In muscle cells, fermentation produces: Lactate and NAD+.
Fermentation is the process by which energy is absorbed from the fuels and the nutrients in the absence of oxygen. Using the anaerobic process of fermentation, only the glycolysis of glucose takes place to form ATP, which is quite less as compared to the amount of energy produced by aerobic form of respiration.
Fermentation may occur in some prokaryotes as the only means of respiration or in the multicellular organisms like human beings at the time of stress when there is lack of oxygen. Thus, fermentation can be lactic acid fermentation or alcohol fermentation.
Lactic acid fermentation is the mechanism that occurs in the muscle cells. When the body has too little oxygen to continue aerobic respiration, like at times of strenuous exercise, glucose is broken down anaerobically to form lactic acid which causes soreness of the muscles after exercise.
What is Fermentation?
Fermentation is the process of breaking down of glucose to release energy in the absence of oxygen. It involves the first step of cellular respiration, which is the conversion of glucose into pyruvate and the release of 2 molecules of ATP.
The energy produced is quite less compared to the entire process of aerobic respiration, however, anaerobic fermentation works as a saviour in the conditions when oxygen cannot be obtained.
This anaerobic process is the only form of respiration in some unicellular organisms like yeast, while in the multicellular organisms, it occurs at the times of need when body is under stress and there is lack of oxygen.
When the demand arises, glucose is broken down anaerobically to provide instant energy and the byproducts are saved to be used again later for aerobic respiration when there is availability of oxygen.
In yeasts and unicellular organisms, fermentation is called alcohol fermentation. In this process, glucose is first broken down into pyruvate.
In the next step, carboxyl group is removed from the pyruvate to release carbon dioxide and form acetaldehyde. Later, NADH passes its electron to acetaldehyde to form ethanol. This method is used in industries for production of alcohol.
In the processes like curdling of milk or in the muscles of human beings while exercising, the type of fermentation is called lactic acid fermentation. The end product of this reaction is lactic acid which gets accumulated in the muscles and causes soreness.
Lactic Acid Fermentation in the muscle cells
Muscle cells respire aerobically at most times, but at the times of strenuous exercise when the body has been exercising very hard and there is lack of oxygen, the muscle cells perform lactic acid fermentation.
This is an anaerobic process, hence it can be carried out in the absence of oxygen to release energy or when the supply of oxygen is not enough to fulfil the fast needs.
Steps in Lactic Acid Fermentation
Lactic acid fermentation takes place in two steps. The first step is glycolysis and it is similar to the process of glycolysis in aerobic respiration, while the second step involves the conversion of pyruvate into lactic acid.
In this step of the process, one molecule of glucose is broken down to form two three-carbon molecules of pyruvate.
Along with the breakdown of glucose, two molecules of ATP are produced that become the source of instant energy. It is at this step that the NAD+ is reduced to NADH by the process of reduction.
C6H12O6 + 2 NAD+ + 2 ADP + 2 P —–> 2CH3(C=O)COOH + 2 ATP + 2 NADH + 2 H+
Formation of lactic acid
This is the step which is specific to lactic acid fermentation. In this step, the pyruvate molecules are converted into the molecules of lactate.
Lactate is the deprotonated form of lactic acid. Alongside, at this step, NAD+ is regenerated from the molecules of NADH and they are returned to the cycle for further glycolysis reactions.
2CH3(C=O)COOH + 2 NADH ————> CH3CH(OH)COOH + 2NAD+
When a lot of lactic acid has built up in the cytoplasm, the fermentation reaction stops. Because of this no more NADH are converted to NAD+ and soon the glycolysis reaction that produces ATP also comes to a stop.
Effect of lactic acid formation on the muscles
As discussed, lactic acid fermentation takes place in the human muscle cells when the body is used extensively for exercise.
At that time, the oxygen supply is unable to meet the oxygen requirement due to the stress. This is when the muscle cells start respiring anaerobically to fulfil the energy requirement.
The end product of lactic acid fermentation, lactic acid, gets accumulated in the muscle cells for as much as one to three minutes and causes the fatigue and soreness of the muscles. It can lead to severe pain and cramping of the muscles.
Lactic acid keeps accumulating in the muscles and in the presence of acidity of the lactic acid, glucose metabolism is slowed down. This is body’s natural mechanism to prevent permanent damage to the muscles from exertion.
The lactic acid formed is then transported to the liver through the bloodstream, where it gets converted to pyruvate again. The pyruvate molecules again serve as the substrate for the aerobic respiration to occur when the sufficient oxygen supply is restored to the body.
Therefore, breathing heavily after strenuous exercise helps to reestablish the oxygen supply and the conversion of lactic acid to pyruvate to aerobic respiration pathway.
Thus, we can conclude that in muscle cells, at the times of heavy exercise when the demand of oxygen is more than the supply, lactic acid fermentation takes place.
As a result of lactic acid fermentation, two molecules of ATP are formed to provide instant energy and lactic acid s formed as a byproduct.
This lactic acid gets accumulated in the muscles and causes fatigue and pain. However, it also gets eliminated from the muscles as soon as the normal oxygen supply is restored when the body slows down.
- Douglas Wilkin and Barbara Akre, Lactic Acid Fermentation: CK-12 Foundation (December 6, 2014).
- Prema Puganenthran, What is the Fermentation that happens when Lactic Acid Builds Up?: Chron.
- Stephen M. Roth, why does lactic acid build up in muscles?: Scientific American (january 23, 2006).
- Mara Pesacreta, How to Distinguish between Aerobic Respiration & Fermentation: Sciencing.
- Douglas Wilkin, Jean Brainard: An Overview of Fermentation, Grades 9 and 10 (2012).