Exergonic 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.