Mitochondria range in diameter from 0.5 to 1 micrometer in diameter. Mitochondria are typically round or oval in shape. They are found in the cytoplasm of nearly all eukaryotic cells, which are cells with clearly defined nuclei.
The primary function of mitochondria is to produce large amounts of energy in the form of ATP, or adenosine triphosphate. Mitochondria also generate heat, store calcium and mediate both cell growth and death. In humans, mitochondria are not present in all cells, and the amount per cell varies. For instance, red blood cells do not contain any mitochondria, and liver cells can contain hundreds up to thousands of mitochondria.