Let us now look at another aspect of respiration. As you know, during aerobic respiration, O2 is consumed and CO2 is released. 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 = (volumeof CO2 evolved)/ (volumeof O consumed)
The respiratory quotient depends upon the type of respiratory substrate used during respiration. When carbohydrates are used as substrate and are completelyoxidised, the RQ will be 1, because equal amounts of CO2 and O2 are evolved and consumed, respectively, as shown in the equation below :
C6H12O6 + 6O2 ----> 6CO2 + 6H2O + ENERGY
RQ = 6CO2/6O2 = 1.0
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:
2(C51H98O6)+145O2 ----> 102CO2 + 98 H2O + ENERGY
RQ = 102CO2 / 145O2 =0.7
When proteins are respiratory substrates the ratio would be about 0.9. What is important to recognise is that in living organisms respiratory substances are often more than one; pure proteins or fats are never used as respiratory substrates.