On p.143 Purves talks about the energy transfer in redox
reactions. The energy goes from the reducing agent to the reduced
product. I know the electrons flow in this direction as well, so are the
electrons representing the energy?
In any oxidation reaction there must also be a reduction. The free energy
change could be positive or negative, depending on the relative stabilities
of the reactants and products, as written. Thus when electrons are
transferred from NADH2 to pyruvate (in fermentation) there is a negative
free energy change. But when electrons are transferred from Gal-3-P to NAD,
there is also a negative free energy change. So in this example, electrons
are flowing either to NAD or away from NADH2, yet free energy is released in
both reactions. So yo cannot make a simple general rule about direction of
electron flow and free energy without a specific reference to all the
reactants and products.
Why does step 6 in glycolysis (a phosphorylation) need NAD but steps
1 and 3 do not, needing an ATP instead?
Each reaction in a biochemical pathway involves a small specific change
as a reactant is converted into a product. The chemical changes are diverse:
some are phosphorylation, some are isomerization, hydration, hydrolyses, etc.
Step 6 in glycolysis needs NAD because it involves an oxidation of an aldehyde
into an acid. That is the change represened by this step. Electrons must be removed
and so there must be an electron acceptor, and NAD fulfills this role here. As to
WHY these changes are orchestrated in this particualr sequence, the answer lies in
the evolution of a series of steps that will allow a cell to produce net ATP from ADp + Pi
via the breakdown of glucose. Thsi may not be theonly way to doit, but it is an
efficient way, and once it got established (presumably billions of years ago), it
spread throughout the living world, such that it now remains a universal biochemical
pathway.