BOT 11
Botany
2–3 Q/year
★★★★★ High Priority
Respiration in Plants
NCERT Class 11 Ch. 14. Glycolysis, Krebs cycle, ETC, oxidative phosphorylation, fermentation. ATP yield calculations — 2–3 questions every year, numerically demanding.
Cellular Respiration — Complete Pathway
From glucose to ATP — every step, every enzyme, every location. Follow the carbon and the electrons.
Glycolysis (Cytoplasm)
Location: Cytoplasm (cytosol). Occurs in ALL living cells, both aerobic and anaerobic.
Substrate: 1 Glucose (6C) → 2 Pyruvate (3C each).
Net yield per glucose: 2 ATP (net), 2 NADH, 2 Pyruvate.
Key enzymes: Hexokinase (glucose→G6P, first step), Phosphofructokinase (PFK — rate-limiting/regulatory step), Pyruvate kinase (last step).
Energy investment: 2 ATP used → 4 ATP produced = net 2 ATP. 2 NAD+ reduced to 2 NADH.
Pyruvate Oxidation (Link Reaction)
Location: Mitochondrial matrix.
Reaction: 2 Pyruvate (3C) → 2 Acetyl-CoA (2C) + 2 CO₂ + 2 NADH.
Enzyme: Pyruvate dehydrogenase complex.
This step is irreversible — once pyruvate enters mitochondria and is decarboxylated, it cannot return to glucose.
Krebs Cycle / TCA Cycle (Mitochondrial Matrix)
Runs TWICE per glucose (once per acetyl-CoA).
Per turn: 2 CO₂ released, 3 NADH, 1 FADH₂, 1 GTP (=ATP).
Per glucose (2 turns): 4 CO₂, 6 NADH, 2 FADH₂, 2 GTP.
Key intermediates: Citrate (6C), Isocitrate, α-Ketoglutarate, Succinate, Fumarate, Malate, Oxaloacetate (4C — regenerated).
Key enzyme: Isocitrate dehydrogenase (regulatory). Succinyl-CoA synthetase (GTP production).
Electron Transport Chain (ETC) + ATP Synthesis
Location: Inner mitochondrial membrane.
Complexes: I (NADH dehydrogenase), II (succinate dehydrogenase), III (cytochrome bc₁), IV (cytochrome c oxidase — final e⁻ acceptor is O₂ → H₂O).
ATP synthase (Complex V): Chemiosmosis — H⁺ gradient drives ATP synthesis. 1 NADH → ~2.5 ATP. 1 FADH₂ → ~1.5 ATP.
Overall yield per glucose (aerobic): ~30–32 ATP (modern estimate) or 36–38 ATP (older textbook value — NEET uses 36 ATP total: 2 glycolysis + 2 Krebs + 32 ETC).
Fermentation (Anaerobic Respiration)
Occurs when O₂ is absent. Glycolysis proceeds → pyruvate → fermentation products.
Lactic acid fermentation: Pyruvate → Lactic acid. Enzyme: Lactate dehydrogenase. Occurs in RBCs (no mitochondria), muscle cells under intense exercise. NAD+ regenerated.
Alcoholic fermentation: Pyruvate → Acetaldehyde (CO₂ released) → Ethanol. Enzymes: Pyruvate decarboxylase + Alcohol dehydrogenase. Occurs in yeast, some plant tissues (germinating seeds, roots in waterlogged soil).
Net yield: Only 2 ATP (from glycolysis). Very inefficient — no Krebs, no ETC.
Respiratory Quotient (RQ): RQ = CO₂ evolved / O₂ consumed. Carbohydrate = 1.0. Fat < 1 (0.7 for fat). Protein ≈ 0.9. Organic acids > 1.
Respiration Fact Vault
Numbers, locations, and yields — all the direct recall facts for NEET.
ATP Yield Summary (per glucose)
Glycolysis: 2 ATP (net) + 2 NADH
Link reaction: 2 NADH
Krebs (×2): 2 ATP + 6 NADH + 2 FADH₂
ETC: ~28 ATP
Total: ~30–32 ATP (modern)
NEET textbook: 36 ATP
NEET 2024+ uses modern values (~30 ATP). Older papers used 36–38. Know both.
Locations
Glycolysis: Cytoplasm
Link reaction: Mitochondrial matrix
Krebs cycle: Mitochondrial matrix
ETC: Inner mitochondrial membrane
Fermentation: Cytoplasm
All steps after glycolysis require mitochondria. Fermentation = entirely in cytoplasm
Respiratory Quotient (RQ)
Glucose (carbohydrate): RQ = 1.0
Fats: RQ = 0.7
Proteins: RQ = ~0.9
Organic acids (succinate): RQ > 1
Anaerobic: RQ = ∞ (no O₂)
RQ of malic acid = 1.33, oxalic acid = 4. Succinate RQ is also >1 (lots of O relative to C)
Fermentation Products
Yeast (fungi): Ethanol + CO₂
Muscle cells (anaerobic): Lactic acid
RBCs: Lactic acid (no mitochondria)
Waterlogged roots: Ethanol + CO₂
Key enzyme in alcoholic fermentation: Pyruvate decarboxylase (removes CO₂) + Alcohol dehydrogenase (NADH → NAD+ regenerated)
Worked Examples
Respiration numericals and conceptual questions — pattern recognition is key.
EasyHow many ATP molecules are produced from one molecule of NADH through oxidative phosphorylation?▾
Old textbook: 1 NADH → 3 ATP; 1 FADH₂ → 2 ATP (P/O ratio method). Modern chemiosmotic calculation: 1 NADH → 2.5 ATP; 1 FADH₂ → 1.5 ATP. NEET exams: The older values (3 and 2) are more commonly used in questions. Know both.
Answer: 3 ATP per NADH (textbook); 2.5 ATP per NADH (modern)
MediumThe Respiratory Quotient of a germinating fatty seed will be: (A) 1.0 (B) 0.7 (C) More than 1 (D) Infinity▾
Fatty seeds (like castor, sunflower) primarily use FATS as respiratory substrate. RQ for fat ≈ 0.7 (fat has proportionally less O compared to C, so more O₂ is needed relative to CO₂ produced). Carbohydrate RQ = 1.0. Organic acids like succulents >1. Anaerobic = infinity (no O₂ consumed).
Answer: (B) 0.7
HardHow many ATP molecules are produced by complete oxidation of one glucose molecule via aerobic respiration (old P/O ratio)?▾
Calculate step by step:
Glycolysis: 2 ATP + 2 NADH (×3=6 ATP)
Link reaction: 2 NADH (×3=6 ATP)
Krebs (×2): 2 GTP/ATP + 6 NADH (×3=18 ATP) + 2 FADH₂ (×2=4 ATP)
Total: 2+6+6+2+18+4 = 38 ATP
Note: 10 total NADH × 3 = 30 ATP + 2 FADH₂ × 2 = 4 ATP + 4 substrate level ATP = 38 ATP. Some textbooks give 36 ATP (because cytoplasmic NADH from glycolysis gives only 2 ATP each, not 3, due to transport cost into mitochondria).
Answer: 38 ATP (or 36 ATP accounting for mitochondrial transport cost)
Mistake DNA
These respiration errors appear in NEET answer choices designed to trap students.
❌ Saying glycolysis only happens in aerobic organisms
Glycolysis occurs in ALL living cells — both aerobic AND anaerobic. It happens in the CYTOPLASM and does NOT require oxygen. The difference is what happens to pyruvate AFTER glycolysis. In aerobic organisms, pyruvate enters mitochondria. In anaerobic, it undergoes fermentation.
Fix: Glycolysis = universal, cytoplasmic, O₂-independent. The aerobic/anaerobic split happens AFTER pyruvate is formed.
❌ Mixing up Krebs cycle and glycolysis locations
Glycolysis = CYTOPLASM. Krebs cycle = MITOCHONDRIAL MATRIX. ETC = INNER MITOCHONDRIAL MEMBRANE. A very common error is placing Krebs cycle in the cytoplasm or confusing the matrix with the membrane.
Fix: Krebs = matrix (fluid inside mitochondria). ETC = on the membrane. ATP synthase is on the inner membrane.
❌ Thinking fermentation produces more ATP than aerobic
Fermentation only produces 2 ATP per glucose (from glycolysis only). Aerobic respiration produces ~36–38 ATP. Fermentation is 18–19× LESS efficient. The advantage of fermentation is SPEED and ability to work without O₂, not energy efficiency.
Fix: Fermentation = fast but 2 ATP only. Aerobic = slower setup but 36–38 ATP. Efficiency ratio = ~1:19.
Chapter Intelligence
PYQ Frequency
Glycolysis steps/ATP: 1 Q/year
Krebs cycle intermediates: 1 Q/year
RQ calculations: 1 Q/2 years
Fermentation: 1 Q/2 years
2026 Prediction
High: ATP yield calculation (modern vs old values)
Expected: Location of each stage question
Watch: RQ of fats vs carbohydrates question
