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Most students memorise three numbers for respiratory quotient: 1 for carbohydrate, 0.7 for fat, 0.9 for protein. Then a question gives a formula they have never seen, or a reading from germinating seeds, and the memorised numbers stop helping. This post explains why RQ takes the values it does, gives you a one-line formula that works for any fuel, and works three examples that are not in your textbook.
Is respiratory quotient still in the syllabus?
Yes. The NEET (UG) 2026 syllabus lists "Respiratory quotient" in the respiration unit, next to amphibolic pathways. The topic is section 12.7 of NCERT Class 11 Chapter 12, Respiration in Plants (reprint 2026-27). Check the syllabus for your own exam year.
NCERT defines RQ as the volume of CO2 evolved divided by the volume of O2 consumed. It works out glucose (RQ = 6/6 = 1.0) and the fat tripalmitin (2C51H98O6 + 145O2 → 102CO2 + 98H2O, so RQ = 102/145 = 0.7). It says protein gives about 0.9, and that living cells usually respire a mixture, never pure fat or protein.
Why does the fuel's oxygen content decide RQ?
At the same temperature and pressure, equal volumes of gas hold equal numbers of molecules. So the coefficients in a balanced equation give the ratio of volumes directly.
Now count the oxygen a fuel CxHyOz needs:
Each carbon becomes one CO2, so it needs one O2. That is x molecules of O2.
Every four hydrogen atoms become two H2O, which needs one more O2. That is y/4.
Oxygen already inside the fuel covers part of this. That saves z/2.
So O2 used = x + y/4 − z/2, CO2 made = x, and RQ = x / (x + y/4 − z/2).
Carbohydrates have the formula (CH2O)n, so y/4 exactly equals z/2 and RQ is 1. Fats are long hydrocarbon chains with very little oxygen. Their hydrogen needs extra O2, so RQ falls below 1. Check NCERT's tripalmitin: 51 / (51 + 24.5 − 3) = 51/72.5 = 0.70, the same as 102/145.
Worked example 1: a different fat
Triolein, another fat, is C57H104O6. Find its RQ.
O2 needed = 57 + 104/4 − 6/2 = 57 + 26 − 3 = 80.
Balanced equation: C57H104O6 + 80O2 → 57CO2 + 52H2O.
Check oxygen: left 6 + 160 = 166; right 114 + 52 = 166. Check hydrogen: 104 on both sides.
RQ = 57/80 = 0.71.
Two different fats, two answers close to 0.7. That is why NCERT can quote one value for fat.
Worked example 2: a fuel with RQ above 1
This one goes beyond NCERT. Malic acid, an organic acid, is C4H6O5.
O2 needed = 4 + 1.5 − 2.5 = 3.
RQ = 4/3 = 1.33.
The molecule is so rich in oxygen that it needs less O2 than the CO2 it gives off. Oxalic acid, C2H2O4, is more extreme: O2 needed = 2 + 0.5 − 2 = 0.5, so RQ = 2/0.5 = 4.
The opposite limit is fermentation. NCERT notes that yeast converts pyruvic acid to CO2 and ethanol without oxygen. CO2 is released while no O2 is used, so the ratio has no finite value. Do not write zero.
Worked example 3: reading a measurement (sample numbers)
Suppose a flask of germinating seeds takes up 12.0 mL of O2 and gives off 10.2 mL of CO2 in an hour. These are sample numbers for practice.
RQ = 10.2/12.0 = 0.85.
That sits between 0.7 and 1.0, so the seeds are probably respiring a mixture.
If only carbohydrate (1.0) and fat (0.7) are involved, let f be the share of the O2 used on carbohydrate. Then 1.0f + 0.7(1 − f) = 0.85, so f = 0.5.
About half the oxygen is going to each fuel. The same idea is used on people. Frayn (1983) reviewed how rates of carbohydrate and fat oxidation are calculated from O2 consumption, CO2 production and urinary nitrogen, and showed that processes such as lipogenesis and gluconeogenesis distort the result unless you correct for them.
Which mistakes cost marks?
Inverting the ratio. RQ is CO2 over O2. Writing O2/CO2 turns 0.7 into 1.43.
Counting water as a gas. Only CO2 and O2 enter the ratio.
Giving the wrong reason for fat. The cause is low oxygen content in the molecule.
Treating 0.9 as exact. NCERT says protein gives "about" 0.9.
Calling anaerobic RQ zero. No O2 is used, so the ratio is undefined.
If you keep slipping on the balancing step, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning, so you can see which atom count went wrong.
A practice routine
Derive RQ = x/(x + y/4 − z/2) on a blank page without notes.
Use it for glucose, sucrose (C12H22O11), tripalmitin, triolein and malic acid. You should get 1, 1, 0.70, 0.71 and 1.33.
Fully balance one fat equation and check every atom.
Invent a reading between 0.7 and 1, and work out the share of O2 used on each fuel.
Revise the pathway itself with our guide to counting ATP in respiration in plants.
Tonight, write the RQ formula from memory and test it on sucrose.
