide clues about extra-terrestrial life. There has been some research done suggesting that the archaebacteria could survive space travel by meteorite, so there is potential for life on other planets.

   Researcher Top

The Endosymbiotic Theory of eukaryote evolution was first suggested by Dr. Lynn Margulis [Figure 6] in the 1960s, and officially in her book, 'Symbiosis in Cell Evolution' in 1981. Her ideas were initially ridiculed by her fellow biologists, but through research and persistence her theory was eventually accepted and is now regarded as the most credible explanation of eukaryote evolution.
Figure 6: Dr. Lynn Margulis [available from wiki/File: Lynn_Margulis.jpg]

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She is best known for her theory of symbiogenesis, which expanded on the aspect of The Endosymbiotic Theory in which the relationship between the prokaryotic cells becomes so strong that the two grew to be dependent on one another. Margulis suggested that this process may have also occurred at other times during evolution. This theory challenges Darwin's idea that mutations occur by genes being passed down from parents to offspring, rather than the genetic material of unrelated organisms being brought together.

   Authors Top

Cleodie Swire is doing Biology, Chemistry, Physics, Further Maths and Spanish at AS Level, and has already taken French. She is currently at The King's School Canterbury, and hopes to do Medicine at University. She enjoys doing sport - especially hockey - and travelling.
How to cite this article:
Swire C. The Endosymbiotic Theory. Young Scientists J 2011;4:61-4

How to cite this URL:
Swire C. The Endosymbiotic Theory. Young Scientists J [serial online] 2011 [cited 2014 Sep 2];4:61-4. Available from:


  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]


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