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Respiratory Quotient

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All living things can create the energy they need to survive through a series of chemical processes known as respiration. Air travels between the tissues and cells of the species and the outside environment through a biochemical process. Carbon dioxide gas is exhaled and oxygen is inhaled during respiration. A metabolic process occurs when an organism converts nutrients into energy and releases waste in the process.

Respiratory Quotient (RQ)

The respiratory quotient or respiratory ratio is the proportion of oxygen consumed to the carbon dioxide exhaled during breathing. RQ value is influenced by the oxidation of respiratory substrates.

RQ = (Volume OF CO2 Eliminated)/(Volume of O2 Consumed)

Respiratory Ratio sometimes referred to as Respiratory Quotient, is abbreviated as RQ. It is defined as the proportion of carbon dioxide released during respiration to oxygen received over the same period and weight at a constant temperature and pressure. In the respiration process, the respiratory substrate is broken down to liberate energy. The two basic forms of respiration that take place in the presence or absence of molecular oxygen are aerobic and anaerobic respiration. The most prevalent respiratory substrate in plants is glucose.

Respiratory Quotient Meaning

O2 is ingested and CO2 is exhaled during aerobic respiration. The respiratory quotient is the ratio of the quantity of O2 used for respiration to the amount of CO2 evolved (R.Q.). An equipment called a respirometer is used to measure both R.Q. and the rate of breathing. A diet with a mixture of carbs, lipids, and proteins has an R.Q. of around 0.85.

Factors affecting Respiratory Quotient 

The following list of variables influences R.Q. value:

  1. The respiratory substrate’s composition.
  2. The quantity of oxygen in the substrate for respiration.
  3. How much of the respiratory substrate is destroyed?
  4. The variations in the respiratory substrates’ degrees of oxidation and reduction
  5. Whether or not all of the absorbed oxygen is utilized for respiration determines the value of R.Q.

Respiratory Quotient of Carbohydrates

The primary respiratory substrate in plants is glucose, followed by fructose, starch, sucrose, and other sugars. When hexose carbohydrates like glucose or fructose are completely oxidized during aerobic respiration, an equal amount of CO2 is released and oxygen is consumed. The Respiratory Quotient (R.Q.) value in this situation is determined to be one of unity.

C6H12O6 + 6O2(Glucose)→6CO2 + 6H2O + Energy 

R.Q.= 6 CO2/ 6 O2=1 or unity.

RQ of Carbohydrates is 1

Respiratory Quotient of Fats

As a kind of store nourishment for plants, fats are often found in seeds. Compared to other substrates, fats require a greater amount of oxygen to be converted into energy. In the aerobic respiration of fats, the Respiratory Quotient (R.Q.) value is smaller than one or unity.

2C51H98O6(Tripalmitin) + 145O2→102CO2 + 98H2O + Energy 

R.Q. = Vol.ofCO2/Vol.ofO2 = 102145 = 0.7 (less than one)

RQ of Fat is approximately 1

Respiratory Quotient of Proteins and their derivatives

The building components of proteins are known as amino acids. When cells are about to fall into famine, for example, proteins can function as respiratory substrates. Amino acid sequence differences affect how proteins are produced. Therefore, depending on the structure of different amino acids, R.Q. also fluctuates from 0.8 to 0.9.

2C3H7O2N(Alanine)+6O2→CO(NH2) 2(Urea)+5CO2+5H2O+Energy 

R.Q.=56=0.83 (less than one)

RQ of Proteins is less than 1

Organic Acid Respiratory Quotient

The value of R.Q. is larger than one when organic acids are oxidized during respiration because they are rich in oxygen and require less oxygen for oxidation.

C4H6O5 (Malic acid)+3O2→4CO2+3H2O+Energy 

R.Q.=43=1.3 (more than one)

RQ of Organic Acid is more than 1

Succulents Respiratory Quotient

Some succulent plants, like Opuntia and Bryophyllum, do not completely oxidize their carbohydrates. Organic acids are created in the dark without CO2 development due to incomplete oxidation. In succulent plants, stomata stay open at night. Thus, the carbohydrates are partly oxidized to organic acids without CO2 being produced at night when oxygen is absorbed. R.Q.’s value, therefore, stays at zero. Because photosynthesis consumes the CO2 generated during respiration as a substrate, the value of RQ stays 0 even during the day.

2C6H12O6 (Glucose)+3O2 → 3C4H6O5 (Malicacid)+3H2O+Energy

R.Q.=03=0 (zero)

RQ of Succulent is 0

Importance of Respiratory Quotient

The following is a list of the respiratory quotient’s significance:

  1. R.Q. aids in identifying the type of respiration being carried out 
  2. R.Q. provides information on several key food material transformations.
  3. The R.Q. data may be used to determine the kind of respiratory substrate that oxidized the substrate’s transformation as well as the biochemical process of respiration.
  4. R.Q. is useful in determining metabolic rate.
  5. R.Q. aids in the diagnosis of several clinical diseases, including acidosis and alkalosis.
  6. R.Q. can be used to predict weight gain in diabetes individuals who are not insulin-dependent.
  7. In situations of severe chronic obstructive pulmonary illness, R.Q. is quite useful.
  8. In severe instances of chronic obstructive pulmonary disease, R.Q. is quite useful.

FAQs on Respiratory Quotient

Question 1: What is a respiratory quotient, exactly?


During aerobic respiration, CO2 is exhaled and O2 is ingested. The respiratory quotient is the ratio of CO2 generated to the quantity of O2 used during breathing (R.Q.).

Question 2: What will the R.Q. be? if tripalmitin serves as a respiration substrate?


Tripalmitin is a lipid, hence the R.Q. value is going to be 0.7.

Question 3: Mention the succulents’ respiratory capacity.


Succulents have no respiratory quotient.

Question 4: In what ways do plants breathe?


Cellular respiration is the mechanism by which all green plants breathe. In this process, energy is created from nutrients that are collected from the soil and are used for various cellular functions.

Question 5: Are plants able to breathe at night?


Yes, plants do breathe throughout their whole lives, both at night and during the day. Oxygen + glucose = carbon dioxide + water + heat energy is the chemical formula for cellular respiration.

Question 6: Identify the breathing system found in woody stems.


Respiration or the exchange of gases occurs through lenticels in hard, woody stems. They are the tiny pores that are dispersed throughout the bark of all trees. 

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Last Updated : 09 Jan, 2023
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