Key Fact
Fermentation and RQ
Anaerobic: pyruvate → ethanol + CO2 (yeast) or lactate (muscle). RQ = CO2 released / O2 consumed. Carbohydrates RQ = 1; fats < 1 (~0.7); proteins ~0.9; organic acids > 1.
-- NCERT Class 11 Biology, Chapter 12, p. 157Respiration Fermentation
8 MCQs5 revision cards9-step worked example
Source: NCERT Plant PhysiologyPYQ coverage: NEET 2022Official key: NTA-verifiedLast updated: 25 Sep 2026
Respiration Fermentation, explained for NEET
Fermentation is anaerobic respiration's incomplete cousin — it regenerates NAD⁺ from NADH without an electron transport chain, allowing glycolysis to continue when oxygen is absent or limiting. The two types tested in NEET are alcoholic fermentation (in yeast: pyruvate → acetaldehyde → ethanol + CO₂) and lactic acid fermentation (in animal muscle and certain bacteria: pyruvate → lactic acid, no CO₂ released).
Key facts from NCERT Class 11 Biology Chapter 12, page 157: fermentation yields only 2 ATP net per glucose (from glycolysis alone), the end-products are incompletely oxidized, and the process is hazardous to organisms at high concentrations (ethanol above ~13% kills yeast).
Respiratory Quotient (RQ) quantifies substrate usage:
RQ = CO₂ evolved / O₂ consumed
- Carbohydrates: RQ = 1.0 (e.g., glucose: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O)
- Fats: RQ < 1 (typically ~0.7; more O₂ needed for the longer carbon chain)
- Organic acids (like malic acid): RQ > 1 (partially oxidized substrate needs less O₂)
- Anaerobic fermentation: RQ = infinity (CO₂ released but zero O₂ consumed)
A common NEET confusion: students assign RQ = 0 to fermentation (confusing "no oxygen involvement" with "no CO₂ release"). In alcoholic fermentation, CO₂ IS released — with no O₂ consumed, the ratio is undefined/infinite. In lactic acid fermentation, neither CO₂ nor O₂ is involved, so RQ is not applicable (sometimes stated as zero or undefined depending on the source — NEET typically asks about alcoholic fermentation's RQ).
Watch out: questions that give a substrate and ask you to predict RQ are testing whether you recall the balanced equation, not computation skill. Know the three benchmark values (1.0, <1, >1) and their substrate associations cold.
Can you answer these Respiration Fermentation MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
The net ATP yield per glucose molecule during fermentation is:
Show answer and why every option is right or wrong
Answer: B. Fermentation involves only glycolysis (2 ATP net) with no further oxidative phosphorylation. NCERT Class 11 Biology Chapter 12, page 162 confirms this value.
Why A is wrong: A — 38 ATP is the theoretical maximum from complete aerobic respiration (glycolysis + Krebs + ETC), not fermentation.
Why C is wrong: C — 8 ATP has no basis in any standard pathway yield.
Why D is wrong: D — 4 ATP is the gross yield of glycolysis before subtracting the 2 ATP invested in the preparatory phase; the NET yield is 2.
MCQ 2Easy RecallPractice
Which molecule is the immediate starting substrate for BOTH alcoholic fermentation and lactic acid fermentation?
Show answer and why every option is right or wrong
Answer: A. A is correct. Both pathways pick up where glycolysis ends, taking pyruvate and reducing it so that NADH can be re-oxidised to NAD⁺ and glycolysis can continue without oxygen. Note that the two pathways share their SUBSTRATE, not their products: alcoholic fermentation yields ethanol and CO₂, lactic acid fermentation yields lactate and no CO₂. The only thing both produce in common is regenerated NAD⁺, which is the point of doing it at all.
Why B is wrong: B — CO₂ is released during alcoholic fermentation (decarboxylation of pyruvate to acetaldehyde) but NOT in lactic acid fermentation.
Why C is wrong: C — Ethanol is produced only in alcoholic fermentation (yeast), not in lactic acid fermentation.
Why D is wrong: D — Lactic acid is the product of lactic acid fermentation only, not of alcoholic fermentation.
MCQ 3Easy RecallPractice
The respiratory quotient (RQ) of a carbohydrate substrate like glucose is:
Show answer and why every option is right or wrong
Answer: D. For glucose: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. RQ = 6CO₂/6O₂ = 1.0. This is the standard NCERT reference value for carbohydrate substrates.
Why A is wrong: A — RQ of 0.7 is characteristic of fat oxidation, where more O₂ is consumed relative to CO₂ released.
Why B is wrong: B — RQ = 0 would mean no CO₂ is produced; glucose oxidation always produces CO₂.
Why C is wrong: C — RQ > 1 occurs with organic acid substrates (e.g., malic acid) that are already partially oxidized.
MCQ 4Direct ApplicationPractice
An organism is respiring with RQ = 0.7. The most likely respiratory substrate is:
Show answer and why every option is right or wrong
Answer: A. Fats have RQ values around 0.7 because their long hydrocarbon chains require proportionally more O₂ for complete oxidation relative to the CO₂ produced. Tripalmitin is a common fat used in textbook RQ calculations.
Why B is wrong: B — Sucrose is a carbohydrate (disaccharide); its RQ is also 1.0.
Why C is wrong: C — Glucose gives RQ = 1.0 (6CO₂ / 6O₂).
Why D is wrong: D — Oxalic acid and other organic acids give RQ > 1 because they are already partially oxidized and need less O₂.
MCQ 5Direct ApplicationPractice
During alcoholic fermentation in yeast, pyruvate is first converted to acetaldehyde by the enzyme:
Show answer and why every option is right or wrong
Answer: C. Pyruvate decarboxylase removes CO₂ from pyruvate to form acetaldehyde (the first step of alcoholic fermentation). Alcohol dehydrogenase then reduces acetaldehyde to ethanol in the second step.
Why A is wrong: A — Alcohol dehydrogenase catalyzes the SECOND step (acetaldehyde → ethanol), not the decarboxylation of pyruvate.
Why B is wrong: B — Lactate dehydrogenase operates in lactic acid fermentation (pyruvate → lactate), not alcoholic fermentation.
Why D is wrong: D — Pyruvate kinase catalyzes the last step of glycolysis (PEP → pyruvate), not any fermentation step.
MCQ 6Direct ApplicationPractice
A tissue sample shows RQ > 1. Which substrate is most likely being oxidized?
Show answer and why every option is right or wrong
Answer: B. Organic acids like malic acid are already partially oxidized — they contain proportionally more oxygen in their structure, so less external O₂ is needed while CO₂ output remains high, giving RQ > 1. For malic acid (C₄H₆O₅), RQ ≈ 1.33.
Why A is wrong: A — Palmitic acid is a fat; fats give RQ < 1 (~0.7) because their long carbon chains demand more O₂.
Why C is wrong: C — Proteins typically give RQ ~0.8–0.9, not greater than 1.
Why D is wrong: D — Starch is a carbohydrate polymer; its RQ = 1.0 (same as glucose upon hydrolysis).
MCQ 7Concept TrapPractice
During vigorous exercise, human skeletal muscle accumulates lactic acid. Why does this NOT release CO₂ unlike alcoholic fermentation?
Show answer and why every option is right or wrong
Answer: D. In lactic acid fermentation, pyruvate is reduced directly to lactate by lactate dehydrogenase using NADH. There is no decarboxylation step (no removal of a carboxyl group), so no CO₂ is produced. In contrast, alcoholic fermentation first decarboxylates pyruvate to acetaldehyde, releasing CO₂.
Why A is wrong: A — Both pathways use the same initial substrate: pyruvate (from glycolysis). The difference is in what happens TO pyruvate, not what precedes it.
Why B is wrong: B — Lactate dehydrogenase reduces pyruvate to lactate; it does not interact with CO₂ at all. No CO₂ is formed in the first place.
Why C is wrong: C — The reason for anaerobic respiration in muscle IS oxygen deficit; oxygen does not inhibit CO₂ release — this reverses causality.
MCQ 8CalculationPractice
A sealed flask contains yeast in glucose solution under strictly anaerobic conditions. After complete fermentation of 1 mole of glucose, what is the total moles of CO₂ released?
Show answer and why every option is right or wrong
Answer: C. Alcoholic fermentation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. Glycolysis splits glucose into 2 pyruvates; each pyruvate is decarboxylated once (releasing 1 CO₂) then reduced to ethanol. Total = 2 moles CO₂ per mole glucose.
Why A is wrong: A — 6 moles of CO₂ is the yield from AEROBIC respiration (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O). Fermentation is incomplete oxidation.
Why B is wrong: B — 3 moles has no stoichiometric basis; each pyruvate releases exactly 1 CO₂, and there are 2 pyruvates per glucose.
Why D is wrong: D — 1 mole would imply only one pyruvate is decarboxylated; but glycolysis produces 2 pyruvates from each glucose, both of which are processed.
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Respiration Fermentation: quick recall before you leave
How do you solve a Respiration Fermentation question? A worked example
Pattern: Recall + direct application (RQ calculation from substrate equation, per P.BIO.U04.UNIT_BUNDLE pattern — similar-term-confusion distractor type)
- 1
Given
A germinating seed is respiring using tripalmitin (a fat) as substrate. The balanced equation for tripalmitin oxidation is:
2C₅₁H₉₈O₆ + 145O₂ → 102CO₂ + 98H₂O - 2
Required
Calculate the respiratory quotient (RQ) of tripalmitin.
- 3
Concept
RQ is defined as the ratio of CO₂ released to O₂ consumed during cellular respiration. It indicates the nature of the respiratory substrate.
- 4
Formula
RQ = Volume (or moles) of CO₂ evolved / Volume (or moles) of O₂ consumed
- 5
Substitution
RQ = 102 / 145
- 6
Calculation
RQ = 102 / 145 = 0.703 (rounded to three decimal places)
Note: The stoichiometric coefficients (2, 145, 102, 98) are exact integers from the balanced equation and do not limit significant figures. - 7
Final answer
RQ of tripalmitin ≈ 0.7
This confirms the benchmark: fats have RQ < 1 because their high hydrogen content demands proportionally more oxygen for complete combustion. - 8
Common trap
Students sometimes invert the ratio (O₂/CO₂ instead of CO₂/O₂), which would give ~1.42 — a value that looks like an organic acid RQ. Always remember: RQ has CO₂ in the numerator.
- 9
Similar NEET-style question
"A respiratory substrate gives RQ = 1.33 upon complete oxidation. Identify the most likely substrate category."
Answer: Organic acid (e.g., malic acid, succinic acid) — RQ > 1 indicates a substrate already partially oxidized, needing less external O₂.
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What to remember before solving Respiration Fermentation questions
More in Plant Physiology: 5 exam traps and mistakes · 1 question pattern from its other lessons.
Respiration Fermentation questions from past NEET papers
1 question from NEET 2022. Answers verified against NTA official keys.
NEET 2022
What amount of energy is released from glucose during lactic acid fermentation?
1Less than 7%
2Approximately 15%
3More than 18%
4About 10%
NTA Answer: Option 1(final)
Sources
NCERT refs: Class 11 Biology Chapter 12, p.157
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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