Explain what is meant by coupled reactions” and describe how exergonic reactions can be used to push or pull endergonic reactions in order to make them proceed.
Glycolysis is the breakdown of glucose to pyruvic acid, with nine intermediate products formed, each catalyzed by an enzyme. Glycolysis has two key functions: it generates ATP from the free energy available from the rearrangement of atoms in monosaccharides, particularly glucose. It also partially breaks down glucose and provides a starting point for the complete oxidation of glucose by another pathway to carbon dioxide and water, generating much ATP. Glycolysis is a perfect example of a coupled reaction” involving exergonic and endergonic reactions.
Exergonic reactions release energy. The bond energy of the product or products is lower than that of the reactants. Endergonic reactions require energy input. The energy of the products is higher than that of the reactants. An exergonic reaction can drive endergonic reactions, such as food breakdowns and movement. This is how they perform coupled reactions. Endergonic reactions have a negative standard free energy change.
These reactions don’t proceed spontaneously in the direction of concentrations of all reactants and products. Since the sign of a standard free-energy change is negative, the conversion of glucose 1-phosphate to glucose 6-phosphate is an exergonic process. An initial reaction for glycolysis (a molecule of ATP donates its phosphate group to the glucose): Coupled reaction occurs in glycolysis when it tries to convert glucose-6-phosphate into a nearly identical compound fructose-6-phosphate. The positive G shows that it is an uphill”, endergonic reaction, one that couldn’t have happened spontaneously. This was because the coupled reaction (they shared a common intermediate molecule), glucose-6-phosphate, the product of step 1 and the reactant of step 2, can proceed as a single reaction.
0 kcal/mole broken down in step 1 is combined with the +0.4 kcal/mole taken in by step 2 to yield a net percent change of -3.6 kcal/mole. These two together are strongly exergonic, so the reaction proceeds. The glycolytic pathway is an arrangement of these kinds of coupled reactions, where exergonic steps push or pull endergonic steps. The favorable net free energy change of the steps taken together allows the series of reactions to continue.
