Mitochondria literally make it possible for us to move and think. While it is true that energy production is one function of mitochondria, this is only a small part of what they do. Liver mitochondria are specialized for other duties. They contain, for examples, enzymes that allow them to detoxify ammonia, a waste product of protein metabolism. These enzymes are not made in the heart.
Mitochondria also differ in the fuels they can bum.
For example, mitochondria in the heart cannot use sugar for energy. They are entirely dependent on the metabolism of fats to meet their energy needs. In contrast, mitochondria in the liver can use both fats and sugars. Their symptoms are a reflection of the tissues that need the function of that gene or genes most. Mitochondrial DNA is the only DNA that we inherit exclusively from our mothers.