Quiz: The Respiratory Quotient Quiz
The respiratory quotient (RQ) is one of the most elegant diagnostic numbers in plant physiology. It is defined as the ratio of the volume of ##\ce{CO2}## evolved to the volume of ##\ce{O2}## consumed during aerobic respiration, and its value reveals which fuel a tissue is oxidising.
In this quiz you will apply the definition to carbohydrates, fats and proteins, and interpret the blended values measured in living organisms.
- Carbohydrates: RQ = 1, since ##\ce{CO2}## and ##\ce{O2}## are exchanged in equal amounts.
- Fats: RQ < 1, for example tripalmitin gives ##\dfrac{102}{145} \approx 0.7##.
- Proteins: RQ is about 0.9.
- Living tissues: substrates are mixed, so the measured RQ is a blended value.
Work carefully with the balanced equations — the arithmetic behind each value is transparent once the definition is clear.
Q1. The respiratory quotient (RQ) is defined as the ratio of:
The respiratory quotient is the ratio of the volume of ##\ce{CO2}## evolved to the volume of ##\ce{O2}## consumed in respiration. It is also called the respiratory ratio.
Q2. The respiratory quotient is also known by which alternative name?
The lesson states that the ratio of the volume of ##\ce{CO2}## evolved to the volume of ##\ce{O2}## consumed is called the respiratory quotient (RQ) or respiratory ratio.
Q3. The value of the respiratory quotient depends primarily on:
The respiratory quotient depends upon the type of respiratory substrate used during respiration. This is the central insight of the lesson.
Q4. When carbohydrates are used as the respiratory substrate and are completely oxidised, the RQ is:
When carbohydrates are completely oxidised, equal amounts of ##\ce{CO2}## and ##\ce{O2}## are evolved and consumed, so the RQ is exactly 1.
Q5. For complete oxidation of glucose, ##\ce{C6H12O6 + 6O2 -> 6CO2 + 6H2O}##, the respiratory quotient is calculated as:
Six volumes of ##\ce{CO2}## are evolved for six volumes of ##\ce{O2}## consumed, so ##\text{RQ} = \dfrac{6}{6} = 1##.
Q6. Why is the RQ for carbohydrates exactly 1?
In carbohydrate oxidation the volumes of ##\ce{CO2}## evolved and ##\ce{O2}## consumed are equal, which makes the ratio exactly 1.
Q7. When fats are used as respiratory substrates, the RQ is:
The lesson states that when fats are used in respiration, the RQ is less than 1. Tripalmitin, for example, gives about 0.7.
Q8. For tripalmitin, the balanced equation gives 102 volumes of ##\ce{CO2}## evolved for 145 volumes of ##\ce{O2}## consumed. The RQ is therefore:
The RQ is ##\dfrac{\text{volume of } \ce{CO2} \text{ evolved}}{\text{volume of } \ce{O2} \text{ consumed}} = \dfrac{102}{145} \approx 0.7## for tripalmitin.
Q9. The RQ value of approximately 0.7 is characteristic of which respiratory substrate?
The worked calculation for the fatty acid tripalmitin gives ##\text{RQ} = \dfrac{102}{145} \approx 0.7##, a value typical of fats.
Q10. When proteins act as respiratory substrates, the RQ is about:
The lesson states that when proteins are respiratory substrates the ratio would be about 0.9.
Q11. In living organisms, pure fats or pure proteins are never used as respiratory substrates. As a result, the measured RQ is:
In living organisms respiratory substrates are often more than one, so the measured quotient is a blended value reflecting a mixture of fuels.
Q12. A plant tissue respiring with an RQ of about 0.9 is most likely oxidising:
An RQ of about 0.9 corresponds to proteins, and in living tissues the value is blended because substrates are mixed.
Q13. Which set of RQ values correctly matches carbohydrate, fat (tripalmitin) and protein respectively?
Carbohydrates give an RQ of 1, tripalmitin gives about 0.7, and proteins give about 0.9.
Q14. The respiratory quotient is a dimensionless pure number because:
Because gases are measured under the same conditions of temperature and pressure, their volumes are directly proportional to moles, so the quotient is a pure number.
Q15. Which statement about the respiratory quotient is correct?
Once the balanced equation for a substrate is known, the RQ can be computed directly from the ratio of ##\ce{CO2}## evolved to ##\ce{O2}## consumed.