Introduction
Ever
wonder how yeast ferment barley malt into beer? Or how your muscles keep
working when you're exercising so hard that they're very low on oxygen?
Both
of these processes can happen thanks to alternative glucose breakdown pathways
that occur when normal, oxygen-using (aerobic) cellular respiration is not
possible—that is, when oxygen isn't around to act as an acceptor at the end of
the electron transport chain. These fermentation pathways
consist of glycolysis with some extra reactions tacked on at the end.
In yeast, the extra reactions make alcohol, while in your muscles, they make
lactic acid.
Fermentation
is a widespread pathway, but it is not the only way to get energy from
fuels anaerobically (in the absence of oxygen). Some living
systems instead use an inorganic molecule other than \text {O}_2O2O,
start subscript, 2, end subscript, such as sulfate, as a final electron
acceptor for an electron transport chain. This process, called anaerobic
cellular respiration, is performed by some bacteria and archaea.
In
this article, we'll take a closer look at anaerobic cellular respiration and at
the different types of fermentation.
Anaerobic
cellular respiration
Anaerobic
cellular respiration is
similar to aerobic cellular respiration in that electrons extracted from a fuel
molecule are passed through an electron transport chain, driving \text{ATP}ATPA,
T, P synthesis. Some organisms use sulfate (\text {SO}_4^{2-})(SO42−)left
parenthesis, S, O, start subscript, 4, end subscript, start superscript, 2,
minus, end superscript, right parenthesis as the final electron acceptor
at the end ot the transport chain, while others use nitrate (\text
{NO}_{3}^-)(NO3−)left parenthesis, N, O, start subscript, 3, end subscript,
start superscript, minus, end superscript, right parenthesis, sulfur, or one of
a variety of other molecules^11start superscript, 1, end superscript.
What
kinds of organisms use anaerobic cellular respiration? Some
prokaryotes—bacteria and archaea—that live in low-oxygen environments rely on
anaerobic respiration to break down fuels. For example, some archaea called
methanogens can use carbon dioxide as a terminal electron acceptor, producing
methane as a by-product. Methanogens are found in soil and in the digestive
systems of ruminants, a group of animals including cows and sheep.
Similarly,
sulfate-reducing bacteria and Archaea use sulfate as a terminal electron
acceptor, producing hydrogen sulfide (\text H_2\text S)(H2S)left
parenthesis, H, start subscript, 2, end subscript, S, right parenthesis as
a byproduct. The image below is an aerial photograph of coastal waters, and the
green patches indicate an overgrowth of sulfate-reducing bacteria.
Aerial
photograph of coastal waters with blooms of sulfate-reducing bacteria appearing
as large patches of green in the water.
Fermentation
Fermentation
is another anaerobic (non-oxygen-requiring) pathway for breaking down glucose,
one that's performed by many types of organisms and cells. In fermentation,
the only energy extraction pathway is glycolysis, with one or two extra
reactions tacked on at the end.
Fermentation
and cellular respiration begin the same way, with glycolysis. In fermentation,
however, the pyruvate made in glycolysis does not continue through oxidation
and the citric acid cycle, and the electron transport chain does not run.
Because the electron transport chain isn't functional, the \text{NADH}NADHN,
A, D, Hmade in glycolysis cannot drop its electrons off there to turn back
into \text {NAD}^+NAD+N, A, D, start superscript, plus, end superscript
The
purpose of the extra reactions in fermentation, then, is to regenerate the
electron carrier \text{NAD}^+NAD+N, A, D, start superscript, plus, end
superscript from the \text{NADH}NADHN, A, D, H produced in
glycolysis. The extra reactions accomplish this by letting \text{NADH}NADHN,
A, D, H drop its electrons off with an organic molecule (such as pyruvate,
the end product of glycolysis). This drop-off allows glycolysis to keep running
by ensuring a steady supply of \text{NAD}^+NAD+N, A, D, start superscript,
plus, end superscript.
Lactic
acid fermentation
In lactic
acid fermentation, \text{NADH}NADHN, A, D, H transfers its
electrons directly to pyruvate, generating lactate as a byproduct. Lactate,
which is just the deprotonated form of lactic acid, gives the process its name.
The bacteria that make yogurt carry out lactic acid fermentation, as do the red
blood cells in your body, which don’t have mitochondria and thus can’t perform
cellular respiration.
Diagram
of lactic acid fermentation. Lactic acid fermentation has two steps: glycolysis
and NADH regeneration.
During
glycolysis, one glucose molecule is converted to two pyruvate molecules,
producing two net ATP and two NADH.
During
NADH regeneration, the two NADH donate electrons and hydrogen atoms to the two
pyruvate molecules, producing two lactate molecules and regenerating NAD+.
Muscle
cells also carry out lactic acid fermentation, though only when they have too
little oxygen for aerobic respiration to continue—for instance, when you’ve
been exercising very hard. It was once thought that the accumulation of lactate
in muscles was responsible for soreness caused by exercise, but recent research
suggests this is probably not the case.
Lactic
acid produced in muscle cells is transported through the bloodstream to the
liver, where it’s converted back to pyruvate and processed normally in the
remaining reactions of cellular respiration.
Alcohol
fermentation
Another
familiar fermentation process is alcohol fermentation, in
which \text{NADH}NADHN, A, D, Hdonates its electrons to a derivative of
pyruvate, producing ethanol.
Going
from pyruvate to ethanol is a two-step process. In the first step, a carboxyl
group is removed from pyruvate and released in as carbon dioxide, producing a
two-carbon molecule called acetaldehyde. In the second step, \text{NADH}NADHN,
A, D, H passes its electrons to acetaldehyde, regenerating \text{NAD}^+NAD+N,
A, D, start superscript, plus, end superscript and forming ethanol.
Diagram
of alcohol fermentation. Alcohol fermentation has two steps: glycolysis and
NADH regeneration.
During
glycolysis, one glucose molecule is converted to two pyruvate molecules,
producing two net ATP and two NADH.
During
NADH regeneration, the two pyruvate molecules are first converted to two
acetaldehyde molecules, releasing two carbon dioxide molecules in the process.
The two NADH then donate electrons and hydrogen atoms to the two pyruvate
molecules, producing two lactate molecules and regenerating NAD+.
Alcohol
fermentation by yeast produces the ethanol found in alcoholic drinks like beer
and wine. However, alcohol is toxic to yeasts in large quantities (just as it
is to humans), which puts an upper limit on the percentage alcohol in these
drinks. Ethanol tolerance of yeast ranges from about 555 percent
to 212121percent, depending on the yeast strain and environmental
conditions.
Facultative
and obligate anaerobes
Many
bacteria and archaea are facultative anaerobes, meaning they can
switch between aerobic respiration and anaerobic pathways (fermentation or
anaerobic respiration) depending on the availability of oxygen. This approach
allows lets them get more ATP out of their glucose molecules when oxygen is
around—since aerobic cellular respiration makes more ATP than anaerobic
pathways—but to keep metabolizing and stay alive when oxygen is scarce.
Other bacteria and
archaea are obligate anaerobes, meaning they can live and grow only
in the absence of oxygen. Oxygen is toxic to these microorganisms and injures
or kills them on exposure. For instance, the Clostridium bacteria
that are responsible for botulism (a form of food poisoning) are obligate
anaerobes. Recently, some multicellular
animals have even been discovered in deep-sea sediments that are free of oxygen.
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