Free Respiratory Quotient OCR A Level Biology revision notes – covering specification points 5.2.2 (j) and 5.2.2 (k).
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The respiratory quotient (RQ) is a ratio reported as a single number that indicates which respiratory substrate is (most likely) being used by an organism.
The RQ ratio represents how much carbon dioxide is produced relative to how much oxygen is used.
The RQ is calculated using the formula:
RQ = CO₂ produced ÷ O₂ consumed
The table below shows typical RQ values for different respiratory substrates:
| Respiratory Substrate | RQ Value | Explanation |
|---|---|---|
| Carbohydrates | 1.0 | Equal amounts of CO₂ produced and O₂ consumed. |
| Lipids | ~0.7 | More O₂ is required to oxidise hydrogen-rich lipids. |
| Proteins | ~0.9 | Depends on the amino acid. |
Uses of RQ Values
RQ values can be used to:
- Identify the respiratory substrate being used (e.g. carbohydrates, lipids, proteins).
- Detect the presence of anaerobic respiration.
- Monitor metabolic shifts during exercise, starvation, medical treatment or fasting.