8: Thermodynamic Controls
- Page ID
- 131866
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“The concept of a thermodynamic ladder has over the past three decades almost indelibly colored the environmental scientist’s view of the distribution of microbial activity in groundwater flows.”
— Bethke et al. (2011)
Cells use energy for life functions, including growth and cell maintenance (Russell, 2007; Russell and Cook, 1995; von Stockar et al., 2006). Growth requires energy to drive anabolic reactions that synthesize biological molecules (Russell, 2007). Maintenance requires energy for at least three functions: motility, turnover of biological molecules, and re-establishment of proton gradients across the cell membrane (Russell, 2007). These functions make up much of the work carried out by cells.
To supply energy for these needs, chemotrophs catalyze chemical reactions that release energy. We can assess the net energy yield of a chemical reaction using Gibbs free energy calculations. Thus, Gibbs free energy calculations can tell us which redox reactions within an environment might be useful as sources of energy for microorganisms. Moreover, the free energy yield of a reaction also has the potential to influence rates of microbial reactions. In this chapter, we primarily focus on describing how to carry out Gibbs free energy calculations and how we can use these relationships to construct a framework for evaluating what microbial reactions are occurring in an environment. In Chapter 9, we consider microbial reaction kinetics, including the link to reaction thermodynamics.
- 8.1: Mass action
- The definition, calculation, and reaction equilibrium significance of reaction quotients. Activity and fugacity
- 8.2: Gibbs free energy
- Definition of Gibbs free energy and its use to determine if reactions are exergonic or endergonic. Methods for calculating Gibbs free energy of a reaction under given conditions.
- 8.3: Impacts of changes in product and reactant activities
- How changes in the activity of products and reactants can impact the free energy yield of a reaction, illustrated through examples.
- 8.4: Available, captured, and usable energy
- Quantifying the energy thresholds that must be cleared for a reaction to be useful for catabolism and the limits on the energy a cell can capture from a reaction.
- 8.5: Redox zonation
- Effects of zonation in electron acceptor use, and reasons for why a redox zonation model for predicting microbial processes is an oversimplification.
- 8.6: Characterizing redox processes in aqueous systems
- Analysis of routinely collected water chemistry data to estimate the redox processes in aqueous environments.