Respiratory Substrates and the Respirometer

Spec Mapping — OCR H420 Module 5.2.2 — Respiration, content statements covering respiratory substrates (carbohydrates, lipids, proteins) and their relative energy yields, the calculation and interpretation of respiratory quotient (RQ), and the use of a respirometer to measure rate of respiration in small organisms and tissues (refer to the official OCR H420 specification document for exact wording).

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 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.

The respirometer itself has a distinguished pedigree. Otto Warburg (Berlin, 1920s) developed the Warburg manometer, a constant-volume gas-pressure apparatus that allowed tissue slices, microbial cultures and isolated mitochondria to be studied quantitatively. Paraphrasing Warburg's school of thought, the rate of oxygen consumption was the master variable of cellular bioenergetics — visible directly through gas-volume change. Warburg's manometric work led to the discovery of the cytochrome respiratory enzymes (for which he received the 1931 Nobel Prize) and to his still-cited observation that tumour cells preferentially use glycolysis even in the presence of oxygen (the "Warburg effect"). Modern respirometry has shifted from manometers to Clark-type oxygen electrodes and computerised gas-exchange systems, but the underlying principle — measuring O₂ consumption and CO₂ production with CO₂-absorbers and control tubes — is essentially unchanged.

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.

Learning objectives — by the end of this lesson you should be able to:

• Compare carbohydrates, lipids and proteins as respiratory substrates in terms of energy density, entry point and ATP yield.
• Explain, quantitatively, why lipids yield so much more energy per gram than carbohydrates.
• Define, calculate and interpret the respiratory quotient (RQ) for different substrates.
• Describe and evaluate the use of a respirometer to measure respiration rate and RQ, including controls.
• Process respirometer data quantitatively, with correct units and error awareness.

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Why Lipids Yield So Much ATP — A Full Worked Calculation

The statement that "fat gives more than twice the energy of carbohydrate per gram" is easy to assert but far more convincing — and more examinable — when you can derive the ATP yield of a fatty acid explicitly. This is a genuine A* quantitative skill and demonstrates exactly why the highly-reduced hydrocarbon chain of a fatty acid is such a rich fuel.

Take palmitate, the 16-carbon saturated fatty acid (palmitic acid is the commonest fatty acid in animal fat). Once activated to palmitoyl-CoA (at an up-front cost of the equivalent of 2 ATP, because ATP is split to AMP + 2Pᵢ), it is oxidised by β-oxidation. Each round of β-oxidation removes one 2-carbon acetyl-CoA and produces 1 reduced FAD and 1 reduced NAD. A 16-carbon chain is cut into 8 acetyl-CoA molecules, which requires 7 rounds of β-oxidation (the last cut yields two acetyl-CoA at once):

• From β-oxidation itself (7 rounds): 7 reduced FAD and 7 reduced NAD.
• From the 8 acetyl-CoA entering the Krebs cycle (8 turns): each turn yields 3 reduced NAD, 1 reduced FAD and 1 ATP → 24 reduced NAD + 8 reduced FAD + 8 ATP.

Now total the reduced coenzymes and feed them through oxidative phosphorylation using the modern yields (~2.5 ATP per reduced NAD; ~1.5 ATP per reduced FAD):

Source	Reduced NAD	Reduced FAD	Direct ATP
β-oxidation (7 rounds)	7	7	0
Krebs cycle (8 turns)	24	8	8
Total	31	15	8

$\text{ATP} = (31 \times 2.5) + (15 \times 1.5) + 8 = 77.5 + 22.5 + 8 = 108\,\text{ATP}$ATP=(31×2.5)+(15×1.5)+8=77.5+22.5+8=108ATP

Subtracting the 2-ATP activation cost gives a net yield of ~106 ATP per palmitate. Compare this with the ~32 ATP from a single 6-carbon glucose: even allowing that palmitate has more than twice the carbons, its yield per carbon (~6.6 ATP) exceeds that of glucose (~5.3 ATP), because the fatty acid's carbons are far more reduced (rich in C–H bonds) and so deliver more hydrogen to the electron transport chain. This is the quantitative heart of why triglyceride is the body's long-term store: the same mass stores far more retrievable ATP, and (being anhydrous, unlike hydrated glycogen) it is lighter to carry — critical for migrating birds, hibernating mammals and oil-storing seeds.

Note on figures: using the classical yields (3 ATP/reduced NAD, 2 ATP/reduced FAD) gives ~129 ATP for palmitate — the same range quoted for C18 stearate elsewhere in this course. As with glucose, OCR accepts either the modern or classical stoichiometry provided the method is shown clearly.

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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

OCR practical activity group 12 requires you to use a respirometer to measure the rate of respiration of small organisms (e.g. germinating seeds, maggots, woodlice) or cell samples.

The Apparatus

flowchart LR
    O[Organism] --> T1[Test tube with KOH]
    O -.CO2 absorbed.-> KOH[Potassium hydroxide solution]
    T1 --> CT[Capillary tube with coloured manometer fluid]
    CT --> C[Control tube - no organism - compensates for pressure/temperature changes]
    WB[Thermostatic water bath]

Principle

• Place the organism in a sealed tube containing a CO₂ absorber (KOH or soda lime) so that any CO₂ released is immediately absorbed.
• As the organism respires, it consumes O₂ — this reduces the gas volume in the tube.
• The reduction in volume causes the coloured fluid in the capillary manometer to move towards the organism.
• Use a scale to measure how far the fluid has moved.
• The volume of O₂ consumed per unit time is the rate of respiration.

Why Use KOH or Soda Lime?

• CO₂ is produced by the organism but must be absorbed so that it does not cancel out the decrease in gas volume from O₂ consumption.
• KOH + CO₂ → K₂CO₃ + H₂O (or equivalent with soda lime).
• If KOH were not present, the CO₂ would replace some of the O₂ in the gas volume, and the fluid would not move as much — you would underestimate the rate of respiration.

Controls

• A control tube with glass beads (instead of an organism) is set up identically. This controls for changes in pressure or temperature not caused by respiration.
• The water bath keeps the temperature constant (typically 20°C or 25°C).
• Equilibrate for ~5 minutes before starting measurements.

Calculating the Rate

If the fluid moves by distance d in the capillary (in mm), and the capillary has cross-sectional area A (in mm²), then:

$\text{Volume of O}_2 \text{ consumed} = A \times d$Volume of O2​ consumed=A×d

Rate = volume ÷ time. Units: cm³ O₂ per minute per gram of tissue.

Measuring RQ with a Respirometer

To measure RQ, run the experiment twice:

1. First run (with KOH): measures O₂ consumption only. The manometer fluid moves because O₂ is consumed and CO₂ is absorbed.
2. Second run (without KOH): CO₂ is not absorbed, so the net change in gas volume = O₂ consumed − CO₂ produced.
3. Subtract one result from the other to get CO₂ produced.
4. Calculate RQ = CO₂ / O₂.

Variables

• IV: temperature, substrate type, age of organism.
• DV: volume of O₂ consumed per unit time.
• Controlled: mass/number of organisms, species, temperature, pressure, light.

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Factors Affecting Respiration Rate in Living Organisms

• Temperature — higher temperature = faster rate, up to an optimum, after which enzymes denature.
• Activity level — exercise or locomotion increases ATP demand, raising respiration rate.
• Availability of substrate — starved organisms respire less.
• Availability of oxygen — anaerobic conditions slow the rate dramatically and also alter the RQ.
• Life stage — germinating seeds and actively growing tissues respire faster than dormant or quiescent cells.
• Species and metabolic rate — small mammals (e.g. shrews) have very high mass-specific respiration rates.

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Exam Tip

OCR often asks you to calculate a rate of respiration from respirometer data. Always include units (cm³ O₂ min⁻¹ g⁻¹ is the standard) and show your working. If you are given the diameter of the capillary, remember to use A = πr² to find the cross-sectional area, not the diameter. A common trap is forgetting to halve the diameter.

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Common Exam Mistakes