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Anaerobic respiration

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Anaerobic respiration is a metabolic process that allows cells to produce ATP energy without using oxygen. There are two distinct forms wherein either ethanol or lactic acid are produced as byproducts. These forms of cellular respiration produces less ATP from each glucose molecule than aerobic respiration, but is useful during strenuous activity when the body cannot produce oxygen fast enough because it is a much quicker process and does not require oxygen. However, as a negative consequence, lactic acid can build up in the muscles causing fatigue and soreness during exercise.

Types

Lactic Acid Fermentation

A sample of lactic acid, a product of lactic acid fermentation.

Both anaerobic and aerobic respiration (requiring oxygen) begin with a process called glycolysis. During this reaction, glucose is converted into pyruvate, which becomes part of a fermentation reaction. Lactic acid fermentation creates lactate and NAD+ from the pyruvate formed during glycolysis.[1] This reaction that is catalyzed by the enzyme lactate dehydrogenase causes a hydrogen to be transferred from NADH to the pyruvate molecule. This causes the bond between the hydrogen and oxygen to change from a double bond to a single bond.[2]

One of the products of lactic acid fermentation, lactate, forms lactic acid.[2] Lactic acid is poisonous to cells. Usually, it is removed from the muscle fibers and diffuses into the blood.[3] The bloodstream carries the lactic acid to the liver, where it can be changed back into pyruvate.[4] It can also be converted to glucose during gluconeogenesis.[5] In order to accomplish this, the body needs to use more ATP, which it will produce through anaerobic respiration. However, sometimes anaerobic respiration occurs too fast for all the lactic acid to be expelled. When this happens, lactic acid will accumulate in the muscles. This is what causes muscles to feel tired and sore during strenuous exercise.[3]

Alcoholic Fermentation

Alcoholic fermentation is very similar to lactic acid fermentation except ethanol is produced during anaerobic respiration instead of lactic acid in the fermentation reaction.[2] It occurs in some types of plants and yeast.[6] In this process, the enzyme pyruvate decarboxylase takes carbon dioxide from pyruvate, forming acetaldehyde. Then, alcohol dehydrogenase gives the NADH's hydrogen atom to the acetaldehyde. This reaction produces NAD and ethanol.[2]

Alcoholic fermentation can form several products useful for humans. The ethanol and carbon dioxide formed by the alcoholic fermentation of yeast and bacteria allow products such as yogurt, vinegar, and liquor to be made.[7][8] This process is also useful in baking. The carbon dioxide produced during the anaerobic respiration of yeast is responsible for making bread rise.[6] In addition, the ethanol produced by bacteria when they break down glucose can be used in gasoline.[8]

Differences Between Anaerobic and Aerobic Respiration

Both anaerobic and aerobic respiration are forms of cellular respiration that produce energy for cells. The main difference between these two processes is that aerobic respiration uses oxygen while anaerobic respiration does not.[1] Aerobic respiration is more complicated than anaerobic respiration and consists of three steps. The glycolysis that occurs in anaerobic respiration can also act as the first part of aerobic respiration.[3] Anaerobic respiration takes placed when the body cannot produce enough oxygen to complete aerobic respiration.[9]

Aerobic and anaerobic respiration also differ in their location. Anaerobic respiration occurs in the cytoplasm of the cell instead of in the mitochondria like in aerobic respiration.[10] Also, aerobic and anaerobic respiration produce different amounts of energy from each glucose molecule. Aerobic respiration can make thirty-eight ATP molecules while anaerobic respiration produces a total of only two ATP molecules. However, anaerobic respiration can create energy more quickly than aerobic respiration. This makes it useful for high-intensity exercise over a short period of time.[9] In addition, anaerobic respiration is used primarily by prokaryotes and muscle cells while aerobic respiration takes place in the majority of plant and animal cells.[10]

Glycolysis

The process of glycolysis.

Both aerobic and anaerobic respiration begin with glycolysis, a chemical reaction that does not require oxygen.[1] In this reaction, glucose is converted into two molecules of pyruvic acid and four adenosine triphosphate molecules.[3] These ATP molecules contain energy.[11] Since this process uses two ATPs, the cell gains a total of two ATP molecules during glycolysis.[3] Glycolysis consists of nine steps. First, a phosphate group is added to the glucose molecule using an enzyme called hexokinase to form glucose-6-phosphate. In the second step, the enzyme phosphoglucose isomerase reorganizes the molecule and changes its structure from a six-membered ring to a five-membered ring. Next, the molecule, now fructose-6-phosphate gains another phosphate group with the help of phosphofructokinase, creating fructose- 1,6-bisphosphate. During the fourth step of glycolysis, another enzyme, aldolase, breaks the fructose- 1,6-biphosphate into two carbon-3 molecules.[12]

In the next step of glycolysis, the carbon-3 molecules are oxidized by an NAD molecule, which takes a hydrogen atom from the carbon-3. In addition, the carbon-3 molecules receive another phosphate group with glyceraldehyde-3-phosphate dehydrogenase serving as a catalyst, resulting in 1,3 bisphoglycerate. After this the enzyme phosphoglycerate kinase is involved in a reaction in which an ADP molecule receives a phosphate group from each 1,3 bisphoglycerate molecule, forming two ATPs and two 3 phosphoglycerate molecules. An enzyme called phosphoglycerate mutase then rearranges the placement of the phosphates on the 3 phosphoglycerate to make 2 phosphoglycerate. Next, enolase takes a water molecule from the 2 phosphoglycerate and forms the product phosphoenolpyruvate. Finally, the enzyme pyruvate kinase catalyzes a reaction that moves a phosphate group from the phosphoenolpyruvate molecules to ADP molecules, creating two more ATPs and pyruvic acid, the final products of glycolysis.[13]

Video

References

  1. 1.0 1.1 1.2 Nave, Carl. Cellular Respiration HyperPhysics. Web. Accessed on November 19, 2015.
  2. 2.0 2.1 2.2 2.3 SparkNotes Editors.Anaerobic Respiration: Homolactic Fermentation SparkNotes. Web. Accessed on November 21, 2015.
  3. 3.0 3.1 3.2 3.3 3.4 Wile, Jay L., and Shannon, Marilyn M. The Human Body: Fearfully and Wonderfully Made!. Cincinnati: Apologia Educational Ministries, Inc., 2001. 147-148. Print.
  4. Anaerobic Cellular Respiration Boundless.com. Web. Accessed on November 21, 2015. Author unknown.
  5. What is Lactic Acid? WiseGEEK. Web. Accessed on November 21, 2015. Author unknown.
  6. 6.0 6.1 What Is Anaerobic Respiration? eHow. Web. Accessed on November 24, 2015. Author unknown.
  7. Obtaining energy BBC. Web. Accessed on November 24, 2015. Author unknown.
  8. 8.0 8.1 Anaerobic Respiration Sussex Technical High School. Web. Accessed on November 24, 2015. Author unknown.
  9. 9.0 9.1 Terry, Ryan. Anaerobic Respiration CK-12. Web. Accessed on November 25, 2015 .
  10. 10.0 10.1 Manohar, Uttara. Aerobic and Anaerobic Respiration Buzzle. Web. Last updated on January 8, 2013.
  11. Wile, Jay L., and Shannon, Marilyn M. The Human Body: Fearfully and Wonderfully Made!. Cincinnati: Apologia Educational Ministries, Inc., 2001. 145. Print.
  12. SparkNotes Editors.Glycolysis Stage 1: Glucose Breakdown SparkNotes. Web. Accessed on November 19, 2015.
  13. SparkNotes Editors.Glycolysis Stage 2: Conversion to Pyruvate SparkNotes. Web. Accessed on November 19, 2015.