Respiratory quotient (RQ)
Respiratory quotient = CO₂ released ÷ O₂ used. It is 1 for carbohydrates, less than 1 for fats and proteins, more than 1 for organic acids, and infinite without oxygen.
What RQ tells us
The respiratory quotient (RQ) is the ratio of the volume of CO₂ released to the volume of O₂ consumed during respiration. RQ tells us which food (substrate) a cell is respiring. Fats have little oxygen in their molecules, so they need more O₂ to break down, and their RQ is less than 1. Organic acids are rich in oxygen, so they need less O₂, and their RQ is more than 1. In anaerobic respiration by yeast, CO₂ is released but no O₂ is used, so the RQ is infinite (∞).
RQ of different substrates
| Substrate | Example | RQ |
|---|---|---|
| Carbohydrate | Glucose | 1 |
| Fat | Tripalmitin | About 0.7 (less than 1) |
| Protein | Proteins in general | Less than 1 |
| Organic acid | Malic acid | About 1.33 (more than 1) |
| Anaerobic respiration (yeast) | Glucose → ethanol + CO₂ | Infinity (no O₂ used) |
RQ of glucose
Find the RQ when glucose is respired aerobically.
- Equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy
- CO₂ released = 6 molecules (6 volumes)
- O₂ consumed = 6 molecules (6 volumes)
- RQ = CO₂ released / O₂ consumed = 6/6
RQ of glucose = 1.
RQ of a fat and of an organic acid
Find the RQ of (i) tripalmitin, a fat: 2C₅₁H₉₈O₆ + 145O₂ → 102CO₂ + 98H₂O (ii) malic acid: C₄H₆O₅ + 3O₂ → 4CO₂ + 3H₂O
- (i) RQ = 102/145
- 102 ÷ 145 = 0.703, which rounds to 0.7 (less than 1)
- (ii) RQ = 4/3
- 4 ÷ 3 = 1.333, which rounds to 1.33 (more than 1)
Tripalmitin: RQ about 0.7 (0.703 rounded to one decimal place). Malic acid: RQ about 1.33 (1.333 rounded to two decimal places).
RQ in anaerobic respiration
Yeast respires without oxygen: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. Find the RQ.
- CO₂ released = 2 volumes
- O₂ consumed = 0
- RQ = 2/0, which has no finite value
The RQ is infinite (∞).
Doctors and sports scientists measure the gases you breathe out to work out your RQ. A value near 0.7 shows that the body is mostly burning fat, as after a long fast. A value near 1 shows it is burning carbohydrates such as rice and idli.
2 marks: “Define respiratory quotient. What is the RQ of glucose?” RQ = volume of CO₂ released / volume of O₂ consumed. For glucose, 6CO₂/6O₂ = 1.
2 marks: “Why is the RQ of fat less than one?” Fats contain little oxygen, so they use more O₂ than the CO₂ they give out (for tripalmitin, 102/145 = 0.7).
4 marks: “What is RQ? Find the RQ of glucose and malic acid, and say what the values show.” Definition; glucose 6/6 = 1; malic acid 4/3 = 1.33; RQ shows which substrate is being respired.
RQ is 'out over in': CO₂ out ÷ O₂ in. Carbohydrates 1, Fats below 1, Acids above 1.