Respiration Fermentation

8 MCQs5 revision cards9-step worked example
Source: NCERT Animal KingdomPYQ coverage: NEET 2020, 2021, 2022, 2023, 2024, 2025Official key: NTA-verifiedLast reviewed: May 2026

Lesson

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 14, page 275: 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.

Practice 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:

MCQ 2Easy RecallPractice

Which end-product is common to BOTH alcoholic fermentation and lactic acid fermentation?

MCQ 3Easy RecallPractice

The respiratory quotient (RQ) of a carbohydrate substrate like glucose is:

MCQ 4Direct ApplicationPractice

An organism is respiring with RQ = 0.7. The most likely respiratory substrate is:

MCQ 5Direct ApplicationPractice

During alcoholic fermentation in yeast, pyruvate is first converted to acetaldehyde by the enzyme:

MCQ 6Direct ApplicationPractice

A tissue sample shows RQ > 1. Which substrate is most likely being oxidized?

MCQ 7Concept TrapPractice

During vigorous exercise, human skeletal muscle accumulates lactic acid. Why does this NOT release CO₂ unlike alcoholic fermentation?

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?

Quick recall before you leave

Worked Example

Pattern: Recall + direct application (RQ calculation from substrate equation, per P.BIO.U04.UNIT_BUNDLE pattern — similar-term-confusion distractor type)

  1. 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. 2

    Required

    Calculate the respiratory quotient (RQ) of tripalmitin.

  3. 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. 4

    Formula

    RQ = Volume (or moles) of CO₂ evolved / Volume (or moles) of O₂ consumed

  5. 5

    Substitution

    RQ = 102 / 145

  6. 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. 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. 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. 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. ---

Before solving, remember these

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, Ch. 14, p. 275

Exam Traps & Common Mistakes

These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.

Trap

C3 vs C4 first-product carbon count swapped

Category: Similar Terms

C3: 3-PGA (3-carbon) is first stable product. C4: oxaloacetate (4-carbon) is first stable product.

When it triggers

Question asks first stable product of CO2 fixation in C3/C4 plant.

How to avoid

Pathway name = carbons in first product. C3 → 3C; C4 → 4C.

Trap

PS I vs PS II event order reversed

Category: Negative Marking

Light reactions: water splits at PS II (P680) FIRST, electrons flow PS II → PS I (P700). PS II numbered AFTER PS I in discovery, but reaction order is PS II → PS I.

When it triggers

Question on Z-scheme order, water photolysis, electron source.

How to avoid

Discovery order ≠ reaction order. Photolysis at PS II; PS II reduces PS I.

Mistake

Places glycolysis in mitochondria.

Root cause: concept gap

Correction

Glycolysis is in CYTOPLASM (cytosol). Krebs cycle in mitochondrial matrix; ETC in inner membrane.

Past Year Questions

38 questions from NEET 2020, 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.

NEET 2025

Given below are two statements : Statement I : Fig fruit is a non-vegetarian fruit as it has enclosed fig wasps in it. Statement II : Fig wasp and fig tree exhibit mutual relationship as fig wasp completes its life cycle in fig fruit and fig fruit gets pollinated by fig wasp. In the light of the above statements, choose the most appropriate answer from the options given below :

1Statement I is incorrect but statement II is correct
2Both statement I and statement II are correct
3Both statement I and statement II are incorrect
4Statement I is correct but statement II is incorrect
NTA Answer: Option 3(final)
NEET 2025

Which are correct: A. Computed tomography and magnetic resonance imaging detect cancers of internal organs. B. Chemotherapeutics drugs are used to kill non-cancerous cells. C. -interferon activate the cancer patients’ immune system and helps in destroying the tumour. D. Chemotherapeutic drugs are biological response modifiers. E. In the case of leukaemia blood cell counts are decreased. Choose the correct answer from the options given below:

1A and C only
2B and D only
3D and E only
4C and D only
NTA Answer: Option 1(final)
NEET 2024

Given below are two statements: Statement I: In C plants, some O binds to RuBisCO, hence CO fixation is decreased. 3 2 2 Statement II: In C plants, mesophyll cells show very little photorespiration while bundle sheath cells do not 4 show photorespiration. In the light of the above statements, choose the correct answer from the options given below:

1Both Statement I and Statement II are true
2Both Statement I and Statement II are false
3Statement I is true but Statement II is false
4Statement I is false but Statement II is true
NTA Answer: Option 3(final)
NEET 2023

Given below are two statements : One is labelled as Assertion A and the other is labelled as Reason R : Assertion A : ATP is used at two steps in glycolysis. Reason R : First ATP is used in converting glucose into glucose-6-phosphate and second ATP is used in conversion of fructose-6-phosphate into fructose-1, 6-diphosphate. In the light of the above statements, choose the correct answer from the options given below :

1Both A and R are true but R is NOT the correct explanation of A.
2A is true but R is false.
3A is false but R is true.
4Both A and R are true and R is the correct explanation of A.
NTA Answer: Option 4(final)
NEET 2021

Which of the following statements is correct ? cambial ring

1Some of the organisms can fix atmospheric Answer (4) nitrogen in specialized cells called sheath cells 138. Plasmid pBR322 has Pstl restriction enzyme site within gene ampR that confers ampicillin resistance.
2Fusion of two cells is called Karyogamy If this enzyme is used for inserting a gene for β-galactoside production and the recombinant
3Fusion of protoplasms between two motile on non-motile gametes is called plasmogamy plasmid is inserted in an E.coli strain (1) It will be able to produce a novel protein with
4Organisms that depend on living plants are dual ability called saprophytes (2) It will not be able to confer ampicillin resistance
NTA Answer: Option 3(final)
NEET 2020

•ÁŸflÊÿ ¸ÃàflÊ •Ê⁄Ò U ¬ÊŒ¬Ê ◊ ©Ÿ∑§ ∑§ÊÿÊ Z ∑§ Áfl ÿ ◊ ÁŸêŸÁ‹Áπà 102. Match the following concerning essential elements and their functions in plants : ∑§Ê ‚È◊ Á‹Ã ∑§ËÁ¡∞ — (a) Iron (i) Photolysis of water (a) ‹Ê „ (i) ¡‹ ∑§Ê ¬˝∑§Ê‡Ê •¬ÉÊ≈UŸ (b) Zinc (ii) Pollen germination (b) Á¡∑ § (ii) ¬⁄Uʪ ∑§Ê • ∑ȧ⁄UáÊ (c) Boron (iii) Required for chlorophyll (c) ’Ê⁄ UÊŸÚ (iii) Ä‹Ê ⁄UÊ Á»§‹ ∑ § ¡Òfl ‚ ‡‹ áÊ biosynthesis ∑ § Á‹∞ •Êfl‡ÿ∑§ (d) Manganese (iv) IAA biosynthesis (d) ◊Ò ªŸË¡ (iv) •Ê߸.∞.∞. ¡Òfl ‚ ‡‹ áÊ Select the correct option : ‚„Ë Áfl∑§À¬ øÈÁŸ∞ — (a) (b) (c) (d) (a) (b) (c) (d)

1(iv) (i) (ii) (iii) (1) (iv) (i) (ii) (iii)
2(ii) (i) (iv) (iii) (2) (ii) (i) (iv) (iii)
3(iv) (iii) (ii) (i) (3) (iv) (iii) (ii) (i)
4(iii) (iv) (ii) (i) (4) (iii) (iv) (ii) (i)
NTA Answer: Option 4(final)
NEET 2020

⁄UÊÁòÊ ◊ ÿÊ ¬Íáʸ ¬˝Ê×∑§Ê‹ ◊ ÉÊÊ‚ ∑§Ë ¬ÁûÊÿÊ ∑ § ‡ÊË ¸ ‚ ¡‹ ∑ § 134. The process responsible for facilitating loss of water Œ˝fl •flSÕÊ ◊ ÁŸ∑§‹Ÿ ∑§Ê ‚Ȫ◊ ’ŸÊŸ ◊ ∑§ÊÒŸ ‚Ë ¬˝Á∑˝§ÿÊ in liquid form from the tip of grass blades at night ©ûÊ⁄UŒÊÿË „Ê ÃË „Ò? and in early morning is :

1¡ËflŒ˝√ÿ∑È §øŸ (1) Plasmolysis
2flÊc¬Êà ‚¡Ÿ¸ (2) Transpiration
3◊Í‹Ëÿ ŒÊ’ (3) Root pressure
4•Ã —‡ÊÊ áÊ (4) Imbibition
NTA Answer: Option 3(final)
NEET 2020

ÁŸêŸÁ‹Áπà ◊ ‚ ∑§ÊÒŸ ∞∑§ ’Ë¡ ¬˝‚ÈÁåà ÁŸÿ ÁòÊà ∑§⁄UŸ flÊ‹Ê 135. Which of the following is not an inhibitory ÁŸ⁄UÊ œ∑§ ¬ŒÊÕ¸ Ÿ„Ë „Ò? substance governing seed dormancy ?

1¬Ò⁄UÊ-∞ S∑§ÊÚÁ’¸∑§ •ê‹ (1) Para-ascorbic acid
2Á¡’⁄ UÁ‹∑§ •ê‹ (2) Gibberellic acid
3∞é‚ËÁ‚∑§ •ê‹ (3) Abscisic acid
4Á»§ŸÊ Á‹∑§ •ê‹ (4) Phenolic acid Hindi+English 31 H3
NTA Answer: Option 2(final)

How NEET usually asks this

Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.

Mineral nutrition, photosynthesis, respiration, growth, hormones, photoperiodism

RecallMedium

Common distractors

photosynthesis pathway product confusion

C3 and C4 pathways share the Calvin cycle; students who cannot anchor the first stable product in each pathway invert the carbon counts: 3-PGA (3-carbon) is the C3 first stable product, OAA (4-carbon) is the C4 first stable product. RuBisCO has dual carboxylase and oxygenase activity; students who recall only one role select incorrect statements about its function, including denying that it causes photorespiration.

ps i ps ii assignment reversal

PS I and PS II are numbered in order of discovery, not order of action in the Z-scheme. PS II acts first (water photolysis, O2 release, P680 reaction centre at 680 nm); PS I acts second (P700 reaction centre at 700 nm, NADPH production). Students who use Roman numeral order assign water photolysis to PS I and attribute the 700 nm absorption maximum to PS II.

respiration compartment swap

Glycolysis feeds the mitochondrial Krebs cycle, creating a mental link: students place glycolysis in the mitochondrial matrix. The anchor -- glycolysis occurs in the cytosol because it predates mitochondria and operates in anaerobes -- is lost under time pressure.

rubisco function confusion

RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) has dual activity: carboxylase drives CO2 fixation in the Calvin cycle; oxygenase drives photorespiration. Questions give multiple statements about RuBisCO and ask which set is correct; students who have not anchored the dual-function name omit or deny the oxygenase role, selecting option sets that describe it as carboxylase only.

assertion reason unchecked reason

A/R questions in plant physiology present a true Assertion (e.g., ATP is used at two steps in glycolysis) alongside a Reason that sounds mechanistically plausible but may be factually imprecise or not the correct explanation. Students accept the Reason without independently checking whether its specific biochemical claim is accurate.

Sources

NCERT refs: Class 11 Biology Chapter 14, p.275

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