Anaerobic respiration is the process by which incomplete oxidation of respiratory substrate takes place. In this case, it occurs in the absence of oxygen resulting the end products of ethyl alcohol and CO2 in plants and lactic acid (in animals) with very slight energy.
Anaerobic respiration is observed in certain bacteria, yeast and other fungi, endoparasites and animal muscles cells. It is also known as fermentation. The common reaction of anaerobic respiration is:
Features of Anaerobic Respiration
- Complete process of anaerobic respiration takes place in the cell cytoplasm only.
- In this case, mitochondria are not involved.
- Oxygen is not used.
- Cytric acid cycle, ETS or oxidative phosphorylation are not involved.
Phase of Anaerobic Respiration
There are two definite phases of anaerobic respiration:
1. Glycolysis: The first phase of anaerobic respiration is glycolysis in which 2 molecules of pyruvic acid and 4H+ are formed from a molecule of glucose from the same reaction of glycolysis(EMP pathway) found in aerobic respiration.
2. Fermentation: The second phase of anaerobic respiration is fermentation which consists of decarboxylation and reduction reactions converting the pyruvic acid into either ethyl alcohol with the evolution of carbon dioxide (CO2).
- Ethanol Fermentation: Two steps involve in this process. At first pyruvic acid undergo carboxylation in the presence of pyruvic carboxylase enzyme and produce acetaldehyde and CO2. Then the acetaldehyde dehydrogenated by NADH2 into ethanol in presence of dehydrogenase enzyme.
- Lactic acid Fermentation: The pyruvic acid dehydrogenated by NADH2 into lactic acid in anaerobic condition of cell and in presence of dehydogenase enzyme. Beside anaerobes lactic acid is formed in muscle cell of higher animal. Higher plants do not produce any lactic acid.
Energetic of Anaerobic Respiration
Two molecules of NADH2 and two molecules of ATP are formed in glycolysis. During fermentation two molecules of NADH2 are used. Only two molecules of ATP are used to produce slight energy of 20 Kcal.
Application of Anaerobic Respiration
Various microorganisms take part in the fermentation process and produce highly useful end products. These useful end products make benefit to the mankind in many ways. Some notable fermentation activities in the industrial sectors are given below:
- Different antibiotics and vitamins are obtained from both bacterial and mold fermentation.
- There are many microorganisms such as Bacillus subtilis, Clostridium histolyticum, Trichoderma konigi. They are used in commercial production of enzymes through fermentation.
- Wine and bear are produced through the fermentation of fruit juice and grain, respectively. In this case, bear is the product of yeast fermentation in grains while wine is the product of yeast fermentation in fruit juice.
- Industrial alcohol such as ethanol is produced from rice, barley, flour and molasses through fermentation process.
- Bread and yeast-based bakery foods are produced through yeast fermentation. In this case, the produced foods have appealing flavor and light, porous texture which largely depend on yeast fermentation.
- Various dairy products are fermented with the activities of lactic acid bacteria such as Lactobacillus, Lactococcus and Leuconostoc. Many cultured milk products such as cheese, butter, sour cream, yogurt, Lassi, kunis, kefir, etc can be found around the world through the process of fermentation.
- In tannery industries, fermented enzymes are used in tanning process to soften lather.
- Certain bacteria such as Bacillus megatherium is used to cure the leaves of tea, tobacco, beans of coffee, and coca, etc.
- Various organic compounds such as lactic acid and acetic acid are produced though the fermentation action of various bacteria. In this case, Lactobacillus helps to produce lactic acid while Acetobacter aceti produces acetic acid/vinegar through fermentation process.