Anaerobic respiration occurs in man in muscle at the time of excessive exercising.Blood supply does not meet the oxygen demand of the exercising body.At that time in muscle, lactic acid is produced as a result of anaerobic respiration.In yeast also, anaerobic respiration occurs in the condition of absence of oxygen.

Respiration is a complex oxidative process in which there is formation of several intermediate molecules occurs.In the end water molecule is produced and ATP molecules are consumed in the glycolysis and Krebs cycle, there is also production of more molecules of ATP, thus a positive balance of ATP molecules is obtained.

Following are the reasons:
1. Incomplete breakdown of respiratory substrate.
2. Some of the products of anaerobic respiration can be oxidized further to release energy which shows that anaerobic respiration does not liberate the whole energy contained in the respiratory substrate.
3. NADH₂ does not produce ATP as electron transport is absent.
4. Oxygen is not utilized for securing electrons and protons.

ATP synthatase consists of F0F1 particles. Fo acts as channel through which H+ passes.
F1 contains the site for synthesis of ATP.

It is ratio of volume of CO₂ evolved and volume of O₂ consumed.
R.Q. = Volume of CO₂ evolved / Volume of O2 consumed
For proteins: R.Q. remains 0.9 and the R.Q. for fat is 0.7
2C₅₁H₉₈O₆ + 145O₂→ 102CO₂ + 98H₂O + Energy
R.Q. = 102/ 145 = 0.7