Cellular respiration is the process most cells use to convert food molecules into energy. In a variety of multicellular organisms, from humans to trees, cellular respiration takes place in the mitochondria. These important organelles and the high-energy molecules of ATP they produce power virtually every biochemical reaction that takes place -- both in your body and in the plants and animals around you.

Mitochondria are the source of ATP, so in order for a cell to supply itself with sufficient levels of ATP, it must contain a sufficient number of mitochondria. A cell in the outermost layer of the skin with relatively low energy demands may contain a few hundred mitochondria. By comparison, a muscle cell may have several thousand of the organelles, which together comprise more than 25 percent of the cell's total volume. Mitochondria in these types of cells are also usually quite large and have many more inner folds, or cristae, than do cells whose activity levels are much lower.