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Locomotion is the way that animals explore their environment in an active method. All animals use the contraction of muscles in order to move. Animals use the chemical energy of ATP to supply force in order to oppose the force of gravity. When an ATP molecule is split into ADP and inorganic phosphate (Pi), 7.3 kcal of energy per mole of ATP is made available to do the work of movement. Although some protests use ATP to wave cilia and sweep themselves from place to place, animals use ATP to shorten the muscle cells. When many muscle cells shorten at the same time, they can exert a great deal of force. Locomotion is the result when this force is used to move bones at the animals¡¦ joints. The bones are a structural material of the vertebrate skeleton. The bone is a special form of connective tissue in which an organic extracellular matrix containing collagen fibers is imbedded with small, needle-shaped crystals of hydroxyapatite (which is formed from calcium phosphate and calcium hydroxide). The collagen fibers run in various directions, but the hydoxyapatite crystals are aligned parallel to the long axes and the curved ends of bones. Hydroxyapatite is brittle but rigid, thus giving bones a lot of strength; the compo