All human cells can be considered biological machines surrounded by a membrane bilayer (plasma membrane). The average thickness or diameter of a nonmuscle cell is approx 10-20 μm. The encapsulating membrane is studded with various receptors for hormones or other circulating biochemicals (Fig. 1). The plasma membrane also contains a number of ion-specific pumps and channels. The interior of each cell contains enzymes and organelles specialized to support a wide array of biological functions. Key organelles include the nucleus (contains the genetic blueprint for cellular function), mitochondria (converts various energy sources to adenosine triphosphate, a general energy currency), and the endoplasmic reticulum and the Golgi apparatus (supports protein synthesis) (Fig. 1). Muscle cells are similar in that they use some of these common organelles, but different in that they are specialized to generate force.