The mitochondrion is an organelle appearing in eukaryotic cells that creates adenosine triphosphate (ATP) for use as the cell's energy source.

The mitochondrion has its own genome that is distinct from that of the cell, and it has its own particular dialect for translating the genetic code. It is believed that the mitochondrion was once an endosymbiotic bacterium whose role became so fundamental to the eukaryotic cell that it became an inseparable component.

A cross-section of the mitochondrion is shown below.