bioenergetics

Meaning: Bioenergetics is the study of how cells transform and transfer energy, using thermodynamics. Cells drive endergonic reactions (ΔG > 0) by coupling them to exergonic ones (ΔG < 0), such as ATP hydrolysis or fuel oxidation, so the summed ΔG is negative. Energy from fuels is captured in ATP and in reduced carriers such as NADH and FADH₂.

Equation: ΔG = ΔG°′ + RT ln Q, where ΔG is the free energy change (J/mol), ΔG°′ is the standard value at pH 7 (J/mol), R = 8.314 J/(mol·K), T is temperature (K), and Q is the reaction quotient (unitless).

On the MCAT: Passages give ΔG°′ values for steps of a pathway and ask whether a step is favorable, whether coupling to ATP hydrolysis lets it proceed, or how changing reactant or product concentration shifts ΔG. ΔG values of coupled reactions add. At equilibrium ΔG = 0 and Q = Keq.

Common confusion: A negative ΔG means a reaction is spontaneous, not fast; enzymes lower activation energy but do not change ΔG or the direction. Also, ATP's energy is not stored in the phosphate bond itself. Breaking a bond takes energy, and the release comes from the products being more stable than the reactants.