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Glucose is the classic respiratory substrate, but in reality cells can respire lipids, proteins and other carbohydrates too. Each substrate yields a different amount of ATP and a different ratio of CO₂ produced to O₂ consumed — the respiratory quotient (RQ). OCR specification module 5.2.2(g)–(h) requires you to know the main substrates, their relative energy content, the concept of RQ and how to measure respiration rate and RQ using a respirometer. This lesson combines biochemistry with a key OCR practical.
Key Definitions:
- Respiratory substrate — any organic molecule that can be oxidised by cells to release energy.
- Respiratory quotient (RQ) — the ratio of CO₂ produced to O₂ consumed during respiration (RQ = CO₂/O₂).
- β-oxidation — the pathway that breaks fatty acids into 2-carbon fragments (acetyl CoA) for entry into the Krebs cycle.
- Deamination — the removal of an amino group from an amino acid, producing urea and a keto acid.
- Respirometer — a piece of apparatus used to measure the rate of oxygen consumption by a small organism or sample of tissue.
Fatty acids are highly reduced (lots of C–H bonds, few C=O bonds). Oxidation releases a lot of energy — each C-H bond is a rich source of electrons for the ETC. A typical 18-carbon fatty acid (stearate) yields roughly 120 ATP through β-oxidation and the Krebs cycle, compared to ~32 for a single 6-carbon glucose.
flowchart LR
FA[Fatty acid - CoA] --> B1[Remove 2C as acetyl CoA]
B1 -->|Reduce NAD and FAD| KR[Krebs cycle]
B1 --> FA2[Shortened fatty acid]
FA2 --> B1
KR --> ETC[ETC -> ATP]
Each round of β-oxidation removes a 2-carbon unit, producing: 1 acetyl CoA + 1 reduced NAD + 1 reduced FAD. A long fatty acid goes round many times.
Using proteins as fuel has a cost: it depletes the body's own muscle and enzyme proteins, and the excretion of urea requires water and ATP. It is a last resort.
| Substrate | Energy density (kJ/g) | ATP per glucose equivalent | Typical use |
|---|---|---|---|
| Glucose | ~17 | ~32 | Default, immediate energy |
| Lipids (stearic acid) | ~39 | ~120 per C₁₈ fatty acid | Long-term storage, endurance |
| Proteins (amino acids) | ~17 | Variable; ~15–30 per amino acid | Starvation, severe stress |
RQ is defined as:
RQ=volume of O2 consumedvolume of CO2 produced
The RQ depends on the chemical composition of the substrate — specifically, how many hydrogens it has per carbon, because more hydrogens means more oxygen is needed relative to the carbon content.
| Substrate | Equation | RQ |
|---|---|---|
| Carbohydrate | C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O | 1.0 |
| Lipid (e.g. triolein) | C₅₇H₁₀₄O₆ + 80O₂ → 57CO₂ + 52H₂O | ~0.7 |
| Protein | Variable (amino acids) | ~0.9 |
| Mixed diet | — | ~0.85 |
If a cell consumes 20 cm³ of O₂ and produces 15 cm³ of CO₂, then:
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