  Molecular machines composed of coupled subsystems transduce free energy
between different external reservoirs, in the process internally transducing
energy and information. While subsystem efficiencies of these molecular
machines have been measured in isolation, less is known about how they behave
in their natural setting when coupled together and acting in concert. Here we
derive upper and lower bounds on the subsystem efficiencies of a bipartite
molecular machine. We demonstrate their utility by estimating the efficiencies
of the $\mathrm{F}_\mathrm{o}$ and $\mathrm{F}_1$ subunits of ATP synthase and
that of kinesin pulling a diffusive cargo.
