Anaerobic Respiration (Fermentation):

In the absence of oxygen, glucose is incompletely oxidized with less amount of energy released. In anaerobic respiration, the first phase is exactly similar to that of aerobic respiration. A molecule of glucose is broken down into two molecules of pyruvic acid. But in the second phase, Pyruvic acid is not completely oxidized (due to the absence of oxygen). It is transformed into ethyl alcohol or lactic acid. In this way, many of the C-H bonds are left unbroken in the products. Anaerobic respiration is further classified as:

A- Alcoholic fermentation:

It occurs in bacteria, yeast, etc. In this type of anaerobic respiration, pyruvic acid is further broken down into alcohol (C₂H₅OH) and CO₂.

2(C₃H₄O₃) + 4H → 2(C₂H₅OH) + 2CO₂

Pyruvic acid Hydrogen → Ethyl Alcohol + Carbon dioxide

B- Lactic acid fermentation:

It occurs in the skeletal muscles of humans and other animals during extreme physical activities when oxygen cannot be transported to the cells as rapidly as it is needed. This also happens in the bacteria present in milk. In this type of anaerobic respiration, each pyruvic acid molecule is converted into lactic acid (C₂H₆O₃).

2(C₃H₄O₃) + 4H = 2(C₂H₆O₃)  

Pyruvic acid Hydrogen - Lactic acid