https://goo.gl/sZBS83
Mitochondria are tiny, ranging from 0.5 to 1.0 micrometers in diameter, with considerable observable variations in the structure and size of the organelle, and are sometimes described as “the powerhouses of the cell” because they generate most of the cell’s supply of adenosine triphosphate (ATP), used as a source of chemical energy, and are involved in various other tasks, such as signaling, cellular differentiation, and cell death, as well as maintaining control of the cell cycle and cell growth.
The term “Mitochondrial disease” refers to a broad range of disorders, each of which involves a mitochondrial dysfunction, with many more believed to have yet to be discovered. Because mitochondria perform so many different functions in different tissues, there are literally hundreds of different mitochondrial diseases that result from either inherited or spontaneous mutations in mtDNA or nDNA, which lead to altered functions of the proteins or RNA molecules that normally reside in mitochondria.