(94d) A Brief Thermodynamic Analysis of the Photosynthetic Reaction

Increased atmospheric carbon dioxide concentration has led to a global environmental focus on methods to capture and reduce this gas. At the same time, an interest and demand has been sparked to develop alternative fuel sources that encompass sustainability and a clean-burning character. In nature, the process of photosynthesis in biological systems is relevant to both these concerns, as the reaction itself is a means of capturing and therefore reducing carbon dioxide. At the same time, the biomass created by the reaction can be processed to contribute to the production of biofuels. A common example of this is the large-scale growth of algae in order to produce algal fuels such as biodiesel. The benefit of the photosynthetic process is its reliance on light energy from the sun, which can be considered as a ?free?, long-lasting energy source. By analysing the photosynthetic reaction thermodynamically at ambient temperature, the light energy or photon requirements relative to carbon dioxide uptake and carbohydrate production can be determined. This allows a mathematical relationship to be established between available light energy and the amount of biomass that can be grown. By looking at the energy and entropy balances over the photosynthetic reaction, as well as the change in Gibbs Free Energy across this reaction, necessary photon amounts are calculated at different wavelengths, since energy per photon is variable. Comparing these amounts with experimental results from literature, discrepancies are revealed, and so the issue of exothermic heat or energy transfer becomes a point of interest for discussion.

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