A Thermodynamic Analysis of the Photosynthesis Reaction by Assigning Photons a Carnot Temperatue

Increasing concentrations in atmospheric carbon dioxide have led to concerns over the use of fossil fuels. These concerns have led to a surge in the development of sustainable alternative fuels. Photosynthesis offers many possibilities since the basic reaction captures carbon dioxide and produces biomass. This biomass can be converted into a various forms of biofuels such as biodiesel. Photosynthetic processes offer the benefit that they are based on sunlight which is a long lasting and freely available energy source. A look at the thermodynamics of a generalised photosynthetic reaction which produces a range of organic compounds shows there is a relationship between the enthalpy (energy) and Gibbs Free Energy (work) over the process. This relationship shows that, for a reversible process, the work and energy needs can be satisfied by a high quality energy source with a Carnot temperature near the temperature of the Sun. By looking at the energy balance over the photosynthetic reaction, the required number of photons can be calculated at different wavelengths, since the energy per photon is determined by its wavelength. A similar calculation can be performed with the Gibbs Free Energy across the reaction and treating the photons as a high quality energy source with a Carnot temperature equal to the surface temperature of the sun. Comparing these results with values obtained from literature discrepancies are observed. The result is that exothermic heat transfer becomes an important issue for real processes.