7.15: Types of Fermentation

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Learning Objectives

Describe the process of lactic acid fermentation
Describe the process of alcohol fermentation

Lactic Acid Fermentation

The fermentation method used by animals and certain bacteria, like those in yogurt, is lactic acid fermentation (Figure 1). This type of fermentation is used routinely in mammalian red blood cells and in skeletal muscle that has an insufficient oxygen supply to allow aerobic respiration to continue (that is, in muscles used to the point of fatigue). In muscles, lactic acid accumulation must be removed by the blood circulation and the lactate brought to the liver for further metabolism. The chemical reactions of lactic acid fermentation are the following:

$\text{Pyruvic acid}+\text{NADH}\longleftrightarrow\text{lactic acid}+\text{NAD}^+$

The enzyme used in this reaction is lactate dehydrogenase (LDH). The reaction can proceed in either direction, but the reaction from left to right is inhibited by acidic conditions. Such lactic acid accumulation was once believed to cause muscle stiffness, fatigue, and soreness, although more recent research disputes this hypothesis. Once the lactic acid has been removed from the muscle and circulated to the liver, it can be reconverted into pyruvic acid and further catabolized for energy.

Practice Question

This illustration shows that during glycolysis, glucose is broken down into two pyruvate molecules and, in the process, two NADH are formed from NAD^{+}. During lactic acid fermentation, the two pyruvate molecules are converted into lactate, and NADH is recycled back into NAD^{+}. — Figure 1. Lactic acid fermentation is common in muscle cells that have run out of oxygen.

Tremetol, a metabolic poison found in the white snake root plant, prevents the metabolism of lactate. When cows eat this plant, it is concentrated in the milk they produce. 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?

[practice-area rows=”2″][/practice-area]
[reveal-answer q=”453242″]Show Answer[/reveal-answer]
[hidden-answer a=”453242″]The illness is caused by lactate accumulation. Lactate 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.[/hidden-answer]

Alcohol Fermentation

This photo shows large cylindrical fermentation tanks stacked one on top of the other. — Figure 2. Fermentation of grape juice into wine produces CO₂ as a byproduct. Fermentation tanks have valves so that the pressure inside the tanks created by the carbon dioxide produced can be released.

Another familiar fermentation process is alcohol fermentation (Figure 3) that produces ethanol, an alcohol (because of this, this kind of fermentation is also sometimes known as ethanol fermentation). There are two main reactions in alcohol fermentation.

The first reaction is catalyzed by pyruvate decarboxylase, a cytoplasmic enzyme, with a coenzyme of thiamine pyrophosphate (TPP, derived from vitamin B1 and also called thiamine). A carboxyl group is removed from pyruvic acid, releasing carbon dioxide as a gas. The loss of carbon dioxide reduces the size of the molecule by one carbon, making acetaldehyde.

The second reaction is catalyzed by alcohol dehydrogenase to oxidize NADH to NAD⁺ and reduce acetaldehyde to ethanol. The fermentation of pyruvic acid by yeast produces the ethanol found in alcoholic beverages. Ethanol tolerance of yeast is variable, ranging from about 5 percent to 21 percent, depending on the yeast strain and environmental conditions.

In alcohol fermentation. One glucose molecule breaks down into two pyruvates via glycolysis (1). The energy from these exothermic reactions is used to bind inorganic phosphates to ADP and convert NAD+ to NADH. The two pyruvates are then broken down into two Acetaldehyde and give off two CO2 as a waste product (2). The two Acetaldehydes are then reduced to two ethanol, and NADH is oxidized back into NAD+ (3). — Figure 3. Diagram of alcohol fermentation

Other Types of Fermentation

Other fermentation methods occur in bacteria. Many prokaryotes are facultatively anaerobic. This means that they can switch between aerobic respiration and fermentation, depending on the availability of oxygen. Certain prokaryotes, like Clostridia, are obligate anaerobes. Obligate anaerobes live and grow in the absence of molecular oxygen. Oxygen is a poison to these microorganisms and kills them on exposure.

It should be noted that all forms of fermentation, except lactic acid fermentation, produce gas. The production of particular types of gas is used as an indicator of the fermentation of specific carbohydrates, which plays a role in the laboratory identification of the bacteria. Various methods of fermentation are used by assorted 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.

Learning Objectives

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.

Practice Questions

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?

[reveal-answer q=”946670″]Show Answer[/reveal-answer]
[hidden-answer a=”946670″]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.

[/hidden-answer]

When muscle cells run out of oxygen, what happens to the potential for energy extraction from sugars and what pathways do the cell use?

[reveal-answer q=”143132″]Show Answer[/reveal-answer]
[hidden-answer a=”143132″]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.[/hidden-answer]

Contributors and Attributions

Biology. Provided by: OpenStax CNX. Located at: http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8. License: CC BY: Attribution. License Terms: Download for free at http://cnx.org/contents/185cbf87-c72...f21b5eabd@10.8
Ethanol fermentation. Authored by: Davidcarmack. Located at: https://en.Wikipedia.org/wiki/File:Ethanol_fermentation-1.svg. License: CC BY-SA: Attribution-ShareAlike