NCERT Grounding
Section 12.7 of NCERT Class 11 Biology (Chapter 12, Respiration in Plants) introduces the Respiratory Quotient as follows: "The ratio of the volume of CO₂ evolved to the volume of O₂ consumed in respiration is called the respiratory quotient (RQ) or respiratory ratio." The text then provides the balanced equations for glucose and tripalmitin oxidation, explicitly stating RQ = 1 for carbohydrates and RQ ≈ 0.7 for tripalmitin. Proteins and organic acids are mentioned qualitatively. NIOS Biology (Chapter 12) reinforces this by listing RQ as a diagnostic tool for the type of substrate and connects it to factors affecting the rate of respiration.
"The respiratory quotient depends upon the type of respiratory substrate used during respiration."
NCERT Class 11 Biology, Section 12.7
Definition and Formula
During aerobic respiration, a cell consumes molecular oxygen (O₂) and releases carbon dioxide (CO₂). The Respiratory Quotient quantifies this gas exchange in a single number that is independent of the absolute rate of respiration — it captures only the ratio of volumes exchanged.
Respiratory Quotient — Formula
RQ = Volume of CO₂ evolved ÷ Volume of O₂ consumed
Measured under identical temperature and pressure conditions (so volumes are directly proportional to moles). The ratio is dimensionless.
Because the volumes of gases are compared at the same temperature and pressure, the ratio equals the molar ratio of CO₂ produced to O₂ consumed — which is determined entirely by the empirical formula of the substrate being oxidised.
Substrate-by-Substrate Analysis
Carbohydrates — RQ = 1.0
Glucose (C₆H₁₂O₆) is the commonest respiratory substrate. Its complete aerobic oxidation is:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy
RQ = 6 CO₂ / 6 O₂ = 1.0
The carbon and oxygen atoms in glucose are already partially oxidised — the molecule's formula places C, H, and O in a 1:2:1 ratio, matching what is needed to produce CO₂ and H₂O in equal proportions of gas. The result is a "balanced" exchange: one litre of O₂ consumed produces exactly one litre of CO₂. All carbohydrates (sucrose, starch, fructose) produce an RQ of 1.0 when completely oxidised because they share the same empirical CHO ratio.
Fats — RQ less than 1 (approximately 0.7)
Fats are highly reduced molecules — they are rich in C–H bonds and contain proportionally much less oxygen than carbohydrates. More molecular oxygen must be imported to complete oxidation; this inflates the denominator of the RQ fraction while the numerator grows less steeply.
NCERT uses tripalmitin (C₅₁H₉₈O₆) as the canonical example. This is a triacylglycerol (glycerol esterified with three palmitic acid chains). Its combustion equation:
2C₅₁H₉₈O₆ + 145O₂ → 102CO₂ + 98H₂O + Energy
RQ = 102 / 145 ≈ 0.7
The ratio 102:145 simplifies to approximately 0.703. Notice that tripalmitin has 102 carbon atoms in total (51 × 2) that all end up as CO₂, but it requires 145 moles of O₂ — far more than the 102 that would suffice for carbohydrate of equivalent carbon content. The surplus oxygen demand arises from the high degree of reduction of fatty acid chains, which carry predominantly C–H bonds rather than C–OH or C=O bonds.
Proteins — RQ approximately 0.9
Proteins are more complex: they contain nitrogen and, in some cases, sulphur in addition to C, H, and O. Their deamination before entry into the Krebs cycle alters the oxygen balance. The resulting RQ falls between those of carbohydrates and fats — approximately 0.9. In practice, proteins are never used as the sole respiratory substrate, but germinating protein-rich seeds (e.g., pulse seeds) show an RQ approaching this value.
Organic Acids — RQ greater than 1
Organic acids such as oxalic acid (C₂H₂O₄) are highly oxidised molecules — they already contain a large number of C–O and C=O bonds relative to their carbon content. Consequently they require very little additional O₂ from the atmosphere for complete combustion, but they still liberate CO₂. The numerator of the RQ fraction therefore exceeds the denominator.
2C₂H₂O₄ + O₂ → 4CO₂ + 2H₂O
RQ = 4 / 1 = 4.0
Succulents (CAM plants) accumulate malic acid overnight and respire it by day, producing RQ values well above 1. Any substrate with an empirical formula richer in oxygen relative to carbon will display RQ greater than 1.
RQ Reference Table
Figure 1. Comparative RQ values for the five major respiratory substrate categories. Bars truncated at 4 units for organic acids and anaerobic cases; actual values can be much higher or undefined.
| Substrate | Example | Equation (summary) | RQ value | Category |
|---|---|---|---|---|
| Carbohydrates | Glucose C₆H₁₂O₆ | 6O₂ consumed, 6CO₂ released | 1.0 | RQ = 1 |
| Proteins | Mixed amino acids | Variable; N excreted as urea/ammonia | ~0.9 | RQ < 1 |
| Fats | Tripalmitin C₅₁H₉₈O₆ | 2C₅₁H₉₈O₆ + 145O₂ → 102CO₂ + 98H₂O | ≈ 0.7 | RQ < 1 |
| Organic acids | Oxalic acid C₂H₂O₄ | 2C₂H₂O₄ + O₂ → 4CO₂ + 2H₂O | > 1 (e.g. 4) | RQ > 1 |
| Anaerobic / fermentation | Glucose → ethanol + CO₂ | C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂; O₂ = 0 | ∞ | RQ = ∞ |
Anaerobic Special Case
During alcoholic fermentation by yeast, glucose is incompletely oxidised to ethanol and CO₂ with no consumption of molecular oxygen:
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
O₂ consumed = 0
RQ = 2 CO₂ / 0 O₂ = ∞ (undefined / infinity)
The physical interpretation is straightforward: CO₂ continues to escape from the cell, but no O₂ enters. The ratio's denominator is zero, making RQ mathematically infinite. This is the one case where RQ ceases to function as a substrate indicator — instead it is a binary marker for the complete absence of aerobic respiration in that tissue.
Lactic acid fermentation (as in muscle cells or certain bacteria) also consumes no O₂, but produces no CO₂ either (pyruvate → lactate). In this case RQ = 0/0, which is indeterminate rather than infinite. NEET questions on this topic universally refer to alcoholic fermentation when they invoke RQ = ∞.
Significance of RQ
Principle: RQ is a window into cellular metabolism — it reveals which fuel is being consumed without requiring extraction and chemical analysis of cellular contents.
Substrate Identification
RQ = 1
Carbohydrate burning
Equal volumes of O₂ and CO₂ exchanged; glucose or starch is the active substrate.
Fat Mobilisation
RQ < 1
Fats or proteins burning
More O₂ consumed than CO₂ released; stored fat or protein being catabolised.
NEET 2019 — tripalmitin ≈ 0.7Organic Acid Oxidation
RQ > 1
Highly oxidised substrate
Less O₂ needed than CO₂ produced; organic acids (malic, oxalic, citric) are the substrate.
Anaerobic Condition
RQ = ∞
No O₂ consumed
CO₂ is released but zero O₂ is consumed; fermentation (alcoholic) is occurring.
In seed germination physiology, RQ tracking is particularly informative. Oil-rich seeds (castor, mustard) begin germination with RQ values well below 1 (fat being oxidised). As stored fats are converted to sucrose and then consumed, the RQ rises toward 1.0. Protein-rich seeds (pulses) show intermediate RQ around 0.8–0.9 during early germination. This time-course shift is a practical application of RQ measurement in plant biochemistry.
Worked Examples
A plant tissue releases 44 mL of CO₂ and consumes 44 mL of O₂ under standard conditions during one hour of respiration. What is the RQ, and what substrate is being respired?
Solution: RQ = Volume CO₂ / Volume O₂ = 44 / 44 = 1.0. An RQ of 1.0 indicates a carbohydrate (most likely glucose) is being completely oxidised.
In an experiment, 102 mL of CO₂ is evolved and 145 mL of O₂ is consumed. (i) Calculate the RQ. (ii) Name the substrate and write the balanced equation for its oxidation.
Solution: (i) RQ = 102 / 145 ≈ 0.7. (ii) The substrate is a fat; the specific example given in NCERT is tripalmitin (C₅₁H₉₈O₆). Balanced equation: 2C₅₁H₉₈O₆ + 145O₂ → 102CO₂ + 98H₂O. The RQ is less than 1 because fat is more reduced (hydrogen-rich, oxygen-poor) than carbohydrate and demands proportionally more O₂.
A sealed chamber containing yeast and glucose shows continuous CO₂ evolution but no detectable O₂ consumption. What is the RQ, and what process is occurring?
Solution: RQ = CO₂ evolved / O₂ consumed = any positive number / 0 = infinity (∞). The process is alcoholic fermentation (anaerobic respiration): C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. Because no molecular O₂ is consumed, the RQ denominator is zero.
Oxalic acid (C₂H₂O₄) undergoes complete aerobic oxidation. Calculate the RQ and state whether it is greater than or less than that of glucose.
Solution: Balanced equation: 2C₂H₂O₄ + O₂ → 4CO₂ + 2H₂O. RQ = 4 / 1 = 4.0. This is much greater than the RQ of glucose (1.0). Oxalic acid is highly oxidised — it has a high oxygen-to-carbon ratio — so it needs very little additional O₂ but still releases CO₂ from its two carbon atoms.
Common Confusion & NEET Traps
Fats (RQ < 1)
0.7
Tripalmitin — NEET direct target
- Fat = reduced, H-rich, O-poor molecule
- Needs more O₂ than CO₂ produced
- Denominator (O₂) > numerator (CO₂)
- RQ = 0.7 → below 1
Organic Acids (RQ > 1)
> 1
Oxalic, malic, citric acids
- Organic acid = oxidised, O-rich molecule
- Needs less O₂ than CO₂ produced
- Numerator (CO₂) > denominator (O₂)
- RQ > 1 → above 1