Respiratory Quotient

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What is Respiratory Quotient?

Respiratory Quotient is abbreviated as RQ and also known as Respiratory Coefficient or Respiratory Ratio, and it is defined as the ratio of volume of carbon dioxide evolved to the volume of oxygen consumed in respiration per unit time per unit weight at standard temperature and pressure. Therefore, we can represent Respiratory Quotient Formula as follows:

RQ= Volume of Carbon Dioxide / Volume of Oxygen Consumed (per unit time and per unit weight at standard temperature and pressure)

The instrument used for measuring Respiratory Quotient is Respirometer.

RQ for Different Respiratory Substrates

Depending upon the Respiratory Quotient, we can know what kinds of respiratory substrate is used for respiration.

1. RQ for Carbohydrates

(In Aerobic respiration)

C6H12O6 + 6 O2 → 6 CO2+ 6 H2O. Thus, the RQ= 6 CO2/ 6 O2=1

(In anaerobic respiration)

C6H12O6→ 2Ethanol + 2CO2 , RQ = Infinity

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2. RQ of Fats/Lipids 

Similarly, with the help of the formula for respiratory quotient, RQ for triolein, it is calculated as 0.71 and for tripalmitin, RQ=0.7

3. RQ of Protein= 0.9

4. RQ for Organic Acid

Using the formula, we get RQ for malic acid= 1.3

5. For Mixed Diet, RQ value= 0.8

Characteristics of Respiratory Quotient

  • The respiratory coefficients differ in organisms for metabolic balance and it ranges from 1.0 to 0.7 ((the former value representing the value for pure carbohydrate oxidation and the latter value expected for pure fat oxidation). 

  • In general, molecules which are more oxidized for example, glucose, require very less oxygen to be fully metabolized and, therefore, they have higher respiratory quotients. On the other hand, molecules that are less oxidized for example fatty acids require more oxygen for their complete metabolism and therefore, they have lower respiratory quotients. 

  • Respiratory Quotient value refers to the caloric value for each liter (L) of Carbon dioxide produced. If Oxygen consumption numbers are available, they are usually used directly, and they are more direct and reliable estimates of energy production.

  • Respiratory Quotient is measured as a contribution from the energy produced from protein. However, due to the complexity of the various ways in which varied amino acids can be metabolized, no single Respiratory Quotient can be assigned to the oxidation of protein in the diet.

  • Insulin, which is known to increase lipid storage and decrease fat oxidation, is thought to be positively associated with increase in the respiratory quotient. A positive energy balance then leads to an increased respiratory quotient.

  • Respiratory Coefficient is a dimensionless number which is used to calculate the basal metabolic rate and it is estimated with the help of the ratio from carbon dioxide production to oxygen absorption.


Terms Related to Respiratory Quotient

1. Cellular Respiration

Cellular respiration is a set of processes and metabolic reactions taking place in the cells of organisms that helps in converting chemical energy from oxygen molecules or nutrients into carbon dioxide, water and waste products with the release of ATP (adenosine triphosphate). It helps in sustaining the lives or living organisms like humans and animals.  

2. Aerobic Respiration

Cellular respiration is the process where cells break down food to use it as an energy. In aerobic cellular respiration, the respiration process makes use of oxygen to yield many more ATP molecules as compared to the anaerobic cellular respiration. It is believed that in terms of efficiency, aerobic metabolism is up to 15 times more stronger than anaerobic metabolism

3. Anaerobic Respiration

Here, cellular respiration occurs in the absence of oxygen to yield only two ATP molecules. Instead of oxygen, organic or inorganic molecules are used in the anaerobic respiration for the final electron acceptors. Some of the examples of anaerobic respiration are alcohol fermentation, decomposition of organic matter and fermentation of lactic acid. The equation is represented as: glucose + enzymes = carbon dioxide + lactic acid/ethanol

Application of Respiratory Quotient

  • A major application of the respiratory quotient is found in severe cases of chronic obstructive pulmonary disease.

  • It is used as an indicator for underfeeding or overfeeding.

  • It is used in analysis of the functioning of the liver.

  • It is also used to predict the weight gain in non-insulin-dependent patients suffering from diabetics.

  • It is also used in diagnosing the patients who are suffering from liver cirrhosis and also for non-protein respiratory quotients.

FAQ (Frequently Asked Questions)

1. Define RQ. What is the Respiratory Quotient of Protein?

RQ stands for respiratory quotient and it is the ratio of the volume of carbon dioxide released to the volume of oxygen used in the cellular respiration.  It is estimated per unit time per unit weight at standard temperature and pressure. The respiratory quotient of protein is known to be 0.9. 

2. What is the Importance of Respiratory Coefficient?

Respiratory quotient is also known as the respiratory coefficient and it is a huge contributor in  dealing with animal metabolism as it helps in comparing the volume of oxygen consumed by an organism helps in conveniently gaining information about the source of energy a living organism (human or animal) is using.

3. What is the Range of Respiratory Quotients? How is it Calculated?

Respiratory ratio or respiratory quotient is found to be in the range of 0.7 to 1.0 depending upon the source of energy. The source of energy can be lipids or fats, proteins, organic acids and carbohydrates. It is calculated with the help of a formula 

RQ = Volume of carbon dioxide released/ Volume of oxygen consumed