mitochondria (thing)

Mitochondria are self-replicating double-walled organelles in all eukaryotic cells. These cells have between 1 and 10,000 mitochondria, but most cells average around 200. The typical liver cell has over 1,000 mitochondria.

Some people have theorized that the mitochondria was a primitive independent life form which formed a symbiotic relationship with a host cell, and have been an important part of eukaryotic cells ever since.