Key Concepts of Section 2.4

• The change in free energy, ΔG, is the most useful measure for predicting the potential of chemical reactions to occur spontaneously in biological systems. Chemical reactions tend to proceed spontaneously in the direction for which ΔG is negative. The magnitude of ΔG is independent of the reaction rate. A reaction with a negative ΔG is referred to as thermodynamically favorable.

• The chemical free-energy change, ΔG°′, equals –2.3 RT log K_eq. Thus the value of ΔG°′ can be calculated from the experimentally determined concentrations of reactants and products at equilibrium.

• The rate of a reaction depends on the activation energy needed to energize reactants to a transition state. Catalysts such as enzymes speed up reactions by lowering the activation energy of the transition state.

• A chemical reaction having a positive ΔG can proceed if it is coupled with a reaction having a negative ΔG of larger magnitude.

• Many otherwise energetically unfavorable cellular processes are driven by the hydrolysis of phosphoanhydride bonds in ATP (see Figure 2-31).

• Directly or indirectly, light energy captured by photosynthesis in plants, algae, and photosynthetic bacteria is the ultimate source of chemical energy for nearly all cells on Earth.

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  An oxidation reaction (loss of electrons) is always coupled with a reduction reaction (gain of electrons).

• Biological oxidation and reduction reactions are often coupled by electron-carrying coenzymes such as NAD⁺ and FAD (see Figure 2-33).

• Oxidation-reduction reactions with a positive ΔE have a negative ΔG and thus tend to proceed spontaneously.