Biochemistry - The Chemical Reactions of Living Cells, Volume 1 - D. Metzler 1980

Bioenergetics of Biochemical Reactions
Thermodynamics and Vital Processes
Bacterial Growth Efficiency

In parallel with rather unsuccessful attempts to formulate a generalized thermodynamic theory applicable to living systems, purely empirical observations of growth processes and energy consumption in living systems have revealed A number of fascinating facts. Anaerobic Fermentation processes, during which Cells harness the energy of Chemical Reactions to synthesize ATP, have been studied in considerable detail (Chap. 9). As a rule, the stoichiometry of these reactions is well established, making it possible to estimate with high precision The amount of ATP synthesized per given amount of fermented substrate. It is also straightforward to measure the biomass generated during fermentation; for instance, one can harvest a culture of rapidly growing Bacteria, wash, dry, and weigh it. It turns out that, regardless of the specific substrate being fermented (with few exceptions), the $Y_{\text{ATP}}$ value—defined as the dry weight of cells in grams per mole of synthesized ATP—remains remarkably constant [22, 31], hovering around 10.5. Another notable finding is that for bacteria whose growth and division (under aerobic conditions) yield exclusively $\text{CO}_2$ and Water, $40\pm5\%$ of the consumed carbon and hydrogen is oxidized to $\text{CO}_2$ and water, while $60\pm5\%$ is assimilated by the cells. Notably, this percentage of assimilated material is significantly higher than that observed in anaerobic fermentation, where the overwhelming majority of the substrate is fermented rather than assimilated. As we will discuss later, this difference stems from the fact that oxidation yields a substantially higher amount of ATP than fermentation does.

Of the Free energy of substrate oxidation, $\sim62\%$ represents the free energy of combustion of the components that make up the dried bacterial mass. Thus, $-\Delta G_f \simeq \Delta H_c \simeq 22\text{ kJ per gram of dry bacterial weight [31]}$.