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.
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.
(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.