For the handout graph of energy levels in glycolysis, what does the
y-axis represent? The y-axis represents changes in standard
free energy for the reacions numbered in the glycolysis handout. The scale
is set arbitrarily with glucose at zero. To determine the free energy change
for a particular reaction, subtract the level of theproduct from the level
of the reactant. Connecting lines going up represent a positive delta Go
(unfavorable) whereas stepping down represents a favorable deltaGo (release
of free energy, a negative delta Go).
The energy-level handout shows that rxn's 1, 3, 7, and 10 are favorable
while rxn's 2, 4, 5, 6, 8, and 9 are unfavorable. On Purves fig 7-7 (p.
150), rxn's 1-4 are unfavorable and rxn's 5-10 are favorable. Is the
chart in Purves just a simplification?
I find the Purves chart confusing (Fig. 7.7 on the Web, Fig. 7.12 int he
text). First. he refers to delta G, not delta Go. One advantage of using
delta Go's for comparisons is that we will all be on the same page
(standard conditions) even if this is not the condition in the cell. I prefer to use
delta Go for comparisons of reactions, and then worry about cell
conditions.
Purves takes the other (valid) attitude that one should consider the cell
conditions from the start, and not bother with the delta Go's. However, I
have no idea what conditions he is using for Q in the equation for delta,
so cannot easily see if there is any discerpancy between our numbers (if I
would want to). Second, Purves considers just parts of the reactions, not always
taking into account all of the reactants in a coupled reaction. So his
reaction 6 (the oxidation) has a tremendous drop in free energy associated
with it, whereas my characterization of that reaction shows a modest
absorption of free energy. The difference here is that Purves does not
take into account the free energy absorbed in reducing 2 NAD's to 2 NADH2's
(106 kcal/mole). His way of looking at it has the advantage of showing the
great amount of free energy that is being released from glucose at this step.
So the two charts are showing two different views of energy issues in
glycolysis and complement each other if you can keep from being confused.
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