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State Board · Class 11 · Biology

Lesson 13: Respiration and Energy Transfer

7 concepts · 56 practice questions with answers · Free to read

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Basics of respiration and ATP formation

Link to Std 9–10

The previous lesson made sugar; here we see how that sugar is broken down to release energy as ATP.

From daily life

A phone runs on a charged battery; the cell's battery is ATP and respiration is its charging.

Simple definition

Respiration = oxidising complex organic substrates (mainly glucose) to release biological energy (ATP). Making ATP is called phosphorylation.

Respiration and ATPRespiration = catabolic; ATP formation = phosphorylationPhoto-phosph.Substrate-levelOxidativein light; chloroplastdirect P transfercytoplasm + matrixNADH + FADH₂inner mito. membraneATPH₂OADP + Pi← energy released (usable)Respiration: complex organic fuel oxidised to simple productsTypes: anaerobic (fermentation) and aerobicATP hydrolysis = energy for metabolismRespiratory substrate: mostly glucosePrimary act: removal of hydrogen (dehydrogenases)Hydrogen finally handed to O₂ → water

In the figure : Respiration and the three modes of ATP formation.

Rule 1 : Types of phosphorylation — Photophosphorylation: in light (chloroplast). Substrate-level: phosphate passed directly from a substrate to ADP; in cytoplasm and mitochondrial matrix. Oxidative: energy from oxidation of NADH+H⁺ and FADH₂; only on the inner mitochondrial membrane. When energy is needed, ATP is hydrolysed.

  • Easy : Substrate-level = direct; oxidative = via the chain.
  • Medium : Oxidative only on the inner membrane.
  • Exam-type (CET) : ATP hydrolysis releases energy.

Rule 2 : Types of respiration and hydrogen removal — Respiration is catabolic: anaerobic (fermentation; no O₂) and aerobic (with O₂). The primary act is removal of hydrogen (dehydrogenases); acceptors take it, and in aerobic respiration it finally goes to O₂ → water. So O₂ does not unite directly with the fuel.

  • Easy : Catabolic = breaking down.
  • Medium : O₂ accepts hydrogen.
  • Exam-type (CET) : Acceptors = NAD⁺, FAD.
Remember

P-S-O: Photo-, Substrate-level, Oxidative phosphorylation.

Common mistake and CET trap

‘O₂ unites directly with glucose’ is wrong. Trap: ‘oxidative in the cytoplasm’ is wrong.

Shortcut

Memory clue : "ATP = Always The Power".

Spot the difference

TypeSiteSource
Photo-ChloroplastLight
Substrate-levelCytoplasm, matrixSubstrate
OxidativeInner membraneNADH, FADH₂

Practice questions

Easy
  1. 1What is phosphorylation?
  2. 2Name its three types.
  3. 3Where does oxidative occur?
Medium
  1. 4Other name for anaerobic.
  2. 5Why is respiration catabolic?
  3. 6Which enzymes remove hydrogen?
Challenging
  1. 7Fate of hydrogen?
  2. 8Why hydrolyse ATP?
Show answers (try them yourself first)
  1. 1Formation of ATP.
  2. 2Photo-, substrate-level, oxidative.
  3. 3Inner mitochondrial membrane.
  4. 4Fermentation.
  5. 5It breaks down substrates.
  6. 6Dehydrogenases.
  7. 7Combines with O₂ to make water.
  8. 8To release energy.

Glycolysis

From daily life

Like breaking a big note into change, glucose splits into two small pyruvate molecules.

Simple definition

Glycolysis: in the cytoplasm, 10 steps convert 1 glucose (6C) → 2 pyruvic acid (3C); the first stage common to anaerobic and aerobic respiration.

GlycolysisGlycolysis (cytoplasm)Glucose (6C)Glucose-6-PFructose-1,6-bisP2 × PGAL (3C)2 × 1,3-BPG2 × Pyruvate (3C)−1 ATP−1 ATP+2 NADH+4 ATPsplit → 2 PGALTwo phasesPreparatorysteps 1–5; costs 2 ATPPay-offsteps 6–10; 4 ATP + 2 NADHNet: 2 ATP + 2 NADH + 2 pyruvateGlucose + 2 ATP → 2 pyruvate + 4 ATP + 2 NADHIn all organisms; cytoplasm; no O₂ neededATP by substrate-level phosphorylationEnzymes: hexokinase, PFK, pyruvate kinaseControl: ATP demand + hormones (insulin, glucagon)

In the figure : The two phases of glycolysis.

Rule 1 : Preparatory phase (steps 1–5) — Glucose is phosphorylated twice (costs 2 ATP) → fructose 1,6-bisphosphate. It splits into PGAL and DHAP (both 3C, isomers); DHAP converts to PGAL → total 2 PGAL. Enzymes: hexokinase, phosphofructokinase (PFK), aldolase.

  • Easy : Costs 2 ATP.
  • Medium : The 1,6-bisphosphate splits.
  • Exam-type (CET) : PGAL = glyceraldehyde-3-phosphate.

Rule 2 : Pay-off phase (steps 6–10) — 2 PGAL → 2 × 1,3-bisphosphoglycerate (oxidation + inorganic phosphate; 2 NADH) → … → 2 pyruvate. 4 ATP by substrate-level phosphorylation. Net: 2 ATP + 2 NADH + 2 pyruvate. Overall: glucose + 2 ATP + 2 Pi + 4 ADP + 2 NAD⁺ → 2 pyruvate + 2 ADP + 4 ATP + 2 NADH+H⁺ + 2 H₂O. Brain and red blood cells depend only on glycolysis.

  • Easy : 4 made, 2 spent; net 2.
  • Medium : 2 NADH formed.
  • Exam-type (CET) : Red blood cells = glycolysis only.
Remember

Spend 2, Make 4, Net 2 (S-M-N).

Common mistake and CET trap

‘Glycolysis in mitochondria’ is wrong (cytoplasm). Trap: ‘net 4 ATP’ is wrong (net 2).

Shortcut

Memory clue : "Glyco-lysis = sugar-splitting".

Spot the difference

PointPreparatoryPay-off
Steps1–56–10
ATP2 spent4 made
NADH—2

Practice questions

Easy
  1. 1Where does glycolysis occur?
  2. 2End product?
  3. 3How many steps?
Medium
  1. 4Net ATP?
  2. 5NADH formed?
  3. 6What are PGAL and DHAP?
Challenging
  1. 7How is ATP formed?
  2. 8How is it controlled?
Show answers (try them yourself first)
  1. 1Cytoplasm.
  2. 22 pyruvic acid.
  3. 310.
  4. 42.
  5. 52.
  6. 63C isomers.
  7. 7Substrate-level.
  8. 8ATP demand, enzymes, hormones.

Fermentation (anaerobic respiration)

From daily life

Sore muscles after a sprint and dough that rises – both come from fermentation.

Simple definition

Fermentation: without O₂, incomplete oxidation of pyruvate after glycolysis → lactic acid or ethanol + CO₂; only 2 ATP.

FermentationFermentationGlucoseGlycolysisPyruvateLactic acid (muscle)Pyruvate + NADH → lactate + NAD⁺No CO₂; fatigue (cramps)At rest back to pyruvateAlcoholic (yeast)Pyruvate → CH₃CHO + CO₂CH₃CHO + NADH → ethanolCO₂ escapes; dough risesNAD⁺ regenerated = glycolysis keeps goingAnaerobic respiration needs no atmospheric O₂Energy: only 2 ATP (incomplete oxidation)Lactic: human muscle; alcoholic: yeastIndustry: dairy, bakery, distillery, antibiotics

In the figure : Lactic and alcoholic fermentation.

Rule 1 : Lactic acid fermentation — In muscle NADH+H⁺ donates a proton and two electrons to pyruvate, regenerating NAD⁺; pyruvate → lactic acid. After vigorous exercise lactic acid builds up and muscles tire. At rest it returns to pyruvate and enters the aerobic path. White fibres (low myoglobin) use glycolysis for fast work.

  • Easy : Lactic: no CO₂.
  • Medium : Regenerated NAD⁺ keeps glycolysis going.
  • Exam-type (CET) : Red fibres: more myoglobin; endurance.

Rule 2 : Alcoholic fermentation — In yeast pyruvate → acetaldehyde + CO₂; acetaldehyde is reduced by NADH+H⁺ to ethanol. Too much ethanol kills yeast; with O₂ yeast can respire aerobically. Uses: bakery, alcohol, dairy, antibiotics, organic acids.

  • Easy : Alcoholic: CO₂ released.
  • Medium : Ethanol = reduced acetaldehyde.
  • Exam-type (CET) : Yeast respires both ways.
Remember

Muscle → Lactic; Yeast → Ethanol + CO₂.

Common mistake and CET trap

‘Fermentation yields 38 ATP’ is wrong (2). Trap: ‘CO₂ in lactic’ is wrong.

Shortcut

Memory clue : "Lactic = tired legs; alcoholic = airy bread".

Spot the difference

PointLacticAlcoholic
OrganismMuscleYeast
ProductLactic acidEthanol + CO₂
ATP22

Practice questions

Easy
  1. 1What is fermentation?
  2. 2Why does fatigue occur?
  3. 3Yeast products?
Medium
  1. 4Importance of NAD⁺?
  2. 5ATP from fermentation?
  3. 6Why does dough rise?
Challenging
  1. 7Why do white fibres tire?
  2. 8Two uses.
Show answers (try them yourself first)
  1. 1Respiration without O₂.
  2. 2Lactic acid accumulation.
  3. 3Ethanol + CO₂.
  4. 4Keeps glycolysis running.
  5. 52.
  6. 6Because of CO₂.
  7. 7Lactic acid builds up.
  8. 8Bakery, alcohol.

Link reaction and Krebs cycle

From daily life

A plant extracts energy step by step; the Krebs cycle does the same for acetyl groups.

Simple definition

Link reaction: pyruvate → acetyl CoA + CO₂ (PDH complex). Krebs (TCA / citric acid) cycle: stepwise oxidation of acetyl CoA in the mitochondrial matrix.

Link + KrebsLink reaction + Krebs cycle (matrix)Pyruvate 3CPDHAcetyl CoA 2CNADH + CO₂Oxaloacetate 4CCitrate 6Cα-KG 5CSuccinyl-CoA 4CSuccinate 4CMalate 4CKrebsTCA / citricPer turn: 3 NADH + 1 FADH₂ + 1 GTP/ATP + 2 CO₂Acetyl CoA (2C) + OAA (4C) → citrate (6C)Amphibolic: both catabolism and anabolismPDH needs thiamin (B1)Link step: mitochondria (eukaryotes)Fatty acids (β-oxidation) → acetyl CoA4 oxidation steps (dehydrogenases)Substrate-level: succinyl-CoA → succinate

In the figure : Link reaction and the Krebs cycle.

Live figure

Link reaction and Krebs cycle

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Rule 1 : Link reaction — Pyruvate (3C) entering mitochondria undergoes oxidative decarboxylation: CO₂ leaves; the remaining 2C group joins coenzyme A as acetyl CoA; NAD⁺ → NADH+H⁺. The PDH complex needs thiamin (vitamin B1); deficiency causes lactic acidosis. β-oxidation of fatty acids also gives acetyl CoA.

  • Easy : Pyruvate 3C → acetyl 2C + CO₂.
  • Medium : Link = bridge between glycolysis and Krebs.
  • Exam-type (CET) : No B1, no PDH.

Rule 2 : Krebs cycle — Acetyl CoA (2C) + oxaloacetate (4C) → citrate (6C); stepwise oxidation releases CO₂ and regenerates oxaloacetate. Four oxidation steps (dehydrogenases; NAD⁺ or FAD). Each turn: 3 NADH + 1 FADH₂ + 1 GTP/ATP (substrate-level) + 2 CO₂. The cycle is amphibolic: catabolism and anabolism (intermediates for fatty acids, glutamate, aspartate).

  • Easy : 6C → 5C → 4C; 2 CO₂ per turn.
  • Medium : Amphibolic = both ways.
  • Exam-type (CET) : 1 glucose = 2 turns.
Remember

Krebs per turn: 3-1-1-2 (NADH-FADH₂-ATP-CO₂).

Common mistake and CET trap

‘Krebs in the cytoplasm’ is wrong (matrix). Trap: ‘Krebs is only catabolic’ is wrong.

Shortcut

Memory clue : "Citrate 6, ketoglutarate 5, succinate 4: 6-5-4".

Spot the difference

PointLinkKrebs
SiteMitochondriaMatrix
CO₂1 per pyruvate2 / turn
ProductAcetyl CoA + NADHNADH, FADH₂, ATP

Practice questions

Easy
  1. 1Link reaction products?
  2. 2Vitamin needed by PDH?
  3. 3How does Krebs start?
Medium
  1. 4Site of Krebs?
  2. 5Products per turn?
  3. 6What is amphibolic?
Challenging
  1. 7Substrate-level ATP in Krebs?
  2. 8How do fats enter Krebs?
Show answers (try them yourself first)
  1. 1Acetyl CoA, CO₂, NADH.
  2. 2Thiamin (B1).
  3. 3Acetyl CoA + oxaloacetate → citrate.
  4. 4Mitochondrial matrix.
  5. 53 NADH, 1 FADH₂, 1 ATP, 2 CO₂.
  6. 6Catabolic + anabolic.
  7. 7Succinyl-CoA → succinate.
  8. 8Via acetyl CoA.

Electron transport system

From daily life

Like a ball bouncing down steps, electrons release energy as they pass down a chain of carriers.

Simple definition

ETS: complexes I–V on the inner mitochondrial membrane; NADH and FADH₂ are oxidised; O₂ is the final acceptor; ATP by chemiosmosis.

ETSElectron transport system (inner membrane)Inner mitochondrial membraneOuter chamber (high H⁺)MatrixIIICoQIIICyt cIVe⁻VF₀F₁ADP+Pi → ATP½O₂ + 2H⁺ → H₂ONADHFADH₂NADH → complex I → CoQ; FADH₂ → complex II → CoQCoQ → III → cytochrome c → IV (cyt a + a₃) → O₂IV = terminal oxidation; O₂ final acceptorH⁺ pumped out → back via F₀ → F₁ makes ATPChemiosmosis (Peter Mitchell) = oxidative phosph.1 NADH = 3 ATP; 1 FADH₂ = 2 ATP

In the figure : Electron transport chain and ATP synthase.

Rule 1 : Path of electrons — NADH+H⁺ → complex I (NADH dehydrogenase) → ubiquinone (CoQ). FADH₂ → complex II (succinate dehydrogenase) → CoQ. Ubiquinol → complex III (cyt bc₁) → cytochrome c (mobile carrier) → complex IV (cyt a, a₃) → O₂ + H⁺ → water (terminal oxidation). Protons are released into the outer chamber.

  • Easy : NADH: I; FADH₂: II.
  • Medium : Cyt c = mobile carrier.
  • Exam-type (CET) : O₂ = final acceptor.

Rule 2 : ATP formation and significance — H⁺ from the outer chamber enter through F₀ of complex V (F₀F₁); F₁ makes ATP from ADP + Pi = oxidative phosphorylation (chemiosmosis, Peter Mitchell). 1 NADH → 3 ATP; 1 FADH₂ → 2 ATP. ETS gives 34 of 38 ATP, regenerates NAD⁺/FAD, forms water and releases energy gradually.

  • Easy : F₀ = channel; F₁ = ATP maker.
  • Medium : NADH 3; FADH₂ 2 ATP.
  • Exam-type (CET) : CO poisoning blocks the cytochromes.
Remember

I-II → CoQ → III → cyt c → IV → O₂.

Common mistake and CET trap

‘ETS in the cytoplasm’ is wrong. Trap: ‘FADH₂ = 3 ATP’ is wrong (2).

Shortcut

Memory clue : "NADH 3, FADH₂ 2: N3-F2".

Spot the difference

ComplexRole
INADH → CoQ
IIFADH₂ → CoQ
III / IVcyt c → O₂
VATP synthase

Practice questions

Easy
  1. 1Site of ETS?
  2. 2Final acceptor?
  3. 3What is complex IV?
Medium
  1. 4Role of cyt c?
  2. 5Who proposed chemiosmosis?
  3. 6ATP per NADH?
Challenging
  1. 7ATP per FADH₂?
  2. 8Two significances of ETS.
Show answers (try them yourself first)
  1. 1Inner mitochondrial membrane.
  2. 2O₂.
  3. 3Cytochrome c oxidase.
  4. 4Mobile carrier between III and IV.
  5. 5Peter Mitchell.
  6. 63.
  7. 72.
  8. 834 ATP; regenerates NAD⁺/FAD.

ATP balance sheet and stepwise oxidation

From daily life

Spending income gradually wastes less – and so does gradual release of energy.

Simple definition

As per the textbook, aerobic oxidation of 1 glucose gives 38 ATP (net); anaerobic only 2. Stepwise oxidation captures more energy as ATP.

ATP balanceATP balance: 1 glucose (as per textbook)StepSubstrateNADHFADH₂ATPGlycolysis+4 −2 = 22 × 3 = 6—8Pyr → AcCoA—2 × 3 = 6—6Krebs2 × 1 = 26 × 3 = 182 × 2 = 424Total38Of 38 ATP, 34 come from the ETSAnaerobic: only 2 ATP; aerobic: 38 ATPStepwise release: more of the energy captured as ATPEach step can be controlled by enzymesIntermediates feed other biomolecule synthesisActual ATP yield varies with conditions

In the figure : ATP balance table.

Rule 1 : Counting 38 ATP — Glycolysis: 4 − 2 = 2 ATP + 2 NADH (6 ATP) = 8. Pyruvate → acetyl CoA: 2 NADH = 6. Krebs: 2 ATP + 6 NADH × 3 = 18 + 2 FADH₂ × 2 = 4 → 24. Total 8 + 6 + 24 = 38. (Actual numbers vary with conditions.)

  • Easy : 8 + 6 + 24 = 38.
  • Medium : 34 from ETS; 4 substrate-level.
  • Exam-type (CET) : Anaerobic: 2; aerobic: 38.

Rule 2 : Benefits of stepwise oxidation — 1. Gradual energy release → more of it is captured as ATP. 2. Enzymes at each step can be switched up or down → rate and energy output follow need. 3. Intermediates can feed synthesis of amino acids and other biomolecules. 4. Avoids damaging the cell.

  • Easy : Gradual release = less waste.
  • Medium : Control at each step.
  • Exam-type (CET) : Intermediates = building blocks.
Remember

8-6-24 = 38 (glyco-link-Krebs).

Common mistake and CET trap

‘All ATP from Krebs’ is wrong. Trap: ‘glycolysis net 4’ is wrong.

Shortcut

Memory clue : "8-6-24 = 38".

Spot the difference

StageATP
Glycolysis8
Link6
Krebs24
Total38

Practice questions

Easy
  1. 1Aerobic ATP (textbook)?
  2. 2Anaerobic ATP?
  3. 3ATP from glycolysis?
Medium
  1. 4ATP from Krebs?
  2. 5ETS contribution?
  3. 6Benefit of stepwise.
Challenging
  1. 7NADH in link step?
  2. 8NADH from Krebs?
Show answers (try them yourself first)
  1. 138.
  2. 22.
  3. 38 (2 + 6).
  4. 424.
  5. 534.
  6. 6More ATP; control.
  7. 72 → 6 ATP.
  8. 86 → 18 ATP.

Respiratory quotient, experiments and significance

From daily life

The ratio of CO₂ out to O₂ in tells which fuel the body is burning.

Simple definition

RQ = volume of CO₂ released ÷ volume of O₂ consumed. Carbohydrate = 1; fats ≈ 0.7; proteins ≈ 0.9; anaerobic = ∞.

RQ and experimentsRQ = CO₂ / O₂1.0Carb.0.9Protein0.7FatRQ = 1RQC₆H₁₂O₆ + 6O₂→ 6CO₂ + 6H₂O6 CO₂ / 6 O₂ = 1Fat/protein: RQ < 1Anaerobic: RQ = ∞(CO₂ out, no O₂ in)Test: yeast + glucose + oil layer → CO₂ (lime water milky), alcohol smellGerminating seeds (mercury): KOH absorbs CO₂ → mercury risesSignificance: energy, growth, repair, movementBalances CO₂/O₂ with photosynthesisPhotosynthesis and respiration are roughly oppositeKrebs intermediates make fatty and amino acids

In the figure : RQ and simple experiments.

Rule 1 : Respiratory quotient — Glucose: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O; RQ = 6 ÷ 6 = 1. Fats and proteins need more O₂, so RQ < 1 (fat ≈ 0.7, protein ≈ 0.9). In anaerobic respiration CO₂ is released with no O₂ used → RQ infinite.

  • Easy : RQ = CO₂ ÷ O₂.
  • Medium : Fat: low, needs more O₂.
  • Exam-type (CET) : RQ 1 = carbohydrate.

Rule 2 : Experiments and significance — Yeast: 10% glucose + yeast, oil layer on top, warm place → lime water turns milky (CO₂), alcohol smell after two days = anaerobic. Germinating seeds (over mercury): gas collects; KOH absorbs it → anaerobic CO₂. Significance: energy (synthesis, growth, repair, movement); intermediates for biomolecules; CO₂–O₂ balance in air; industries.

  • Easy : Milky lime water = CO₂.
  • Medium : Oil layer = shuts out air.
  • Exam-type (CET) : Respiration ↔ photosynthesis roughly opposite.
Remember

RQ: carb 1, protein 0.9, fat 0.7 (CPF = 1-0.9-0.7).

Common mistake and CET trap

‘Fat RQ = 1’ is wrong (0.7). Trap: ‘anaerobic RQ = 0’ is wrong (infinite).

Shortcut

Memory clue : "C-P-F: 1 - 0.9 - 0.7".

Spot the difference

FuelRQ
Carbohydrate1
Protein≈ 0.9
Fat≈ 0.7
Anaerobic∞

Practice questions

Easy
  1. 1Define RQ.
  2. 2RQ of glucose?
  3. 3RQ of fat?
Medium
  1. 4RQ of protein?
  2. 5RQ in anaerobic?
  3. 6Why does lime water turn milky?
Challenging
  1. 7Role of KOH?
  2. 8Two significances.
Show answers (try them yourself first)
  1. 1CO₂ volume ÷ O₂ volume.
  2. 21.
  3. 3≈ 0.7.
  4. 4≈ 0.9.
  5. 5Infinite.
  6. 6Because of CO₂.
  7. 7Absorbs CO₂.
  8. 8Energy; CO₂–O₂ balance.

Quick revision of the whole lesson

ConceptKey fact
PhosphorylationPhoto, substrate-level, oxidative
GlycolysisCytoplasm; net 2 ATP + 2 NADH
FermentationLactic / ethanol + CO₂; 2 ATP
KrebsMatrix; 3 NADH + 1 FADH₂ + 1 ATP
ETSInner membrane; NADH 3, FADH₂ 2
Balance8 + 6 + 24 = 38
RQCarb 1; fat 0.7; protein 0.9

🎯 Most important points for the exam

  • Types of phosphorylation.
  • Glycolysis steps and ATP.
  • Lactic and alcoholic fermentation.
  • Krebs cycle and amphibolic role.
  • ETS and chemiosmosis.
  • ATP balance sheet.
  • RQ values.
Study mantra

Understand the concept → draw the figure → memorise the key points → solve questions yourself.