Of course, the role they play as cellular furnaces, converting nutrients and oxygen into energy, is immensely important. Every cell needs ATP—the chemical fuel generated by mitochondria—and some cells are particularly demanding: a muscle cell that pushes the leg into a sprint, a beta cell in the pancreas that synthesizes the hormone insulin, a brain cell that fires a signal to help create a thought. Each of these kinds of cells contains as many as 10,000 mitochondria, and it's no secret that those tiny organelles keep the home fires burning.

Photo: Mark Hooper