Respiratory Quotient - A short Study

RESPIRATORY QUOTIENT

During the process of  aerobic respiration, the oxygen  is consumed and carbon dioxide is released. The ratio of the volume of carbon dioxide released  to the volume of oxygen used  in respiration is called the respiratory quotient (RQ) or respiratory ratio.

RQ = volume of carbon dioxide released / volume of oxygen used 

The respiratory quotient depends mainly on the respiratory substrate used during respiration. 

Carbohydrates used as respiratory substrate

When carbohydrates are used as substrates then carbohydrates  are completely oxidised during respiration therefore  the RQ for carbohydrates is one. During the oxidation of carbohydrates, equal amounts of Carbon dioxide  and oxygen are evolved and consumed respectively. During the oxidation of carbohydrates, six molecules of carbon dioxide are released and six molecules of oxygen are used.

C6H12O6 + 6O2 → 6CO2 + 6H2O

      RQ=6CO2/6O2 =1

Fat used as respiratory substrate

When fats  are used as substrates then  the RQ for fats is less than one . During the oxidation of a fat like tripalmitin,102 molecules of carbon dioxide are released and 145 molecules of oxygen are used.

C51H98O6 + 102O2 → 198CO2 + 98H2O

   RQ=102CO2/198O2 =0.7

Protein used as respiratory substrate

When proteins  like Albumin are used as the  respiratory substrates the  RQ is about 0.88. 

C72H112N18O22S +77 O2 →63CO2 + 38H2O + SO3+ 9CO(NH2)2

RQ=77CO2/63O2 =0.88

For Ideal respiratory substrate in living organisms, respiratory quotients  are often more than one therefore pure proteins or fats are never used as respiratory substrates.

Respiratory Quotient can be used as an indicator  of feeding habits . If the feeding habit is under control then it  stimulates the body to utilize  the fat stored inside the body. As a result ,the RQ will less than the  respiratory quotient. In case of over feeding, the body encourages the synthesis of fat . Underfeeding is marked by a respiratory quotient below 0.85, while a respiratory quotient greater than 1.0 indicates overfeeding.