Respiratory Quotient (RQ)
The Respiratory Quotient (RQ) is defined as the ratio of the volume of carbon dioxide (CO₂) evolved to the volume of oxygen (O₂) consumed during respiration.
Formula:
RQ = Volume of CO₂ evolved / Volume of O₂ consumed
RQ gives a clear idea about the type of respiratory substrate (carbohydrate, fat, protein, etc.) being oxidised in an organism.
RQ Values for Different Respiratory Substrates
1. Carbohydrates (e.g., Glucose)
- Carbohydrates contain carbon, hydrogen, and oxygen in balanced proportions.
- During complete oxidation, the amount of CO₂ released is equal to the amount of O₂ consumed.
Reaction:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy
RQ = 6 / 6 = 1.0
RQ = 1 indicates carbohydrate respiration.
2. Fats
- Fats contain very little oxygen compared to carbon and hydrogen.
- Hence, they require more oxygen for oxidation, but produce comparatively less CO₂.
Example (Tripalmitin):
2(C₅₁H₉₈O₆) + 145O₂ → 102CO₂ + 98H₂O + Energy
RQ = 102 / 145 ≈ 0.7
RQ < 1 is characteristic of fat respiration.
3. Proteins
- Proteins are first deaminated, and their carbon skeletons are respired.
- Due to complex composition, protein respiration gives an RQ value less than 1.
Average RQ ≈ 0.9
4. Organic Acids
- Organic acids contain more oxygen relative to carbon.
- Less oxygen is required for their oxidation, but more CO₂ is released.
Example (Malic Acid):
C₄H₆O₅ + 3O₂ → 4CO₂ + 3H₂O + Energy
RQ = 4 / 3 ≈ 1.33
RQ > 1 indicates respiration of organic acids.
5. Anaerobic Respiration (Fermentation)
- Oxygen is not consumed, but CO₂ may be released (alcoholic fermentation).
Reaction:
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + Energy
RQ = 2 / 0 = ∞ (infinite)
RQ is infinite in anaerobic respiration.
RQ Under Special Conditions
- Anaerobic respiration: RQ = ∞
- Germinating oil seeds: RQ < 1
- Succulent plants (organic acids): RQ > 1
Significance of Respiratory Quotient
- Helps identify the nature of respiratory substrate
- Indicates metabolic condition of the organism
- Useful in physiological and ecological studies
Important NCERT Notes
- RQ has no unit
- RQ varies with substrate and physiological condition
- RQ cannot be calculated if CO₂ evolution does not occur
Memory Capsules – Respiratory Quotient (RQ)
RQ Formula:
RQ = CO₂ / O₂
Key RQ Values:
- Carbohydrates → RQ = 1
- Fats → RQ ≈ 0.7
- Proteins → RQ ≈ 0.9
- Organic acids → RQ > 1
- Anaerobic respiration → RQ = ∞
Trick to Remember:
- Less oxygen in substrate → More O₂ needed → RQ < 1
- More oxygen in substrate → Less O₂ needed → RQ > 1
💡 Questions and Answers
Q1. What is Respiratory Quotient (RQ)?
A1:
Respiratory Quotient is the ratio of carbon dioxide released to oxygen consumed during respiration. It helps us understand which food material is being used for energy.
Q2. Why is the RQ of carbohydrates equal to 1?
A2:
Because during carbohydrate respiration, the amount of CO₂ released is exactly equal to the amount of O₂ consumed.
Q3. Why is the RQ of fats less than 1?
A3:
Fats contain very little oxygen, so more oxygen is required for their oxidation, resulting in less CO₂ release compared to oxygen consumption.
Q4. Why is the RQ infinite in anaerobic respiration?
A4:
In anaerobic respiration, oxygen is not consumed, so the denominator becomes zero, making the RQ value infinite.
Q5. What does an RQ greater than 1 indicate?
A5:
An RQ greater than 1 indicates that organic acids are being used as respiratory substrates.