Respiratory Quotient

BiologyRespiration in PlantsFor NEET aspirants

The respiratory quotient (RQ) is the ratio of the volume of CO₂ evolved to the volume of O₂ consumed in respiration. This page covers its definition and formula, why the respiratory quotient depends on the substrate, the worked values for carbohydrates (1.0), the fat tripalmitin (0.7) and proteins (about 0.9), and why real organisms respire mixed substrates. It follows the NCERT Class 11 chapter Respiration in Plants. NEET asks RQ values, quick ratio calculations and statement questions on substrates.

On this page1Definition2RQ of carbohydrates3RQ of fats4RQ of proteins5Mixed substrates6Exam essentials7Quick revision8Solved examples9Practice

Key Points at a Glance

★ Must learn Respiratory quotient (RQ), or respiratory ratio = volume of $C O_{2}$ evolved / volume of $O_{2}$ consumed.
RQ is measured during aerobic respiration, when $O_{2}$ is consumed and $C O_{2}$ is released.
★ Must learn RQ depends on the type of respiratory substrate.
★ Must learn Carbohydrates (completely oxidised): RQ = 1, since equal volumes of $C O_{2}$ and $O_{2}$ are evolved and consumed.
★ Must learn Fats: RQ less than 1; tripalmitin gives 102 $C O_{2}$ / 145 $O_{2}$ = 0.7.
Proteins: RQ about 0.9.
Real organisms often respire more than one substrate; pure proteins or fats are never used alone.
Order of RQ: fats (0.7) < proteins (about 0.9) < carbohydrates (1.0).

1. What Is Respiratory Quotient?

During aerobic respiration, $O_{2}$ is consumed and $C O_{2}$ is released.
Comparing these two volumes tells us which substrate a tissue is respiring.

★ Very important Respiratory quotient (RQ): the ratio of the volume of $C O_{2}$ evolved to the volume of $O_{2}$ consumed in respiration. It is also called the respiratory ratio.

RQ = \frac{Volume of C O _{2} evolved}{Volume of O _{2} consumed}

★ Exam imp The respiratory quotient depends upon the type of respiratory substrate used during respiration.
RQ is a ratio of two gas volumes, so it has no unit.

Memory Trick

"C over O": $C O_{2}$ (what Comes out) goes on top, $O_{2}$ (what is taken in) goes below. Read RQ as "Carbon dioxide Out per Oxygen In".

Key idea

RQ = CO₂ evolved / O₂ consumed, and its value reveals the respiratory substrate.

2. RQ of Carbohydrates

When carbohydrates are the substrate and are completely oxidised, the RQ is 1.
This is because equal amounts of $C O_{2}$ and $O_{2}$ are evolved and consumed.
For glucose:

C_{6} H_{12} O_{6} + 6 O_{2} ⟶ 6C O_{2} + 6 H_{2} O + Energy

RQ = \frac{6 C O _{2}}{6 O _{2}} = 1.0

★ Exam imp Six molecules of $O_{2}$ are used and six molecules of $C O_{2}$ are released, so the ratio is exactly 1.

Key idea

Glucose uses one O₂ for every CO₂ it releases, so its RQ is 1.0.

3. RQ of Fats

★ Exam imp When fats are used in respiration, the RQ is less than 1.
The calculation for tripalmitin, a fat, used as a substrate:

2 (C_{51} H_{98} O_{6}) + 145 O_{2} ⟶ 102C O_{2} + 98 H_{2} O + Energy

RQ = \frac{102 C O _{2}}{145 O _{2}} = 0.7

Two molecules of tripalmitin consume 145 molecules of $O_{2}$ and release 102 molecules of $C O_{2}$ .
So more $O_{2}$ is consumed than $C O_{2}$ is released, and the RQ (102/145 = 0.70) is below 1.

★ Very important RQ values to learn: carbohydrates 1.0, proteins about 0.9, fats 0.7 (tripalmitin, 102/145).

Extra Depth: why RQ moves away from 1

Fats contain much less oxygen than carbohydrates for the same carbon and hydrogen, so more $O_{2}$ is needed and RQ falls below 1.
Oxygen-rich substrates such as organic acids give RQ above 1: malic acid ( $C_{4} H_{6} O_{5} + 3 O_{2}$ gives $4C O_{2} + 3 H_{2} O$ ) has RQ 4/3, about 1.33.
In anaerobic respiration, $C O_{2}$ may be released with no $O_{2}$ consumed.

Memory Trick

Read the tripalmitin numbers from its formula $C_{51} H_{98} O_{6}$ . Two molecules hold 102 carbons, so 102 $C O_{2}$ ; they hold 196 hydrogens, so 98 $H_{2} O$ . Only the 145 $O_{2}$ has to be learnt.

Quick Recall: tap to check

Is the RQ of fats more or less than 1?

Less than 1 (0.7 for tripalmitin).

How many

O_{2}

molecules do two tripalmitin molecules consume?

145.

Is RQ written as

C O_{2}

over

O_{2}

O_{2}

over

C O_{2}

C O_{2}

evolved over

O_{2}

consumed.

Memory Trick

Fat is Far from 1. Fats give the lowest RQ (0.7) because they need the most $O_{2}$ . Remember the tripalmitin pair "102 over 145".

Key idea

Fats need extra oxygen to be oxidised, so their RQ (0.7 for tripalmitin) is below 1.

4. RQ of Proteins and Mixed Substrates

4.1 Proteins

★ Exam imp When proteins are the respiratory substrates, the RQ is about 0.9.

4.2 Substrates in living organisms

In living organisms, respiratory substrates are often more than one.
★ Exam imp Pure proteins or fats are never used as respiratory substrates.
So a measured RQ usually reflects a mixture of substrates, lying between the values of the individual substrates.

Respiratory substrate	RQ	Compared with 1	Reason
Carbohydrates (glucose), completely oxidised	1.0	Equal to 1	Equal volumes of $C O_{2}$ evolved and $O_{2}$ consumed
Proteins	About 0.9	Less than 1	Slightly more $O_{2}$ consumed than $C O_{2}$ evolved
Fats (tripalmitin)	0.7	Less than 1	102 $C O_{2}$ evolved for 145 $O_{2}$ consumed

Memory Trick

Arrange substrates by RQ with "Fats, Proteins, Carbohydrates: 7, 9, 10". Read the values as 0.7, 0.9 and 1.0, rising in the order F, P, C.

Tips and Tricks

For an RQ calculation, simply divide the $C O_{2}$ figure by the $O_{2}$ figure. Then match the answer: 1.0 means carbohydrate, about 0.9 means protein, and about 0.7 means fat.

NEET Focus

RQ is CO₂ over O₂, never the reverse. Statement traps invert the ratio, give fats an RQ above 1, or claim that pure fats or proteins are respired. Also note: carbohydrates give RQ 1 only when completely oxidised.

Quick Recall: tap to check

Write the formula for RQ.

RQ = volume of

C O_{2}

evolved / volume of

O_{2}

consumed.

What is the RQ of tripalmitin, and why?

0.7, because 2 tripalmitin molecules release 102

C O_{2}

while consuming 145

O_{2}

Which substrate has an RQ of about 0.9?

Proteins.

Why does a measured RQ rarely match one pure substrate?

Living organisms respire more than one substrate; pure proteins or fats are never used.

Key idea

Proteins give about 0.9; real tissues respire mixtures, so measured RQ values fall between the pure values.

5. Exam Essentials

Pairs to Match

List I	List II
Respiratory quotient	Volume of $C O_{2}$ evolved / volume of $O_{2}$ consumed
Respiratory ratio	Another name for RQ
Carbohydrates (completely oxidised)	RQ = 1.0
Glucose	6 $C O_{2}$ / 6 $O_{2}$
Fats	RQ less than 1
Tripalmitin	102 $C O_{2}$ / 145 $O_{2}$ = 0.7
Proteins	RQ about 0.9
Living organisms	Respire more than one substrate

Exceptions

RQ is $C O_{2}$ / $O_{2}$ ; it is not $O_{2}$ / $C O_{2}$ .
Carbohydrates give RQ = 1 only when completely oxidised.
Fats and proteins give RQ less than 1, not more.
Pure proteins or fats are never used as respiratory substrates in living organisms.

Numbers to Remember

1.0: RQ of carbohydrates (glucose: 6 $C O_{2}$ and 6 $O_{2}$ ).
0.7: RQ of fats (tripalmitin: 102 $C O_{2}$ and 145 $O_{2}$ for 2 molecules).
About 0.9: RQ of proteins.
145 $O_{2}$ consumed and 102 $C O_{2}$ and 98 $H_{2} O$ formed when 2 tripalmitin molecules are oxidised.
Molecular formula of tripalmitin: $C_{51} H_{98} O_{6}$ .

Examples to Remember

Substrate	RQ
Glucose	Carbohydrate substrate, RQ 1.0
Tripalmitin	Fat substrate, RQ 0.7
Proteins	RQ about 0.9

6. Quick Revision

RQ (respiratory ratio) = volume of $C O_{2}$ evolved / volume of $O_{2}$ consumed in respiration.
During aerobic respiration $O_{2}$ is consumed and $C O_{2}$ is released.
RQ depends on the type of respiratory substrate.
Completely oxidised carbohydrates: RQ = 1, since equal volumes of $C O_{2}$ and $O_{2}$ are involved.
Glucose: $C_{6} H_{12} O_{6}$ + 6 $O_{2}$ gives 6 $C O_{2}$ + 6 $H_{2} O$ + energy; RQ = 6/6 = 1.0.
Fats: RQ less than 1. Tripalmitin: 2 $C_{51} H_{98} O_{6}$ + 145 $O_{2}$ gives 102 $C O_{2}$ + 98 $H_{2} O$ ; RQ = 102/145 = 0.7.
Proteins: RQ about 0.9.
Increasing order of RQ: fats, proteins, carbohydrates.
Real organisms often respire more than one substrate; pure proteins or fats are never used.
RQ has no unit because it is a ratio of two gas volumes.

7. Solved Examples

Solved Example 1

Two molecules of tripalmitin are oxidised as: 2

C_{51} H_{98} O_{6}

+ 145

O_{2}

gives 102

C O_{2}

+ 98

H_{2} O

+ energy. What is the RQ?
(A) 1.42
(B) 1.0
(C) 0.7
(D) 0.9

Solution:

Answer: (C). RQ = volume of $C O_{2}$ evolved / volume of $O_{2}$ consumed = 102/145 = 0.70. Option (A), 1.42, is the inverted ratio (145/102).

Solved Example 2

Match List I with List II.
List I: A. Carbohydrates, completely oxidised, B. Tripalmitin, C. Proteins, D. Ratio of

C O_{2}

evolved to

O_{2}

consumed
List II: I. About 0.9, II. 1.0, III. Respiratory quotient, IV. 0.7
Choose the correct answer:
(A) A-II, B-IV, C-I, D-III
(B) A-IV, B-II, C-I, D-III
(C) A-II, B-I, C-IV, D-III
(D) A-III, B-IV, C-I, D-II

Solution:

Answer: (A). Carbohydrates 1.0 (II), tripalmitin 0.7 (IV), proteins about 0.9 (I), and the ratio itself is the respiratory quotient (III).

Solved Example 3

Read the statements.
A. RQ = volume of

O_{2}

consumed / volume of

C O_{2}

evolved.
B. RQ depends on the type of respiratory substrate.
C. The RQ of fats is less than 1.
D. Living organisms commonly respire pure fats.
E. RQ is also called the respiratory ratio.
Choose the correct answer:
(A) B, C and E only
(B) A, B and C only
(C) A, D and E only
(D) B, D and E only

Solution:

Answer: (A). A is wrong: RQ is $C O_{2}$ evolved over $O_{2}$ consumed. D is wrong: pure proteins or fats are never used as respiratory substrates.

Solved Example 4

Arrange the respiratory substrates in increasing order of their RQ.
A. Carbohydrates
B. Fats
C. Proteins
Choose the correct answer:
(A) B, C, A
(B) C, B, A
(C) A, C, B
(D) B, A, C

Solution:

Answer: (A). Fats 0.7 < proteins about 0.9 < carbohydrates 1.0.

Solved Example 5

Germinating seeds take in 50 mL of

O_{2}

and give out 35 mL of

C O_{2}

in the same time. What is the RQ, and what is the most likely substrate?
(A) 1.43; carbohydrate
(B) 0.7; fat
(C) 1.0; carbohydrate
(D) 0.9; protein

Solution:

Answer: (B). RQ = 35/50 = 0.7, which matches the value for fats. Such a value is typical of seeds that store fat.

Solved Example 6

Which of the following is NOT correct?
(A) The RQ of completely oxidised carbohydrates is 1
(B) The RQ of proteins is about 0.9
(C) The RQ of tripalmitin is about 1.4
(D) In living organisms, respiratory substrates are often more than one

Solution:

Answer: (C). Tripalmitin has an RQ of 0.7 (102/145). The value 1.4 comes from inverting the ratio.

8. Practice Questions

Practice Questions

Calculate the RQ for glucose from: $C_{6} H_{12} O_{6}$ + 6 $O_{2}$ gives 6 $C O_{2}$ + 6 $H_{2} O$ + energy.
(A) 0.7
(B) 0.9
(C) 1.0
(D) 6.0Answer: (C). RQ = 6 $C O_{2}$ / 6 $O_{2}$ = 1.0.
Match List I with List II. List I: A. RQ = 1.0, B. RQ = 0.7, C. RQ about 0.9, D. Mixed RQ. List II: I. Proteins, II. Tripalmitin, III. Glucose, IV. Several substrates respired together. Options:
(A) A-III, B-II, C-I, D-IV
(B) A-II, B-III, C-I, D-IV
(C) A-III, B-I, C-II, D-IV
(D) A-IV, B-II, C-I, D-IIIAnswer: (A). Glucose 1.0, tripalmitin 0.7, proteins about 0.9, and real tissues respire mixtures.
Statement I: When fats are used in respiration, the RQ is less than 1. Statement II: In living organisms, pure fats are commonly the only respiratory substrate. Options:
(A) Both Statement I and Statement II are correct
(B) Both Statement I and Statement II are incorrect
(C) Statement I is correct but Statement II is incorrect
(D) Statement I is incorrect but Statement II is correctAnswer: (C). Pure proteins or fats are never used as respiratory substrates.
Which of the following is NOT correct about respiratory quotient?
(A) It is the ratio of $C O_{2}$ evolved to $O_{2}$ consumed
(B) It depends on the respiratory substrate
(C) It is 1 for completely oxidised carbohydrates
(D) It is more than 1 for fatsAnswer: (D). Fats give an RQ less than 1 (0.7 for tripalmitin).
Arrange in decreasing order of RQ: A. Proteins, B. Carbohydrates, C. Fats. Options:
(A) B, A, C
(B) A, B, C
(C) C, A, B
(D) B, C, AAnswer: (A). Carbohydrates 1.0 > proteins about 0.9 > fats 0.7.
Which statements are correct? A. RQ has no unit. B. RQ depends on the respiratory substrate. C. For tripalmitin, 145 $O_{2}$ are consumed for 102 $C O_{2}$ released (2 molecules). D. Proteins give an RQ above 1. Options:
(A) A, B and C only
(B) A and D only
(C) B, C and D only
(D) A, B, C and DAnswer: (A). D is wrong: the RQ of proteins is about 0.9.
Define RQ. What is its value for fats?Answer: RQ (respiratory quotient) is the ratio of the volume of $C O_{2}$ evolved to the volume of $O_{2}$ consumed in respiration. For fats it is less than 1; for tripalmitin it is 102/145, about 0.7.

Common Mistakes to Avoid

Watch out

Writing RQ as $O_{2}$ consumed / $C O_{2}$ evolved. The correct ratio is $C O_{2}$ evolved / $O_{2}$ consumed.
Giving fats an RQ above 1 (for example 1.4). That value comes from inverting the ratio; fats give about 0.7.
Stating that all carbohydrates always give RQ = 1. This holds when they are completely oxidised.
Mixing up the protein and fat values. Proteins give about 0.9; fats give about 0.7.
Thinking living organisms respire pure fats or pure proteins. Substrates are usually mixed.
Calling tripalmitin a carbohydrate or a protein. It is a fat, with formula $C_{51} H_{98} O_{6}$ .
Writing RQ with a unit such as mL. It is a ratio of two volumes, so it has no unit.

Frequently Asked Questions

What is respiratory quotient?

Respiratory quotient (RQ), also called the respiratory ratio, is the ratio of the volume of carbon dioxide evolved to the volume of oxygen consumed in respiration. Its value depends on the type of respiratory substrate, so it indicates whether carbohydrates, fats or proteins are being respired.

What is the RQ of carbohydrates?

When carbohydrates are completely oxidised, the RQ is 1. For glucose, six molecules of oxygen are consumed and six molecules of carbon dioxide are released, so equal volumes of the two gases are involved and the ratio 6/6 equals 1.0.

Why is the RQ of fats less than 1?

Fats need more oxygen for their oxidation than the volume of carbon dioxide they release. For example, two molecules of tripalmitin consume 145 molecules of oxygen and release 102 molecules of carbon dioxide, giving an RQ of 102/145, about 0.7.

What is the RQ of proteins?

When proteins are the respiratory substrates, the RQ is about 0.9. This lies between the value for fats (0.7) and the value for completely oxidised carbohydrates (1.0). Proteins are never respired alone in living organisms, so a measured value usually reflects a mixture of substrates.

How is RQ calculated from a balanced equation?

Divide the number of carbon dioxide molecules (or volumes) produced by the number of oxygen molecules (or volumes) consumed in the balanced equation of oxidation. For glucose this is 6/6 = 1.0, and for tripalmitin 102/145 = 0.7. The answer has no unit.

Why does RQ depend on the respiratory substrate?

Different substrates need different amounts of oxygen for complete oxidation compared with the carbon dioxide they release. Carbohydrates use equal volumes, so RQ is 1; fats need relatively more oxygen, so RQ is below 1; proteins give about 0.9. So the RQ value indicates which substrate is being respired.

Do plants respire only one substrate at a time?

No. In living organisms, respiratory substrates are often more than one, and pure proteins or fats are never used as respiratory substrates. A measured RQ therefore usually reflects a mixture of substrates and lies between the values for carbohydrates, proteins and fats.

What should I remember about RQ for NEET?

As in the NCERT text, remember the definition (carbon dioxide evolved over oxygen consumed), that RQ depends on the substrate, the values 1.0 for carbohydrates, 0.7 for fats (tripalmitin, 102/145) and about 0.9 for proteins, and that real organisms respire mixed substrates.

Previous year questions on Respiratory Quotient

2 questions from past papers, each with a step-by-step solution.

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