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 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
A) 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.
B) 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.
C) 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 of free energy, enthalpy and entropy
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 process.