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.