Exergonic and Endergonic Reactions

• Chemical reactions in cells are classified as exergonic or endergonic based on changes in Gibbs free energy (ΔG).

1) Exergonic Reactions (Energy-Releasing Reactions)

1. Definition: The products have lower free energy than the reactants', meaning energy is released.

2. ΔG Value: Negative (ΔG < 0) → Spontaneous reaction.

3. Biological Importance: Energy released is harnessed to power cellular processes (e.g., ATP synthesis).

   1. Example:

4. Cellular Respiration (Glucose Breakdown)

5. C6H12O6 + 6O2 → 6CO2 + 6H2O, ΔG=−686kcal/mol

6. 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.