Mitocondria are cell organelles (structure of a cell) which can be viewed as an energy factory of the cell.
It generates the energy required for a cell to function: called Adenosine triphosphate (ATP).
They are composed of an external straight membrane and an inner jagged membrane.
It probably was an external organism which was integrated in cells during cell evolution. (particularly in Eucaryotes). Indeed more energy was required for more complex cells to function.
One clue to this is that mitochondria have their own genetic material (DNA).
It might be involved in skin diseases such as vitiligo (to read more, click HERE)