"Inside cells there are proteins that convert chemical energy into useful work. For example, kinesins and dyneins haul cargo around the cell. Myosin molecules can bind to actin filaments and exert forces, which is how our muscles work. Other molecules rotate, such as the protein that creates the molecules that are the prime fuel of our cells. The general method by which these molecular motors operate is through a Brownian ratchet mechanism. However, whereas the classic Brownian ratchet does not actually work, molecular motors harness molecular binding energies to satisfy the second law of thermodynamics. Typically, binding of an ion or molecule (such as ATP) to the motor leads to a conformational change in the protein. This conformational change can act like a power-stroke in the motor. Hydrolysis of ATP or release of the bound ion then returns the motor to its original state, thereby completing a cycle (or, in the case of rotational motors, a binding and release event typically only produces a substep of a complete rotation)." from Does cell biology need physicists?, by
Charles W. Wolgemuth,
http://physics.aps.org/articles/v4/4
"The busy life in living cells involves a great deal of transport activities and mechanical tasks, which are undertaken by motor proteins* —molecular machines that convert chemical energy into mechanical work. In recent years, these remarkable machines have inspired artificial devices that deliver mechanical work or propel themselves in a viscous environment. We do not yet understand the mechanism behind the complex mechanochemical coupling in motor proteins. Standard rules used in macroscale engineering do not work at the nanoscale. New strategies are needed for the development of artificial nanoscale machines." http://physics.aps.org/articles/v3/108
*Motor proteins are a class of molecular motors that are able to move along the surface of a suitable substrate. They are powered by the hydrolysis of ATP and convert chemical energy into mechanical work. more
http://en.wikipedia.org/wiki/Motor_protein
