Why is the RQ of fats about 0.7? And how is the respiratory pathway "amphibolic"?
Aarav Kulkarni11th Sample
29 Sept 2026 · 1 min read
Making a checklist of RQ for tomorrow's test and I got stuck on the reason part, not the numbers.
NCERT: RQ = volume of CO2 evolved / volume of O2 consumed. Glucose: , so RQ = 1. Fine. For tripalmitin: and . I can do the arithmetic but I don't get the why of it being less than 1.
I also tried malic acid, , which gives . Is that correct? And for anaerobic respiration I wrote 0 in a worksheet since no O2 is used, but the answer key says infinity. I'm confused about that too.
Second part: NCERT calls the respiratory pathway "amphibolic". I made a list of how fats and proteins enter, and which intermediates are drawn off for synthesis, but I'm not sure if it's complete. Also, in an actual cell where mixed substrates are being used, is RQ just somewhere between?
8 answers & replies
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[+][–]Zoya Khan12thSample· 20d ago
The reason is hidden in the molecular formula. A carbohydrate like glucose already carries enough oxygen in its own structure (the H:O ratio is 2:1, same as water), so the oxygen you breathe in is basically used to burn the carbon, and every O2 used gives one CO2. Fats are very poor in oxygen compared with the amount of carbon and hydrogen. A lot of hydrogen in the fat has to be oxidised to water using external O2, and that O2 does not leave as CO2. So you consume more O2 for each CO2 released, so the ratio drops below 1. Your tripalmitin numbers show it nicely: 145 O2 in, only 102 CO2 out.
[+][–]Rahul DubeyDropperSample· 20d ago
1.33 for malic acid is right. Organic acids are already rich in oxygen, so they need less external O2 for each CO2, so RQ goes above 1. Same idea as in Zoya's answer, other direction.
So the tidy picture is: carbohydrates = 1, fats less than 1 (about 0.7), proteins about 0.9, organic acids more than 1.
[+][–]Sara Fernandes11thSample· 20d ago
I think zero for anaerobic is the logical answer? Like no gas is exchanged at all, no O2 used, so RQ = 0. The answer key may have a typo 😅 Also for the amphibolic bit I'd just remember it as "it breaks things down AND builds things up", that's what amphi means.
[+][–]Siddharth RaoDropperSample· 20d ago
Sara, it can't be zero. Anaerobic respiration releases CO2 but uses no O2, so you get CO2 / 0 = infinity (undefined, really). Zero would mean O2 consumed but no CO2 given out. Key is right, your logic got flipped.
[+][–]Pooja Iyer12thSample· 20d ago
mnemonic time!! "Fats Fall, Acids Ascend". Carbs sit at 1 in the middle, fats fall below (0.7), proteins slightly below 1 (0.9), organic acids ascend above 1. And anaerobic is off the chart, infinity. If your teacher says RQ of carbs is 1 you are 1 mark up already 😌
[+][–]Tanvi Deshmukh12thSample· 20d ago
Amphibolic list!! this is how I have it in my notes:
Catabolic (breakdown) side
- Carbohydrates: glucose goes through glycolysis, link reaction, TCA cycle, ETS
- Fats: fatty acids become acetyl CoA and enter the cycle; glycerol goes to PGAL
- Proteins: amino acids are deaminated and the carbon skeletons enter at pyruvate / acetyl CoA / TCA cycle intermediates
Anabolic (building) side
- Acetyl CoA is also used to make fatty acids
- TCA intermediates (like alpha-ketoglutarate and OAA) can be taken out to make amino acids
- PGAL / glycerol link to fat synthesis
So the same pathway runs in both directions of supply. Your list was probably complete, just add the arrows going out.
[+][–]Aarav Kulkarni11thSample· 20d agoAuthor
this is the table I needed, thank you. And ok I was wrong on anaerobic, infinity, writing it in red.
[+][–]Nisha Verma12thSample· 20d ago
For the last question: yes, in a real cell it's a mix, so the measured RQ is in between. Germinating oily seeds give a value below 1, for example, but never exactly 0.7 as other substrates are also being used.
