Respiratory Quotient (RQ): Study Notes

Respiratory Quotient (RQ): Study Notes!

Respiratory quotient is the ratio of the volume of carbon dioxide produced to the volume of oxygen consumed in respiration over a period of time. Its value can be one, zero, more than 1 or less than one.

RQ = Volume of C0₂ evolved/Volume of 0₂ absorbed

RQ Equal to Unity:

Respiratory quotient is equal to unity if carbohydrates are the respiratory substrate and the respiration is aerobic.

C₆H₁₂0₆ + 60₂ → 6C0₂ + 6H₂0 RQ = 6C0₂/60₂ = 1

RQ Less than Unity:

RQ is less than one when respiration is aerobic but the respiratory substrate is either fat or protein. RQ is about 0.7 for most of the common fats. It occurs during germination of fatty seeds.

C₅₇H₁₀₄O₆ + 80 0₂ → 57C0₂ + 52H₂0 RQ = 57C0₂/800₂ = 0.71 triolein

2(C₅₁H₉₈0₆) + 1450₂ → 102C0₂ + 98H₂0 RQ = 102C0₂/1450₂ = 0.7 tripalmitin

RQ is about 0.9 in case of proteins, peptones, etc.

RQ Zero:

Succulents do not evolve carbon dioxide during night (when their stomata are open) as the same is used in carbon fixation. They also change carbohydrates to organic acids which utilise oxygen but do not evolve carbon dioxide.

2C₆H₁₂0₆ + 30₂ —» 3C₄H₆0₅ + 3H₂0 RQ = Zero C0₂/30₂ = Zero

RQ More than Unity:

(a) RQ slightly more than unity is found when organic acids are broken down as respiratory substrates under aerobic conditions, e.g.,

2(COOH)₆ + 0₂ —» 4C0₂ + 2H₂0 RQ = 4 C0₂/1 0₂ = 4.0 oxalic acid

C₄H₆0 + 3 0₂ —» 4C0₂ + 3H₂0 RQ = 4 C0₂/3 0₂ or 1.3 malic acid

2C₄H₆0₄ + 7 0₂ —» 8C0₂ + 6H₂0 RQ = 8 C0₂/7 0₂ or 1.14 succinic acid

(b) In anaerobic respiration there is no consumption of oxygen. Carbon dioxide is produced in most of the cases. Therefore, respiratory quotient is infinity. Carbohydrate is the usual substrate.

An intermediate value is obtained where an organ is undergoing both aerobic and anaerobic modes of respiration.

Importance:

(i) Knowledge of respiratory quotient helps in determining respiratory substrate.

(ii) It helps in knowing the type of respiration being performed,

(iii) It provides some information about major transformation of food materials.