Anaerobic respiration:
Anaerobic respiration is a process of release of energy in enzymatically controlled incomplete degradation of organic food without oxygen being used as oxidant. It is an exclusive mode of respiration in some parasitic worms and microorganism for example (bacteria, moulds).
Anaerobic respiration occurs in the roots of some waterlogged plants, muscles of animals, and as a supplementary mode of respiration in massive tissues.
Fermentation:
Fermentation is a kind of anaerobic respiration carried out primarily by fungi and bacteria. Louis Pasteur (1822-1895) concluded that fermentation occurred only when living yeast cells were present. However, Buchner (1897) found that living yeast cells were not necessary for fermentation.
He demonstrated that extract prepared by crushing yeast cells could ferment sugar. This extract contains an enzyme, zymase capable of fermenting hexose sugar. Besides zymase, yeast cell also contains enzymes like sucrase and maltase, which can ferment sucrose and maltose respectively. Direct fermentation of starch by yeast is not possible as it lacks amylase enzyme. Because of the enzyme zymase, fermentation is also termed as zymosis.
Mechanism of anaerobic respiration/fermentation:
Fermentation is the anaerobic breakdown of carbohydrates and other organic compounds into alcohols, organic acids, gases etc., with the help of microorganisms or their enzymes. The mechanism of anaerobic respiration is similar to common pathway of aerobic respiration up to glycolysis.
Glycolysis breaks down glucose enzymatically in several steps to form two molecules of pyruvate. Pyruvate is anaerobically broken down to yield various products depending upon the organism and the types of tissue. The two common products are ethyl alcohol and lactic acid.
Ethyl alcohol fermentation:
When oxygen is not available, yeast and some other microbes convert pyruvic acid into ethyl alcohol. It is quite common in fungi (e.g., Rhizopus, yeast) and bacteria. Yeast can respire both aerobically and anaerobically. Anaerobic respiration occurs in sugary solution if the fungus is not in contact with the atmosphere. It causes fermentation.
The process consists of following two steps:
In the first step, pyruvate is broken down to form acetaldehyde releasing carbon dioxide in the presence of enzyme pyruvate decarboxylase and TPP (thiamine pyrophosphate).
Pyruvate→Acetaldehyde+carbon dioxide.
In the second step, acetaldehyde is reduced to ethyl alcohol or ethanol. This reaction also requires an enzyme i.e., alcohol dehydrogenase. Hydrogen is obtained from NADH2 produced during oxidation of glyceraldehyde 3 phosphate to 1, 3 diphosphoglycerate in glycolysis.
Acetaldehyde+NADH→C2H5OH+NAD+
Fig: Schematic representation of Alcoholic fermentation
Lactic acid fermentation:
In this process the lactose sugar that is present in the milk, is converted into lactic acid which provides a distinctive sour taste to the milk. The fermentation occurs in lactic acid bacteria (e.g., Lactobacillus), some fungi and muscles. Lactic acid that is produced in muscles is sent to liver to regenerate glucose.
In lactic acid fermentation pyruvate, which is produced in glycolysis is directly reduced by NADH2 to form lactic acid. There is no evolution of carbon dioxide . The enzyme is lactic dehydrogenase, which requires FMN (Flavin mononucleotide) and Zn2+for the process.
Pyruvic acid+NADH2 → lactic acid+NAD+
Anaerobic respiration produces very little energy in comparison to aerobic respiration. Following are the reasons:
- There is incomplete breakdown of respiratory substrate.
- At least one of the products of anaerobic respiration is organic, which on further oxidation releases energy.
- NADH2 produced during glycolysis is often re-utilized.
- ATP formation does not occur during regeneration of NAD+.
- Oxygen is not used for receiving electrons and protons.
Difference between Anaerobic respiration and Aerobic respiration
Following are the differences between anaerobic and aerobic respiration:
| Anaerobic respiration | Aerobic respiration |
| 1. It is the normal mode of respiration in some parasitic worms and microorganisms such as bacteria, mould etc. | 1. Aerobic respiration is the normal mode of respiration of plants and animals. |
| 2. No exchange of gases occurs. | 2. It involves exchange of gases between the organism and the environment. |
| 3. No use of oxygen in the breakdown of respiratory substrate. | 3. It uses oxygen in the breakdown of respiratory materials into simple substances. |
| 4. There is an incomplete breakdown of respiratory material. | 4. Complete oxidation of respiratory material. |
| 5. Anaerobic respiration consists of two steps i.e., glycolysis and incomplete breakdown of pyruvic acid. | 5. Aerobic respiration consists of three steps i.e.,glycolysis, Krebs cycle and terminal oxidation. |
| 6. Anaerobic respiration does not involve electron transport. | 6. Aerobic respiration involves electron transport. |
| 7. At least one of the end products is organic i.e., ethanol. It may or may not produce inorganic substances. | 7. The end products are inorganic i.e., CO2 and water. |
| 8. It does not require mitochondria. | 8. It requires the assistance of mitochondria. |
| 9. It evolves less quantity of CO2. | 9. It evolves large quantity of CO2. |
| 10. It produces very little energy (i.e., per gm mole of glucose produces only 39-50 Kcal of energy) | 10. Energy released is comparatively more (i.e., per gm mole of glucose produces 686 Kcal of energy). |
Importance of anaerobic respiration
Anaerobic respiration is important during periods of oxygen deficiency. Following are some of the uses of anaerobic respiration.
In baking Industry:
Alcoholic fermentation has an important role in baking industry . In the bread making process, yeast undergoes cellular respiration. As a result, it releases CO2 and water helping the dough to rise making it soft and puffy.
In the alcohol Industry:
Alcoholic fermentation is used in brewing (i.e., the production of alcoholic drinks by fermentation) industry. In brewing anaerobic respiration is very important. It is because, it turns the glucose into beer. It is also used for the production of various types of whisky and other wines.
In dairy industries:
The dairy industry depends upon the action of lactic acid bacteria. Hence, the lactic acid bacteria coverts milk sugar to lactic acid. The milk contains sugar in it i.e., lactose. The bacterial fermentation thus, converts lactose into lactic acid. As a result, the lactic acid coagulates the milk protein casein and the droplets of milk fat fuse.
In the production of vinegar and organic acids:
Fermentation activity of acetic acid bacteria produces vinegar. Acetic acid bacteria act on the alcoholic beverages such as wine, thus producing vinegar. Vinegar helps in cleaning. It also helps in preserving food. Salad dressings, soups, sauces, drinks also require vinegar.
Microbial fermentation produces citric acid. Citric acid acts as flavouring agent in foods and beverages such as drinks, wines, Jam, jellies, frozen fruits, desserts etc. It also acts as an antioxidant and preserves the flavours of food. It is good for cleaning purpose.