Respiration is of two types : aerobic and anaerobic respiration.

(1) Aerobic respiration : "When free or molecular oxygen participates in the respiratory breakdown of organic substrate, it is called aerobic respiration." (In this process, free molecular oxygen acts as the final electron acceptor.)

Aerobic respiration is represented by the following chemical equation.

C₆H₁₂O₆ + 6O₂ ® 6CO₂ + 6H₂O + Energy

(2) Anaerobic respiration : "When the respiratory breakdown of organic substrate takes place without participation of free molecular oxygen, it is called anaerobic respiration." (In this process, free or molecular oxygen is not the final electron acceptor.)

Anaerobic respiration is represented by the following chemical equation.

C₆H₁₂O₆ ® 2C₂H₅OH + 2CO₂ + Energy

The process of respiration is completed in two phases. Reactions in Phase I are called glycolysis and are common to both types of respiration. Reactions in Phase II depend upon whether O₂ is utilized or not.