  Biomolecular machines are protein complexes that convert between different
forms of free energy. They are utilized in nature to accomplish many cellular
tasks. As isothermal nonequilibrium stochastic objects at low Reynolds number,
they face a distinct set of challenges compared to more familiar
human-engineered macroscopic machines. Here we review central questions in
their performance as free energy transducers, outline theoretical and modeling
approaches to understand these questions, identify both physical limits on
their operational characteristics and design principles for improving
performance, and discuss emerging areas of research.
