Muscle cells are found in muscle tissue, and are responsible for providing motion in specialized ways; for example, skeletal muscle moves the skeleton, allowing the body to move against outside pressure and gravity, and against its own inertia. Muscle cells expand and contract to provide their motion, and so they require a great deal of energy, much more than the other cells of the body. Since the most efficient source of biological energy is adenosine-5'-triphosphate (ATP), it is important for muscle cells to have a lot of mitochondria, many more than in other cells. This allows the fast and efficient conversion of glucose to ATP, which is used by the cell almost instantly. Without that fast conversion, the muscles would be sluggish and unresponsive, and incapable of fast and strong movements. The reason that muscles must produce their own ATP is that muscle cells are specialized for movement, not for storage, and ATP does not store for long periods of time. Muscle cells cannot store lipids like adipose cells; adipose cells store energy in lipid form, but are not called on to convert these lipids into ATP as often, and so they have fewer mitochondria than muscle cells.