Although the mechanism of cytoplasmic streaming is not completely understood, it is thought to be mediated by “motor” proteins—molecules made up of two proteins that use adenosine triphosphate (ATP) to move one protein in relation to the other. If one of the proteins remains fixed on a substrate, such as a microfilament or a microtubule, the motor proteins can move organelles and other molecules through the cytoplasm. Motor proteins often consist of actin filaments, long protein fibres aligned in rows parallel to the streaming just inside the cell membrane. Myosin molecules attached to cellular organelles move along the actin fibres, towing the organelles and sweeping other cytoplasmic contents in the same direction.