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Explain what is meant by “coupled reactions” and describe how exergonic reactions
can be used to push or pull endergonic reactions in order to get them to proceed.
Glycolysis is the breakdown of glucose to pyruvic acid, and there are nine
intermeddaite products formed, and each one catalyzed by an enzyme.Glycolysis has two
key functions: It generate some ATP from the free energy available from the
rearrangement of the atoms in monosacharides (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 with the generation of much ATP.
Glycolysis is a perfect example of a “coupled reaction”, involving exergonic and
energonic 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 ( for food breakdowns and movement) - this is how they
perfrom coupled reactions.
ndergonic is when chemical reactions with a negative standard free energy change.
These reactions dont proceed spontaneously in the direction concentartions of all reactants
and products. Since the sign of a standrad 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 ( for example)
glucose-6-phosphate into a nearly identical compound fructose-6-phosphate:
The positive G shows that it is an “uphill”, endergonic reaction, one that couldnt have
happend spontaneously.This was because the coupled reaction ( they shared a common
intermeddiate molecule), glucose-6-phsphate, the product of step 1 and the reactant of
step 2 - can proceed as a single reaction. The -4.0 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, with the favorable net free energy change of
the steps taken together, allowing the series of reactions to go on.
Word Count: 385
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