Mitochondria are often referred to as the powerhouses of the cell. These important organelles transform glucose, a simple sugar, into molecules of an energy-rich substance called ATP. In a multistep process called cellular respiration, mitochondria can convert a single molecule of glucose into as many as 38 molecules of ATP. This conversion of one large molecule into many smaller molecules allows cells to parcel out the energy they take in. A tiny biochemical reaction that requires only a tiny amount of energy can thus be powered by just a few molecules of ATP without wasting the energy of an entire glucose molecule. Indeed, ATP drives virtually every cellular process that takes place in your body -- from the smallest to the largest.

Although ATP is critical to the survival and functioning of every cell, different types of cells have dramatically different energy requirements. Just as the activity levels of workers in, say, a bakery vary from person to person, depending on each worker's specific job and on the time of day, the activity levels of your body's cells vary from one type of cell to another. Compare a living cell from the outer layer of your skin with a cell from a muscle in your arm, for example. While both types of cells serve important functions, your outermost skin cells, which serve mostly a protective role, perform very little active work. Muscle cells, in contrast, are often required to contract relentlessly. In return, muscle cells and other highly active cells, including nerve and liver cells, demand huge amounts of ATP.

Because mitochondria are the source of ATP, the simplest way for a cell to supply itself with sufficient levels of ATP is to contain sufficient numbers of mitochondria. A cell in the outermost layer of the skin 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.

When the demands on a cell regularly deplete ATP levels, mitochondria are able to divide and multiply their numbers independent of cell division. This is because mitochondria contain their own DNA, a circular chromosome much like the genetic material found inside bacteria. This chromosome regulates many of the organelles' functions and allows them some level of independence. They are stimulated to divide when levels of ATP in the cell become too low.