Summary
The ratio of carbon dioxide produced to oxygen consumed, indicating which fuel is being used.
In depth
Measured by respirometers. Values: carbohydrate 1.0, protein 0.8, fat 0.7. Mixed diets give about 0.8 to 0.85. It indicates metabolic state, for example in starvation or ketogenic diets.
RQ values reflect substrate. High-carb diets raise RQ; fasting lowers it. RQ above 1 can mean lipogenesis.
Example: RQ is measured using a respirometer with soda lime to absorb CO2.
The respiratory quotient (RQ) is the ratio of CO2 produced to O2 consumed: RQ = CO2 / O2. It indicates the fuel being oxidised: carbohydrates give 1.0, fats about 0.7, proteins about 0.8 to 0.9, with mixed diets about 0.85.
For glucose, six CO2 and six O2 give 1. For the fatty acid palmitate, 16 CO2 per 23 O2 gives 0.70, because fats contain less oxygen so need more oxygen to oxidise. Values above 1 suggest fat synthesis from carbohydrate or anaerobic respiration, and below 0.7 ketone production. Measured using a respirometer or indirect calorimetry.
Example: germinating oily seeds (castor) have a low RQ.
Remember: RQ is a ratio of gas exchange, not an energy value.
Frequently asked questions
What is Respiratory Quotient?
The ratio of carbon dioxide produced to oxygen consumed, indicating which fuel is being used.
Which area of biology covers Respiratory Quotient, and how advanced is it?
Respiratory Quotient is one of the 53 topics in Metabolism & Bioenergetics. It is rated Intermediate, so it suits learners who already know the core vocabulary. Related topics in this area include Respiratory Equation, Ketone Body Metabolism, Protein Catabolism.
Can you give an example of Respiratory Quotient?
Germinating seeds with fats have a lower RQ than those with starch.
What should I learn before Respiratory Quotient?
Work through the Beginner topics in Metabolism & Bioenergetics first, such as Metabolism, Anabolism and Catabolism, ATP, then return to Respiratory Quotient. Intermediate topics assume the Beginner vocabulary.
What topics are related to Respiratory Quotient?
The closest topics to Respiratory Quotient are Respiratory Equation, Ketone Body Metabolism, Protein Catabolism, Beta-Oxidation. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Respiratory Quotient?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Respiratory Quotient. To practise the whole of Metabolism & Bioenergetics, use the Metabolism & Bioenergetics quiz.
How is Respiratory Quotient different from Respiratory Equation?
Respiratory Quotient: The ratio of carbon dioxide produced to oxygen consumed, indicating which fuel is being used. Respiratory Equation: Glucose plus oxygen yields carbon dioxide, water and energy released as ATP and heat. Read both side by side and note what each one explains.
How should I study Respiratory Quotient step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Respiratory Equation and explain the difference aloud. Finally, take the Metabolism & Bioenergetics quiz and come back to this page for any question you miss.