In anaerobic conditions there is no overall oxidation of the
monosacharide so why is there a free energy change big enough to be used
to make ATP? The answer comes from comparing the structure of
glyceraldehyde with the structure of lactate. The conversion of
glyceraldehyde to lactate is the net result of the last two stages of
glycolysis. This conversion involves a simple swap of atoms.
(Although achieved in several individual reactions.)
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As you know, oxidation is loss of electrons. In aerobic respiration
it is molecular oxygen which ultimately accepts electrons from 'fuel'
molecules. Although there is no loss of electrons in anaerobic
glycolysis, there is a certain amount of partial transfer of electrons
from the carbon and hydrogen atoms to the oxygen atoms within the fuel
molecule. In essence, some atoms partly 'oxidise' others and are
themselves 'reduced'.
When the atoms of glyceraldeyhde are rearranged to form lactate a
-COOH group is formed. The bonding electrons in this group are more
mobile than in most bonds of organic molecules and so the
electronegative oxygen atoms are able to pull them towards their nuclei.
In a way there is an internal, partial redox. This is also the
reason why the group is acidic - the electrons in the O-H bond are
pulled away from the hydrogen nucleus so hard that the bond breaks.
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When electrons move closer to atoms which attract them there is
a drop in potential energy and so there is energy available to do work.
i.e. make ATP from ADP and phosphate. It's possible that stages two
and three evolved earlier than stage one in anaerobic organisms which
utilised sugars simpler than glucose.
Please note that the animation is a rough summary of several individual
reactions.
It fails to take into account the change in chirality at carbon atom 2 and omits
the acid dissociation which will occur at physiological pH.
