Question

R.Q. of an actively photosynthesizing tissue is
A. Infinity.
B. <1.
C. >1.
D. Zero.

Answer 1

Respiratory quotient refers to the ratio of carbon dioxide production to oxygen consumption and the value of RQ (respiratory quotient) depends on the type of food substances that are metabolized.

Complete answer:
During aerobic respiration, oxygen is being consumed and carbon dioxide is released. Then the ratio of carbon dioxide which is given out to the volume of oxygen that is absorbed in respiration is known as a respiratory ratio or respiratory quotient (R.Q.).

R.Q. = volume of carbon dioxide evolved/volume of oxygen consumed

- Where incomplete oxidation of carbohydrates occurs the value of RQ is zero. In these cases oxygen absorbed does not oxidise the carbohydrates completely into carbon dioxide and water but it produces an intermediate product of the partial oxidation. This takes place especially in the respiration of succulent plants like cacti (Opuntia). In these plants respiration of carbohydrates occurs at night when stomata are open and this results in the production of organic acids which utilize oxygen but do not evolve carbon dioxide.
- During carbohydrate metabolism, one molecule of carbon dioxide is produced for every molecule of oxygen consumed by the tissue which means during carbohydrate metabolism amount of carbon dioxide produced in tissue is equal to the amount of oxygen consumed and therefore the value of RQ is one.
- In the case of anaerobic respiration, there is no consumption of oxygen. Carbon dioxide is produced in most cases therefore the value of RQ is infinite.
- When fat is utilized then the oxygen reacts with fats and a large amount of oxygen combines with hydrogen ions to form water instead of carbon dioxide. So, carbon dioxide output is less than the oxygen consumption, and therefore the value of RQ is less.
In the case of actively photosynthesizing tissue, the net volume of carbon dioxide evolved that is the volume of carbon dioxide produced in respiration minus that is utilized in photosynthesis) is equal to the net volume of oxygen that is the volume of the oxygen consumed in respiration minus that is evolved in photosynthesis).

Hence, the correct answer is option (D).

Note: The value of the respiratory quotient depends upon the respiratory substrate and its oxidation. It’s a dimensionless number that is used in a calculation for basal metabolic rate when estimated from the production of carbon dioxide to oxygen consumption.