What is Respiratory Quotient?
Energy is the driving force for all living organism. Energy in biosystem is present in the form of ATP.
Energy is obtained when the biomolecules are broken down. The process of oxidizing the biomolecules for the energy is termed as respiration.
Internal and External respiration are two types of respiration involving the interaction of external environment with the internal environment through responsible organs.
The air filled with oxygen is transported to the internal tissues which is absorbed by the cell to drive the metabolic pathway that yield carbon dioxide and energy as the end products.
This carbon dioxide is released from the tissues to the external environment.
The amount of O2 consumed and CO2 released will provide the metabolic rate of any organisms along with the nature of component can also be detected.
To measure the metabolic rate of any organism the oxygen consumption and carbon dioxide release is experimentally determined.
The ratio between the O2 consumption and CO2 release is the respiratory quotient of the particular substrate uptake. Expressed as:
RQ = volume of CO2 released / volume of O2 consumed.
The quotient lacks dimension and unit. The respiratory quotient is also termed as Respiratory Ratio.
Features of Respiratory Quotient
1. RQ is substrate specific and species specific
2. RQ is dimensionless hence lacks unit.
3. RQ changes according to the external environmental factors such as pH, Temperature etc.,
4. RQ determines type of respiration – aerobic or anaerobic
5. Basal Metabolic Rate of the body can be determined.
Plants and animal utilize oxygen at different rate according to their needs and external environment to produce energy. But the basic concept of respiratory quotient and substrate specificity remains the same.
Condition and Interpretation of Respiratory Quotient
There are certain condition and interpretation for RQ.
1. When, RQ = 1
Considering Glucose molecule,
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O
RQ = 6/6 = 1
Interpretation: Respiration is aerobic.
2. When, RQ < 1
C57H104O6 + 80 O2 → 57 CO2 + 52 H2O
RQ = 57/80 = 0.7
Interpretation: Respiration is still aerobic but the substrate is either fat or protein
3. When, RQ = 0
Carbohydrates are transformed to organic acids consuming Oxygen with no release of CO2.
2C6H1206 + 302 → 3C4H605 + 3H20
RQ = 0/3 = 0
Interpretation: carbon dioxide is not released This takes place in succulents during night.
4. When, RQ > 1
C4H60 + 3 02 → 4C02 + 3H20
RQ = 4/3 = 1.3
Interpretation: Organic acids breakdown under aerobic
C6H12O6 → C4H5OH + 2 CO2
RQ= 2/0 = ∞
Interpretation: Anaerobic respiration taking place
The respiratory quotient of any animal is of the average 0.8. this is because any organism will not consume either one of the biomolecules at a particular time. The substrate hence remains mixed this yields the reduced RQ.
Factors Affecting Respiratory Quotient
1. Role of diet: From the above conditions the RQ for carbohydrate remains is one indicating the O2 consumption and CO2 release are the same. In fats they consume a lot oxygen and reduce the CO2 release rate. Hence diet influences the RQ.
2. Effect of Interconversion: In interconversion of glucose to fat and fat to glucose. The interconversion increases High CO2 and Low O2
3. Alkalosis and Acidosis: In alkalosis reduced less CO2 is released. Acidosis will increase the O2 consumption
4. Rise of Body Temperature: Results in excessive loss of CO2
Importance of Respiratory Quotient
1. Low value of RQ means
a. Aerobic respiration takes place
b. CO2 is absorbed in them.
2. High value of RQ means
a. Anaerobic respiration
b. Carbohydrates converts to fats
c. Food storage process takes place.
3. Respiratory Quotient is used to determine the BMR – Basal Metabolic Rate
4. Quality of respiratory organ
Respiratory Quotient Citations
- Body composition, respiratory quotient, and weight maintenance. Am J Clin Nutr . 1995 Nov;62(5 Suppl):1107S-1117S.
- Respiratory quotient during exercise. J Appl Physiol . 1961 Jul;16:606-10.
- Respiratory quotient during dialysis. N Engl J Med . 1977 Jul 7;297(1):56-7.
- Food quotient, respiratory quotient, and energy balance. Am J Clin Nutr . 1993 May;57(5 Suppl):759S-764S; discussion 764S-765S.
- Effects of an individualized nutrition intervention on the respiratory quotient of patients with liver failure. Asia Pac J Clin Nutr . 2019;28(3):428-434.