Biochemistry Notes on Bioenergetics, Thermodynamics, and Reactions

Bioenergetics

• Definition:
  • Bioenergetics refers to the biological energy flow and transformation processes within living systems.
• Significance:
  • Involves processes that meet energy demands such as digestion and metabolism.
  • Concentrates on the transformation of energy in biological and environmental contexts.
  • Focuses on initial and final energy states of reactants rather than the mechanisms of reactions.

Thermodynamics and Biological Systems

• Thermodynamics: Principles governing the flow of heat, energy, and matter.
  • System: The part of the universe under study.
  • Surroundings: Everything outside the system.
• Types of Systems:
  • Isolated: Exchanges neither matter nor energy.
  • Closed: Exchanges energy but not matter.
  • Open: Exchanges both matter and energy.

Laws of Thermodynamics

1. First Law:
   • Total energy of a system remains constant; energy can be transformed but not created or destroyed.
2. Second Law:
   • Entropy of a closed system always increases, leading to spontaneous processes.

Gibbs Free Energy (G)

• Definition: Energy associated with reactants/products in a reaction; predicts reaction feasibility.
• Forms of Gibbs Free Energy:
  • ΔG: Change in Gibbs Free Energy for a reaction.
  • ΔG°: Standard Gibbs Free Energy change, measured under standard conditions.
• Gibbs Free Energy Behavior:
  • If ΔG < 0: Exergonic (spontaneous, energy is released).
  • If ΔG > 0: Endergonic (non-spontaneous, energy is absorbed).
  • If ΔG = 0: Reaction is at equilibrium.

Factors Affecting ΔG

• ΔG Determined by:
  • Enthalpy (ΔH): Total heat content of the reactants/products.
  • Entropy (ΔS): Degree of disorder or randomness in the system.
• Equation:
  • $\Delta G = \Delta H - T \Delta S$
    Where T is temperature in Kelvin (°K = °C + 273).

Gibbs Free Energy in Gas Reactions

• Equation at Constant Pressure and Temperature:
  • $\Delta G = \Delta G° + RT \ln\left(\frac{[B]}{[A]}\right)$
  • R: Ideal gas constant (8.315 J/mol/K).
• At equilibrium,
  • $\Delta G = 0$ implies
  • $\Delta G° = - RT \ln K_{eq}$.

Biochemical Pathways

• Series of reactions characterized by individual Gibbs Free Energys (ΔG).
• Thermodynamic Feasibility:
  • ΔG pathway can be additive; pathways are feasible if the sum of ΔGs is negative.

Role of Enzymes

• Enzymes catalyze reactions, lowering the activation energy needed.
• Biological reactions may be coupled to overcome positive ΔGs, involving common intermediates.

Adenosine Triphosphate (ATP)

• Energy Currency: Universal energy coupler in cells.
• Important for energy transfer and storage in biochemical reactions.
  • Terminal phosphate bonds are considered high-energy and release energy upon hydrolysis.
  • Hydrolysis:
  • $\text{ATP} + H<em>2O \leftrightarrow \text{ADP} + P</em>i$
  • Energy yield: $\Delta G°' = -31 kJ/mol$.

Coupling Reactions Involving ATP

• ATP allows the coupling of exergonic (energy-releasing) and endergonic (energy-requiring) reactions.
• Example:
  • Conversion of glucose to glucose-6-phosphate coupled with ATP hydrolysis.

Anabolism and Catabolism

• Anabolic Reactions: Synthesize complex molecules from simpler ones (endergonic).
• Catabolic Reactions: Break down complex molecules into simpler ones (exergonic).
• ATP serves as a bridge between these two metabolic types.

Redox Reactions

• Redox Reactions: Coupled reactions involving electron transfer.
  • Oxidation: Loss of electrons.
  • Reduction: Gain of electrons.
• Measurement:
  • Reduction potential (E) indicates the tendency of species to gain electrons, affecting Gibbs Free Energy.
  • Equation:
  • $\Delta G = -nF\Delta E$ where:
    • n: Number of electrons transferred
    • F: Faraday’s constant (96,480 J/V/mol).
• Positive E favors a negative G (forward reaction); negative E favors a positive G (reverse reaction).

Conclusion

• Understanding the principles of bioenergetics, thermodynamics, and Gibbs Free Energy is essential in biochemistry to predict the feasibility and direction of chemical reactions in biological systems.