ABSTRACT
The common cliche that “equilibrium is death” underscores that life is fundamentally a nonequilibrium phenomenon. A truly nonequilibrium situation would, however, be a likely death to any tractable physical theory purporting to rationalize the behavior of biological systems, as even the most basic concepts of, say, pressure or temperature do not apply to nonequilibrium systems. Should a doctor be ridiculed for taking your temperature on the grounds that the notion of temperature is, technically, inapplicable to a nonequilibrium system such as the human body? Probably not. The fact that our body’s temperature is well defined for any practical purposes has microscopic implications. We expect, for example, the velocities of the atoms composing our body to obey the Maxwell-Boltzmann distribution corresponding to this temperature. The existence of the Maxwell-Boltzmann distribution was essential for our derivation of the microscopic expression for a reaction rate (see Chapter 5), which has led to an Arrhenius-type expression for the rate coefficient, k = A exp[−E/(kB T )]. The validity of this expression is widely (and justifiably) adopted by the scientists who study chemical transformations in living organisms. It is then clear that living systems must be, at least, in a state of partial equilibrium [1], where some of the degrees of
freedom (e.g., the momenta of all the atoms) display the behavior prescribed by the laws of equilibrium statistical mechanics and thermodynamics. The assumption of partial equilibrium was implicit in Chapter 4 in our discussion of stochastic models of molecular dynamics, such as the Langevin equation or the master equation. Indeed, those models assume that the motion of the molecule itself does not disrupt thermal equilibrium of the surroundings. For example, the statistical properties of the random force R(t) in the Langevin equation are not affected by the molecule’s motion and depend only on the temperature, T , of the surroundings.
