How to Determine the Value of Respiratory Quotient ? | Plants

The below mentioned article will guide you about how to determine the value of respiratory quotient by using Ganong’s Respirometer.

The apparatus:

Ganong’s Respirometer (Fig. 16.20) consists of a glass-bulb connected with a graduated glass tube. The neck of the bulb bears a hole. The lower part of the glass stopper which fits in the neck of bulb also bears a similar hole. The graduated tube is connected with a leveling glass tube filled with mercury by means of a rubber-tube. Whole of the apparatus is fixed in a stand.

Method:

The respiring material is placed in glass-bulb and the stopper is rotated to bring its hole in front of the hole of the glass bulb so that the air inside the bulb communicates with outer atmosphere. (If the respiring material is a green tissue, the bulb must be covered by black cloth). By adjusting the leveling tube, the mercury in both the tubes is brought at the same level. Now the stopper is again rotated so that its hole does not lie in front of the hole of the bulb and the internal atmosphere of bulb is disconnected from the outer atmosphere.

Level of mercury in graduated tube is noted. After a few hours a second reading is taken:

(i) In case, the carbohydrates are respiratory substrates, there will be no change in mer­cury level because equal vol. of CO₂ and O₂ have been released and absorbed respectively. Now, a KOH crystal is inserted into the graduated tube through the open end of leveling tube which will absorb all the CO₂ of the bulb and the level of mercury will rise up. This rise in mercury level will be equal to the Vol. of CO₂ released. It will also be the Vol. of O₂ taken in respiration.

(ii) In case, the respiring material contains fats etc., as the respiratory substrates, the mercury level will rise up in the graduated tube because more O₂ has been absorbed than CO₂ released (Fig. 16.21). Suppose this is V₁ c.c. It will represent the excess Vol. of O₂ absorbed. Again, KOH is added which will absorb CO₂ and the mercury will further rise, suppose by V₂ c.c.

The value of R.Q. will be:

Vol. of CO₂/ Vol of O₂ = V₂ c.c./ V₁ c.c. + V₂ c.c.

(iii) In case the respiratory substrate is rich in oxygen as in case of organic acids, the level of mercury will fall (Fig. 16.22) suppose by V₁ c.c., because more CO₂ has been given off than O₂ absorbed. Thus V, c.c. will be excess vol. of CO₂ given (or this much lesser vol. of O₂ has been taken in respiration). Addition of KOH will raise the mercury level suppose by V₂ c.c.

Then the value of R.Q. will be:

Vol. of CO₂/ Vol of O₂ = V₂ c.c. V₂-V₁ c.c.