- The ratio of the volume of CO2 evolved to the volume of O2 consumed in respiration is called the respiratory quotient (RQ) or respiratory ratio.
RQ = Volume of CO2 evolved / volume of O2 utilized
- The respiratory quotient depends upon the type of respiratory substrate used during respiration.
- When carbohydrates are used as substrate and are completely oxidised, the RQ will be 1, because equal amounts of CO2 and O2 are evolved and consumed, respectively.
C6H1206 + 6O2 à 6CO2 + 6H2O + energy
RQ = 6/6 = 1
- When fats are used in respiration, the RQ is less than 1. Calculations for a fatty acid, tripalmitin, if used as a substrate is shown:
2C51H98 O6 + 145O2 à 102 CO2 + 98H20 + energy
RQ = 102/145 = 0.4
- When proteins are respiratory substrates the ratio would be about 0.9.
- Pure proteins or fats are never used as respiratory substrates.
Fig. Respiratory quotient