Respiration in Plants · Lesson 8 of 9
Respiratory Quotient and Substrate Identification
“RQ definition, gas volumes, glucose RQ 1, fat ~0.7, protein ~0.9, organic-acid extensions and caveats.”
• Define respiratory quotient (RQ) as CO₂ evolved divided by O₂ consumed. • Calculate RQ from balanced substrate oxidation equations and measured gas volumes. • Explain RQ 1 for carbohydrate and less than 1 for fat-rich substrates. • Interpret the textbook's tripalmitin calculation and protein approximation. • Explain why mixed substrates and concurrent photosynthesis complicate RQ measurement.
What gases reveal about the respiratory fuel
Different organic fuels contain different ratios of carbon, hydrogen and oxygen, so complete oxidation of the same number of carbon atoms can require different quantities of molecular O₂. A plant respiring carbohydrate may consume a volume of oxygen close to the volume of carbon dioxide released, while oxidation of a more reduced fatty substance usually requires more O₂ per unit of CO₂. The respiratory quotient (RQ) captures this relationship as a ratio.
The volume of CO₂ evolved in respiration divided by the volume of O₂ consumed in the same interval under comparable conditions.
RQ has no unit because it divides two volumes with the same unit, provided gas volumes refer to comparable temperature and pressure. In experiments one can calculate it using rates as well: volume of CO₂ released per minute divided by volume of O₂ taken up per minute. The definition applies to oxygen-consuming processes and must be treated carefully when O₂ uptake is zero or when another process, such as photosynthesis, affects net gas exchange.
Complete glucose oxidation has RQ = 1
The balanced overall reaction for glucose is C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + energy. One mole of glucose oxidised releases six moles of CO₂ and consumes six moles of O₂. At the same temperature and pressure, equal molar amounts of ideal gases occupy equal volumes. The RQ is therefore 6/6 = 1. This is the source's standard carbohydrate reference.
An RQ of exactly one is useful but not unique proof that a tissue is using only carbohydrate. Mixed substrates and other physiological processes can give similar overall values. A measured gas ratio is evidence to interpret with context, not a complete molecular identification test.
Why fats typically give RQ below one
Many fats contain a high proportion of carbon and hydrogen relative to the oxygen already present in the molecule. Fully oxidising these elements requires substantial molecular oxygen. Consequently, O₂ uptake exceeds CO₂ release, giving RQ below one. The source illustrates this with tripalmitin, a triglyceride containing three palmitic acid-derived chains; it is not a free single fatty-acid molecule.
The balanced source equation is 2 C₅₁H₉₈O₆ + 145 O₂ → 102 CO₂ + 98 H₂O + energy. Therefore RQ = 102/145 ≈ 0.703, commonly rounded to 0.7. An individual lipid or a mixed dietary fat may yield a somewhat different quotient, so 0.7 is an illustrative fat-oxidation value.
Protein RQ and the effect of mixed fuels
The source gives protein as a respiratory substrate with an approximate RQ of 0.9. Protein metabolism is more complicated than complete combustion of a simple carbohydrate because proteins contain nitrogen and their amino-acid carbon skeletons differ. Consequently, one should not pretend that every protein has one fixed chemical equation or one universal RQ. In living tissues several fuels are usually oxidised simultaneously, so an observed RQ often reflects a mixture.
Respiration of organic acids can yield RQ greater than one because their carbon skeletons already contain relatively much oxygen. As a supplementary illustration, full oxidation of malic acid (C₄H₆O₅) is C₄H₆O₅ + 3 O₂ → 4 CO₂ + 3 H₂O, giving RQ 4/3 ≈ 1.33. This example extends the source's explicit glucose, tripalmitin and protein comparisons.
| Substrate | Example or indicative RQ | Interpretation |
|---|---|---|
| Glucose/carbohydrate | 1.0 | CO₂ volume equals O₂ volume under complete oxidation |
| Tripalmitin/fat | 102/145 ≈ 0.70 | More O₂ consumed than CO₂ evolved |
| Protein | ~0.9 (source approximation) | Depends on mixture and nitrogen handling |
| Malic acid (supplementary) | 4/3 ≈ 1.33 | More CO₂ evolved than O₂ consumed in full oxidation |
How to calculate RQ from a gas-volume observation
Suppose a sample consumes 30 mL of oxygen and releases 21 mL of CO₂ over the same period under matched conditions. RQ = 21/30 = 0.70. This suggests a relatively reduced substrate such as fat is contributing strongly, but it does not prove the sample uses pure fat. A combination of fuels and processes can influence the same ratio.
If a tissue consumes 25 mL O₂ and releases 25 mL CO₂, the measured RQ is 1. If it releases 16 mL CO₂ while consuming 12 mL O₂, the ratio is 1.33, consistent with some oxygen-rich organic-acid metabolism. Always place evolved CO₂ in the numerator and consumed O₂ in the denominator; reversing the ratio gives a different quantity.
Balanced equations require atom conservation
When constructing an RQ equation, first balance the carbon atoms to set CO₂, then balance hydrogen atoms to set H₂O, and finally determine oxygen atoms needed from O₂. For glucose, six carbons give six CO₂ and twelve hydrogens give six water molecules; oxygen balance requires six O₂. For tripalmitin, using two molecules avoids fractions in the coefficients and produces 102 CO₂ and 98 H₂O, requiring 145 O₂.
RQ is a ratio derived from the fully balanced reaction, not a memorised property of the number of carbon atoms alone. A simplified equation may be difficult for protein because nitrogen-containing products must also be accounted for. The source therefore gives only an approximate protein RQ.
Respiratory quotient is not defined for oxygen-free fermentation
If an anaerobic fermentation produces CO₂ but no O₂ is consumed, the denominator in the usual respiratory quotient is zero. The classical aerobic RQ is then not a finite useful value. Do not describe anaerobic ethanol fermentation as having 'infinite RQ' in a way that implies it is a normal substrate quotient; the oxygen-based measure simply ceases to function as intended.
Why green leaves complicate measurements
An illuminated leaf can consume CO₂ and evolve O₂ through photosynthesis while respiring mitochondria consume O₂ and release CO₂. A sensor measuring only net CO₂ and O₂ at the leaf surface therefore combines opposing processes. A measurement intended to study respiratory RQ requires conditions or experimental methods that distinguish respiration from photosynthesis and account for other gas exchanges.
Tissue age, temperature, water availability, substrate mix and oxygen supply can also affect measured rates. For example, dark germinating seeds can be useful for observing gas uptake, but a respirometer still needs controls for gas pressure, temperature and any absorbent used to remove CO₂. The measured change should not be interpreted as a pure oxygen uptake if carbon dioxide is simultaneously entering the gas phase.
An inquiry using substrate mixtures
Suppose two germinating-seed preparations differ in stored reserves, one starch-rich and one oil-rich. A hypothesis is that under conditions favouring full oxidation, the starch-rich preparation may have a quotient nearer one while the oil-rich preparation may show a value below one. To test it, compare matched mass, developmental stage, temperature and gas measurement conditions, and consider the fact that each seed also uses mixed metabolites. Report observed ratios without assuming they must exactly equal theoretical pure-substrate values.
Problem
A chamber sample consumes 40 mL of O₂ and releases 28 mL of CO₂. Another consumes 18 mL of O₂ and releases 24 mL of CO₂. Calculate each RQ and interpret cautiously.
- 1.First RQ = 28/40 = 0.70, compatible with substantial oxidation of a more reduced fuel such as fat.
- 2.Second RQ = 24/18 ≈ 1.33, compatible with metabolism of some oxygen-rich organic acids.
- 3.These ratios do not uniquely identify substrates if multiple pathways operate; gas measurements must be appropriately controlled.
Quick Quiz
Quiz
What is the formula for RQ?
What is RQ for complete oxidation of glucose?
Why is fat oxidation typically associated with RQ below one?
What RQ follows from 102 CO₂ divided by 145 O₂?
Which approximate RQ does the source give for protein substrates?
What does an RQ around 1.33 suggest under controlled complete oxidation?
Why should RQ from illuminated leaves be interpreted cautiously?
What happens to conventional aerobic RQ when O₂ uptake is zero?
Practice Problems
- Define respiratory quotient and explain its units.
- Calculate RQ for glucose using the balanced equation.
- Calculate fat RQ from the source tripalmitin equation.
- Explain why RQ cannot always identify an exact respiratory substrate.
- Explain why photosynthesis and anaerobic fermentation complicate RQ.
RQ = volume of CO₂ evolved / volume of O₂ consumed for the same interval and comparable gas conditions; because both are volumes the quotient is dimensionless.
Key Takeaways
• RQ is CO₂ evolved divided by O₂ consumed under comparable measurement conditions. • Complete glucose oxidation has RQ = 6/6 = 1. • Tripalmitin oxidation in the source has RQ = 102/145 ≈ 0.7, typical of fats. • Protein RQ is approximately 0.9 in the source, but varies with substrate and nitrogen metabolism. • Oxidation of oxygen-rich organic acids can yield RQ greater than one, as with malic acid (~1.33). • A tissue using mixed substrates need not show a pure-substrate quotient. • RQ in illuminated leaves may be distorted by simultaneous photosynthesis, and standard aerobic RQ is not finite when O₂ uptake is zero.