NEET Biology Respiration In Plants R. Q.

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

Value of R.Q. varies with substrate. Thus the measurement of R.Q. gives some idea of the nature of substrate being respired in a particular tissue.

When carbohydrates are completely oxidised the value of R.Q. is unity (=one). If fats and proteins are the substrate the value of R.Q. is less than unity (0.5 to 0.9) and when organic acids are substrate the value is more than unity (1.3 to 4.0).

In succulent like Opuntia, Bryophyllum where there is incomplete oxidation of carbohydrates no CO₂ is produced, hence the value of R.Q. is zero.

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.)

                                                           \[\underset{\text{Glucose}}{\mathop{{{C}_{6}}{{H}_{12}}{{O}_{6}}}}\,+6{{O}_{2}}\to 6C{{O}_{2}}+{{H}_{2}}O\]  ; \[\frac{C{{O}_{2}}}{{{O}_{2}}}=\frac{6}{6}=1\] (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)

(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)