The Nonequilibrium Statistical Thermodynamics of Biological Cycles

Abstract

Biological processes are sustained by forces that prevent relaxation to equilibrium. These nonequilibrium forces (e.g., a gradient in ATP concentration) create currents that, driven by simple physical laws, propagate through a network of interactions to produce remarkable biological phenomena. We use statistical thermodynamics to model the response of these outputs--a load-carrying molecular motor, the organization of cellular structures, kinetic proofreading, etc.--with respect to such simple forces. We present a formalism that extends the general framework of Onsager to model this response arbitrarily far from equilibrium for cyclic processes. This presentation will focus on several specific nonequilibrium processes such as the processive motion of molecular motors, the equilibrium-shifting action of chaperons, kinetic proofreading, and metabolic cycles. These processes necessarily produce entropy in the surrounding environment, and by quantifying this dissipation we consider cellular efficiency and how the cell decides where to spend its limited energetic resources. This includes a discussion of efficiency for processes that have no measurable thermodynamic output.