Relationship between free energy, enthalpy, and entropy

• The relationship between free energy (G), enthalpy (H), and entropy (S) is fundamental in biochemistry, determining the spontaneity and direction of chemical reactions in biological systems.
• These relationship between free energy (G), enthalpy (H), and entropy (S)thermodynamic parameters are interconnected through the Gibbs free energy equation, which predicts reaction feasibility.

Gibbs Free Energy Equation

• The relationship between these parameters is expressed as:
• ΔG = ΔH − TΔS
• Where:
  • ΔG = Change in free energy
  • ΔH = Change in enthalpy
  • T = Absolute temperature (Kelvin)
  • ΔS = Change in entropy
• This equation explains how enthalpy and entropy influence the spontaneity of a reaction.

Key Thermodynamic Parameters

1. Free Energy (G)

   • Measures the usable energy available for work in a system under constant temperature and pressure.
   • Determines reaction spontaneity:
     • ΔG < 0 → Spontaneous (Reaction proceeds without external energy).
     • ΔG > 0 → Non-spontaneous (Requires energy input).

2. Enthalpy (H)

   • Represents the total heat content of a system, including internal energy and pressure-volume interactions.
   • Indicates heat exchange during a reaction:
     • ΔH > 0 → Endothermic (Heat absorbed from surroundings).
     • ΔH < 0 → Exothermic (Heat released to surroundings).

3. Entropy (S)

   • Measures system disorder or randomness.
   • Determines molecular arrangement and energy distribution:
     • ΔS > 0 → Increased disorder (More randomness).
     • ΔS < 0 → Decreased disorder (More order).

Interplay Between ΔH, ΔS, and ΔG

1. Spontaneous Reactions (ΔG < 0)

   • Occur when the system releases heat (ΔH < 0) and/or increases disorder (ΔS > 0).
   • Example: Cellular respiration, where energy is released, and molecular disorder increases.

2. Non-Spontaneous Reactions (ΔG > 0)

   • Require energy input, often characterized by heat absorption (ΔH > 0) and/or decreasing disorder (ΔS < 0).
   • Example: Photosynthesis, which requires sunlight energy to drive an ordered process.

3. Role of Temperature (T):

   • Higher T amplifies the impact of ΔS in determining ΔG.
   • Some reactions become spontaneous only at high temperatures when TΔS outweighs ΔH.

Biological Significance

• Controls metabolic reactions (e.g., ATP hydrolysis, enzyme catalysis).
• Governs energy storage and release in biochemical pathways.
• Regulates homeostasis and cellular function by managing reaction spontaneity.
• This thermodynamic relationship is essential for understanding how biological systems efficiently manage energy and maintain life processes.2

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