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The below mentioned article provides a study note on the respiratory quotient.
The ratio of the volume of CO2 released to the volume of O2 taken in respiration is called as respiratory quotient and is denoted as R.Q.
R.Q. = Vol. of CO2 /Vol. of O2
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Value of Respiratory Quotient:
The value of R.Q. depends upon the nature of the respiratory substrate (the organic food matter oxidised in respiration) and its oxidation.
(1) When carbohydrates such as hexose sugars are oxidised in respiration, the value of R.Q. is 1 or unity because vol. of CO2 evolved equals to the vol. of O2 absorbed as is shown by the following equation:
C6H12O6 + 6O2 (Glucose) → 6CO2 + 6H2O
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R.Q. = Vol. of CO2/ Vol. of O2 = 6/6 = 1 or unity
(2) When fats are the respiratory substrates, the value of R.Q. becomes less than one because the fats are poorer in oxygen in comparison to carbon and they require more O2 for their oxidation which is obvious from the following equation:
2C51H98O6 (Tripalmitin) + 145O2 → 102CO2 + 98H2O
R.Q. = Vol. of CO2/ Vol. of O2 = 102/145 = 0.7 (less than one)
(Fats are oxidised in respiration usually during the germination of fatty seeds).
The value of R.Q. is also less than one when proteins are the respiratory substrates
(3) When organic acids are oxidised in respiration the value or R.Q. becomes more than one. It is because organic acids are rich in O2 and require less O2 for their oxidation e.g.,
C4H6O5 (Malic acid) + 3O2 → 4CO2 + 3H2O
R.Q. = Vol. of CO2/ Vol. of O2 = 4/3 = 1.3 (more than one)
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(4) Partial oxidation of carbohydrates:
In some succulent plants like Opuntia, Bryophyllum etc carbohydrates are incompletely oxidised to organic acids in dark without the evolution of CO2, hence the value of R.Q. remains O.
2C6H12O6 (Glucose) + 3O2 → 3C4H6O5 (Malic acid) + 3H2O
R.Q. = Vol.of CO2/ Vol. of O2 = 0/3 = 0
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(5) During anaerobic respiration, due to the absence of O2 the value of R.Q. is always very high rather infinite.
C6H12O6 (Glucose) → 2CO2 + 2C2H5OH (Alcohol)
R.Q. = Vol. of CO2/ Vol. of O2 = 2/0 = 
(infinite)
Significance of Respiratory Quotient:
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By determining the value of R.Q. the nature of respiratory substrate can be known. For example, if the value of R.Q. is one, it indicates that carbohydrates are being oxidised during respiration. Similarly if the value is less than one it will be concluded that organic matter like fats constitute the respiratory substrates.
