  Maintained by environmental fluxes, biological systems are thermodynamic
processes that operate far from equilibrium without detailed-balance dynamics.
Yet, they often exhibit well defined nonequilibrium steady states (NESSs). More
importantly, critical thermodynamic functionality arises directly from
transitions among their NESSs, driven by environmental switching. Here, we
identify constraints on excess thermodynamic quantities that ride above the
NESS housekeeping background. We do this by extending the Crooks fluctuation
theorem to transitions among NESSs, without invoking an unphysical dual
dynamics. This and corresponding integral fluctuation theorems determine how
much work must be expended when controlling systems maintained far from
equilibrium. This generalizes feedback control theory, showing that Maxwellian
Demons can leverage mesoscopic-state information to take advantage of the
excess energetics in NESS transitions. Altogether, these point to universal
thermodynamic laws that are immediately applicable to the accessible degrees of
freedom within the effective dynamic at any emergent level of hierarchical
organization. By way of illustration, this readily allows analyzing a
voltage-gated sodium ion channel whose molecular conformational dynamics play a
critical functional role in propagating action potentials in mammalian neuronal
membranes.
