Chemical reactions in cells are classified as Exergonic and Endergonic Reactions based on the change in Gibbs free energy (ΔG).
1.Exergonic Reactions (Energy-Releasing Reactions)
In exergonic reactions, the products have lower free energy than the reactants, meaning energy is released during the reaction.
The ΔG for exergonic reactions is negative (G<0), indicating the reaction can occur spontaneously.
These reactions are often used to power cellular processes, as the released energy can be harnessed to do work (e.g., ATP synthesis).
Example: The breakdown of glucose in cellular respiration is exergonic, as it releases energy stored in glucose molecules:
C6H12O6 + 6O2 → 6CO2 + 6H2O, ΔG=−686 kcal/mol
2.Endergonic Reactions (Energy-Consuming Reactions)
In endergonic reactions, the products have higher free energy than the reactants, meaning energy is required to drive the reaction.
The ΔG\Delta GΔG for endergonic reactions is positive (ΔG>0), indicating the reaction is non-spontaneous and needs an input of energy to proceed.
These reactions are often coupled with exergonic reactions in biological systems to ensure they occur.
Example: The synthesis of glucose in photosynthesis is endergonic, requiring energy input from sunlight:
6CO2 + 6H2O → C6H12O6 + 6O2, ΔG = +686 kcal/mol
Coupling of Exergonic and Endergonic Reactions
In biological systems, exergonic and endergonic reactions are often coupled to allow energy transfer.
The energy released from an exergonic reaction can be used to drive an endergonic one.
A common example is the coupling of ATP hydrolysis (an exergonic reaction) with endergonic cellular processes, such as muscle contraction, active transport, or biosynthesis.
When ATP is broken down into ADP and an inorganic phosphate (Pi), it releases energy (ΔG=−7.3kcal/mol), which can be used to drive reactions that require energy input.