Which statement about anaerobic respiration is true?

A. Anaerobic respiration prevents muscle fatigue.

B. Anaerobic respiration generates a high yield of ATP.

C. Anaerobic respiration is used commercially in the production of beer, wine, bread, yogurt, and cheese.

D. Anaerobic respiration can provide energy to animals for several days in the absence of oxygen.

The correct answer is C: Anaerobic respiration is used commercially in the production of beer, wine, bread, yogurt, and cheese.

Anaerobic respiration is cellular respiration that occurs in the absence of oxygen. Although anaerobic respiration does produce some energy due to ATP being formed during glycolysis, it does not produce a very large quantity of energy.

It certainly does not produce a high yield of ATP such as that which is produced by the electron transport chain of aerobic cellular respiration. Aerobic respiration is respiration that occurs in the presence of oxygen.

During glycolysis, which occurs in the cytoplasm of the cell, NADH2 is formed from NAD and ATP is formed from ADP in reduction reactions. A reduction reaction is a reaction in which electrons and hence energy, are added to a molecule.

Anaerobic respiration is also known as fermentation and occurs in the cytoplasm of the cell. Fermentation can be used commercially to provide some energy when oxygen is absent.

Some food items and drinks are produced by the process of fermentation; this includes, for instance, cheese, yogurt, and bread. Drinks formed by fermentation include alcoholic beverages such as beer and wine.

Animals and some bacteria undergo anaerobic respiration as well. This can be a short-term solution when oxygen is absent.

Skeletal muscles can run out of oxygen at which time the cells switch from aerobic to anaerobic respiration. This causes the build-up of lactic acid which causes muscle fatigue.

Once oxygen returns, the cells switch back to the aerobic respiration. It is important to realize that anaerobic respiration is only a short-term solution and that animal cells will not live for days without oxygen. Some bacteria though can survive and may only use anaerobic respiration for metabolism.

Glycolysis

Glycolysis is the first part of anaerobic respiration that also does occur in aerobic respiration. The process of glycolysis involves several steps during which a glucose molecule is converted to various intermediates. Several enzymes are used to catalyze reactions at each step of the process

Energy from ATP is used to make glucose 6-phosphate from glucose. At a later stage, a glyceraldehyde-3-phosphate molecule is formed. Eventually, two pyruvate molecules are formed in preparation for the next stages of cellular respiration.

What is important to realize is that some energy is formed during glycolysis in the form of ATP and NADH2. However, this is not a high level of energy.

In fact, only a few molecules of ATP and NADH2 are formed. Glycolysis includes the energy producing reactions of anaerobic respiration.

Ethanol fermentation

Ethanol fermentation is also called alcohol fermentation and is, of course, used to produce beer and wine. The process occurs naturally in many plants and yeasts and includes glycolysis in the cytoplasm and then further reactions to produce ethanol.

The ethanol is the final molecule formed at the end of the process of ethanol fermentation, hence the name ethanol fermentation.

Glycolysis is the first set of reactions that occur. These occur in the cytoplasm of the cell and produce two pyruvate molecules, which have three carbons each.

The pyruvates are produced from a single molecule of glucose that is split during several enzymatic reactions that occur during glycolysis. During glycolysis, ATP is produced as is the NADH2.

Ethanol production

Ethanol fermentation has two steps, during which time NAD is produced as NADH2. is oxidized (loses electrons). The NAD is recycled back to be used again during glycolysis. A single glucose molecule that is broken down results in 2 ATP, 2 NADPH2, and 2 pyruvate molecules.

Pyruvate is first converted to acetaldehyde by the enzyme pyruvate decarboxylase, during this time carbon dioxide is also released.

Note that this is a decarboxylation reaction in which carbons are being removed in the process. The carbons are part of the carboxyl group that is removed in the reaction.

The acetaldehyde that is formed is then converted into ethanol with the help of an alcohol dehydrogenase enzyme. An electron is removed from the NADH2 during the reaction and added to the acetaldehyde.

The final products of alcohol fermentation are two ethanols which each contain two carbon atoms. Two molecules of carbon dioxide are released during the process.

Lactic acid fermentation

The first step is once again glycolysis in which two pyruvate molecules are formed in the cytoplasm. The enzyme lactate dehydrogenase then converts the pyruvate molecules into lactate or what is known as lactic acid. During this process, the NADH2 is oxidized to NAD which is recycled back into glycolysis.

Sore muscles are often because of lactic acid building up. If you have exercised and your muscles feel sore afterward then this is probably because your cells temporarily switched to anaerobic respiration and produced lactic acid. It is the lactic acid that is making your muscles feel sore.

Once oxygen is available to your cells again, the cells switch back to aerobic respiration. Animals are facultative anaerobes. In other words, animal cells can for a short time, survive without oxygen, but they will need oxygen to continue to live.

Some bacteria only use anaerobic cellular respiration to produce energy. These are known as obligate anaerobes. Fungi and bacteria can be used to produce food and drink items by fermentation.

Commercial application of fermentation

Certain bacteria are used commercially to make food items; these include the Streptococcus, Lactobacillus and Leuconostoc genera of bacteria.

Lactic acid fermentation creates an acidic environment which causes proteins to break down (denature), to form some food items.

Some bacteria can produce a combination of ethanol and lactic acid when they undergo fermentation. These are heterolactic bacteria such as Leuconostoc mesenteroides, which is actually used to produce sauerkraut and pickles by fermentation of cabbage and cucumbers.

Homolactic bacteria only produce lactic acid as a byproduct of fermentation. For instance,  Streptococcus thermophile and Lactobacillus lbrueckii, are homolactic species of bacteria.

Beer is often made using a combination of hops, malt, and yeast. The yeast is a fungus which carries out the ethanol fermentation process. Wine is also made using yeast to ferment a plant, but grapes are used in the case of wine.

Today some people even make fruit wines where they use some other type of fruit. Winemaking is quite complex, and some people use bacteria that produce lactic acid as well. These can help to improve the taste of wine.

References

  1. PH Raven, RF Evert, SE Eichhorn (1987). Biology of plants. Worth Publishers.
  2. N Parker, AT Tu, BM. Forster, P Lister (2017). Microbiology.  Rice University.
  3. Editors of Encyclopedia Britannica (2018). Glycolysis. Retrieved from Britannica.com.
  4. Editors of Encyclopedia Britannica (2018). Fermentation. Retrieved from Britannica.com.
  5. Editors of Encyclopedia Britannica (2018). Cellular respiration. Retrieved from Britannica.com.

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