🌧 Respiratory Quotient (RQ): Formula, Values and Exam Applications

From Field to Lab — Measuring What a Seed Breathes

Quick answer: Respiratory Quotient (RQ) is the ratio of volume of CO₂ evolved to volume of O₂ consumed during respiration. In exams, remember: carbohydrates = 1.0, fats = 0.7, proteins = about 0.8, organic acids > 1, and anaerobic respiration = infinity because no oxygen is consumed.

Search Query / Exam Prompt	Direct Answer
RQ formula	RQ = volume of CO₂ evolved / volume of O₂ consumed
RQ of glucose / carbohydrate	1.0
RQ of fat / oilseed	0.7 or < 1
RQ of protein	~0.8
RQ of organic acids	> 1
RQ of anaerobic respiration	Infinity
RQ instrument	Ganong's Respirometer

In the previous lesson, we traced the step-by-step mechanism of respiration — glycolysis, the link reaction, Krebs cycle, and ETS — and arrived at a total of 38 ATP per glucose. Now we examine the efficiency of that energy capture, learn how to identify which substrate is being respired using the Respiratory Quotient, and explore the external factors that speed up or slow down respiration in living tissues.

A plant breeder wants to know whether stored groundnut seeds are respiring fats or carbohydrates. The answer lies in the Respiratory Quotient (RQ) — the ratio of CO₂ released to O₂ absorbed. If RQ is less than 1, the seeds are burning fats (which need extra oxygen). If RQ equals 1, they are burning carbohydrates. This simple measurement, done with a Ganong's Respirometer, reveals the metabolic state of the seed — and helps predict how long it will remain viable in storage.

This lesson covers:

1. Efficiency of respiration — how much energy is captured vs lost as heat
2. Respiratory substrates — entry points for fats, proteins, and carbohydrates
3. Respiratory Quotient (RQ) — values for different substrates and their agricultural meaning
4. Factors affecting respiration — temperature, oxygen, moisture, and more

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Efficiency of Respiration

How efficiently does respiration convert food energy into usable ATP? The answer matters practically — the 58% lost as heat is why grain heaps warm up in storage and why compost piles steam.

When glucose is completely oxidised aerobically, 686 kcal of energy is released. Of this, 38 ATP molecules are produced.

Parameter	Value
Total energy per glucose	686 kcal
ATP molecules produced	38
Energy per ATP	7.6 kcal
Efficiency	38 × 7.6 / 686 = 42%
Energy lost as heat	58%

Practical observations:

• Germinating seeds in a test tube cause temperature to rise (fast respiration)
• Dormant seeds have slow respiration (low moisture keeps enzymes inactive)

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Solved Problem

Q: During aerobic respiration, a plant cell released 42 molecules of CO₂. How many ATP are produced?

Solution: 6 CO₂ per glucose = 38 ATP. For 42 CO₂ = 42/6 × 38 = 7 × 38 = 266 ATP.

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Respiratory Substrates — Entry Points

Cells do not always burn glucose. Depending on tissue type and nutritional status, fats or proteins may be the primary fuel. Each substrate enters the respiratory pathway at a different point and yields a different amount of energy — which directly determines the RQ value measured in experiments.

Substrate	Breakdown	Entry Point	Energy Yield
Glucose (carbohydrate)	Direct	Glycolysis (start)	Standard (686 kcal)
Fats	Glycerol → PGAL; Fatty acids → Acetyl CoA	Mid-glycolysis; Krebs	More per gram (9 kcal/g vs 4 kcal/g)
Proteins	Deamination → carbon skeletons	Various Krebs cycle points	Last resort (starvation)

Key Metabolic Connections

Intermediate	Connects To	Significance
PGAL	Glycerol → fats/lipids	Glycolysis intermediate for fat synthesis
PGA	Serine, Glycine, Cysteine (amino acids)	Links to protein synthesis
Pyruvate	Alanine (amino acid)	Transamination
Acetyl CoA	Common link: fats ↔ carbohydrates ↔ proteins	Central metabolic hub

• Beta-oxidation = sequential removal of 2-carbon units from fatty acids as Acetyl CoA, producing NADH and FADH₂
• This is why respiration is amphibolic — intermediates feed both breakdown and synthesis pathways

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Respiratory Quotient (RQ)

Now that we know different substrates require different amounts of O₂ and produce different amounts of CO₂, we can use their ratio as a diagnostic tool. The RQ (also called Respiratory Ratio) reveals what substrate is being respired — a single number that tells you the metabolic state of any tissue.

RQ = Volume of CO₂ released / Volume of O₂ absorbed

• Measured by Ganong's Respirometer
• NCERT's respiration-in-plants syllabus explicitly includes amphibolic pathways and respiratory quotient, so RQ is a core plant physiology topic, not an optional laboratory detail.

Substrate	Equation	CO₂	O₂	RQ	Meaning
Carbohydrates (glucose)	C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O	6	6	1.0	Equal CO₂ and O₂
Fats (tripalmitin)	C₅₁H₉₈O₆ + 72.5O₂ → 51CO₂ + 49H₂O	51	72.5	0.7	More O₂ needed (fats have less O)
Proteins	—	—	—	0.8	Between fats and carbs
Organic acids (malic acid)	C₄H₆O₅ + 3O₂ → 4CO₂ + 3H₂O	4	3	1.33	Already partly oxidised
Anaerobic respiration	C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂	2	0	∞ (infinity)	No O₂ absorbed

What RQ Tells Us

RQ Value	Substrate Being Respired	Agricultural Significance
= 1.0	Carbohydrates	Normal respiration in most tissues
< 1.0	Fats	Germinating oilseeds (groundnut, mustard)
0.8	Proteins	Protein-rich seeds; starvation conditions
> 1.0	Organic acids	Succulents, CAM plants
= ∞	Anaerobic respiration	Waterlogged roots, fermentation

RQ Interpretation for Crop and Storage Questions

Situation	Likely RQ	Why It Matters
Germinating wheat seed	Around 1.0	Starch is the main reserve; carbohydrate oxidation dominates
Germinating groundnut / mustard seed	< 1.0	Oil reserves need more oxygen for oxidation
Starving tissue	< 1.0	Fats and proteins are increasingly used as substrates
Waterlogged root	∞ if anaerobic	Oxygen uptake stops; fermentation products injure roots
Succulent tissue rich in organic acids	> 1.0	Organic acids are already oxygen-rich, so less O₂ is needed

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Factors Affecting Respiration

Respiration rate is not constant — it responds to environmental conditions. Understanding these factors is directly applicable to post-harvest management, seed storage, cold chain logistics, and waterlogging tolerance in crops.

Factor	Effect	Agricultural Relevance
Temperature	Rate increases with temperature (Q₁₀ = 2); optimal 30°C	Cold storage slows respiration, preserves produce
Oxygen supply	Aerobic respiration needs O₂; without O₂ → fermentation	Waterlogging reduces O₂ → root death
CO₂ concentration	High CO₂ inhibits respiration	Modified atmosphere storage uses high CO₂
Water content	Low moisture → slow respiration (dormant seeds)	Seed drying to 12% moisture for safe storage
Substrate availability	More substrate → more respiration (up to enzyme saturation)	Well-nourished plants respire more
Injury / Wounding	Increases respiration rate (wound respiration)	Harvesting damage → faster post-harvest deterioration
Light	Indirectly increases respiration (more photosynthate available)	—

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Summary Table — Key Facts at a Glance

Fact	Answer
Respiration efficiency	42%
Energy per glucose	686 kcal
ATP per glucose (aerobic)	38 ATP
Energy per ATP	7.6 kcal
RQ of carbohydrates	1.0
RQ of fats	0.7
RQ of proteins	0.8
RQ of organic acids	1.33
RQ of anaerobic respiration	Infinity (∞)
RQ measured by	Ganong's Respirometer
Common metabolic link	Acetyl CoA
Fat oxidation pathway	Beta-oxidation
Germinating oilseed RQ	< 1.0 (fats being respired)
Dormant seed respiration	Slow (low moisture)
Q₁₀ for enzymes	2

Summary Cheat Sheet

Fact	Answer
Total energy released per glucose molecule	686 kcal
Number of ATP produced per glucose (aerobic)	38 ATP
Energy stored per ATP molecule	7.6 kcal
Efficiency of aerobic respiration	42%
Energy lost as heat during respiration	58%
RQ formula	CO₂ released / O₂ absorbed
Instrument to measure RQ	Ganong's Respirometer
RQ of carbohydrates (glucose)	1.0
RQ of fats (tripalmitin)	0.7
RQ of proteins	0.8
RQ of organic acids (malic acid)	1.33
RQ of anaerobic respiration	Infinity (∞)
RQ of germinating oilseeds (groundnut, mustard)	Less than 1.0 (fats)
Central metabolic hub linking fats, carbs, and proteins	Acetyl CoA
Fatty acid breakdown pathway producing Acetyl CoA	Beta-oxidation
Glycerol from fat breakdown enters glycolysis as	PGAL
Respiration is called amphibolic because	It serves both catabolic and anabolic pathways
Q₁₀ value for respiratory enzymes	2 (rate doubles per 10°C rise)
Safe moisture content for seed storage	12%
High CO₂ effect on respiration	Inhibits respiration (used in modified atmosphere storage)