Ask A Scientist© Molecular Biology Archive

name        Stefanie D.
status      student
age         16

Question -  How does Gibb's Free energy tie in with reduction
reactions and glycolysis and equilibrium???

  Hidden in your question is a far deeper one, "What is thermodynamics all
about?"
The Gibbs Free Energy is a measure of the amount of work that can be
extracted from some process operating at constant pressure. In the
literature it is given the symbol "G" or "F". You will see both. The term
"Free Energy" should be understood in the context of "available" not in the
context of "getting something without any cost".

The Gibbs Free Energy is comprised of two terms: G = H - T*S, where 'H' is
the "enthalpy", "T" is the absolute temperature, and "S" is the "entropy".
The change in Free Energy call it
dG = dH - TdS at a constant temperature. The dH term is the energy change
(at constant pressure), and the dS term is the entropy change (a measure of
the change in the amount of disorder produced by the process.

If dG < 0, (which means dH < 0 and/or dS > 0) that process, say a chemical
reaction will occur spontaneously. Here again, you need to be careful about
the definition of the term "spontaneously". In the context here it means
that the process, given a pathway, occurs spontaneously; it says nothing
about the rate at which the process might occur. So for example, you could
mix H2 and O2 gas in a balloon, and if you are very careful, the two gases
will mix without any appreciable formation of H2O. None the less, that
reaction is still considered "spontaneous" in the context of thermodynamics,
because if you provide a pathway -- a spark, a match, or a wandering cosmic
ray -- the reaction to form water occurs explosively.

A negative value of dG occurs if dH < 0, that means the process releases
energy in some form to the surroundings. The H2 + O2 reaction above is an
example. If the process tends to lead to a final state (or products) that is
more disorganized than the starting state (or reactants), the process
(reaction) will also tend to occur spontaneously. A simple example of this
is the mixing of two gases (A and B) initially in different containers
separated by a barrier. When the barrier (valve or whatever) is removed or
opened the two gases will mix to form a uniform mixture of
A+B, even if the pressure and temperature of the gasses are the same and the
gases do not undergo a chemical reaction.

If dG should happen to be zero, dG = 0, the process (or reaction) is at
equilibrium and no change in the system will/can occur. So the criterion for
determine if a system (process, chemical reaction) will occur, is at
equilibrium, or will not occur is a matter of determining whether dG < 0, dG
= 0, dG > 0, respectively. There are ways of doing this, but that is to much
detail for a forum such as this.

Returning now to your question about the reduction reactions and glycolysis.
Those are just two chemical reactions to which the same general principles
of Gibbs Free Energy apply. If dG < 0, the reaction occurs (assuming some
pathway or catalyst is present); if dG = 0 the reaction is at equilibrium,
and if dG > 0, the reaction, as written will not occur.

I hope you followed this through. Like so many questions we receive, the
question is easy to ask, but whose explanation is much more complicated. We
always run the risk of "over explaining" a question. I hope I didn't do that
here.

Vince Calder
========================================================
The direction taken and equilibrium point approached by any chemical
reaction is governed by the second law of thermodynamics, which states that
total entropy must never decrease. An equilibrium point is when entropy
stays constant, and any deviation would lower the entropy.

The Gibbs free energy function is a convenient way to determine which way a
reaction carried out at constant pressure will go, without having to keep
track of total entropy. (Knowing total entropy requires knowing about both
the system of interest and the surroundings with which it exchanges heat;
the Gibbs free energy allows you to worry only about the system of
interest.) Basically, the second law of thermodynamics combined with the
definition of the Gibbs free energy states that the Gibbs free energy for a
spontaneous process carried out at constant pressure must decrease. For
reduction reactions and the reactions of glycolysis, the Gibbs free energy
must decrease. At equilibrium, the Gibbs free energy is at a minimum: small
changes in any direction will increase it.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
========================================================
Gibb's Free Energy ties into these reactions the same way it does to every
reaction. Gibb's Free Energy is a concept which allows us to predict if a
reaction is thermodynamically favorable, i.e., will the reaction proceed as
written? It combines two important factors, the difference in the enthalpy
between reactants and products (= the energy change at constant pressure),
and the difference in entropy, or randomness, between reactants and products.
     While space does not permit, the calculation of the difference in free
energy for reactions is reasonably straightforward. The bottom line is this:
if the products have less free energy than the reactants, the reaction is
thermodynamically favorable. If conditions are right, e.g., a suitable
activation energy is overcome, the reaction will "go" as written.
     In glycolysis, a six-carbon sugar is broken down to 2 three-carbon
pyruvate molecules through a series of reactions. Picture the intact sugar as
a tennis ball at the top of the stairs. Every step it bounces on as it
bounces down the stairs is a step in the breakdown of the sugar. Each step is
at a lower free energy than the previous steps. The product pyruvate is
represented by the bottom step that the ball ends up resting on. Now in
glycolysis our stairway looks a little strange. Some steps are only an inch
lower than the previous step, and other steps are several feet lower than the
previous step. This represents small and large changes in free energy that
occur in the series of reactions that make up glycolysis. As long as the
overall direction is "down", the overall pathway is thermodynamically
favorable.
     Now, as for equilibrium... Imagine a large amount of pyruvate being
present. This has the effect of shifting the free energy term towards the
reverse reaction. (Any good biochem book will have a section on concentration
and free energy). In our analogy, large amounts of pyruvate result in the
"slope" of our stairway being changed, so that the "bottom" of the stairs is
now at the same level as the "top" of the stairs. Now there is no net force
driving the ball either up or down the stairs; the forward and reverse
reactions are equally favored. The reactants and products are in equilibrium,
and no NET movement will occur.

Paul Mahoney, Ph.D.
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