Respiratory substrates and RQ
Respiratory substrates and RQ
- A respiratory substrate is a molecule broken down to release energy.
- Different substrates release different amounts of energy.
- The gases they use tell us which one is being respired.
Practice
Which respiratory substrate releases the most energy per gram?
Lipids release the most energy per gram (most hydrogen); then proteins, then carbohydrates.
Energy per gram
- Per gram, lipids release the most energy (they have the most hydrogen).
- Proteins are next.
- Carbohydrates release the least.
The respiratory quotient (RQ)
$$\text{RQ} = \frac{\text{molecules of CO}_2 \text{ produced}}{\text{molecules of O}_2 \text{ taken in}}$$
- Typical values: carbohydrate ≈ 1.0, lipid ≈ 0.7, protein ≈ 0.9.
- You can work out RQ from a respiration equation.
Practice
An organism produces 4 molecules of CO₂ and takes in 5 molecules of O₂. What is its RQ?
RQ = CO₂ produced ÷ O₂ taken in = 4 ÷ 5 = 0.8.
Practice
A respiratory quotient of about 1.0 suggests the substrate is mainly:
Carbohydrate gives RQ ≈ 1.0; lipid ≈ 0.7; protein ≈ 0.9.
The respirometer
- A respirometer measures the oxygen taken in by living things — e.g. germinating seeds or small invertebrates.
- From the gases exchanged, you can find their RQ, or test how temperature changes the rate of oxygen uptake.
Practice
A respirometer is used to measure:
A respirometer measures oxygen uptake (e.g. of germinating seeds or invertebrates), used to find RQ.
You've got it
Key idea
- respiratory substrate = molecule broken down for energy; per gram lipid > protein > carbohydrate
- $\text{RQ} = \dfrac{\text{CO}_2 \text{ produced}}{\text{O}_2 \text{ used}}$
- RQ values: carbohydrate ≈ 1.0, lipid ≈ 0.7, protein ≈ 0.9
- a respirometer measures oxygen uptake (germinating seeds / invertebrates) to find RQ