Non-equilibrium thermodynamics of light absorption

Geza Meszena and Hans V. Westerhoff

Journal of Physics A:Mathematics and General 32: 301-311 (1999)



Abstract

     Free energy transduction, the basic physical process underlying the phenomena of life
has been described by non-equilibrium thermodynamics. However, the possibility of a 
thermodynamic description of the very first step, the conversion
of solar energy to the free energy of the living material has been questioned on the basis
of an apparent lack of the microscopic reversibility of light absorption/emission. We
develop a description of light absorption that is consistent with non-equilibrium
thermodynamics.

    Two complementary, but equivalent ways are presented to overcome the problem with
microscopic reversibility. The first one roots in Quantum Electrodynamics (which is a time
reversal invariant theory per se) but interfaces with chemical thermodynamics in a non-trivial
way. The other one regards light as a classical, but non-ideal gas of photons. An activity
function for this non-ideal gas is postulated to account for the Planck distribution and an infinite
number of absorption/emission processes (all of them are reversible) to account for the observed
kinetics.

     Both approaches allow to include the first step of photosynthesis in the canonical description of 
biochemical processes. Non-linear flow-force relationship of the process is discussed.