A mitochondrion is a small, membrane-bound organelle found in the cells of most eukaryotic organisms, including animals, plants, and fungi. Often referred to as the "powerhouse of the cell," mitochondria are responsible for producing energy in the form of adenosine triphosphate (ATP) through a process called cellular respiration. Mitochondria have their own DNA, which is distinct from the nuclear DNA found in the cell's nucleus. This unique feature supports the endosymbiotic theory, which suggests that mitochondria originated from free-living bacteria that were engulfed by ancestral eukaryotic cells, leading to a mutually beneficial relationship.