R.Q. is the ratio of the volume of CO2 released to the volume of oxygen taken in respiration R.Q. is the ratio of the volume of CO2 released to the volume of oxygen taken in respiration.

# Value of R.Q. varies with substrate. Thus the measurement of R.Q. gives some idea of the nature of substrate being respired in a particular tissue.
- When carbohydrates are completely oxidised the value of R.Q. is unity (=one). If fats and proteins are the substrate the value of R.Q. is less than unity (0.5 to 0.9) and when organic acids are substrate the value is more than unity (1.3 to 4.0).
- In succulent like Opuntia, Bryophyllum where there is incomplete oxidation of carbohydrates no CO2 is produced, hence the value of R.Q. is zero.

#R.Q. is usually measured by Ganong's respirometer.