Coupled Reactions

Coupled Reactions: An Overview

• Coupled reactions are a pair of chemical reactions in which one of the reactions provides the energy needed for the second reaction to occur.
• These reactions are typical in cells where the energy released from exergonic reactions (spontaneous reactions that release energy) drives endergonic reactions (nonspontaneous reactions that require energy input).

Understanding Exergonic and Endergonic Reactions

• Exergonic reactions are energy-releasing reactions. They are generally catabolic, breaking down complex molecules into simpler ones.
• Endergonic reactions, on the other hand, require energy to proceed and are considered energy-absorbing reactions. These reactions are often anabolic, building up complex molecules from simpler ones.

Adenosine Triphosphate (ATP) and Coupled Reactions

• ATP, or adenosine triphosphate, often acts as the “energy currency” of the cell, and is typically involved in coupled reactions.
• The hydrolysis of ATP into adenosine diphosphate (ADP) and inorganic phosphate (Pi) is an exergonic process, which releases energy that can be used to fuel endergonic reactions.

Calculating the Free Energy Change (∆G)

• The value of ∆G, or the change in Gibbs Free Energy, determines whether a reaction is exergonic (∆G < 0) or endergonic (∆G > 0).
• For coupled reactions, the overall ∆G value is the sum of the ∆G values of the individual reactions. If the total ∆G is negative, the coupled reaction is spontaneous.

Importance of Coupled Reactions

• Without coupled reactions, many necessary and vital biochemical reactions in organisms would not occur, leading to life ceasing to exist.
• Enzymes often facilitate these reactions, allowing the survival of cells and thus, organisms.