12.7 Respiratory Quotient

As you know, during aerobic respiration, \(\mathrm{O}_2\) is consumed and \(\mathrm{CO}_2\) is released. The ratio of the volume of \(\mathrm{CO}_2\) evolved to the volume of \(\mathrm{O}_2\) consumed in respiration is called the respiratory quotient \((\mathrm{RQ})\) or respiratory ratio.
\(
\mathrm{RQ}=\frac{\text { volume of } \mathrm{CO}_2 \text { evolved }}{\text { volume of } \mathrm{O}_2 \text { consumed }}
\)
The respiratory quotient depends upon the type of respiratory substrate used during respiration.
When carbohydrates are used as substrate and are completely oxidised, the \((\mathrm{RQ})\) will be 1, because equal amounts of \(\mathrm{CO}_2\) and \(\mathrm{O}_2\) are evolved and consumed, respectively, as shown in the equation below:
\(
\begin{gathered}
\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6+6 \mathrm{O}_2 \longrightarrow 6 \mathrm{CO}_2+6 \mathrm{H}_2 \mathrm{O}+\text { Energy } \\
\mathrm{RQ}=\frac{6 \mathrm{CO}_2}{6 \mathrm{O}_2}=1.0
\end{gathered}
\)
When fats are used in respiration, the \((\mathrm{RQ})\) is less than 1 . Calculations for a fatty acid, tripalmitin, if used as a substrate is shown:
\(
2\left(\mathrm{C}_{51} \mathrm{H}_{98} \mathrm{O}_6\right)+145 \mathrm{O}_2 \longrightarrow 102 \mathrm{CO}_2+98 \mathrm{H}_2 \mathrm{O}+\text { energy }
\)
Tripalmitin
\(
\mathrm{RQ}=\frac{102 \mathrm{CO}_2}{145 \mathrm{O}_2}=0.7
\)
When proteins are respiratory substrates the ratio would be about 0.9 .
In living organisms respiratory substrates are often more than one; pure proteins or fats are never used as respiratory substrates.