11th Class Biology Respiration In Plants Respiratory Quotient / Respiration Ratio

R.Q. is the ratio of the volume of \[C{{O}_{2}}\]released to the volume of oxygen taken in respiration and is written as

\[R.Q.=\frac{Volume\,of\,C{{O}_{2}}\,evolved}{Volume\,of\,{{O}_{2}}\,absorbed}=\frac{C{{O}_{2}}}{{{O}_{2}}}\]

R.Q. is usually measured by Ganong's respirometer.

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

\[\underset{\text{Glucose}}{\mathop{{{C}_{6}}{{H}_{12}}{{O}_{6}}}}\,+6{{O}_{2}}\to 6C{{O}_{2}}+6{{H}_{2}}O;\,\,\frac{C{{O}_{2}}}{{{O}_{2}}}=\frac{6}{6}=1\]  (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) \[\underset{\text{Stearic}\,\text{acid}}{\mathop{{{C}_{18}}{{H}_{36}}{{O}_{2}}}}\,+26{{O}_{2}}\to 18C{{O}_{2}}+18{{H}_{2}}O;\frac{C{{O}_{2}}}{{{O}_{2}}}=\frac{18}{26}=0.7\](Less than unity)

(ii) \[\underset{\text{Tripalmitin}}{\mathop{2{{C}_{51}}{{H}_{98}}{{O}_{6}}}}\,+145{{O}_{2}}\to 102C{{O}_{2}}+98{{H}_{2}}O;\frac{C{{O}_{2}}}{{{O}_{2}}}=\frac{102}{145}=0.7\] (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)\[\underset{\text{Malic}\,\text{acid}}{\mathop{{{C}_{4}}{{H}_{6}}{{O}_{5}}}}\,+3{{O}_{2}}\to 4C{{O}_{2}}+3{{H}_{2}}O;\frac{C{{O}_{2}}}{{{O}_{2}}}=\frac{4}{3}=1.33\] (More than unity)

(ii) \[\underset{\text{Oxalic}\,\text{acid}}{\mathop{2{{(COOH)}_{2}}}}\,+{{O}_{2}}\to 4C{{O}_{2}}+2{{H}_{2}}O;\frac{C{{O}_{2}}}{{{O}_{2}}}=\frac{4}{1}=4\] (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)

\[{{C}_{6}}{{H}_{12}}{{O}_{6}}\xrightarrow{\text{Zymase}}2{{C}_{2}}{{H}_{5}}OH+2C{{O}_{2}}\];

\[\frac{C{{O}_{2}}}{{{O}_{2}}}=\frac{2}{0}=\infty \](Infinite)