Fermentation – Class 11 | Chapter – 14 | Biology Short Notes Series PDF

Fermentation: It is an anaerobic process in which energy can be released from glucose even if oxygen is not available. It occurs in yeast cells and bacteria and also in the muscles of animals. It is an anaerobic pathway in which glucose is broken down. The respiration that happens at the minute level in our body, viz., in the cell is called the cellular respiration. It occurs in the presence or absence of oxygen. Any type of cellular respiration begins with glycolysis where a 3-C molecule, pyruvic acid is formed as the end product.

Types of Fermentation

  • Homo: only one type of product formation
  • Hetero: more than one product formed

On the basis of the end product formed, it can be categorized as follows:

1. Lactic Acid Fermentation

Lactic acid is formed from pyruvate produced in glycolysis NAD+ is generated from NADH. Enzyme lactate dehydrogenase catalyses this reaction. Lactobacillus bacteria prepare curd from milk via this type of fermentation. During intense exercise when oxygen supply is inadequate, muscles derive energy by producing lactic acid, which gets accumulated in the cells causing fatigue.

Lactic Acid Fermentation

Lactic Acid Fermentation

2. Alcohol Fermentation

This is used in the industrial production of wine, beer, biofuel, etc. The end product is alcohol and CO2. Pyruvic acid breaks down into acetaldehyde and CO2 is released. In the next step, ethanol is formed from acetaldehyde. NAD+ is also formed from NADH, utilized in glycolysis. Yeast and some bacteria carry out this type of fermentation. Enzyme pyruvic acid decarboxylase and alcohol dehydrogenase catalyse these reactions.

Alcohol Fermentation

3. Acetic acid Fermentation

Vinegar is produced by this process. This is a two-step process.

The first step is the formation of ethyl alcohol from sugar anaerobically using yeast.

In the second step, ethyl alcohol is further oxidized to form acetic acid using acetobacter bacteria. Microbial oxidation of alcohol to acid is an aerobic process.

Acetic acid Fermentation

4. Butyric acid Fermentation

This type of fermentation is characteristic of obligate anaerobic bacteria of genus clostridium. This occurs in retting of jute fibre, rancid butter, tobacco processing and tanning of leather. Butyric acid is produced in the human colon as a product of dietary fibre fermentation. It is an important source of energy for colorectal epithelium. Sugar is first oxidized to pyruvate by the process of glycolysis and then pyruvate is further oxidized to form acetyl-CoA by the oxidoreductase enzyme system with the production of H2 and CO2. Acetyl-CoA is further reduced to form butyric acid. This type of fermentation leads to a relatively higher yield of energy. 3 molecules of ATP are formed.

Butyric acid Fermentation


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