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 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 has its 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 which are reversible) to account for the observed kinetics.

Both approaches allow us to include the first step of photosynthesis in the canonical description of biochemical processes. The nonlinear, flow-force relationship of the process is discussed.