Question

The respiratory quotient during cellular respiration would depend on

• A. The nature of enzymes involved
• B. The nature of the substrate
• C. The amount of carbondioxide released
• D. The amount of oxygen utilised

Answer 1

Answer: (B)

Respiratory Quotient:
R.Q. is the ratio of the volume of carbon dioxide released to the volume of oxygen taken in respiration and it is mathematically expressed as:
$R.Q.=\frac{V o l u m e o f C O_{2} e v o l v e d}{V o l u m e o f O_{2} c o n s u m e d}$
The value of R.Q. varies with the substrate. Thus the measurement of R.Q. gives some idea of the nature of the substrate being respired in a particular tissue. R.Q. is usually measured by Ganong's respirometer.
(1) Carbohydrates: When carbohydrates are used as the respiratory substrate like germinating wheat, rhizomes, tubers, paddy grains, or green leaves kept in the dark, etc. R.Q. value is unity.
$C_{6}H_{12}O_{6}+6O_{2} \rightarrow 6CO_{2}+6H_{2}O+Energy$
$R.Q.=\frac{V o l u m e o f C O_{2} e v o l v e d}{V o l u m e o f O_{2} c o n s u m e d}=\frac{6}{6}=1$
(2) Fats/Oils: When fats or oils are used as the respiratory substrate like germinating linseed, castor, mustard, til seeds, etc. R.Q. value is generally less than one around 0.7. For example, in the case of tripalmitin, the reaction can be written as follows:
$2C_{51}H_{98}O_{6}+145O_{2} \rightarrow 102CO_{2}+98H_{2}O+Energy$
$R.Q.=\frac{V o l u m e o f C O_{2} e v o l v e d}{V o l u m e o f O_{2} c o n s u m e d}=\frac{102}{145}=0.70$
(3) Proteins: When proteins are used as the respiratory substrate like germinating gram, pea, bean, mung seeds, etc. R.Q. value is generally less than one around 0.8-0.9.
(4) Organic acids: When organic acids are used as the respiratory substrate like malic acid, oxalic acid, etc. R.Q. value is generally more than one. The breakdown of malic acid is as follows:
$C_{4}H_{6}O_{5}+3O_{2} \rightarrow 4CO_{2}+3H_{2}O+Energy$
$R.Q.=\frac{V o l u m e o f C O_{2} e v o l v e d}{V o l u m e o f O_{2} c o n s u m e d}=\frac{4}{3}=1.33$
(5) In xerophytes: When there is incomplete oxidation of carbohydrates as in the respiration of Bryophyllum, Opuntia, etc. R.Q. value is zero as CO₂ is not released during the night time.
$2C_{6}H_{12}O_{6}+3O_{2} \rightarrow 3C_{4}H_{6}O_{5}+3H_{2}O+Energy$
$R.Q.=\frac{V o l u m e o f C O_{2} e v o l v e d}{V o l u m e o f O_{2} c o n s u m e d}=\frac{0}{3}=0$
(6) Anaerobic respiration: Respiration in the absence of O₂ i.e. in anaerobic respiration, R.Q. value is infinity as O₂ is not consumed.
$C_{6}H_{12}O_{6}\overset{Z y m a s e}{ \rightarrow }2C_{2}H_{5}OH+2CO_{2}+Energy$
$R.Q.=\frac{V o l u m e o f C O_{2} e v o l v e d}{V o l u m e o f O_{2} c o n s u m e d}=\frac{2}{0}=Infinity$