Mitochondria are primarily found within the cytoplasm of eukaryotic cells.
Mitochondria play a vital role in cellular energy production. They are essential for the evolution of complex animals because they are the primary site of ATP (adenosine triphosphate) production, the main energy currency of the cell. According to the provided reference, mitochondria occupy a "substantial portion of the cytoplasmic volume of eucaryotic cells." Without these organelles, animal cells would be reliant on the less efficient process of anaerobic glycolysis for their ATP needs.
