Question

Define ‘Respiratory Quotient’ (RQ) and calculate the Respiratory Quotient for carbohydrate.

Answer 1

Hint: Respiratory quotient (RQ) is the most direct measure, so far devised of, of the relative quantity of food stuffs entering into the metabolism of humans and other animals. It is a dimensionless number used in a calculation for basal metabolic rate when estimated from carbon dioxide elimination to oxygen absorption.

Complete answer:
Aerobic respiration is the significant process most living organisms undergo to use food energy efficiently. In aerobic respiration, oxygen is consumed and carbon dioxide is released.

Respiratory quotient (RQ) or respiratory ratio can be defined as the ratio of the volume of carbon dioxide produced to the volume of oxygen consumed in respiration over a period of time. Its value can be one, zero, more than 1 or less than 1.

\[RQ = \dfrac{{\begin{array}{*{20}{l}}
  {Volume{\text{ }}of{\text{ }}C{O_2}evolved} \\
  \;
\end{array}}}{{\begin{array}{*{20}{l}}
  {Volume{\text{ }}of{\text{ }}{O_2}absorbed} \\
  \;
\end{array}}}\]

RQ is useful because the volumes of CO$_2$ and O$_2$ produced totally depend on which fuel source is being metabolized. Measuring RQ is a convenient method to get information about the source of energy an animal is using. Also, we can compare the metabolism of animals under different environmental conditions by simply comparing RQ. The value of the respiratory quotient depends on the kind of respiratory substrate.

Respiratory quotient of carbohydrates:
Respiratory quotient is unity if carbohydrate is respiratory substrate and respiration is aerobic. Its value is calculated as:
\[{C_6}{H_{12}}{O_{6\;}} + \;6{O_2}\; \to \;6C{O_2} + \;6{H_2}O\]
As the gas exchange in this reaction is equal, the respiratory quotient for carbohydrates is:
\[RQ{\text{ }} = \;\dfrac{{6{O_2}}}{{C{O_2}}}\] $ = 1$

Note: To measure the energy source an animal is using could be very difficult as the chemical content of carbohydrates and lipids is different and the way they are metabolized is different. However, using general rules about the respiratory quotient, it is a fairly simple and informative way to quantify aspects of animal metabolism.