The sources of thermal energy exchange accompanying microbial catabolism

Abstract

Calculations are made of thermal energy exchanges from catabolic reactions in the aqueous state, using thermodynamic properties at 0.001 M, indicated by subscript “ B ”. Heats of reaction are calculated as Δ r H B , 298.15 o ′ = Δ r X B , 298.15 o ′ + Δ r Q ab , B , 298.15 o ′ (1), or as Δ r H B , 298.15 o ′ = Δ r G B , 298.15 o ′ + T Δ r S B , 298.15 o ′ (2), where Δ r X B , 298.15 o ′ and Δ r G B , 298.15 o ′ represent non-thermal, chemical energy converted into thermal energy during a reaction and where Δ r Q ab , B , 298.15 o ′ and T Δ r S B , 298.15 o ′ represent the exchange of absorbed thermal energy as reactants become converted into products. Percentages are tabulated of the thermal energy exchanges contributed to Δ r H B , 298.15 o ′ by Δ r X B , 298.15 o ′ and Δ r Q ab , B , 298.15 o ′ , and by Δ r G B , 298.15 o ′ and T Δ r S B , 298.15 o ′ . Aerobically, for substrates not containing nitrogen, the value of Δ r X B , 298.15 o ′ averages 4.21% more negative than that of Δ r G B , 298.15 o ′ . For substrates containing nitrogen this average drops to 1.80%. For substances not containing nitrogen, the thermal energy contributed by Δ r Q ab , B , 298.15 o ′ to Δ r H B , 298.15 o ′ averages 2.21%, whereas that contributed by T Δ r S B , 298.15 o ′ averages −1.95%. The difference is 4.16%, which is close to the average value of 4.21% that Δ r X B , 298.15 o ′ is more negative than Δ r G B , 298.15 o ′ . This observation indicates that the difference between Δ r X B , 298.15 o ′ and Δ r G B , 298.15 o ′ is due almost entirely to the manner in which exchanges in absorbed thermal energies are measured or calculated for the reaction systems studied. The same applies to the oxidation of substances containing nitrogen. Here, the thermal energy contributed to Δ r H B , 298.15 o ′ by Δ r Q ab , B , 298.15 o ′ averages 5.32%, whereas that contributed by T Δ r S B , 298.15 o ′ averages 3.50%. The difference between them is 1.82%, which is close to the average value of 1.80% that Δ r X B , 298.15 o ′ is more negative than Δ r G B , 298.15 o ′ . Anaerobically, for the processes tested fewer inferences could be drawn. The values of Δ r G B , 298.15 o ′ are much more negative than those of Δ r X B , 298.15 o ′ in three examples out of five and widely variable, quite the opposite of the data on the aerobic processes. One anaerobic Δ r X B , 298.15 o ′ value is positive, indicating that something other than the total free energy change must be driving this particular process.