Anaerobic Respiration: Homolactic Fermentation
After Glycolysis
Glycolysis, as we have just described it, is an anaerobic
process. None of
its nine steps involve the use of oxygen. However, immediately upon finishing
glycolysis, the cell must continue respiration in either an aerobic or
anaerobic direction; this choice is made based on the circumstances of the
particular cell. A cell that can perform aerobic respiration and which finds
itself in the presence of oxygen will continue on to the aerobic citric acid
cycle in the
mitochondria. If a cell able to
perform aerobic respiration is in a situation where there is no oxygen (such as
muscles under extreme exertion), it will move into a type of anaerobic
respiration called homolactic fermentation. Some cells such as yeast are
unable to carry out aerobic respiration and will automatically move into a type
of anaerobic respiration called alcoholic fermentation.
Figure 3.1: Anaerobic vs. Aerobic pathways
More specifically, the differences in aerobic and anaerobic respiration rest on
the different very roles played by the NADH molecule produced in step 5 of
glycolysis. In both aerobic and
anaerobic respiration, the NADH molecule is part of the enzyme complex and must
be restored to its NAD, oxidized state. If there are aerobic conditions,
meaning oxygen is available, the NADH molecule can be transported to the
mitochondria where it can be immediately
converted back to NAD and plays a role in the electron transport
chain. However, under
anaerobic, oxygen-deficient conditions, NADH gets converted back to NAD through
anaerobic mechanisms, whether homolactic or alcoholic fermentation.
Homolactic Fermentation
Instead of being immediately reoxidized after glycolysis step 5 as it would in
aerobic respiration, the NADH molecule remains in its reduced form until
pyruvate has been formed at the end of glycolysis. The pyruvate product of
glycolysis gets further acted upon under anaerobic conditions by the enzyme
lactate dehydrogenase (LDH).
Figure 3.2: Homolactic Fermentation.
In this reaction, the hydrogen from the NADH molecule is transferred to the
pyruvate molecule. This results in its carbon-oxygen double bond being reduced
to a carbon-oxygen single bond with the addition of a hydrogen atom. The result
is the molecule lactate. From the lactate product, lactic acid can be formed,
which causes the muscle fatigue that accompanies strenuous workouts where oxygen
becomes deficient.
Alcoholic Fermentation
There is another way that the NADH molecule can be re-oxidized. Anaerobic
conditions in yeast convert pyruvate to carbon dioxide and ethanol. This occurs
with the help of the enzyme pyruvate decarboxylase which removes a carbon
dioxide molecule from the pyruvate to yield an acetaldehyde. The acetaldehyde
is then reduced by the enzyme alcohol dehydrogenase which transfers the
hydrogen from NADH to the acetaldehyde to yield NAD and ethanol. This enzyme is
not found in humans.
Figure 3.3: Alcoholic Fermentation.
Anaerobic Byproducts
As you can see, both of these anaerobic conditions leads to glycolytic products
other than pyruvate. These different products are necessary because the NADH
molecule must be reoxidized so that it can function in the next round of
glycolysis of newly introduced glucose. If oxygen is not present to help
oxidize it, other reactions, such as those of homolactic and alcoholic
fermentation, must occur.