Notes on Respiratory Quotient | Plants

The below mentioned article provides a note on respiratory quotient.

During aerobic respiration, oxygen is consumed and carbon dioxide is released. The ratio of the volume of carbon dioxide given out to the volume of oxygen absorbed in respiration, i.e., CO₂/O₂ is called respiratory quotient (RQ) or respiratory ratio

RQ = volume of CO₂ evolved/volume of O₂ consumed

The value of respiratory quotient depends upon the nature of respiratory substance used during respiration. This varies for different types of substrates.

Carbohydrates (RQ Equal to Unity):

The value of respiratory quotient is unity (one) when hexose sugars are consumed in respiration. Here equal volumes of CO₂ and O₂ are evolved and consumed, respectively. This is obvious from overall equation of respiration.

Fats (RQ Less than Unity):

When substances poorer in oxygen than carbohydrates, e.g., fats are oxidised in respiration, the value of respiratory quotient is less than one.

It is explained in tripalmitin and triolein as follows:

RQ in above-mentioned examples is less than unity, i.e., 0.7, because fats contain less oxygen than the carbohydrates, and therefore, they require relatively greater amount of oxygen for oxidation.

In case of proteins RQ is 0.5 (approximately).

Succulents (RQ is Zero):

Where incomplete oxidation of carbohydrates takes place RQ is zero. In such cases oxygen absorbed does not oxidise the carbohydrates completely to CO₂ and water but produces intermediate products of partial oxidation.

This occurs especially in respiration of succulent plants, such as cacti (Opuntia). In such plants respiration of carbohydrates occurs at night when stomata are open which results in production of organic acids that utilise O₂ but do not evolve CO₂.

The equation is as follows:

Organic Acids (RQ More than Unity):

When substances rich in oxygen are respired, the value of RQ is more than unity. This occurs in respiration of organic acids. Organic acids contain more oxygen than carbohydrates, and therefore, relatively less amount of oxygen is required for their oxidation.

The examples are shown by following equations:

Here RQ is 4 in case of oxalic acid.

This means, RQ is 1.6 in case of tartaric acid.

In case of malic acid, RQ is 1.3.

Anaerobic Respiration (RQ Infinity):

As we know, in anaerobic respiration CO₂ is evolved but O₂ is not consumed, and therefore RQ is infinite (∞).

Equation is as follows:

Significance of RQ:

The RQ value of a respiring tissue may provide valuable information to an investigation. From an RQ value, one can obtain a rough indication of the nature of substrate being oxidised.

It also helps in knowing the type of respiration being performed. It may also provide some information about major transformation of food materials.

If RQ is less than one, then substrate is of low oxygen content, e.g., fats. In starvation the cell material itself becomes substrate. According to Blackman, in starving conditions, if cellular proteins are the respiratory substrates then it is called protoplasmic respiration.