12.1.412 · Energy and Respiration

Respiratory Quotient (RQ) — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 9 parts Lesson 4 of 18 1 quiz

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Lesson summary

12.1.4 Respiratory Quotient (RQ) — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Define RQ, calculate it from equations and use it to identify the respiratory substrate.Respiratory quotient — the ratio of the volume of CO2 produced to the volume of O2 consumed in a …

What the symbols mean
Why gases can be compared
Balance first
Linoleic acid, C18H32O2, gives 18 ÷ 25
Organic acid
Cause of differences

Part 1 of 7Definition

Definition

Respiratory quotient

Respiratory quotient the ratio of the volume of CO₂ produced to the volume of O₂ consumed in a given time.

Formula

RQ = CO₂ produced ÷ O₂ consumed.
What the symbols mean: RQ = respiratory quotient (no unit) · about 1.0 for carbohydrate, 0.7 for lipid, 0.9 for protein

Units

Units none, because it is a ratio.

Use

Use it shows which substrate is being respired and whether anaerobic respiration is occurring.

Why gases can be compared

Why gases can be compared equal numbers of molecules of any gas occupy the same volume.

Part 2 of 7Calculating RQ from equations

1Method

Method write the respiration equation, balance it, then divide the number of CO₂ by the number of O₂.

2Balance first

Balance first always count all CO₂ in the products and all O₂ in the reactants.

3Glucose

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O, so RQ = 6 ÷ 6 = 1.0.

4Lipid

Lipid oleic acid C₁₈H₃₄O₂ + 25.5O₂ → 18CO₂ + 17H₂O, so RQ = 18 ÷ 25.5 = 0.71.
Linoleic acid, C₁₈H₃₂O₂, gives 18 ÷ 25 0.72.
Lipids need relatively more oxygen to oxidise their hydrogen to water, so RQ is lower.

5Organic acid

Organic acid malic acid C₄H₆O₅ + 3O₂ → 4CO₂ + 3H₂O gives RQ = 4 ÷ 3 = 1.33.
Organic acids are already highly oxidised, giving RQ above 1, as in succulent plants.

Part 3 of 7RQ values for substrates

Substrate	Approximate RQ
Carbohydrate	1.0
Protein	0.8 to 0.9
Lipid	0.7
Mixture of substrates (typical human at rest)	0.8 to 0.85
Anaerobic respiration in yeast	Greater than 1, tending to infinity if no O₂ is used

Cause of differences

Cause of differences the number of C-H bonds differs, so more hydrogen can build a proton gradient and more oxygen is needed to form water.

Mixtures

Mixtures values between 0.7 and 1.0 suggest a mixture of substrates.

Part 4 of 7Interpreting RQ data

Near 1.0

Near 1.0 carbohydrate is the main substrate and respiration is aerobic, as in germinating cereal seeds rich in starch.

Near 0.7

Near 0.7 lipid respiration, as in germinating oil-rich seeds such as sunflower or castor bean.
Some seeds give 0.3 to 0.5 if part of the CO₂ is used in making carbohydrate from lipid.

Above 1

Above 1 some anaerobic respiration alongside aerobic respiration, for example tissue exercising beyond its oxygen supply.

Diet

RQ rises towards 1 after a carbohydrate meal and falls towards 0.7 during fasting as fat is mobilised.

Over- and under-feeding

Over- and under-feeding above 1 suggests carbohydrate is being converted to fat, and below 0.7 suggests underfeeding.

Order of substrate use

Order of substrate use carbohydrates, then lipids, then proteins.

Part 5 of 7Anaerobic respiration and RQ

Anaerobic respiration and RQ

Yeast

Yeast ethanol fermentation releases CO₂ but takes up no oxygen, so RQ is theoretically infinite.

Mixed systems

Mixed systems with some aerobic and some anaerobic respiration, values above 1.0 occur.

Mammalian muscle

Mammalian muscle lactate fermentation produces no CO₂ and uses no O₂, so RQ cannot be calculated.
The lactate is later oxidised using oxygen (the oxygen debt), so RQ in recovery differs from rest.

Breath analysis

Breath analysis after hard exercise lactic acid displaces CO₂ from hydrogencarbonate in blood, giving RQ above 1 not caused by substrate choice.

Part 6 of 7Measuring RQ

Methods

Methods a respirometer, blood gas analysis, or comparing exhaled and inhaled air with gas analysers.

Clinical use

Clinical use doctors estimate the fuel used by ventilated patients.

Link to energy

Link to energy the lower RQ (lipid) needs more oxygen per CO₂ and releases more energy per gram.

Weighted RQ

Weighted RQ a mixture of carbohydrate and lipid gives a value between 0.7 and 1.0.

Part 7 of 7More from the specification

1Respirometric respiratory quotient

Respirometric respiratory quotient calculating carbon dioxide produced divided by oxygen consumed provides a quantitative means to deduce respired substrate.

2Respiratory Quotient (RQ) Calculation

The RQ is the ratio of carbon dioxide molecules produced to oxygen molecules consumed by cells during respiration: RQ = / (volume of O₂ consumed), indicating the type of respiratory substrate being oxidised.
- (volume of CO₂ released)

3Respirometer and Manometer Setup

A respirometer with a manometer measures changes in gas volume;
- consuming oxygen causes manometer fluid displacement when carbon dioxide is absorbed by potassium hydroxide.

4Chemiosmosis and ATP Synthesis

During oxidative phosphorylation, protons are pumped across the inner mitochondrial membrane into the intermembrane space,
and their return down the electrochemical gradient drives the synthesis of ATP from ADP and inorganic phosphate via ATP synthase.

5Respiratory Quotient (RQ) Calculation

The RQ is the ratio of carbon dioxide molecules produced to oxygen molecules consumed by cells during respiration: RQ = / (volume of O₂ consumed), indicating the type of respiratory substrate being oxidised.
- (volume of CO₂ released)

6Respirometer and Manometer Setup

A respirometer with a manometer measures changes in gas volume;
- consuming oxygen causes manometer fluid displacement when carbon dioxide is absorbed by potassium hydroxide.

7Chemiosmosis and ATP Synthesis

During oxidative phosphorylation, protons are pumped across the inner mitochondrial membrane into the intermembrane space,
and their return down the electrochemical gradient drives the synthesis of ATP from ADP and inorganic phosphate via ATP synthase.

8Concentration gradient

The difference in concentration of a substance between two regions.

9Amino acid

The unit from which proteins are made; the sequence decides the protein's shape and function.

10Substrate-linked phosphorylation

Direct transfer of a phosphate from a substrate to ADP.

11Intermembrane space

The gap between the two membranes where protons accumulate.

12Chemiosmosis

ATP synthesis driven by a proton gradient across a membrane.

13Manometer

U-tube whose fluid moves as gas volume changes.

14Hydrolysis

A reaction that splits a molecule by adding water.

Key terms

Term	Meaning
Respiratory quotient (RQ)	CO_{2} produced divided by O_{2} consumed.
Respirometer	Apparatus that measures gas uptake or release by organisms.
Lactate fermentation	Anaerobic pathway turning pyruvate into lactate.
Oxygen debt	Extra oxygen needed after exercise to remove lactate.
Mixed substrates	Respiration of more than one type of substrate at once.
Organic acid	An already-oxidised respiratory substrate that gives RQ above 1.

Avoid these mistakes

Saying RQ has units — it is a ratio.

Saying a lipid RQ is above 1 — it is about 0.7.

Using an unbalanced equation to calculate RQ.

Saying RQ can be found for lactate fermentation — no gas is exchanged.

Last updated: 15 October 2026