Exergonic and Endergonic Reactions

• Exergonic and endergonic reactions are two types of chemical processes classified by their energy changes.
• • An exergonic reaction releases free energy to the surroundings, resulting in a negative ΔG (ΔG < 0).
  • An endergonic reaction requires an input of free energy from the surroundings, resulting in a positive ΔG (ΔG > 0).

1. Exergonic Reactions (Energy-Releasing Reactions)

• Definition: The products have lower free energy than the reactants, meaning energy is released.
• ΔG Value: Negative (ΔG < 0) → Spontaneous reaction.
• Biological Importance: Energy released is harnessed to power cellular processes (e.g., ATP synthesis).
• Example:
  • Cellular Respiration (Glucose Breakdown)
    • C6H12O6 + 6O2 → 6CO2 + 6H2O, ΔG=−686kcal/mol
  • Releases stored energy, which cells use for ATP production.

2. Endergonic Reactions (Energy-Consuming Reactions)

• Definition: The products have higher free energy than the reactants, meaning energy input is required.
• ΔG Value: Positive (ΔG > 0) → Non-spontaneous reaction.
• Biological Importance: Often coupled with exergonic reactions to proceed.
• Example:
  • Photosynthesis (Glucose Synthesis)
    • 6CO2 + 6H2O → C6H12O6 + 6O2, ΔG=+686kcal/mol
  • Requires energy input from sunlight to form glucose.

3. Coupling of Exergonic and Endergonic Reactions

• Concept: Exergonic reactions release energy that drives endergonic reactions.
• Example: ATP Hydrolysis (Exergonic) Powers Cellular Work (Endergonic).
  • ATP hydrolysis:
    • ATP → ADP + Pi, ΔG=−7.3kcal/mol
  • This energy fuels muscle contraction, active transport, and biosynthesis.
• This coupling ensures efficient energy transfer, enabling cells to sustain essential functions.

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