The basis for movement in living cells is contractile proteins, which can convert chemical energy into the mechanical energy of tension and motion.

Muscle cells are highly specialized for contraction, and their primary constituents are contractile proteins. However, proteins with contractile properties have also been extracted from many other types of cells. For example, such proteins are responsible for the migration of some white blood cells from capillaries into peripheral tissues, for the move­ments of mitochondria, and for the movement of the cilia on some epithelial cells.