Describe: Respiratory Quotient.

During aerobic respiration,$O_{2}$ is consumed and $CO_{2}$ is released.
The ratio of the volume of $CO_{2}$ evolved to the volume of $O_{2}$ consumed in respiration is called the respiratory quotient $(RQ)$.
$RQ = \frac{\text{Volume of } CO_{2} \text{ evolved}}{\text{Volume of } O_{2} \text{ consumed}}$
The respiratory quotient depends upon the type of respiratory substrate used during respiration.
$RQ$ of Carbohydrates: When carbohydrates are used as substrate and are completely oxidized,the $RQ$ is $1$,because equal amounts of $CO_{2}$ and $O_{2}$ are evolved and consumed.
Equation: $C_{6}H_{12}O_{6} + 6O_{2} \longrightarrow 6CO_{2} + 6H_{2}O + \text{Energy}$
$RQ = \frac{6CO_{2}}{6O_{2}} = 1.0$
$RQ$ of fat: When fats are used in respiration,the $RQ$ is less than $1$. Calculation for a fatty acid,tripalmitin,if used as a substrate is shown:
$2(C_{51}H_{98}O_{6}) + 145O_{2} \longrightarrow 102CO_{2} + 98H_{2}O + \text{Energy}$
$RQ = \frac{102CO_{2}}{145O_{2}} \approx 0.7$
Protein: When proteins are respiratory substrates,the ratio is about $0.9$.
When organic acids are used for respiration,the $RQ$ remains above $1$.
Example: Equation for respiration of oxalic acid:
$2(COOH)_{2} + O_{2} \longrightarrow 4CO_{2} + 2H_{2}O + \text{Energy}$
$RQ = \frac{4CO_{2}}{1O_{2}} = 4.0$