Download Derived copy of Bis2A 07.2 Fermentation

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Derived copy of Bis2A 07.2
Fermentation
∗
Erin Easlon
Based on Bis2A 07.2 Fermentation† by
OpenStax College
Mitch Singer
This work is produced by OpenStax-CNX and licensed under the
Creative Commons Attribution License 4.0‡
Abstract
By the end of this section, you will be able to:
• Discuss the fundamental dierence between anaerobic cellular respiration and fermentation
• Describe the type of fermentation that readily occurs in animal cells and the conditions that initiate
that fermentation
1 FERMENTATION
1.1 What happens to the NADH from glycolysis
Exercise 1: Thought Question
(Solution on p. 8.)
In glycolysis, NAD+ is converted to NADH; what happens to the NADH produced?
During glycolysis NAD+ is reduced to NADH and glucose is oxidized to pyruvate. During this process the
cells must regenerate NAD+ by a second redox reaction. In respiration, this occurs when NADH is used
as an electron donor to the electron transport chain. If cells lack an electron transport chain, i.e. can not
respire, then cells must use other means to regenerate NAD+ . This is where fermentation comes into play.
In aerobic respiration, the nal electron acceptor is an oxygen molecule, O2 . If aerobic respiration occurs,
then ATP will be produced using the energy of the high-energy electrons carried by NADH or FADH2 to
the electron transport chain. If aerobic respiration does not occur, NADH must be reoxidized to NAD+ for
reuse as an electron carrier for glycolysis to continue. How is this done? Some living systems use an organic
molecule as the nal electron acceptor. Processes that use an organic molecule to regenerate NAD+ from
NADH are collectively referred to as fermentation.
Helpful videos regarding fermentation
Here is a chemwiki link on fermentation reactions1 .
∗ Version
1.1: Jul 17, 2015 2:56 pm -0500
† http://cnx.org/content/m56023/1.2/
‡ http://creativecommons.org/licenses/by/4.0/
1 http://cnx.org/content/m56886/latest/ http://chemwiki.ucdavis.edu/Biological_Chemistry/Metabolism/Glycolysis/Fermentation
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2 Lactic Acid Fermentation
The fermentation method used by animals and some bacteria like those in yogurt is lactic acid fermentation
(Figure 1). This occurs routinely in mammalian red blood cells and in skeletal muscle that has insucient
oxygen supply to allow aerobic respiration to continue (that is, in muscles used to the point of fatigue). In
muscles, lactic acid produced by fermentation must be removed by the blood circulation and brought to the
liver for further metabolism. The chemical reaction of lactic acid fermentation is the following:
Pyruvic acid + NADH ↔ lactic acid + NAD+
(1)
The enzyme that catalyzes this reaction is lactate dehydrogenase. The reaction can proceed in either
direction, but the left-to-right reaction is inhibited by acidic conditions. This lactic acid build-up causes
muscle stiness and fatigue. Once the lactic acid has been removed from the muscle and is circulated to the
liver, it can be converted back to pyruvic acid and further catabolized for energy.
:
Figure 1: Lactic acid fermentation is common in muscles that have become exhausted by use.
Exercise 2
What reactants are used up in glycolysis (need to be replaced)?
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(Solution on p. 8.)
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a.NAD+
b.NADH
c.ATP
d.ADP
e.glucose
f.glycolytic enzymes
g.a, c and d
h.a and e
i.all of the above
Exercise 3
(Solution on p. 8.)
Which reactants that are used up in glycolysis are "replaced" by the act of fermenting?
a.NAD+
b.NADH
c.ATP
d.ADP
e.glucose
f.glycolytic enzymes
g.a, c and d
h.a and e
i.all of the above
Tremetol, a metabolic poison found in white snake root plant, prevents the metabolism of lactate.
When cows eat this plant, Tremetol is concentrated in the milk. Humans who consume the milk
become ill. Symptoms of this disease, which include vomiting, abdominal pain, and tremors, become
worse after exercise. Why do you think this is the case?
3 Alcohol Fermentation
Another familiar fermentation process is alcohol fermentation (Figure 2), which produces ethanol, an alcohol.
The alcohol fermentation reaction is the following:
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Figure 2: The reaction resulting in alcohol fermentation is shown.
In the rst reaction, a carboxyl group is removed from pyruvic acid, releasing carbon dioxide as a gas.
The loss of carbon dioxide reduces the molecule by one carbon atom, making acetaldehyde. The second
reaction removes an electron from NADH, forming NAD+ and producing ethanol from the acetaldehyde,
which accepts the electron. The fermentation of pyruvic acid by yeast produces the ethanol found in alcoholic
beverages (Figure 3). If the carbon dioxide produced by the reaction is not vented from the fermentation
chamber, for example in beer and sparkling wines, it remains dissolved in the medium until the pressure is
released. Ethanol above 12 percent is toxic to yeast, so natural levels of alcohol in wine occur at a maximum
of 12 percent.
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Figure 3: Fermentation of grape juice to make wine produces CO2 as a byproduct. Fermentation tanks
have valves so that pressure inside the tanks can be released.
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4
:
Visit this site2 to see anaerobic cellular respiration in action.
Other fermentation methods occur in bacteria. Many bacteria are facultatively aerobes. This means that
they can switch between aerobic and anaerobic growth depending on the availability of oxygen. Certain
bacteria, like Clostridia bacteria, are obligate anaerobes. Obligate anaerobes live and grow in the absence of
molecular oxygen. Oxygen is a poison to these microorganisms and kills them upon exposure. It should be
noted that many forms of fermentation, an exception is lactic acid fermentation, produce gas, usually CO2
and acids, such as lactate or acetate. The production of particular types of gas is used as an indicator of
the fermentation of specic carbohydrates, which plays a role in the laboratory identication of the bacteria.
The various methods of fermentation are used by dierent organisms to ensure an adequate supply of NAD+
for the sixth step in glycolysis. Without these pathways, that step would not occur, and no ATP would be
harvested from the breakdown of glucose.
2 http://openstaxcollege.org/l/fermentation2
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Exercise 4
7
(Solution on p. 8.)
Alcohol dehydrogenase in yeast serves the same role as lactate dehydrogenase in mammals. This
role is:
a.
b.
c.
d.
To remove excess pyruvate.
To regenerate NAD+from electron transport.
To regenerate NAD+for glycolysis.
To reduce the amount of oxygen needed for growth.
5 Additional Links
Here are some additional links to help you study
YouTube
• Alcohol and lactic acid fermentations3
6 Section Summary
If NADH cannot be metabolized through aerobic respiration, another electron acceptor is used. Most organisms will use some form of fermentation to accomplish the regeneration of NAD+ , ensuring the continuation
of glycolysis. The regeneration of NAD+ in fermentation is not accompanied by ATP production; therefore,
the potential for NADH to produce ATP using an electron transport chain is not utilized.
7 Art Connections
Exercise 5
(Solution on p. 8.)
Figure 1 Tremetol, a metabolic poison found in white snake root plant, prevents the metabolism of
lactate. When cows eat this plant, Tremetol is concentrated in the milk. Humans who consume the
milk become ill. Symptoms of this disease, which include vomiting, abdominal pain, and tremors,
become worse after exercise. Why do you think this is the case?
8 Review Questions
Exercise 6
(Solution on p. 8.)
Which of the following fermentation methods can occur in animal skeletal muscles?
a.
b.
c.
d.
lactic acid fermentation
alcohol fermentation
mixed acid fermentation
propionic fermentation
9 Free Response
Exercise 7
(Solution on p. 8.)
When muscle cells run out of oxygen, what happens to the potential for energy extraction from
sugars and what pathways do the cell use?
3 https://www.youtube.com/watch?v=XREALVgxBEI
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Solutions to Exercises in this Module
Solution to Exercise (p. 1)
Be prepared to discuss this in class.
Solution to Exercise (p. 2)
h
Solution to Exercise (p. 3)
a
Solution to Exercise (p. 7)
c
to Exercise (p. 7)
Figure 1 The illness is caused by lactic acid build-up. Lactic acid levels rise after exercise, making the
symptoms worse. Milk sickness is rare today, but was common in the Midwestern United States in the early
1800s.
to Exercise (p. 7)
A
to Exercise (p. 7)
Without oxygen, oxidative phosphorylation and the citric acid cycle stop, so ATP is no longer generated
through this mechanism, which extracts the greatest amount of energy from a sugar molecule. In addition,
NADH accumulates, preventing glycolysis from going forward because of an absence of NAD+ . Lactic acid
fermentation uses the electrons in NADH to generate lactic acid from pyruvate, which allows glycolysis to
continue and thus a smaller amount of ATP can be generated by the cell.
Glossary
Denition 1: fermentation
the steps that follow the partial oxidation of glucose via glycolysis to regenerate NAD+ ; occurs in
the absence of oxygen and uses an organic compound as the nal electron acceptor
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