Reactions
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.
