A reevaluation of the thermodynamics of growth of Saccharomyces cerevisiae on glucose, ethanol, and acetic acid

Abstract

Statistically determined values for the free energy and enthalpy change per equivalent accompanying the combustion of organic substances have been used to test the hypothesis that the free energy, enthalpy, and entropy changes accompanying anabolism are zero or nearly so. It is found that while this can be true in some cases, it is not always so, because the free energy and entropy changes per available electron accompanying biological combustion are different for different small molecular weight substrates although they are constant for cellular biomass. If the average energy change per equivalent accompanying the biological combustion of the cells is markedly different from that of the substrate, there will be a corresponding change in energy accompanying anabolism that is determined by the difference between them. The efficiency of available electron conservation is proposed as being preferable to free energy, enthalpy, and entropy efficiencies in that the latter parameters all have different values, whereas the former is common to all three of the latter. Free energy, enthalpy, and entropy efficiencies can be calculated from available electron conservation efficiencies if the biological combustion energies of the substrate, cells, and products are known per electron transferred. The methods described are applicable equally to aerobic and anaerobic growth process systems.Key words: yeast growth thermodynamics, electron conservation.