12.1.6—RQ calculations

Calculate respiratory quotient from comparable gas measurements: RQ = volume of CO₂ produced ÷ volume of O₂ consumed. The two volumes must refer to the same time interval and compatible conditions.

1. Set the basis: Use a balanced aerobic respiration equation, or corrected gas-volume changes measured over the same interval. Keep both gas volumes in matching units.
2. Identify the gases: Select the CO₂ output for the numerator and the O₂ uptake for the denominator. Do not use the substrate, water or a raw total displacement as a gas volume.
3. Calculate: Substitute the two matched values into RQ = V(CO₂ produced) / V(O₂ consumed) and simplify the dimensionless ratio.
4. Sense-check: A reversed ratio, mixed time intervals, an unbalanced equation, a leak or an uncorrected control can produce an implausible result. If the denominator is zero, the ordinary aerobic RQ is not defined in the usual finite way.
5. Interpret cautiously: Compare the result with expected substrate patterns only after checking the conditions. An intermediate value can indicate mixed substrate use; unusual values may reflect anaerobic metabolism, measurement error or a non-steady state rather than a new substrate.

Symbolic worked read-off: if the balanced equation gives c units of CO₂ and o units of O₂ for the same reaction basis, then RQ = c / o. The calculation is valid only after balancing the equation and confirming that both coefficients represent gas amounts on the same basis.

RQ is dimensionless, but the input measurements are not interchangeable: CO₂ belongs in the numerator and O₂ in the denominator. This card teaches the calculation logic; respirometer setup, controls and apparatus-derived gas changes belong to 4645.