12.1.5—Respiratory quotient (RQ)

Respiratory quotient (RQ) is the ratio of carbon dioxide produced to oxygen consumed during respiration: RQ = CO₂ produced ÷ O₂ consumed. It is a dimensionless gas-exchange clue, not a direct measure of energy expenditure.

• Why substrates differ: Carbohydrates, lipids and proteins have different molecular compositions, so their oxidation can produce different CO₂:O₂ relationships.
• Using RQ as evidence: A measured RQ can be compared with the typical pattern expected for a respiratory substrate, helping identify which substrate is contributing most under the measured conditions.
• Mixed substrates: A value between typical single-substrate patterns can indicate that more than one substrate is being respired; it should be treated as evidence about the mixture, not as proof of one pure substrate.
• Aerobic boundary: The usual RQ interpretation assumes oxygen uptake is part of the measured respiration. Anaerobic pathways do not all provide a normal finite RQ in the same way, so the context must be stated.
• Interpretation boundary: RQ is a ratio of gases over a defined interval. It is not the same as an energy value, an ATP yield, or a complete diagnosis of cellular metabolism.

Respiratory substrate and pathway → relative CO₂ output and O₂ uptake → measured RQ → cautious inference about substrate use. The inference is strongest when conditions are controlled and the sample is not simultaneously using several pathways or substrates.

Do not invert the ratio or treat one RQ reading as an absolute label. The next card teaches how to calculate RQ from a balanced equation; this card establishes meaning and interpretation only.