A thermodynamic analysis of photosynthesis: Quantum requirement and theoretical biomass growth rates
Year: 2012
Type: dissertation
Abstract: Interest in photosynthesis continues to increase with a global focus on developing \nalternative fuel sources such as biofuels. It is the various forms of biomass \nthat provide the raw materials for the production of biofuels such as bioethanol \nand biodiesel. The vast majority of these materials are grown by the photosynthesis \nprocess. Developing a mathematical model of the process allows for \na clearer understanding of what limits and propels the process, and what this \nreaction might be capable of in terms of biomass growth on an industrial scale. \nThe primary driving force of photosynthesis, solar energy, is a crucial aspect \nof this model. Relating the amount of light to organism growth is done by exploring \nthe concept of quantum requirement, which is essentially the amount of \nphotons needed to drive the photosynthesis reaction. The theoretical quantum \nrequirement calculated is dependent on the wavelength of incoming photons in \nthe photosynthetically active radiation (PAR) spectrum, and varies between \n9.94 moles of photons per mole of biomass (approximated as glucose) at 400nm, \nand 17.8 moles of photons per mole of biomass at 700 nm. Measured, experimental \nvalues of quantum requirement from the literature considered here \naverage a value of 61.7 moles of photons per mole of biomass required. \nThe conclusion is that the theoretical lower limit of operation, which is \ntherefore the absolute minimum quantum requirement, occurs when the photosynthesis \nprocess operates reversibly. It is suggested that the large difference \nbetween theoretical and measured quantum requirement values may indicate \nthat the photosynthesis process is irreversible in nature. By the model developed, \ntwo other observations of interest are made: photon entropy has little \neffect on the calculated quantum requirement; heat must be rejected from the \nprocess, and increases in magnitude with increasing irreversibility. \nIn the final chapter, the theoretical model for quantum requirement is \nutilised in conjunction with spectral irradiance data to obtain estimates of theoretical \nphotosynthetic growth rates. With a reversible photosynthetic process, \na growth rate of 81:0 g=m2:hr of biomass would be possible if all light energy \nin the PAR spectrum were absorbable. Applying the absorption capabilities of \na commonly-occurring pigment such as chlorophyll a, this theoretical growth \nrate is reduced to 20:0g=m2:hr, to indicate the light saturation and absorptionlimited \neffect of a plant’s cell biology. \nBoth values are still largely above experimentally determined growth rates \nfrom literature. These discrepancies are discussed with focus on relevant enzymes \nand proteins in the light-dependent and light-independent reactions of \nphotosynthesis that limit its kinetics and efficiency. The importance of calculating \nmaximum possible biomass growth rates is that they, in essence, set the \nupper limits on biomass production in our environment. With this knowledge, \nlarge-scale estimates can be made, and improved efficiencies can be targeted \nand measured against the ideal, theoretical case.
Author: Heinrich Wilfrid Kok
Language: English
Subfield: Molecular Biology
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W2235541408