  We study the energetics of isothermal ratchets which are driven by a chemical
reaction between two states and operate in contact with a single heat bath of
constant temperature. We discuss generic aspects of energy transduction such as
Onsager relations in the linear response regime as well as the efficiency and
dissipation close to and far from equilibrium. In the linear response regime
where the system operates reversibly the efficiency is in general nonzero.
Studying the properties for specific examples of energy landscapes and
transitions, we observe in the linear response regime that the efficiency can
have a maximum as a function of temperature. Far from equilibrium in the fully
irreversible regime, we find a maximum of the efficiency with values larger
than in the linear regime for an optimal choice of the chemical driving force.
We show that corresponding efficiencies can be of the order of 50%. A simple
analytic argument allows us to estimate the efficiency in this irreversible
regime for small external forces.
