(645d) Thermodynamic Energy and Exergy Efficiency Limits of Photosynthesis

With recent concerns about sustainable resources and environmental protection, growing attention has been focused upon biological sources of chemicals and fuels; however, to analyze such bioprocesses for commercial viability, the theoretical efficiencies must be known as an upper-bound on performance. Since almost all exergy contained in biomass originated from solar radiation, photosynthesis is the gateway to sustainable bioprocess development. Yet, despite the mechanism for photosynthesis being known for decades, there is still no well-established definition for its efficiency. The two types of studies that exist in the literature consider either the thermodynamic/physical effects of the process (evaporation, carbon dioxide sequestration, temperature changes) [1], [2], [3] and ignore the complex biochemical reaction mechanism, or the converse [4] [5]. The disjunction is due to the dichotomy that exists between biology and traditional thermodynamics. This study dissects the complex bio-processes involved in photosynthesis and analyzes their energy and exergy effects, bridging the gap and creating a clearer picture that is acceptable to both thermodynamicists and biologists.  For the light reactions, reduction potentials were used to analyze the flow of excited, high-energy electrons through photosystem II and photosystem I, resulting in an exergy efficiency of 25%. For the dark reactions, free energies of formation were taken from the literature and used to calculate the Gibbs free energy and exergy changes; exergy changes due to concentration changes were assumed to be negligible for both the light and dark reactions. The resulting efficiency of the dark reactions was 89.5%. An exergy balance was formulated for water flows throughout the system to account for exergy destruction due to transpiration. Similarly, an exergy balance was constructed to account for losses due to ATPsynthase as the proton-motive force is used to synthesize ATP. Heuristic values for photorespiration, photo-degradation, and absorbance were taken from the literature and combined with the theoretical analysis above, yielding an overall photosynthetic efficiency of 1.9%. The overall efficiency shows good agreement with recent publications in the literature [1], [2]. Ultimately, the largest losses were due to poor absorbance and electron transfer through the photosystems.

References:

[1] Bisio G, Bisio A. Some Thermodynamic Remarks on Photosynthetic Energy Conversion. Energy Conversion Management 1998; 39(8):741-748.

[2]  Petela R. An approach to the exergy analysis of photosynthesis. Solar Energy 2008; 82: 311-328.

[3] Reis HA, Miguel AF. Analysis of the exergy balance of green leaves. Int. J. Exergy 2006; 3(3): 231-237.

[4] Lems S, van der Kooi HJ, de Swaan Arons J. Exergy analyses of the biochemical processes of photosynthesis. Int. J. Exergy 2010; 7(3): 333-351.

[5] Chain RK, Arnon DI. Quantum Efficiency of Photosynthetic Energy Conversion, Proceeding of the National Academy of Science 1977; 74(8): 3377-3381.