Respiratory Quotient (RQ)

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.

formulae 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.

formulae C6H12O6 1 6O2 6CO2 ↑ 1 6H2O 1 Energy Glucose

RQ of glucose 5 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.

formulae C6H12O6 2CO2↑1 2C2H5OH 1 Energy Glucose Ethyl alcohol }RQ of glucose 5 2 molecules of CO2 Anaerobically zero molecule of O2 5 ∞ (infinity)

3. In some succulent plants like O_puntia, 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.

formulae 2C6H12O6 1 3O2 3C4H6O5 1 3H2O 1 Energy Glucose Malic acid RQ of glucose 5 zero molecule of CO2 in succulents 3 molecules of O2 5 0 (zero)

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

formulae 2(C51H98O6) 1 145O2 102CO2↑1 98H2O 1 Energy Tripalmitin(Fat) RQ of 5 102 molecules of CO2 Tripalmitin 145 molecules of O2 5 0.7 (less than unity)

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

formulae C4H6O5 1 3O2 4CO2 ↑1 3H2O 1 Energy Malic acid RQ of 5 4 molecules of CO2 malic acid 3 molecules of O2 5 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.

Red colour in various parts of plants is due to the presence of anthocyanin, synthesis of which require more O2 than CO2 evolved. RQ will be less than one.

Activity Take a test tube with some germinated seeds and fill with water. Keep this test tube after some time until liberation of CO2. When the carbon dioxide from respiration is mixed to water, carbonic acid (H2CO3) is produced. Therefore, as more carbon dioxide is released, the solution becomes more acidic. You will see changes in pH as an indicator using blue litmus paper changed into red that respiration has occurred CO21H2O H2CO3

The apparatus used for determining respiration and RQ is called Ganong’s Respirometer.

Respiratory quotients of some other substances Proteins : 0.8–0.9 Oleic acid (Fat) : 0.71 Palmitic acid (Fat) : 0.36 Tartaric acid : 1.6 Oxalic acid : 4.0