Respiratory Quotient

BiologyRespiration in PlantsFor NEET aspirants

Respiratory quotient

R.Q. is the ratio of the volume of CO₂ released to the volume of oxygen taken in respiration and is written as

R.Q.= $\frac{V o l u m e o f C O _{2} e v o l v e d}{V o l u m e o f O _{2} ab sor b e d} = \frac{C O _{2}}{O _{2}}$

R.Q. is usually measured by Ganong&aposs respirometer.

(1) When carbohydrates are the respiratory substrate (=germinating wheat, oat, barley, paddy grains or green leaves kept in dark or tubers, rhizomes, etc.)

\mathop {{C_6}{H_{12}}{O_6}}\limits_{{\text{Glucose}}}#xA0+ 6{O_2} \to 6C{O_2} + 6{H_2}O $\frac{C O _{2}}{O _{2}} = \frac{6}{6} = 1$ (Unity)

(2) When fats are the respiratory substrate (=germinating castor, mustard, linseed, til seeds) for fatty substances R.Q. is generally less than one .

(i) \mathop {{C_{18}}{H_{36}}{O_2}}\limits_{{\text{Stearic}}\,{\text{acid}}}#xA0+ 26{O_2} \to 18C{O_2} + 18{H_2}O $\frac{C O _{2}}{O _{2}} = \frac{18}{26} = 0.7$ (Less than unity)

(ii) \mathop {2{C_{51}}{H_{98}}{O_6}}\limits_{{\text{Tripalmitin}}}#xA0+ 145{O_2} \to 102C{O_2} + 98{H_2}O $\frac{C O _{2}}{O _{2}} = \frac{102}{145} = 0.7$ (Less than unity)

(3) When protein are the respiratory substrate (=germinating gram, pea, bean, mung seeds)

value of R.Q. is less than unity (0.5-0.9).

(4) When organic acids are the respiratory substrate

(i)\mathop {{C_4}{H_6}{O_5}}\limits_{{\text{Malic}}\,{\text{acid}}}#xA0+ 3{O_2} \to 4C{O_2} + 3{H_2}O $\frac{C O _{2}}{O _{2}} = \frac{4}{3} = 1.33$ (More than unity)

(ii) \mathop {2{{(COOH)}_2}}\limits_{{\text{Oxalic}}\,{\text{acid}}}#xA0+ {O_2} \to 4C{O_2} + 2{H_2}O $\frac{C O _{2}}{O _{2}} = \frac{4}{1} = 4$ (More than unity)

Some other organic acids and their R.Q. are - Succinic acid (1.14), Taurtric acid (1.6) and Acetic acid (1).

(5) When there is incomplete oxidation of carbohydrates (In the respiration of succulents i.e., Bryophyllum, Opuntia).

$2 C_{6} H_{12} O_{6} + 3 O_{2} \to 3 C_{4} H_{6} O_{5} + 3 H_{2} O$ $\frac{C O _{2}}{O _{2}} = \frac{0}{3} = 0$ (Zero)

(6) Respiration in the absence of O₂ (in anaerobic respiration)

$C_{6} H_{12} O_{6} Zymase 2 C_{2} H_{5} O H + 2 C O_{2}$ $\frac{C O _{2}}{O _{2}} = \frac{2}{0} = \infty$ (Infinite)

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