Respiratory Substrates and the Respirometer

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.

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The Three Main Substrates

1. Carbohydrates

• Primarily glucose, but also starch and glycogen after hydrolysis.
• Enters directly into glycolysis.
• Yield: ~32 ATP per glucose (or equivalent per monosaccharide).
• Energy density: ~17 kJ g⁻¹.
• First choice fuel in most tissues because it is easy to mobilise and enters respiration directly.

2. Lipids (Triglycerides)

• Hydrolysed to glycerol + 3 fatty acids by lipases.
• Glycerol is converted to TP and enters glycolysis.
• Fatty acids enter the mitochondrial matrix and undergo β-oxidation: they are chopped two carbons at a time into acetyl CoA molecules, which enter the Krebs cycle directly.
• β-oxidation itself also reduces NAD and FAD, adding to the ETC input.
• Energy density: ~39 kJ g⁻¹ — more than double that of carbohydrates!
• This high energy density is why fat is the body's long-term energy store.

Why Do Lipids Yield So Much Energy?

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.

β-Oxidation in Outline

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.

3. Proteins

• Used as a respiratory substrate only when carbohydrates and lipids are exhausted, e.g. during starvation or extreme exercise.
• Hydrolysed to amino acids by proteases.
• Amino acids are deaminated in the liver (the amino group is removed as NH₃, then detoxified to urea via the ornithine cycle and excreted).
• The remaining keto acids enter the Krebs cycle at various points depending on the amino acid (e.g. alanine → pyruvate, glutamate → α-ketoglutarate, aspartate → oxaloacetate).
• Energy density: ~17 kJ g⁻¹, similar to carbohydrates.

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.

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Comparing the Substrates

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

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Respiratory Quotient (RQ)

RQ is defined as:

$RQ = \frac{\text{volume of CO}_2 \text{ produced}}{\text{volume of O}_2 \text{ consumed}}$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.

Typical RQ Values

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

Interpreting an RQ

• RQ = 1.0 → pure carbohydrate respiration.
• RQ = 0.7 → pure lipid respiration.
• RQ = 0.85 → mixed substrate (typical human diet).
• RQ > 1.0 → anaerobic respiration may be contributing (extra CO₂ released without corresponding O₂ uptake).
• RQ < 0.7 → unusual; may indicate fixation of CO₂ (e.g. some plants in light conditions) or conversion of carbohydrate to lipid (lipogenesis).

A Worked Example

If a cell consumes 20 cm³ of O₂ and produces 15 cm³ of CO₂, then:

• RQ = 15 / 20 = 0.75
• This is close to the lipid value, suggesting mainly fat is being respired.

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The Respirometer: Required Practical