Respiration

Most of plants and animal cells respire aerobically, that is, in the presence of oxygen. However, there are certain microbes that respire in the absence of free oxygen. This respiration is called anaerobic respiration. It is also called fermentation. Among plants, it takes place in yeast, bacteria such as E.coli, Clostridium tetani, etc. Among animals, only certain cells are temporarily anaerobic (when they are short of oxygen) such as the muscle cells.

Anaerobic respiration is of two types:

• Alcoholic fermentation
• Lactic acid fermentation

Alcoholic Fermentation

It occurs in plants like the yeast (a fungus).

It can be represented as follows:

Glucose 2 ethanol + 2CO₂ + 2ATP

This process also takes place in higher plants for a very short while and only when free oxygen is not available. For example, germinating seeds respire anaerobically.

Lactic Acid Fermentation

It can be represented as follows:

Glucose 2 lactic acid + 2ATP

During this process, no carbon dioxide is released. It is because of the accumulation of lactic acid that there is fatigue and cramps in the muscles after prolonged exercises.

This process takes place when the small store of ATP in the muscles is used up and energy is required immediately. Aerobic respiration takes a longer time and hence, the cells resort to anaerobic respiration.

Some authors prefer to designate fermentation as an extra-cellular process only. That is, only the anaerobic reactions which take place outside the cell and in the medium are called fermentation. Under this definition alcoholic fermentation as carried out by the microbes is called fermentation as it is carried out by yeast in solution. It is also an industrial process. The alcoholic fermentation carried out by the germinating seeds is always called anaerobic respiration.

It must be noted that the first step of respiration - glycolysis is common to both aerobic and anaerobic respiration. Thus, in anaerobic respiration also pyruvic acid is formed. The pyruvic acid is then fermented to ethanol or lactic acid.

The overall reaction for aerobic respiration is

C₆H₁₂O₆ +6O₂ 6CO₂+6H₂O + 38 ATP

Comparing this with the fermentation equations will show that anaerobic respiration is far less efficient than the aerobic respiration. The anaerobic respiration releases only 2 ATP molecules whereas the aerobic respiration releases 38 ATP molecules.

Importance of Anaerobic Respiration

Anaerobic respiration releases less energy, it meets the requirements of the microbes growing in anaerobic conditions. Fermentation is a commercially important process.

It is used in the following processes:

• Manufacture of alcohol
• Curing of tea leaves, tobacco, etc.
• Formation of curd from milk
• Manufacture of vinegar, an industrially important compound.