Respiratory Quotient (RQ) - Significance, formula

 

       Respiratory Quotient (RQ)

Definition:

The ratio of volume of carbon dioxide given out and volume of oxygen taken in during respiration is called Respiratory Quotient or Respiratory ratio. RQ value depends upon respiratory substrates and their oxidation.

RQ = Volume of CO2 liberated / Volume of O2 consumed

1. The respiratory substrate is a carbohydrate, it will be completely oxidised in aerobic respiration and the value of the RQ will be equal to unity.

C₆H₁₂O₆ + 6O₂ → 6CO₂ ↑+ 6H₂O +Energy

RQ of glucose = 6 molecules of CO2 / 6 molecules of O2

= 1 (unity)

2. If the respiratory substrate is a carbohydrate it will be incompletely oxidised when it goes through anaerobic respiration and the RQ value will be infinity.

C₆H₁₂O₆ →  2CO₂↑+ 2C₂H₅OH + Energy

RQ of glucose = 2 molecules of CO₂ / Anaerobically zero molecule of O₂

= ∞ (infinity)

3. In some succulent plants like Opuntia, Bryophyllum carbohydrates are partially oxidised to organic acid, particularly malic acid without corresponding release of CO2 but O2 is consumed hence the RQ value will be zero.

2C₆H₁₂O₆ + 3O₂ → 3C₄H₆O₅ + 3H₂O +Energy

RQ of glucose in succulents = zero molecule of CO2 / 3 molecules of O2

= 0 (zero)

4. When respiratory substrate is protein or fat, then RQ will be less than unity.

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

RQ of Tripalmitin = 102 molecules of CO2 / 145 molecules of O2

= 0.7 (less than unity)

5. When respiratory substrate is an organic acid the value of RQ will be more than unity.

C₄H₆O₅ + 3O₂ →4CO₂↑+3H₂O +Energy

RQ of malic acid = 4 molecules of CO2 / 3 molecules of O2

= 1.33 (more than unity)

Significance of RQ:

1. RQ value indicates which type of respiration occurs in living cells, either aerobic or anaerobic.

2. It also helps to know which type of respiratory substrate is involved.