Overview
This lecture covers Gibbs free energy in bioenergetics, how it defines reaction favorability, the calculation of ΔG and ΔG°', and the connection between equilibrium constants and energy in biochemical reactions, including practice problems.
Gibbs Free Energy and Reaction Favorability
- Gibbs energy (G) measures the potential energy available to do work in a system.
- ΔG (change in free energy) determines if a reaction is thermodynamically favorable.
- Negative ΔG means the reaction is favorable (spontaneous); positive ΔG means unfavorable (non-spontaneous).
- Gibbs equation: ΔG = ΔH - TΔS (where ΔH = enthalpy change, T = temperature in K, ΔS = entropy change).
Standard vs. Actual Free Energy Change
- ΔG (actual) is the energy change under current (cellular) conditions.
- ΔG°' (standard) is the energy change under biochemical standard conditions (pH 7, 1 atm, 25°C, 1M concentrations).
- Standard values are tabulated and used for reference in calculations.
Equilibrium Constant and Gibbs Energy
- At equilibrium, ΔG = 0, and the ratio of products to reactants (Keq) is constant.
- ΔG°' = -RT ln(Keq), where R = gas constant (8.3 x 10^-3 kJ/mol·K), T = temperature in Kelvin.
- A Keq > 1 (favoring products) gives a negative ΔG°'; Keq < 1 gives a positive ΔG°'.
Mass Action Ratio and Non-Equilibrium Reactions
- The mass action ratio (Q) = [products]/[reactants] at any non-equilibrium state.
- Actual ΔG is calculated by: ΔG = ΔG°' + RT ln(Q).
- Cellular reactions rarely occur at equilibrium; Q accounts for actual concentrations in cells.
Experimental Determination and Calculations
- ΔG°' values are measured by allowing reactions to reach equilibrium under standard conditions, then using concentrations to compute Keq and thus ΔG°'.
- Example calculation: Use measured concentrations at equilibrium to find Keq, then apply ΔG°' = -RT ln(Keq).
- For actual ΔG in cells, plug ΔG°', R, T, and the mass action ratio Q into ΔG = ΔG°' + RT ln(Q).
Practice Problems and Biological Implications
- Practice calculating ΔG for biochemical reactions using provided ΔG°' and concentration data.
- Reaction conditions (concentration, temperature) can shift favorability; high reactant/low product concentrations drive reactions forward (Le Chatelier's Principle).
- Biological systems maintain reactions far from equilibrium for energy flow and life.
Key Terms & Definitions
- Gibbs free energy (G) — Energy in a system available to do work at constant temperature and pressure.
- ΔG — Actual change in free energy under cellular conditions.
- ΔG°' — Standard change in free energy at biochemical standard state.
- Enthalpy (ΔH) — Heat content change, primarily from bond changes.
- Entropy (ΔS) — Measure of disorder or randomness.
- Keq — Equilibrium constant; ratio of product to reactant concentrations at equilibrium.
- Q (mass action ratio) — Ratio of products to reactants at any point, not necessarily at equilibrium.
- Exergonic — Reaction releases energy (ΔG < 0).
- Endergonic — Reaction requires energy input (ΔG > 0).
Action Items / Next Steps
- Review and solve practice calculations for ΔG and ΔG°' using sample values.
- Read textbook tables of standard Gibbs free energies for reference.
- Bring a calculator for future quizzes/exams involving Gibbs energy problems.
- Prepare for discussion on how enzymes influence reaction rates but not ΔG.