6: Enzyme Thermodynamics
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- 6.1: Enzymes
- Biological catalysts are called enzymes, and the overwhelming majority of enzymes are proteins. The exceptions are a class of RNA molecules known as ribozymes, of which most act upon themselves (i.e. part of the RNA strand is a substrate for the ribozyme part of the strand). In this book (and most textbooks in this field), unless otherwise specified, the term enzyme refers to one made of protein. Enzymes also confer extraordinary specificity to a chemical reaction.
- 6.3: Endergonic and Exergonic Reactions
- Endergonic and exergonic reactions are defined by Gibbs energy changes. Exergonic reactions occur spontaneously (∆G < 0), while endergonic reactions require energy input (∆G > 0). Chemical equilibrium occurs when forward and reverse reaction rates balance, but most biological reactions avoid equilibrium by continuously using energy.
- 6.4: The Laws of Thermodynamics
- Two fundamental concepts govern energy as it relates to living organisms: the First Law of Thermodynamics states that total energy in a closed system is neither lost nor gained — it is only transformed. The Second Law of Thermodynamics states that entropy constantly increases in a closed system.
- 6.5: Thermodynamics
- Energy in biological systems follows the laws of thermodynamics. The First Law states that energy is conserved and transferred, while the Second Law introduces entropy, which always increases, making energy less available for work. Although local decreases in entropy can occur, the universe’s total entropy must always rise.
- 6.6: Energy in Metabolism
- Living organisms consist of highly organized cells that maintain order despite the universal tendency toward disorder in nonliving systems. This organization requires a continuous input of energy, primarily managed through cellular metabolism. ATP serves as the key energy currency, facilitating biochemical reactions governed by Gibbs free energy (∆G). Key principles include standard free energy change, Le Chatelier’s principle, and energy storage in triphosphates.
