History of the Universe

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The mitochondrion ("my-tow-con-dree-on") is an organelle of a eukaryote cell. That means, it is part of a cell wrapped in its own membrane. Two of them are called mitochondria.

They serve two functions in a cell.

Waste Disposal Unit

They take in the waste products of fermentation and gently burn them to make simpler chemicals like carbon dioxide and water.

Power Station

During this burning some of the chemical energy of these waste products is converted into ATP, the molecule used by all living things to store and transport energy. So mitochondria are also the power stations of the cell.


An animal mitochondrion showing its DNA. Diagram courtesy of Mariana Ruiz Villarreal

Mitochondria occur in both plant and animal cells as sausage shaped bodies closely packed in regions actively using ATP energy. They are about 1 micrometer in length, slightly smaller than a typical bacterium.

If a soccer ball were inflated to the size of the Earth (Soccearth) then a mitochondrion would be between 25 and 75 meters across.


Mitochondria still have a small chromosome of their own and make some of their own proteins, showing that they were once separate bacteria. During reproduction, copies of the mother's mitochondria are passed on inside the egg. They are found in all eukaryotes but do not occur in bacteria. This suggests that the mitochondria were originally symbionts. They evolved from an early aerobe that began to live inside a larger anaerobe.


In time the genes of the host bacterium split into several smaller units called chromosomes. The reason is not clear. These chromosomes were wrapped up in a membrane in the middle of the cell to make the nucleus. Notice we used the same word to describe the central part of an atom, because the word means "center".

All cells more advanced than bacteria are like this. They are called "eukaryotes" ("you-carry-oats"). The name means "well centered".

Some parts of an animal eukaryotic cell

Eukaryotes are much larger and more complex than bacteria, and have many more genes. Eukaryotes appeared about 1.5 billion years ago.

Almost all the plant and animal cells you can see around you are eukaryotic. They contain structures called organelles.

Eukaryote on Soccearth

If a soccer ball was inflated to the size of the Earth (Soccearth) then a single eukaryotic cell can be from 500m to 2km or more across - about the size of a factory.

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History of the Universe eBook
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Written by Wyken Seagrave
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