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Respiratory Ratio or Quotient

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 respiratory quotient (RQ) or respiratory ratio.
RQ =

The respiratory quotient depends on the type of respiratory substrate used during respiration. This is different for different substrates.


When carbohydrates are used as a substrate and are completely oxidized, the RQ will be 1, because equal amounts of CO2 and O2 are evolved and consumed, respectively, as shown in the equation below:

RQ = = 1.0


When fats are used in respiration, the RQ is less than 1. For example, it is explained below with tripalmitin (2 molecules) as a substrate.


RQ == 0.7

The RQ in this case is less than 1 because fats contain less oxygen than carbohydrates. Therefore, they require relatively greater amounts of O2 for oxidation.

Organic acids

When organic acids, such as oxalic acid and malic acid, serve as respiratory substrates, then RQ is more than one. This is because organic acids contain more oxygen than carbohydrates. Therefore, relatively less amount of oxygen is required for their oxidation.

RQ = = 4


That in anaerobic respiration, CO2 is evolved but oxygen is not used. Therefore, RQ, in such a case, will be infinite. For example,

RQ = = Infinity (ยต)

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