Respiratory quotient (RQ)Spec 7.6
In short
The respiratory quotient (RQ) is the ratio of the volume of carbon dioxide produced to the volume of oxygen consumed in a given time. Its value shows which substrate is being respired: about 1.0 for carbohydrate, about 0.7 for lipid and about 0.9 for protein. A value above 1.0 suggests anaerobic respiration.
The respiratory quotient (RQ) compares the carbon dioxide produced with the oxygen consumed by an organism or tissue over the same time.
RQ has no units, because it is a ratio. Its value depends on the substrate being respired, because different substrates contain different proportions of oxygen.
| Substrate respired | Typical RQ | Reason |
|---|---|---|
| Carbohydrate (glucose) | 1.0 | 6CO₂ produced for 6O₂ used |
| Lipid | about 0.7 | Lipids have little oxygen in the molecule, so more O₂ is needed per CO₂ released |
| Protein | about 0.9 | Between lipid and carbohydrate |
A mixture of substrates gives a value between those shown. An RQ greater than 1.0 means that more CO₂ is being produced than O₂ used, so some anaerobic respiration may be taking place in the tissue (for example, in yeast).
Calculating RQ
A sample of germinating seeds uses 40 cm³ of oxygen and produces 36 cm³ of carbon dioxide in 10 minutes. Calculate the RQ and suggest which substrate was respired.
- RQ = volume of CO₂ produced ÷ volume of O₂ consumed
- RQ = 36 ÷ 40
Answer: RQ = 0.9, which suggests that protein (or a mixture of substrates) is being respired.
RQ is a ratio, so no units. Write the working as 'CO₂ produced ÷ O₂ consumed' and state what the answer suggests about the substrate.
Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026