Respiration: (k) the use and interpretation of the respiratory quotient (RQ)

The idea

The respiratory quotient (RQ) is the ratio of carbon dioxide produced to oxygen consumed during respiration: RQ = CO₂ produced ÷ O₂ consumed. It is used to identify which respiratory substrate is being respired. For carbohydrates such as glucose, RQ is about 1.0 because equal volumes of CO₂ are produced and O₂ used. For lipids, RQ is about 0.7 because they contain much hydrogen and little oxygen, so more O₂ is needed to oxidise them. For proteins, RQ is about 0.8 to 0.9. A value between these indicates a mixture of substrates.

What earns the marks

• RQ = CO₂ produced ÷ O₂ consumed, expressed as a ratio.
• An RQ of approximately 1.0 indicates carbohydrate respiration.
• An RQ of approximately 0.7 indicates lipid respiration.
• An RQ of approximately 0.8 to 0.9 indicates protein respiration.
• Intermediate RQ values indicate a mixture of respiratory substrates being respired.
• Anaerobic respiration can raise the apparent RQ because CO₂ is released without oxygen being consumed.

Common mistakes and how to put them right

• Inverting the ratio to O₂ ÷ CO₂.

  Put it right: RQ is CO₂ produced divided by O₂ consumed.

• Assuming a high RQ always means lipid use.

  Put it right: High RQ near 1.0 indicates carbohydrate, while low RQ near 0.7 indicates lipid.

• Ignoring anaerobic respiration when interpreting an unusually high RQ.

  Put it right: Anaerobic respiration releases CO₂ without O₂ uptake, raising the apparent RQ.

Exam tips

• Write the RQ formula before substituting values to avoid inverting the ratio.
• Compare your calculated RQ with the reference values (1.0 carbohydrate, 0.7 lipid, 0.8–0.9 protein) to identify the substrate.
• Check whether anaerobic respiration could be occurring before concluding a substrate from an unusual RQ.