Thermodynamics and Bioenergetics
High-Yield Summary
- First law of thermodynamics: energy is conserved, only transformed between forms. ΔU = q + w (internal energy = heat + work).
- Systems: isolated (no energy or matter exchange), closed (energy only), open (energy and matter). Biological systems are open in principle but often treated as closed for analysis.
- Gibbs free energy: ΔG = ΔH − TΔS. ΔG < 0 = spontaneous, ΔG > 0 = nonspontaneous, ΔG = 0 = equilibrium.
- ΔG° = standard conditions (1 M, 1 atm, 25°C); biochemistry uses ΔG°′ (adds pH 7). Actual cellular ΔG = ΔG° + RT ln Q, accounting for real (non-standard) concentrations.
Key Terms
- Bioenergetics
- The study of how energy is stored, transferred, and utilized in living organisms.
- Enthalpy (ΔH)
- Overall heat change of a reaction, reflecting bond-energy differences between reactants and products.
- Entropy (ΔS)
- Measure of disorder/energy dispersion; positive ΔS = greater disorder.
- Reaction quotient (Q)
- Ratio of product to reactant concentrations (each raised to its stoichiometric coefficient) at a given moment, used to find actual ΔG.
Gibbs Free Energy
ΔG = ΔH − TΔS
- ΔG = Change in Gibbs free energy — determines spontaneity
- ΔH = Change in enthalpy (heat)
- T = Temperature (Kelvin)
- ΔS = Change in entropy
- ΔG < 0: spontaneous. ΔG > 0: nonspontaneous. ΔG = 0: equilibrium.
Actual Cellular Free Energy
ΔG = ΔG° + RT ln Q
- ΔG° = Standard free energy change (ΔG°′ in biochemistry, includes pH 7)
- R = Gas constant
- Q = Reaction quotient (products/reactants at current concentrations)
- Needed because cells never actually operate at 1 M/1 atm/25°C standard conditions.
Enthalpy/Entropy Combinations and Spontaneity
| ΔH / ΔS | Spontaneity |
|---|---|
| Positive / Positive | Spontaneous at high temperatures |
| Positive / Negative | Nonspontaneous at all temperatures |
| Negative / Positive | Spontaneous at all temperatures |
| Negative / Negative | Spontaneous at low temperatures |
Common MCAT Trap
- ΔG tells you spontaneity, NOT rate — a spontaneous reaction (ΔG < 0) can still be slow without a catalyst.
- ΔG°′ (biochemistry standard) includes pH 7, unlike general chemistry's ΔG° — don't treat them as interchangeable.
- Negative/Negative and Positive/Positive cases are temperature-DEPENDENT — read the table direction carefully (high vs. low T).
Quick Recall
What does a negative ΔG indicate?
What extra condition does ΔG°′ add compared to ΔG° in general chemistry?
Write the equation for actual (non-standard) cellular free energy.