state that the respiratory quotient (RQ) is the ratio of the number of molecules of carbon dioxide produced to the number of molecules of oxygen taken in, as a result of respiration

Energy – Respiration

Cellular respiration is the set of metabolic pathways that convert the chemical energy stored in nutrients into adenosine‑triphosphate (ATP), the usable form of energy for the cell. During aerobic respiration, oxygen is taken up and carbon dioxide is released.

Respiratory Quotient (RQ)

The respiratory quotient (RQ) is a dimensionless number that expresses the relationship between the amount of carbon dioxide produced and the amount of oxygen consumed during respiration.

Mathematically,

\$RQ = \frac{\text{moles of } CO2 \text{ produced}}{\text{moles of } O2 \text{ consumed}} = \frac{V{CO2}}{V{O2}}\$

Thus, the RQ is the ratio of the number of molecules (or volume, under constant conditions) of carbon dioxide (\$CO2\$) generated to the number of molecules (or volume) of oxygen (\$O2\$) taken in.

Why RQ is Important

• Indicates which type of substrate (carbohydrate, fat, protein) is being oxidised.
• Helps in estimating metabolic rate and energy expenditure.
• Used in indirect calorimetry to assess nutritional status and exercise physiology.

Typical RQ \cdot alues for Different Substrates

Calculating RQ in the Laboratory

1. Place the subject (human, animal, or plant tissue) in a closed metabolic chamber.
2. Measure the volume of \$O2\$ entering the chamber and the volume of \$CO2\$ exiting over a fixed time period.
3. Convert the volumes to moles (or use the volumes directly if temperature and pressure are constant).
4. Apply the formula \$RQ = \dfrac{V{CO2}}{V{O2}}\$.

Interpretation of RQ \cdot alues

• RQ ≈ 1.0 – Predominant oxidation of carbohydrates.
• RQ ≈ 0.7 – Predominant oxidation of fats.
• RQ ≈ 0.8 – Mixed substrate utilization, with a notable contribution from proteins.
• RQ > 1.0 – May indicate lipogenesis (conversion of excess carbohydrate to fat) or hyperventilation.

Summary

The respiratory quotient (RQ) is a simple yet powerful indicator of metabolic substrate use. By comparing the amount of \$CO2\$ produced to the amount of \$O2\$ consumed, students can deduce whether carbohydrates, fats, or proteins are being oxidised, and can relate these findings to energy expenditure and physiological state.