The term “Machine” invokes familiar images from the macroscopic world, of gears, springs, and levers engaging each other sequentially in a deterministic chain of events. Gravity and inertia are major forces to be reckoned with in the design, or are in fact utilized in the very function, of a device such as a crane, a locomotive, or a centrifuge. The world of molecules this book is concerned with is quite different in many respects. In this water-drenched nanodimension world, mobile parts are in constant jittering motion, powered by random thermal bombardment from the molecules of the aqueous solvent. The forces of gravity and inertia are dwarfed, by orders of magnitude, by those produced by non-covalent interactions, collisions with water molecules, and drag in the solvent. Complicating matters further, biological molecules and the “levers” and “actuators” within them lack the rigidity of materials like steel. Perhaps most difficult to conceive by a mind trained on the experience of the macroscopic world, however, is the disintegration of causal connections into a series of sporadic irreversible chemical events that impart directed motion, separated by stretches of “time” where the molecule has no apparent directionality.

Molecular Machines as a concept existed well before Bruce Alberts’ (1998) programmatic essay in the journal Cell, but his article certainly helped in popularizing the term, and in firing up the imagination of students and young scientists equipped with new tools that aim to probe and depict the dynamic nature of the events that constitute life at the most fundamental level. “Machine” is useful as a concept because the molecular assemblies in this category share important properties with their macroscopic counterparts, such as processivity, localized interactions, and the fact that they perform work toward making a defined product. The concept stands in sharp contrast to the long-held view of the cell as a sack, or compendium of sacks, in which molecules engage and disengage one another more or less randomly. In invoking mechanistic imagery, it is also, finally, a view that invites physicists to employ their tools and creative minds in developing models that take into account the reality of the nanoworld in a quantitative way. Thus Bruce Albert's is, as invitations go, the third in the past century, with Werner Schroedinger and Richard Feynman having been the first and second to usher their colleagues into tackling the grand challenges posed by biology.