D23.4 Gibbs Free Energy in Biological Systems

Biological organisms often couple the product-favored hydrolysis of ATP (adenosine triphosphate) to a reactant-favored reaction. Thus ATP hydrolysis reaction can be used to drive a necessary, but thermodynamically unfavorable, reaction.

ATP + H2O ⇌ ADP + H2PO4          ΔrG° = −30.5 kJ/mol

ATP can be made available in an organism where an endergonic reaction needs to occur. Its hydrolysis can then be coupled with the endergonic reaction to yield a thermodynamically favorable overall reaction.

For example, ATP hydrolysis can be used to drive condensation reactions of amino acids to generate proteins as graphically illustrated by the figure below.

Figure: ATP. Exergonic reactions like burning of glucose drive ATP synthesis. The ATP molecules are used to power other endergonic reactions like protein synthesis.

The figure also shows ATP formation from ADP being initially coupled to the glucose oxidization reaction:

C6H12O6 + 6 O2 → 6 CO2 + 6 H2O          Δr=−2880 kJ/mol

which has close to 100× greater capability to do work than the hydrolysis of a single ATP. Hence, the equilibrium for this reaction so strongly favors the products that a single arrow is typically used in the chemical equation as it is essential not reversible. It may not be surprising that glucose and all sugars are very energetic molecules since they are the primary energy source for life.

Exercise: Characteristics of Exergonic Processes

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Chemistry 109 Fall 2021 by John Moore, Jia Zhou, and Etienne Garand is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.