Study guides › B/B › Biochemistry
Bioenergetics & Thermodynamics of Metabolism
Every metabolic reaction obeys thermodynamics. You will use free energy and equilibrium constants, see how ATP hydrolysis drives unfavorable reactions, read reduction potentials, and explain how cells control flux away from equilibrium.
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What this chapter covers
- Free Energy, Equilibrium & Standard Conditions. The MCAT loves the difference between ΔG and ΔG°′, and the link between ΔG°′ and the equilibrium constant.
- Coupled Reactions, ATP & High-Energy Compounds. ATP drives unfavorable reactions through coupling; expect calculations with ΔG°′ values and comparisons of phosphoryl transfer potentials.
- Redox Reactions & Reduction Potentials. Reduction potentials explain the order of the electron transport chain and why NADH yields more ATP than FADH₂.
- Metabolic Control, Energy Charge & Kinetic vs Thermodynamic Control. Cells live at steady state, not equilibrium. Know how flux is regulated and how kinetics differs from thermodynamics.
Key points to know
- ΔG < 0 favorable; ΔG = 0 equilibrium; sign of ΔG sets direction
- ΔG = ΔG°′ + RT ln Q; at equilibrium ΔG°′ = −RT ln K′eq
- ATP hydrolysis ΔG°′ ≈ −30.5 kJ/mol; cellular ΔG ≈ −50 to −65
- Coupling: add ΔG°′ values of reactions sharing an intermediate
- Electrons flow to the higher (more positive) E°′
- ΔG°′ = −nFΔE°′; positive ΔE°′ means negative ΔG°′
- Cells operate at steady state, far from equilibrium
- Rate-limiting steps are irreversible and are regulated
Try a practice question
A reaction in a cell has ΔG = 0. What is true?
- It is at equilibrium and Q equals K′eq
- It is at standard conditions
- It is irreversible
- It has stopped because the enzyme is inactive
Show the answer
A. ΔG = 0 means no net driving force, so forward and reverse rates are equal and Q = K′eq. It says nothing about enzyme activity or standard conditions.
Common questions
What does this chapter cover?
Bioenergetics & Thermodynamics of Metabolism covers Free Energy, Equilibrium & Standard Conditions, Coupled Reactions, ATP & High-Energy Compounds, Redox Reactions & Reduction Potentials, Metabolic Control, Energy Charge & Kinetic vs Thermodynamic Control.
What should I know first?
ΔG < 0 favorable; ΔG = 0 equilibrium; sign of ΔG sets direction ΔG = ΔG°′ + RT ln Q; at equilibrium ΔG°′ = −RT ln K′eq ATP hydrolysis ΔG°′ ≈ −30.5 kJ/mol; cellular ΔG ≈ −50 to −65
21 original practice questions accompany this chapter in the app. Spaced review brings missed ideas back at the right time; it helps memory but cannot promise you will not miss an idea again.