Thermodynamics of metabolic energy conversion

We show how metabolic energy conversion may be described in the frame of a generalized formulation of standard linear nonequilibrium thermodynamics. To apply Onsager's force-flux formalism to the study of metabolism we resort to the introduction of an effective metabolic equivalent of a thermodynamic working fluid, which is not a physical substance per se but provides a theoretical ground to compute generalized thermoelastic coefficients, effective transport parameters and a metabolic figure of merit. This figure of merit characterizes the ability of a given organism to produce useful mechanical power from the digested food chemical potential, in order to sustain a muscular effort. It depends both on the intrinsic performance of the equivalent working fluid and also on the working conditions that constrains the organism. Finally, we demonstrate that the maximum power principle, frequently invoked in biology, can be understood as a specific configuration of an out-of-equilibrium system, and should not be considered as a fundamental principle.