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.