I. INTRODUCTION & KEY CONCEPTS
A BASICS
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| FEATURE | DETAIL |
|---|---|
| Respiration | Breaking of C-C bonds of complex compounds through oxidation within cells → release of considerable amount of energy ⭐⭐ |
| Respiratory substrates | Compounds oxidised; usually carbohydrates, but also proteins, fats, organic acids under certain conditions ⭐ |
| Energy release pattern | In a series of slow step-wise reactions controlled by enzymes (NOT in a single step) ⭐⭐ |
| ATP | Energy currency of the cell ⭐⭐ |
| Why step-wise? | Not all liberated energy goes as heat; enables coupling of energy to ATP synthesis ⭐ |
| Glucose | Favoured substrate for respiration; all carbohydrates usually first converted to glucose ⭐ |
| Carbon skeleton | Produced during respiration → used as precursors for biosynthesis of other molecules ⭐ |
| Location in eukaryotes | Breakdown begins in cytoplasm (glycolysis) → continues in mitochondria (aerobic respiration) ⭐⭐ |
B COMBUSTION EQUATION
🔑 EQUATION: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy
II. DO PLANTS BREATHE?
A GAS EXCHANGE IN PLANTS
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| FEATURE | DETAIL |
|---|---|
| Specialised respiratory organs | ABSENT in plants (unlike animals) ⭐⭐ |
| Gas exchange structures | Stomata and lenticels (exchange by diffusion) ⭐ |
| Plants require | O₂ for respiration; release CO₂ ⭐ |
B THREE REASONS FOR ABSENCE OF SPECIALISED RESPIRATORY ORGANS
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| REASON | DETAIL |
|---|---|
| 1 | Each plant part takes care of its own gas-exchange needs; very little transport of gases from one part to another ⭐ |
| 2 | Plants respire at rates far lower than animals ⭐ |
| 3 | Most living cells located quite close to the surface of the plant ⭐ |
C ADDITIONAL ANATOMICAL FACTS & D. EARLY ATMOSPHERE
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| FEATURE | DETAIL |
|---|---|
| Stems | Living cells organised in thin layers inside and beneath the bark ⭐ |
| Interior cells of stems | Dead — provide only mechanical support ⭐ |
| Lenticels | Openings in stems for gas exchange ⭐ |
| Air spaces | Loose packing of parenchyma cells → interconnected network of air spaces ⭐ |
| During photosynthesis | O₂ availability not a problem — O₂ released within the cell ⭐ |
| First cells on Earth | Lived in atmosphere that lacked oxygen ⭐ |
| All living organisms retain | Enzymatic machinery to partially oxidise glucose without O₂ ⭐ |
| Anaerobes | Facultative (with/without O₂) or Obligate (require anaerobic conditions) ⭐ |
III. GLYCOLYSIS (EMP PATHWAY)
A OVERVIEW
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| FEATURE | DETAIL |
|---|---|
| Meaning | Greek: glycos = sugar; lysis = splitting ⭐ |
| Also called | EMP pathway (Embden, Meyerhof, Parnas) ⭐⭐ |
| Location | Cytoplasm (cytosol) of ALL living organisms ⭐⭐⭐ |
| Pathway type | Common to both aerobic and anaerobic respiration ⭐⭐ |
| In anaerobic organisms | Only process in respiration ⭐ |
| Process | Glucose undergoes partial oxidation → 2 molecules of pyruvic acid ⭐⭐ |
| Number of reactions | Chain of 10 enzyme-catalysed reactions ⭐ |
| O₂ requirement | Neither use of O₂ nor release of CO₂ ⭐⭐ |
⚡ EXAM TRAP
NEET 2022, 2024, RE-NEET 2026 (reinforced): Glycolysis occurs in cytoplasm; one glucose yields 2 molecules of pyruvic acid in cytoplasm; net ATP = 2.
B SOURCE OF GLUCOSE IN PLANTS
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| FEATURE | DETAIL |
|---|---|
| Derived from | Sucrose (end product of photosynthesis) or storage carbohydrates ⭐ |
| Sucrose → | Glucose + Fructose (by enzyme invertase) ⭐⭐ |
| Phosphorylation | Glucose/Fructose → Glucose-6-phosphate (by enzyme hexokinase) ⭐⭐ |
| Isomerisation | Glucose-6-phosphate → Fructose-6-phosphate ⭐ |
C KEY STEPS OF GLYCOLYSIS
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| STEP | REACTION | KEY EVENT |
|---|---|---|
| 1 | Glucose → Glucose-6-phosphate | ATP consumed (hexokinase) ⭐⭐ |
| 2 | G6P → Fructose-6-phosphate | Isomerisation ⭐ |
| 3 | F6P → Fructose-1,6-bisphosphate | ATP consumed ⭐⭐ |
| 4 | F1,6BP → DHAP (3C) + PGAL (3C) | Splitting into two trioses ⭐ |
| 5 | PGAL → 1,3-BPGA | NAD⁺ → NADH + H⁺; inorganic Pi added ⭐⭐ |
| 6 | BPGA → 3-PGA | ATP synthesised ⭐ |
| 7 | ... → PEP | — |
| 8 | PEP → Pyruvic acid | ATP synthesised ⭐ |
🔑 KEY: ATP used at TWO steps (Steps 1 and 3) — Glucose → G6P AND F6P → F1,6BP
⚡ EXAM TRAP
NEET 2018, 2019, 2023: ATP utilised at two steps in glycolysis — glucose → G6P AND F6P → F1,6BP. Glycolysis operates as long as NAD⁺ is supplied.
D ATP BALANCE OF GLYCOLYSIS
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| PARAMETER | VALUE |
|---|---|
| ATP consumed | 2 ATP ⭐ |
| ATP produced (by SLP) | 4 ATP ⭐ |
| Net ATP gain (by SLP) | 2 ATP ⭐⭐⭐ |
| NADH + H⁺ produced | 2 ⭐⭐ |
| Total ATP gain (incl. NADH via ETS) | 8 ATP (2 SLP + 6 from 2 NADH) ⭐⭐ |
⚡ EXAM TRAP
NEET 2022: Net ATP gain from glycolysis = 2 (by SLP).
E KEY PRODUCT & FATE
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| FEATURE | DETAIL |
|---|---|
| Key product | Pyruvic acid (pyruvate) ⭐⭐ |
| Fate depends on | Cellular need, type of organism, and O₂ availability ⭐⭐ |
| Three major fates | (1) Lactic acid fermentation; (2) Alcoholic fermentation; (3) Aerobic respiration ⭐⭐ |
⚡ EXAM TRAP
NEET 2024: Fate of pyruvate depends on organism type and oxygen availability.
IV. FERMENTATION
A OVERVIEW
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| FEATURE | DETAIL |
|---|---|
| Condition | Anaerobic (without O₂) ⭐⭐ |
| Occurs in | Many prokaryotes, unicellular eukaryotes, and germinating seeds ⭐⭐ |
| Type of oxidation | Incomplete oxidation of glucose ⭐⭐ |
| Energy released | Less than 7% of total energy in glucose ⭐⭐ |
| Net ATP gain | 2 ATP (same as glycolysis — no additional ATP) ⭐⭐ |
| Products | Hazardous — either acid or alcohol ⭐ |
| Reducing agent | NADH + H⁺ (reoxidised to NAD⁺ in both types) ⭐ |
⚡ EXAM TRAP
NEET 2022, 2024: Energy released in fermentation < 7% of glucose energy. Fermentation occurs in germinating seeds under anaerobic conditions.
B TYPES OF FERMENTATION
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| FEATURE | ALCOHOLIC FERMENTATION | LACTIC ACID FERMENTATION |
|---|---|---|
| Organism | Yeast ⭐⭐ | Some bacteria; animal muscle cells (during exercise, when O₂ inadequate) ⭐⭐ |
| Enzymes | Pyruvate decarboxylase + Alcohol dehydrogenase ⭐⭐ | Lactate dehydrogenase ⭐⭐ |
| Products | Ethanol + CO₂ ⭐⭐ | Lactic acid ⭐⭐ |
| CO₂ released? | YES ⭐ | NO ⭐⭐⭐ |
🔑 REACTIONS: Alcoholic: Pyruvic acid → Ethanol + CO₂ (WITH CO₂) | Lactic acid: Pyruvic acid → Lactic acid (WITHOUT CO₂)
⚡ EXAM TRAP
NEET 2014: Lactic acid fermentation — NO CO₂ released!
C YEAST — SPECIAL FACT
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| FEATURE | DETAIL |
|---|---|
| Yeast death | Yeast poison themselves to death when alcohol concentration reaches about 13% ⭐⭐ |
| Maximum natural alcohol | ~13% in naturally fermented beverages ⭐ |
| Higher concentrations? | Obtained by distillation ⭐ |
V. OXIDATIVE DECARBOXYLATION (LINK REACTION)
A OVERVIEW
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| FEATURE | DETAIL |
|---|---|
| Pyruvate transport | From cytoplasm into mitochondria ⭐ |
| Location | Mitochondrial matrix ⭐⭐ |
| Process | Oxidative decarboxylation of pyruvate ⭐⭐ |
| Enzyme | Pyruvate dehydrogenase ⭐⭐ |
| Cofactors required | NAD⁺, Coenzyme A, Mg²⁺ ⭐⭐ |
| Also called | Link reaction (links glycolysis to TCA cycle) ⭐ |
⚡ EXAM TRAP
NEET 2020: Pyruvate dehydrogenase requires Mg²⁺, NAD⁺, CoA.
B REACTION
🔑 REACTION: Pyruvic acid + CoA + NAD⁺ → Acetyl CoA + CO₂ + NADH + H⁺ (Pyruvate dehydrogenase; Mg²⁺)
C PRODUCTS PER GLUCOSE (2 PYRUVATES)
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| PRODUCT | AMOUNT |
|---|---|
| Acetyl CoA | 2 ⭐ |
| CO₂ | 2 ⭐ |
| NADH + H⁺ | 2 ⭐⭐ |
D TWO CRUCIAL EVENTS OF AEROBIC RESPIRATION
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| EVENT | DETAIL | LOCATION |
|---|---|---|
| 1 | Complete oxidation of pyruvate by stepwise removal of all H atoms → 3 CO₂ ⭐ | Matrix of mitochondria ⭐ |
| 2 | Passing electrons to molecular O₂ with simultaneous ATP synthesis ⭐ | Inner membrane of mitochondria ⭐ |
VI. TCA CYCLE (KREBS' CYCLE)
A OVERVIEW
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| FEATURE | DETAIL |
|---|---|
| Full name | Tricarboxylic Acid (TCA) cycle ⭐⭐ |
| Also called | Krebs' cycle (after Hans Krebs) / Citric acid cycle ⭐ |
| Location | Matrix of mitochondria ⭐⭐⭐ |
| Starting reaction | Condensation of acetyl group (from Acetyl CoA) with OAA + H₂O → Citric acid ⭐⭐ |
| Starting enzyme | Citrate synthase (CoA released) ⭐ |
| Number of decarboxylations | TWO (2) per turn ⭐⭐ |
⚡ EXAM TRAP
NEET 2017, 2023, 2024, 2026 (reinforced): TCA cycle occurs in matrix of mitochondria; starts with condensation of acetyl group with OAA → citric acid (NOT pyruvic acid — TRAP!). Two decarboxylations per turn.
B KEY STEPS OF TCA CYCLE
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| STEP | REACTION | C | KEY EVENT |
|---|---|---|---|
| 1 | Acetyl CoA + OAA + H₂O → Citric acid | 2C+4C →6C | Condensation; CoA released ⭐⭐ |
| 2 | Citric acid → Isocitrate | 6C→6C | Isomerisation ⭐ |
| 3 | Isocitrate → α-Ketoglutaric acid + CO₂ | 6C→5C | 1st decarboxylation; NAD⁺ → NADH + H⁺ ⭐⭐ |
| 4 | α-KG → Succinyl-CoA + CO₂ | 5C→4C | 2nd decarboxylation; NAD⁺ → NADH + H⁺ ⭐⭐ |
| 5 | Succinyl-CoA → Succinic acid | 4C→4C | GTP synthesised (SLP); NO oxidation ⭐⭐⭐ |
| 6 | Succinic acid → Malic acid | 4C→4C | FAD → FADH₂ ⭐⭐ |
| 7 | Malic acid → OAA | 4C→4C | NAD⁺ → NADH + H⁺ ⭐⭐ |
⚡ EXAM TRAP
NEET 2017, 2020, 2022, 2024: α-Ketoglutaric acid = ONLY 5C compound in TCA cycle! SLP → Succinyl-CoA → Succinic acid (GTP synthesised; NO oxidation of substrate). NAD⁺ reduced at 3 points; FAD reduced at 1 point.
C PRODUCTS PER TURN OF TCA CYCLE & D. CARBON COMPOUNDS
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| PRODUCT | AMOUNT PER TURN |
|---|---|
| NADH + H⁺ | 3 ⭐⭐ |
| FADH₂ | 1 ⭐⭐ |
| GTP (→ ATP by SLP) | 1 ⭐⭐ |
| CO₂ | 2 ⭐ |
| CARBON NUMBER | COMPOUND(S) |
|---|---|
| 6C | Citric acid, Isocitrate ⭐ |
| 5C | α-Ketoglutaric acid (ONLY one) ⭐⭐⭐ |
| 4C | OAA, Succinyl-CoA, Succinic acid, Malic acid ⭐ |
E TCA SUMMARY EQUATION (PER PYRUVATE, INCL. LINK)
🔑 EQUATION: Pyruvic acid + 4NAD⁺ + FAD + 2H₂O + ADP + Pi → 3CO₂ + 4NADH + 4H⁺ + FADH₂ + ATP (4 NADH = 1 from link + 3 from TCA; 3 CO₂ = 1 from link + 2 from TCA)
| FEATURE | DETAIL |
|---|---|
| OAA | Must be continuously replenished (first member of cycle) ⭐ |
| NAD⁺ and FAD | Must be regenerated from NADH and FADH₂ respectively ⭐ |
VII. ELECTRON TRANSPORT SYSTEM (ETS) & OXIDATIVE PHOSPHORYLATION
A OVERVIEW
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| FEATURE | DETAIL |
|---|---|
| Location | Inner mitochondrial membrane ⭐⭐⭐ |
| Function | Oxidise NADH + H⁺ and FADH₂ → transfer electrons to O₂ → synthesise ATP ⭐⭐ |
| Final electron / hydrogen acceptor | Molecular O₂ (gets reduced to H₂O) ⭐⭐⭐ |
| O₂ role | Limited to the terminal stage only; drives entire process by removing hydrogen ⭐⭐ |
⚡ EXAM TRAP
NEET 2019, 2021, 2024, 2026 (reinforced): ETS present in inner mitochondrial membrane; O₂ = final hydrogen acceptor; reduced to H₂O.
B FIVE ETS COMPLEXES
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| COMPLEX | NAME | KEY FEATURE |
|---|---|---|
| I | NADH dehydrogenase | Oxidises NADH; transfers electrons to ubiquinone ⭐⭐ |
| II | Succinate dehydrogenase | Receives FADH₂ (from succinate oxidation); transfers to ubiquinone ⭐⭐ |
| III | Cytochrome bc₁ complex | Transfers electrons from ubiquinol to Cytochrome c ⭐⭐ |
| IV | Cytochrome c oxidase | Contains Cyt a, a₃ + two copper centres; terminal oxidase ⭐⭐ |
| V | ATP synthase | Synthesises ATP from ADP + Pi ⭐⭐⭐ |
⚡ EXAM TRAP
NEET 2022, 2024, 2025: Know all five complexes and their names; Complex II is succinate dehydrogenase.
C MOBILE ELECTRON CARRIERS
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| CARRIER | DETAIL |
|---|---|
| Ubiquinone (UQ / Co-Q) | Located within inner membrane; receives electrons from both Complex I and II ⭐⭐ |
| Cytochrome c | Small protein; attached to outer surface of inner membrane; mobile carrier between Complex III and IV ⭐⭐ |
⚡ EXAM TRAP
NEET 2022: Mobile electron carriers = Cytochrome c and Ubiquinone (Co-Q).
D ATP YIELD FROM ETS
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| ELECTRON DONOR | ATP PRODUCED |
|---|---|
| 1 NADH | 3 ATP ⭐⭐⭐ |
| 1 FADH₂ | 2 ATP ⭐⭐⭐ |
⚡ EXAM TRAP
NEET 2021: 1 NADH → 3 ATP; 1 FADH₂ → 2 ATP. PYQ swapping values was INCORRECT!
E OXIDATIVE PHOSPHORYLATION
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| FEATURE | DETAIL |
|---|---|
| Definition | ATP synthesis driven by energy released from oxidation-reduction reactions in ETS ⭐⭐ |
| Named because | Energy of oxidation-reduction utilised for phosphorylation ⭐ |
| vs Photophosphorylation | Photophosphorylation uses light energy; Oxidative phosphorylation uses redox energy ⭐⭐ |
| Proton gradient | Formed in intermembrane space of mitochondria ⭐⭐ |
| Mechanism | Chemiosmotic (same principle as in chloroplasts) ⭐ |
⚡ EXAM TRAP
NEET 2016, 2018, 2021, 2024, 2026 (reinforced): Oxidative phosphorylation = ATP from oxidation-reduction energy; occurs on inner membrane; proton gradient / accumulation formed in intermembrane space!
F ATP SYNTHASE (COMPLEX V) STRUCTURE & G. ROLE OF NAD⁺
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| PART | DETAIL |
|---|---|
| F₁ | Peripheral membrane protein complex; contains site for ATP synthesis from ADP + Pi ⭐⭐ |
| F₀ | Integral membrane protein complex; forms channel for protons across inner membrane ⭐⭐ |
| Proton flow direction | Intermembrane space → Matrix (through F₀) ⭐⭐ |
| H⁺ per ATP | 4H⁺ pass through F₀ per ATP produced ⭐⭐ |
| NAD⁺ in respiration | Functions as an electron carrier ⭐ |
VIII. THE RESPIRATORY BALANCE SHEET
A ASSUMPTIONS FOR CALCULATION
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| ASSUMPTION | DETAIL |
|---|---|
| 1 | Sequential, orderly pathway functioning (glycolysis → TCA → ETS one after another) ⭐ |
| 2 | NADH from glycolysis transferred into mitochondria for oxidative phosphorylation ⭐⭐ |
| 3 | None of the intermediates utilised for synthesis of other compounds ⭐ |
| 4 | Only glucose is being respired — no alternative substrates entering at intermediate stages ⭐ |
🔑 KEY POINT: These assumptions are NOT really valid in a living system — all pathways work simultaneously; substrates enter and are withdrawn as needed; ATP utilised as needed.
B ATP BALANCE SHEET — PER GLUCOSE
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| STAGE | LOCATION | NADH | FADH₂ | ATP (SLP) | CO₂ |
|---|---|---|---|---|---|
| Glycolysis | Cytoplasm | 2 | 0 | 2 (net) | 0 |
| Link reaction (×2) | Mito. matrix | 2 | 0 | 0 | 2 |
| TCA cycle (×2) | Mito. matrix | 6 | 2 | 2 | 4 |
| TOTAL | — | 10 | 2 | 4 | 6 |
C ATP FROM OXIDATIVE PHOSPHORYLATION & D. GRAND TOTAL
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| SOURCE | CALCULATION | ATP |
|---|---|---|
| 10 NADH | 10 × 3 | 30 ⭐ |
| 2 FADH₂ | 2 × 2 | 4 ⭐ |
| Total from ETS | — | 34 ⭐ |
| By SLP (substrate level) | — | 4 ⭐ |
| By Oxidative Phosphorylation (ETS) | — | 34 ⭐ |
| NET TOTAL | — | 38 ATP ⭐⭐⭐ |
E FERMENTATION vs AEROBIC RESPIRATION
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| FEATURE | FERMENTATION | AEROBIC RESPIRATION |
|---|---|---|
| Breakdown | Partial (incomplete oxidation) ⭐ | Complete (to CO₂ and H₂O) ⭐ |
| ATP yield | Net 2 ATP only ⭐⭐ | Many more (38 ATP) ⭐⭐ |
| NADH oxidation | Slow ⭐ | Very vigorous ⭐ |
| O₂ requirement | Not required ⭐ | Required ⭐ |
IX. AMPHIBOLIC PATHWAY
A OVERVIEW
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| FEATURE | DETAIL |
|---|---|
| Traditional view | Respiratory pathway = catabolic (breakdown of substrates) ⭐ |
| Correct view | Respiratory pathway involved in BOTH catabolism AND anabolism ⭐⭐ |
| Hence called | Amphibolic pathway (not just catabolic) ⭐⭐⭐ |
| Catabolism | Breaking down processes ⭐ |
| Anabolism | Synthesis processes ⭐ |
⚡ EXAM TRAP
NEET 2022: Respiratory pathway = amphibolic (both catabolic + anabolic).
B ENTRY POINTS OF DIFFERENT SUBSTRATES
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| SUBSTRATE | BROKEN DOWN TO | ENTRY POINT |
|---|---|---|
| Carbohydrates | Glucose | Enters glycolysis at the start ⭐ |
| Fats | Glycerol + Fatty acids ⭐⭐ | — |
| → Glycerol | → PGAL | Enters glycolysis ⭐ |
| → Fatty acids | → Acetyl CoA | Enters TCA cycle ⭐⭐ |
| Proteins | → Amino acids (after deamination) | Enter at various stages (pyruvate, Acetyl CoA, or within TCA cycle) ⭐⭐ |
⚡ EXAM TRAP
NEET 2016, 2023: Acetyl CoA = biomolecule common to respiration-mediated breakdown of fats, carbohydrates, AND proteins. Fatty acids → Acetyl CoA before entering respiratory pathway.
C KEY CONCEPT — WHY AMPHIBOLIC?
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| SCENARIO | DETAIL |
|---|---|
| Breaking down fats | Fatty acids → Acetyl CoA → enters respiratory pathway ⭐ |
| Synthesising fats | Acetyl CoA withdrawn from respiratory pathway → fatty acids synthesised ⭐ |
| Same for proteins | Respiratory intermediates serve as link for both breakdown AND synthesis ⭐ |
X. RESPIRATORY QUOTIENT (RQ)
A DEFINITION
🔑 FORMULA: RQ = Volume of CO₂ evolved / Volume of O₂ consumed
| FEATURE | DETAIL |
|---|---|
| RQ depends on | Type of respiratory substrate used during respiration ⭐⭐ |
| Pure proteins / fats | Never used as respiratory substrates alone (always mixed in living organisms) ⭐ |
B RQ VALUES
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| SUBSTRATE | RQ VALUE |
|---|---|
| Carbohydrates | 1.0 ⭐⭐ |
| Proteins | ~0.9 ⭐ |
| Fats (e.g., Tripalmitin) | 0.7 ⭐⭐ |
| Organic acids | More than 1 ⭐⭐ |
⚡ EXAM TRAP
NEET 2019, 2026 (reinforced): RQ of fats (tripalmitin) = 0.7.
C RQ CALCULATIONS
Carbohydrates (RQ = 1.0) ⭐⭐
🔑 EQUATION: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy | RQ = 6CO₂ / 6O₂ = 1.0
Fats — Tripalmitin (RQ = 0.7) ⭐⭐
🔑 EQUATION: 2(C₅₁H₉₈O₆) + 145O₂ → 102CO₂ + 98H₂O + Energy | RQ = 102CO₂ / 145O₂ = 0.7
XI. RAPID REVISION — KEY COMPARISON TABLES
TABLE 1: Stages of Respiration — Location Summary ⭐⭐⭐
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| STAGE | LOCATION |
|---|---|
| Glycolysis | Cytoplasm (cytosol) |
| Link reaction (Oxidative decarboxylation) | Mitochondrial matrix |
| TCA cycle (Krebs' cycle) | Mitochondrial matrix |
| ETS & Oxidative phosphorylation | Inner mitochondrial membrane |
TABLE 2: Glycolysis — Quick Facts ⭐⭐⭐
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| FEATURE | DETAIL |
|---|---|
| Also called | EMP pathway |
| Location | Cytoplasm |
| Input | 1 Glucose (6C) |
| Output | 2 Pyruvic acid (3C) |
| ATP consumed | 2 |
| ATP produced (SLP) | 4 |
| Net ATP (SLP) | 2 |
| NADH produced | 2 |
| O₂ used | No |
| CO₂ released | No |
| Total ATP (incl. ETS) | 8 |
TABLE 3: Alcoholic vs Lactic Acid Fermentation ⭐⭐⭐
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| FEATURE | ALCOHOLIC | LACTIC ACID |
|---|---|---|
| Organism | Yeast | Bacteria; animal muscles |
| Enzymes | Pyruvate decarboxylase + Alcohol dehydrogenase | Lactate dehydrogenase |
| Products | Ethanol + CO₂ | Lactic acid |
| CO₂ released | YES | NO |
| Net ATP | 2 | 2 |
| Energy released | < 7% | < 7% |
TABLE 4: TCA Cycle — Per Turn ⭐⭐⭐
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| PRODUCT | AMOUNT |
|---|---|
| NADH + H⁺ | 3 |
| FADH₂ | 1 |
| GTP (→ ATP by SLP) | 1 |
| CO₂ | 2 |
| Decarboxylations | 2 |
TABLE 5: Five ETS Complexes ⭐⭐⭐
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| COMPLEX | NAME | KEY FEATURE |
|---|---|---|
| I | NADH dehydrogenase | Oxidises NADH → electrons to UQ |
| II | Succinate dehydrogenase | Receives FADH₂ → electrons to UQ |
| III | Cytochrome bc₁ | Transfers to Cyt c |
| IV | Cytochrome c oxidase | Cyt a, a₃ + 2 Cu centres |
| V | ATP synthase | F₁ + F₀; ATP synthesis |
TABLE 6: Complete ATP Balance Sheet — Per Glucose (Aerobic) ⭐⭐⭐
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| STAGE | NADH | FADH₂ | ATP (SLP) | CO₂ |
|---|---|---|---|---|
| Glycolysis | 2 | 0 | 2 | 0 |
| Link reaction (×2) | 2 | 0 | 0 | 2 |
| TCA cycle (×2) | 6 | 2 | 2 | 4 |
| TOTAL | 10 | 2 | 4 | 6 |
*ATP FROM ETS: 10 NADH × 3 = 30; 2 FADH₂ × 2 = 4 → ETS total = 34 | Grand total = 4 (SLP) + 34 (ETS) = 38 ATP*
TABLE 7: RQ Values ⭐⭐⭐
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| SUBSTRATE | RQ |
|---|---|
| Carbohydrates | 1.0 |
| Proteins | ~0.9 |
| Fats (Tripalmitin) | 0.7 |
| Organic acids | > 1.0 |
TABLE 8: Fermentation vs Aerobic Respiration ⭐⭐⭐
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| FEATURE | FERMENTATION | AEROBIC RESPIRATION |
|---|---|---|
| Oxygen | Not required | Required |
| Breakdown | Partial (incomplete) | Complete (CO₂ + H₂O) |
| ATP yield | 2 | 38 |
| NADH oxidation | Slow | Vigorous |
| End products | Ethanol + CO₂ OR Lactic acid | CO₂ + H₂O |
| Location | Cytoplasm only | Cytoplasm + Mitochondria |
TABLE 9: ATP Synthase — F₁ vs F₀ ⭐⭐⭐
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| PART | DETAIL |
|---|---|
| F₁ | Peripheral membrane protein; site for ATP synthesis |
| F₀ | Integral membrane protein; proton channel |
| Proton flow | Intermembrane space → Matrix |
| H⁺ per ATP | 4 |
TABLE 10: Chloroplast vs Mitochondria — Proton Accumulation ⭐⭐⭐
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| FEATURE | CHLOROPLAST | MITOCHONDRIA |
|---|---|---|
| Proton accumulation | Thylakoid lumen | Intermembrane space |
| ATP synthase name | CF₁ / CF₀ | F₁ / F₀ |
| Proton flow direction | Lumen → Stroma | Intermembrane space → Matrix |
| Energy source for gradient | Light energy | Oxidation-reduction energy |
TABLE 11: Amphibolic Pathway — Entry Points ⭐⭐⭐
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| SUBSTRATE | BROKEN TO | ENTRY POINT |
|---|---|---|
| Carbohydrates | Glucose | Start of glycolysis |
| Glycerol (from fats) | PGAL | Glycolysis |
| Fatty acids (from fats) | Acetyl CoA | TCA cycle |
| Proteins | Amino acids | Pyruvate / Acetyl CoA / TCA intermediates |
TABLE 12: Key Numbers — Quick Reference ⭐⭐⭐
↔ Swipe table sideways to view full columns
| PARAMETER | VALUE |
|---|---|
| Glycolysis reactions | 10 |
| Glycolysis ATP consumed | 2 |
| Glycolysis net ATP (SLP) | 2 |
| Glycolysis total ATP (incl. ETS) | 8 |
| Link reaction NADH per glucose | 2 |
| Link reaction CO₂ per glucose | 2 |
| TCA NADH per turn | 3 |
| TCA FADH₂ per turn | 1 |
| TCA GTP per turn | 1 |
| TCA CO₂ per turn | 2 |
| TCA decarboxylations per turn | 2 |
| Total NADH per glucose | 10 |
| Total FADH₂ per glucose | 2 |
| Total ATP by SLP per glucose | 4 |
| Total CO₂ per glucose | 6 |
| 1 NADH → ATP | 3 |
| 1 FADH₂ → ATP | 2 |
| Total ATP (aerobic, per glucose) | 38 |
| Fermentation ATP | 2 |
| Energy in fermentation | < 7% of glucose |
| Yeast death alcohol level | ~13% |
| H⁺ per ATP (F₀) | 4 |
| RQ Carbohydrates | 1.0 |
| RQ Proteins | ~0.9 |
| RQ Fats (Tripalmitin) | 0.7 |
| RQ Organic acids | > 1 |
| Only 5C compound in TCA | α-Ketoglutaric acid |
| Mobile electron carriers | Cytochrome c + Ubiquinone (Co-Q) |
| ETS location | Inner mitochondrial membrane |
| Link reaction enzyme | Pyruvate dehydrogenase |
| Sucrose → Glucose + Fructose | Invertase |
| Glucose → G6P enzyme | Hexokinase |
XII. COMMON EXAM TRAPS — QUICK REFERENCE
NEET 2025, 2026 & RE-NEET 2026 REINFORCED TRAPS
RE-NEET 2026: One glucose yields 2 molecules of pyruvic acid in cytoplasm.
NEET 2026: Krebs cycle occurs in mitochondrial matrix.
NEET 2026: Electron Transport System is located on inner mitochondrial membrane.
NEET 2025: Complex II of mitochondrial ETS is succinate dehydrogenase.
NEET 2026: Accumulation of protons during ETS occurs in intermembrane space.
NEET 2026: RQ for fat tripalmitin from equation 2(C₅₁H₉₈O₆) + 145O₂ → 102CO₂ is 0.7.
NEET 2026: Glycolysis in cytoplasm, Link reaction and TCA in mitochondrial matrix, ETS on inner mitochondrial membrane.
CONSOLIDATED PYQ Q&A TABLE
↔ Swipe table sideways to view full columns
| TRAP / QUESTION | CORRECT ANSWER |
|---|---|
| Respiration is? | Breaking of C-C bonds through oxidation → energy release ⭐ |
| ATP is? | Energy currency of the cell ⭐ |
| Why no specialised respiratory organs in plants? | 3 reasons: own gas exchange, lower rates, cells near surface ⭐ |
| Glycolysis means? | Sugar splitting (Greek) ⭐ |
| Glycolysis also called? | EMP pathway ⭐ |
| Glycolysis occurs in? | Cytoplasm of ALL living organisms ⭐ |
| Glycolysis common to? | Both aerobic AND anaerobic respiration ⭐ |
| Glucose → in glycolysis? | Partial oxidation → 2 pyruvic acid ⭐ |
| O₂ used in glycolysis? | NO ⭐ |
| CO₂ released in glycolysis? | NO ⭐ |
| Reactions in glycolysis? | 10 ⭐ |
| Sucrose → Glucose + Fructose by? | Invertase ⭐ |
| Glucose phosphorylated by? | Hexokinase ⭐ |
| ATP used at how many steps? | TWO (Glucose→G6P AND F6P→F1,6BP) ⭐⭐ |
| Net ATP gain in glycolysis (SLP)? | 2 ATP ⭐ |
| Total ATP from glycolysis (incl. ETS)? | 8 ATP ⭐ |
| Glycolysis needs continuous supply of? | NAD⁺ ⭐ |
| Key product of glycolysis? | Pyruvic acid (pyruvate) ⭐ |
| Fate of pyruvate depends on? | Organism type AND O₂ availability ⭐ |
| Three fates of pyruvate? | Alcoholic ferm., Lactic acid ferm., Aerobic respiration ⭐ |
| Fermentation occurs under? | Anaerobic conditions ⭐ |
| Fermentation occurs in? | Prokaryotes, unicellular eukaryotes, germinating seeds ⭐ |
| Fermentation = what type of oxidation? | Incomplete oxidation ⭐ |
| Alcoholic fermentation enzymes? | Pyruvate decarboxylase + Alcohol dehydrogenase ⭐ |
| Lactic acid fermentation enzyme? | Lactate dehydrogenase ⭐ |
| CO₂ released in lactic acid ferm.? | NO ⭐⭐ |
| CO₂ released in alcoholic ferm.? | YES ⭐ |
| Energy released in fermentation? | Less than 7% of glucose energy ⭐ |
| Net ATP in fermentation? | 2 ATP ⭐ |
| Yeast die at what alcohol conc.? | ~13% ⭐ |
| Reducing agent in fermentation? | NADH + H⁺ (reoxidised to NAD⁺) ⭐ |
| Pyruvate → Acetyl CoA is called? | Oxidative decarboxylation / Link reaction ⭐ |
| Catalysed by? | Pyruvate dehydrogenase ⭐ |
| Cofactors for link reaction? | NAD⁺, CoA, Mg²⁺ ⭐ |
| Link reaction occurs in? | Mitochondrial matrix ⭐ |
| TCA cycle also called? | Krebs' cycle / Citric acid cycle ⭐ |
| TCA cycle occurs in? | Matrix of mitochondria ⭐ |
| TCA starts with? | Condensation of acetyl group + OAA + H₂O → Citric acid ⭐ |
| TRAP: TCA starts with OAA or pyruvic acid? | OAA (NOT pyruvic acid!) ⭐⭐ |
| Starting enzyme? | Citrate synthase ⭐ |
| Decarboxylations in TCA? | TWO ⭐ |
| Only 5C compound in TCA? | α-Ketoglutaric acid ⭐⭐ |
| SLP in TCA at which step? | Succinyl-CoA → Succinic acid (GTP synthesised) ⭐ |
| TRAP: Does SLP step involve oxidation? | NO ⭐⭐ |
| NAD⁺ reduced at how many points? | THREE ⭐ |
| FAD reduced at how many points? | ONE (Succinic acid → Malic acid) ⭐ |
| Products per TCA turn? | 3 NADH + 1 FADH₂ + 1 GTP + 2 CO₂ ⭐ |
| ETS present in? | Inner mitochondrial membrane ⭐ |
| TRAP: Inner or outer membrane? | Inner membrane ⭐⭐ |
| Complex I? | NADH dehydrogenase ⭐ |
| Complex II? | Succinate dehydrogenase ⭐ |
| Complex III? | Cytochrome bc₁ complex ⭐ |
| Complex IV? | Cytochrome c oxidase (Cyt a, a₃ + Cu centres) ⭐ |
| Complex V? | ATP synthase ⭐ |
| Mobile electron carriers? | Cytochrome c AND Ubiquinone (Co-Q) ⭐ |
| Final electron acceptor? | O₂ (reduced to H₂O) ⭐ |
| O₂ role? | Limited to terminal stage only ⭐ |
| 1 NADH → ? ATP | 3 ATP ⭐⭐ |
| 1 FADH₂ → ? ATP | 2 ATP ⭐⭐ |
| TRAP: NEET 2021 swapped values? | That statement was INCORRECT ⭐ |
| Oxidative phosph. = ? | ATP synthesis from oxidation-reduction energy ⭐ |
| Proton gradient in mitochondria? | Intermembrane space ⭐ |
| ATP synthase parts? | F₁ (peripheral, ATP synthesis) + F₀ (integral, proton channel) ⭐ |
| Proton flow for ATP? | Intermembrane space → Matrix ⭐ |
| H⁺ per ATP? | 4 ⭐ |
| NAD⁺ acts as? | Electron carrier ⭐ |
| Net ATP per glucose (aerobic)? | 38 ATP ⭐⭐ |
| Assumptions for 38 ATP? | Sequential pathways; NADH transferred; no intermediates diverted; only glucose ⭐ |
| Are assumptions valid in reality? | NO — all pathways work simultaneously ⭐ |
| Respiratory pathway is? | Amphibolic (both catabolic + anabolic) ⭐⭐ |
| Why amphibolic? | Intermediates used for BOTH breakdown AND synthesis ⭐ |
| Acetyl CoA common to breakdown of? | Fats, carbohydrates, AND proteins ⭐ |
| Fatty acids enter as? | Acetyl CoA ⭐ |
| Glycerol enters as? | PGAL (into glycolysis) ⭐ |
| Proteins enter as? | Amino acids (after deamination) → various stages ⭐ |
| Pure proteins/fats used as sole substrate? | NEVER (always mixed in living organisms) ⭐ |
| RQ definition? | Volume CO₂ evolved / Volume O₂ consumed ⭐ |
| RQ depends on? | Type of respiratory substrate ⭐ |
| RQ Carbohydrates? | 1.0 ⭐ |
| RQ Proteins? | ~0.9 ⭐ |
| RQ Fats (Tripalmitin)? | 0.7 ⭐⭐ |
| RQ Organic acids? | More than 1 ⭐ |
| Glucose is? | Favoured substrate for respiration ⭐ |