Fermentation: Anaerobic Respiration: Definition, Equation, Types

What Is Fermentation?

This process mainly takes place in the yeast cells and bacteria and the muscle cells of animals. It is an anaerobic pathway of glucose breakdown to yield energy. The cellular respiration process, both aerobic and anaerobic, commences with glycolysis. Glycolysis is the pathway in which glucose gets converted into pyruvic acid, a three-carbon compound.

This molecule is further oxidised in the mitochondria only when oxygen is present. However since, in anaerobic conditions, there is no possibility of oxidation in the mitochondria, so fermentation provides an alternative route for pyruvate metabolism. This pathway allows cells to re-generate NAD+, through which glycolysis can proceed to continue ATP production when O2 is in short supply.

Definition Of Fermentation

Fermentation is an anaerobic process in which energy from glucose can be released if oxygen is not available. Thanks to this mechanism, cells are capable of continuing the production of ATP. Though less efficient than the aerobic route of respiration, production will continue.

Types Of Fermentation

There are three main types of fermentation based on the end products formed:

Lactic Acid Fermentation

In lactic acid fermentation, glucose or any other types of sugar are converted to lactic acid by some bacteria and yeast. This process, however, has a higher occurrence in situations of extensive exercise where the muscle cells require more oxygen to supply energy. Lactic acid builds up in the muscles after exercises and causes them to feel tired and sore, as generally happens a day or two after doing heavy exercise.

Alcohol Fermentation

Alcohol fermentation is a process in which pyruvate is converted into ethanol and carbon dioxide. The application is in the manufacture of ethyl alcohol or wine and beer brewing. The process involves mainly yeast cells, especially Saccharomyces cerevisiae. The conversion of pyruvate into ethanol is through the activity of two enzymes: pyruvic acid decarboxylase and alcohol dehydrogenase.

Acetic Acid Fermentation

Acetic acid fermentation refers to the process of fermentation whereby starch and sugars present in grains and fruits get converted into vinegar and condiments. Apple cider vinegar is one such example. This type of fermentation is significant in the industries of food as it leads to a great deal of variety of fermented products.

Fermentation – Anaerobic Respiration

Anaerobic respiration is cellular respiration performed without oxygen. One typical pathway for anaerobic respiration is realized by many kinds of prokaryotes and unicellular eukaryotes in fermentation. In this process, there is partial oxidation of glucose with the formation of acid, alcohol, and gases.

In yeast, the pyruvic acid formed from glucose is converted into ethanol and carbon dioxide. The process of alcoholic fermentation is catalyzed by particular enzymes. On the other hand, in certain bacteria and animal muscle cells, pyruvic acid reduces into lactic acid under anaerobic conditions and hence is called lactic acid fermentation.

Both alcoholic and lactic acid fermentation utilize the reducing power of NADH and regenerate NAD+ so that glycolysis can proceed. Although fermentation produces much less ATP from glucose compared to aerobic respiration, it does produce some ATP, and more importantly for the cell, it allows glycolysis to continue producing ATP in the absence of oxygen. The complete degradation of a single glucose molecule in fermentation yields only two ATP molecules.