Professor Dave here, let’s get anaerobic.
In the biochemistry series, we looked at aerobic
respiration.
This is the process by which the cells in
our body generate ATP, through glycolysis,
the citric acid cycle, and oxidative phosphorylation.
This third step is one that produces the bulk
of the ATP payout, and it relies on the presence
of molecular oxygen, because oxygen is the
final electron acceptor in the electron transport chain.
So without oxygen, this process can not occur,
hence the term aerobic, which means involving
oxygen.
But there are other methods of ATP production
that do not rely on the presence of oxygen
that we will want to understand now that we
have begun to look at different kinds of organisms.
Those would be anaerobic respiration, and
fermentation.
The latter is used by animals such as ourselves
when the body is not getting enough oxygen
to fuel strenuous activity, and both processes
are also used as a metabolic pathway by certain
microorganisms that exist primarily in environments
without oxygen, so let’s get a closer look
at these now.
With anaerobic respiration, an electron transport
chain is still used, but the final electron
acceptor at the end of the chain is something
other than molecular oxygen.
For example, some bacteria will use a sulfate
ion, and produce hydrogen sulfide instead
of water as a byproduct of this activity,
which produces an odor of rotten eggs that
you may have smelled in certain environments.
Fermentation on the other hand, does not involve
respiration of any kind.
This will always begin with glycolysis, just
as we learned before, as this step does not
require oxygen, and does result in the production
of 2 ATP per glucose molecule.
But the pyruvate that results with oxidation
by NAD+ will not feed into the citric acid cycle.
Instead something else will happen, depending
on which type of fermentation is occurring,
so as to regenerate NAD+ from the NADH that
is produced, allowing glycolysis to continue.
The two types of fermentation are alcohol
fermentation and lactic acid fermentation.
With the first of these, glucose is oxidized
over the ten steps we learned about, to produce
two pyruvate molecules and two ATP molecules,
using two molecules of NAD+ in the process.
Then, each pyruvate will undergo decarboxylation
to produce acetaldehyde.
And finally, acetaldehyde is reduced by NADH
to produce ethanol.
This allows for the regeneration of NAD+,
which can then be used in glycolysis again.
The final product, ethanol, is the kind of
alcohol that humans consume, hence the term
alcohol fermentation, and this process has
been utilized for centuries to produce alcoholic
beverages such as beer and wine.
Yeast, which is a unicellular member of the
fungi kingdom, performs alcohol fermentation,
so this organism is used in these processes
to produce the alcohol we drink, and also
by bakers, where the carbon dioxide gas produced
during the decarboxylation step is what causes
bread to rise.
Next, looking at lactic acid fermentation,
again glycolysis occurs precisely as we have
learned, but this time the pyruvate that is
produced will not decarboxylate, instead it
will be directly reduced by NADH to produce lactate.
This again allows for the regeneration of
NAD+ for use during glycolysis, and also involves
no release of carbon dioxide.
Lactate is the conjugate base of lactic acid,
and it is the production of this compound
by certain fungi and bacteria that allows
them to be used in the industrial production
of cheese and yogurt.
In addition, human muscle cells will resort
to this process when the oxygen supply is
running low, most typically during prolonged
periods of strenuous exercise, when glucose
catabolism is going faster than our ability
to breathe enough oxygen into the bloodstream
to sustain the activity.
In such a case, the cells will switch over
to fermentation, allowing for a brief burst
of additional energy production, but the lactic
acid that builds up in the muscles during
this activity causes muscle fatigue, which
limits the duration that this activity can
be sustained.
So to summarize, all of the forms of energy
production that we have learned, whether aerobic
or anaerobic, begin with glycolysis.
This is therefore the most evolutionarily
ancient method of ATP production, and must
have evolved in very early forms of prokaryotic
life, given that it occurs in the cytosol
and not in any membrane-bound organelle, such
as mitochondria, which did not exist until
eukaryotes came about.
This makes sense with what we know about earth’s
early atmosphere, which did not contain oxygen,
until the evolution of photosynthetic cyanobacteria,
which prompted large-scale oxygenation of
the atmosphere, making the evolution of aerobic
respiration possible.
In every living organism, once glycolysis
is complete, pyruvate may be reduced through
lactic acid fermentation, or broken down and
then reduced through alcohol fermentation.
It can also be fed into the citric acid cycle
to continue with aerobic respiration, or anaerobic
respiration if something other than oxygen
is available to act as an electron acceptor
in the absence of oxygen.
In all of these processes, the NADH that forms
when NAD+ is reduced during glycolysis must
be oxidized to regenerate NAD+, and this is
achieved either when NADH reacts with pyruvate
or acetaldehyde during fermentation, or when
NADH dumps electrons into the electron transport
chain during respiration.
The main difference is the electron transport
chain drives oxidative phosphorylation, which
produces most of the 30 to 32 ATP generated
during cellular respiration, whereas fermentation
produces only 2 ATP, given that it is comprised
only of glycolysis, so it’s a huge difference
in terms of energy production.
And with that, we have a more thorough understanding
of the various ways that cells can generate ATP.
