Overview
This lecture covers bioenergetics, focusing on Gibbs free energy, its calculation, relationship to equilibrium, and application to biochemical reactions in the cell.
Introduction to Bioenergetics and Gibbs Energy
- Bioenergetics studies energy flow in biological systems.
- Gibbs free energy (G) determines if reactions are favorable in biochemistry.
- J. Willard Gibbs defined the concept of potential (free) energy in compounds.
- Changes in Gibbs energy can be used or transferred during reactions.
Types and Measurement of Gibbs Energy
- ΔG (delta G) is the actual change in free energy under current conditions.
- ΔG°′ (delta G naught prime) is the biochemical standard change in free energy, defined at pH 7, 1 atm, 25°C, and 1 M concentrations.
- Negative ΔG means a reaction is favorable; positive ΔG means unfavorable as written.
- The Gibbs equation: ΔG = ΔH − TΔS (enthalpy minus temperature times entropy change).
Equilibrium and the Relationship between ΔG and Keq
- At equilibrium, the ratio of product to reactant concentrations defines the equilibrium constant (Keq).
- ΔG°′ is related to Keq by the equation: ΔG°′ = –RT ln Keq (R = gas constant, T = temperature in Kelvin).
- If Keq > 1, the reaction proceeds spontaneously toward products (negative ΔG°′).
Actual Cellular Conditions and the Mass Action Ratio
- Most cellular reactions do not occur at standard conditions; instead, use the mass action ratio (Q): Q = [products]/[reactants] at actual concentrations.
- The more general equation: ΔG = ΔG°′ + RT ln Q.
- At equilibrium, Q = Keq and ΔG = 0; away from equilibrium, ΔG can be strongly negative.
Standard Gibbs Energy Values and Experimental Determination
- ΔG°′ values are measured experimentally under standard conditions and published in tables.
- To determine ΔG°′, start with 1 M concentrations, let the reaction reach equilibrium, measure concentrations, calculate Keq, and use ΔG°′ = –RT ln Keq.
Examples and Application to Biochemical Reactions
- Calculating ΔG for a reaction requires ΔG°′, the concentrations, and temperature.
- Even if ΔG°′ is positive (unfavorable), actual ΔG can be negative (favorable) in cells due to concentration differences.
- Examples: Enolase and aldolase reactions in glycolysis show this distinction.
- Enzymes (catalysts) speed up reactions but do not change ΔG or Keq.
Key Terms & Definitions
- Gibbs free energy (G) — The energy in a system available to do work at constant temperature and pressure.
- ΔG — Actual change in Gibbs free energy under given conditions.
- ΔG°′ — Standard change in Gibbs free energy at pH 7, 1 atm, 25°C, 1 M.
- Keq — Equilibrium constant; ratio of product to reactant concentrations at equilibrium.
- Q (mass action ratio) — Ratio of products to reactants under current (not equilibrium) conditions.
- Enthalpy (ΔH) — Heat content or bond energy of a system.
- Entropy (ΔS) — Measure of disorder or randomness in a system.
Action Items / Next Steps
- Review tables of standard ΔG°′ values in your textbook.
- Practice using the Gibbs equation to solve for ΔG using given concentrations and temperatures.
- Bring a calculator to class/exams for Gibbs energy calculations.
- Prepare for practice problems on calculating ΔG for specific biochemical reactions.