Let us now look at another aspect of respiration. As you know, during aerobic respiration, O2 is consumed and CO2 is released. The ratio of the volume of CO2 evolved to the volume of O2 consumed in respiration is called the respiratory quotient (RQ) or respiratory ratio.
The respiratory quotient depends upon the type of respiratory substrate used during respiration. When carbohydrates are used as substrate and are completely oxidized, the RQ will be I, because equal amounts of CO2 and O2 are evolved and consumed. When fats are used in respiration, the RQ is less than 1 e.g., calculations for a fatty acid, tripalmitin. When proteins are respiratory substrates the ratio would be about 0.9.
RQ value in succulents is zero. In succulents Opuntia (Cactaceae), Bryophyllum (crassulaceae) carbohydrates are partially oxidized to malic acid. CO2 is taken back and is fixed by RUBP carboxylase in Calvin cycle. RQ value is infinite during anaerobic respiration because there is no utilization of O2 in this process.