■ The change in free energy AG is the most useful measure for predicting the direction of chemical reactions in biological systems. Chemical reactions tend to proceed in the direction for which AG is negative.
■ Directly or indirectly, light energy captured by photosynthesis in plants and photosynthetic bacteria is the ultimate source of chemical energy for almost all cells.
■ The chemical free-energy change AG°' equals -2.3 RT log Keq. Thus the value of AG°' can be calculated from the experimentally determined concentrations of reactants and products at equilibrium.
■ A chemical reaction having a positive AG can proceed if it is coupled with a reaction having a negative AG of larger magnitude.
■ Many otherwise energetically unfavorable cellular processes are driven by hydrolysis of phosphoanhydride bonds in ATP (see Figure 2-24).
■ An oxidation reaction (loss of electrons) is always coupled with a reduction reaction (gain of electrons).
■ Biological oxidation and reduction reactions often are coupled by electron-carrying coenzymes such as NAD + and FAD (see Figure 2-26).
■ Oxidation-reduction reactions with a positive AE have a negative AG and thus tend to proceed spontaneously.
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