The action of motor proteins

There exist special protein molecules, called motor proteins. They perform mechanical work in the cell. The motor protein (myosin) typically "walks" on a specific substratum like the actine filament. The energy comes from the adenosine triphosphate (ATP) molecules. The undesired activity of the myosin is prevented by covering the active sites of the actine filament with the inhibitory protein complexes (tropomyosine and troponine). The calcium ions interact with the troponine and afterwards the coat deforms exposing the active sites where walks the myosin.