Fluctuations When Driving Between Nonequilibrium Steady States

TL;DR

This paper extends fluctuation theorems to transitions between nonequilibrium steady states in biological systems, revealing universal thermodynamic constraints and implications for energy expenditure and control in far-from-equilibrium processes.

Contribution

It generalizes the Crooks fluctuation theorem to NESS transitions without dual dynamics, providing new insights into thermodynamic constraints and control in biological systems.

Findings

Derived constraints on excess thermodynamic quantities in NESS transitions

Extended fluctuation theorems to systems driven far from equilibrium

Applied theory to analyze ion channel conformational dynamics

Abstract

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.