Fermentation: Meaning, Types and Efficiency:

Meaning of Fermentation:

The mechanism of anaerobic respiration or fermentation is similar to common pathway of aerobic respiration upto glycolysis. However, oxidation of pyruvic acid and NADH by mitochondria requires oxygen. When oxygen is limiting, NADH and pyruvic acid begin to accumulate.

Under such condition, plants carry out fermentation (anaerobic respiration) which leads to the formation of CO2 and either ethanol (ethyl alcohol) or lactic acid. In most of cases ethanol is produced.

Types of Fermentation:

Two types of fermentation are common:

a. Alcoholic Fermentation:

In this type, the pyruvic acid is first decarboxylated to acetaldehyde in the presence of enzyme carboxylase. Thiamine pyrophosphate (TPP) is required as co-factor.

Acetaldehyde is then reduced to ethanol (ethyl alcohol) by the enzyme alcohol dehydro­genase. Coenzyme NADH2 (produced in glycolysis) is oxidised.

The overall equation for anaerobic respiration involving alcoholic fermentation is as follows:

b. Lactic acid Fermentation:

This type of fermentation occurs in lactic acid bacteria, certain fungi and muscles. Here, pyruvic acid is converted into lactic acid by the enzyme lactic dehydrogenase. Coenzyme NADH2, produced in glycolysis, is oxidised.

The overall equation for anaerobic respiration (lactic acid fermentation) is as follows:

There is net gain of only 2 ATP molecules (in glycolysis stage) during anaerobic respiration (or fermentation) and most of energy contained in glucose molecule is released as heat.

Importance:

Many of the fermentive processes are of great importance in the household and industry. An example of the household is souring of milk or formation of curd. In the latter category are the alcoholic and acid fermentations.

The ripening of cheese, the retting of flax, sannhemp, ramie, and jute, the curing of tobacco and tea leaves, and the tanning of leather are some very important examples of commercial processes dependent upon fermentations brought about by different species of bacteria.

Alcoholic fermentation is used in brewing industry, where various types of beers, whisky and wines are produced, whereas CO2 of alcoholic fermentation is used in baking industry for making bread and biscuits. Vinegar is produced by fermentation activity of acetic acid bacteria. Cleaning of raw hides is done by fermentive activity of bacteria.

Efficiency of Fermentation Process:

The total energy production from 38 ATP molecules is 1292 kJ. This means, one ATP molecule produces 34 kJ of energy, i.e., 38 × 34 = 1292 kJ energy.

However, energy released by complete oxidation of one molecule of glucose is 2870 kJ. This shows, the efficiency is 45 per cent, i.e., 1292 × 100/2870 = 45.01

Thus, it becomes clear that only a part of this energy is used to make ATP, and more than half of energy generated during respiration is released in the form of heat.