Introduction

Molecular machines (Mavroidis et al. 2004) are devices that convert one form of energy into another. Just like their macroscopic counterparts, molecular machines have an “engine”, an input and an output. Most of the machines I consider in this chapter are motors (Howard 2001, Kolomeisky and Fisher 2007, Schliwa 2003) which are enzymes that convert chemical energy into mechanical work.

In spite of the striking similarities, it is the differences between molecular machines and their macroscopic counterparts that makes the studies of these systems so interesting from the perspective of physicists. Biomolecular machines are usually single proteins or macromolecular complexes comprising several proteins and/or RNAs. These operate in a domain where the appropriate units of length, time, force and energy are nano-meter, milli-second, pico-Newton and kBT, respectively (kB being the Boltzmann constant and T is the absolute temperature). Already in the first half of the twentieth century D’Arcy Thompson, father of modern bio-mechanics, realized the importance of viscous drag and Brownian forces in this domain. He pointed out that (Thompson 1963) “where bacillus lives, gravitation is forgotten, and the viscosity of the liquid, the resistance defined by Stokes’ law, the molecular shocks of the Brownian movement, doubtless also the electric charges of the ionized medium, make up the physical environment and have their potent and immediate influence on the organism. The predominant factors are no longer those of our scale; we have come to the edge of a world of which we have no experience, and where all our preconceptions must be recast”.