Respiratory Quotient

Respiratory Quotient

Respiratory quotient (RQ) or respiratory ratio is the ratio of the volume of carbon dioxide released by a tissue over a period of time to the volume of oxygen absorbed during this period.

RQ =

The respiratory quotient, besides other factors, depends upon the type of respiratory substrate and the completeness of oxidation.

It can be measured with the help of an apparatus called Ganong's respirometer.

The value of RQ may be equal to unity, more than unity or less than unity, depending upon the substrates oxidised.

This can be illustrated by following examples:

(i) In case of carbohydrates : Plant material where carbohydrates serve as the respiratory substrates (e.g., germinating seeds of cereals, leaves kept in dark, flowers and fruits rich in carbohydrates) have RQ equal to unity, i.e., one.

C₆H₁₂O₆ + 6O₂ ® 6CO₂ + 6H₂O

RQ = = 1

(ii) In case of fats: e.g. , In germinating fatty seeds (e.g., castor, sesame, mustard, linseed, etc.) Fats are poor in structural O₂, so the RQ value is less than unity.

2C₅₁ H₉₈O₆ + 145O₂ ® 102CO₂ + 98H₂O

(tripalmitin)

RQ = = 0.7

(iii) In case of proteins -e.g., germinating pulse seeds.

In proteins too, the proportion of carbon is more than oxygen, hence their RQ is also less than unity i.e., (0.5 -0.9).

(iv) In succulents (during night) e.g., Bryophyllum, Opuntia

Incomplete oxidation of carbohydrates occurs during night when stomata are open.

3C₆H₁₂0 ₆ + 3O₂ —® 3C₄H₆O₅ + 3H₂O

RQ = = 0

(v) In case of organic acids. In fleshy plants (i.e., members of Cactaceae and Crassulaceae) the organic acids (such as malic acid, oxalic acid, etc.) are produced during night are oxidised during day time. Organic acids contain high proportion of oxygen as compared to carbohydrates, therefore, less oxygen is absorbed than CO₂ liberated. The RQ value is more than unity.

e.g., The RQ value is 4 for oxalic acid and 1.33 for malic acid

(vi) In case of'anaerobic condition (in the absence of O₂). In the absence of oxygen, tissue respires anaerobically. Thus, CO₂ liberation takes place without any utilization of oxygen.

e.g.,

RQ = = ∞ (Infinite)

(vii) R Q. of maturing fatty seed is > 1.

(viii) RQ. of mixed diet is 0.85.

(ix) RQ. of starved leaves is < 1.

(x) RQ. of coloured petals < 1.