Mitochondria are the cell's power plant. When the cell needs energy, it breaks down molecules of ATP to release the stored energy. The more energy the cell needs, the more mitochondria it contains. One cell can have anywhere from a few mitochondria up to thousands. The highest numbers are found in nerve, muscle, and liver cells.
Some scientists believe that mitochondria are the key to aging.
As we grow older, our mitochondria collect more and more mutations. Eventually, mutations can reduce the energy available to the cell. If the energy drops low enough, the cell can malfunction. If the energy drops even further, the cell will stop working. If muscle cells can't get enough energy through cellular respiration, they have to get energy without oxygen.