During aerobic respiration, 02 is consumed and C02 is released. The ratio of the amount of C02 evolved to the amount of 02 consumed in respiration is called the respiratory quotient (RQ).

The value of the RQ depends upon the type of substrate used for respiration. Generally carbohydrates are used as the substrates for respiration. Complete oxidation of carbohydrates during aerobic respiration consumes 02 and releases C02 in equal amounts as per the following equation:

Therefore, for carbohydrates the RQ is 1.

The chemical composition for fats differs from that of carbohydrates in that fats contain considerably fewer oxygen atoms in proportion to atoms of carbon and hydrogen. The fatty acids, which are formed by the hydrolysis of fats, are used as respiratory substrates. The overall equation for the complete oxidation of a common fatty acid such as palmitic acid during aerobic respiration is given below.


Proteins are composed of amino acids which in turn are composed of carbon, hydrogen, oxygen, nitrogen and also sulfur. The respiratory quotient for protein varies from 0.8 to 0.9.