Respiration

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Respiration — Quick Facts

Cellular respiration is the process of breaking down organic molecules (primarily glucose) to release energy in the form of ATP. It occurs in all living cells — plants, animals, and microorganisms.

Overall Equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP) ΔG° = −2900 kJ mol⁻¹ of glucose

Two Main Types:

1. Aerobic Respiration: Requires oxygen. Occurs in mitochondria. Yields 36–38 ATP per glucose.
2. Anaerobic Respiration: Does not require oxygen. Occurs in cytoplasm. Yields only 2 ATP per glucose (net).

Key Stages:

Stage	Location	ATP Yield (per glucose)
Glycolysis	Cytoplasm	2 ATP (net), 2 NADH
Pyruvate oxidation	Mitochondrial matrix	2 NADH
Krebs cycle (CAC)	Mitochondrial matrix	2 ATP, 6 NADH, 2 FADH₂
Electron transport chain	Inner mitochondrial membrane	~34 ATP

Total theoretical yield: ~38 ATP (practical yield ~36 due to transport costs)

ATP Yield from NADH and FADH₂:

• NADH (from glycolysis, needs to enter mitochondria via malate-aspartate shuttle): yields ~3 ATP
• NADH (from matrix reactions): yields ~3 ATP
• FADH₂: yields ~2 ATP

⚡ Exam tip: In anaerobic respiration in plants, the end product is ethyl alcohol + CO₂ (via pyruvate decarboxylase + alcohol dehydrogenase). In muscles during heavy exercise (oxygen debt), lactate is produced (via lactate dehydrogenase).

⚡ Key formula for RQ: Respiratory Quotient (RQ) = CO₂ released / O₂ consumed. For glucose: RQ = 6/6 = 1.0. For fats: ~0.7. For proteins: ~0.8–0.9.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Respiration — NEET/JEE Study Guide

Glycolysis (EMP Pathway — 10 enzyme-catalysed steps):

Phase 1: Energy Investment (steps 1–5):

1. Glucose → Glucose-6-phosphate (hexokinase, requires ATP)
2. G6P → Fructose-6-phosphate (phosphoglucose isomerase)
3. F6P → Fructose-1,6-bisphosphate (phosphofructokinase, PFK — key regulatory step)
4. F1,6BP → Glyceraldehyde-3-phosphate + DHAP (aldolase)
5. DHAP → G3P (triose phosphate isomerase)

Net: 2 G3P produced from 1 glucose. Cost: 2 ATP invested.

Phase 2: Energy Payoff (steps 6–10): 6. G3P + NAD⁺ + Pi → 1,3-bisphosphoglycerate + NADH (G3P dehydrogenase) 7. 1,3-BPG + ADP → 3-phosphoglycerate + ATP (phosphoglycerate kinase) — substrate-level phosphorylation 8. 3-PG → 2-phosphoglycerate (phosphoglycerate mutase) 9. 2-PG → Phosphoenolpyruvate (enolase) 10. PEP + ADP → Pyruvate + ATP (pyruvate kinase) — substrate-level phosphorylation

Products of Glycolysis (per glucose):

• 2 ATP (net)
• 2 NADH (worth ~6 ATP in aerobic conditions)
• 2 Pyruvate

Link Reaction (Pyruvate → Acetyl CoA): Pyruvate + NAD⁺ + CoA → Acetyl CoA + NADH + CO₂ (Occurs twice per glucose; acetyl CoA enters Krebs cycle)

Krebs Cycle (Citric Acid Cycle / Tricarboxylic Acid Cycle): Acetyl CoA + 3 NAD⁺ + FAD + GDP + Pi → 2 CO₂ + 3 NADH + FADH₂ + GTP + CoA

Key enzymes: Citrate synthase (step 1), isocitrate dehydrogenase (regulatory, produces first CO₂ and NADH), α-ketoglutarate dehydrogenase (produces second CO₂ and NADH), succinyl CoA synthetase (substrate-level phosphorylation).

ETS (Electron Transport Chain) — Complexes I to IV: Complex I (NADH dehydrogenase): NADH → NAD⁺ + H⁺ + e⁻ Complex II (Succinate dehydrogenase): FADH₂ → FAD + H⁺ + e⁻ Coenzyme Q (CoQ): Collects electrons from I and II, passes to Complex III Complex III (cytochrome bc1): Passes electrons to cytochrome c Complex IV (cytochrome c oxidase): Passes electrons to O₂ (terminal electron acceptor) → O₂ + 4H⁺ → 2H₂O

⚡ Common mistake: Some students think ETS produces ATP directly. It doesn’t — it creates a proton gradient across the inner mitochondrial membrane, and ATP synthase (Complex V) uses this gradient’s potential energy to make ATP. This is chemiosmotic theory (Peter Mitchell, Nobel Prize 1978).

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Respiration — Comprehensive Notes

ATP Yield Calculation (Detailed):

Step	Product	ATP-equivalent yield
Glycolysis (net)	2 ATP	+2 ATP
Glycolysis (2 NADH)	2 NADH → ~6 ATP (using malate-aspartate shuttle)	+6 ATP
Pyruvate oxidation (2 NADH)	2 NADH → 6 ATP	+6 ATP
Krebs cycle (2 GTP)	2 ATP	+2 ATP
Krebs cycle (6 NADH)	6 NADH → 18 ATP	+18 ATP
Krebs cycle (2 FADH₂)	2 FADH₂ → 4 ATP	+4 ATP
Total		~38 ATP

In some tissues (e.g., brain, using glycerol-phosphate shuttle), NADH from glycolysis yields only 2 ATP each → total ~36 ATP.

Anaerobic Respiration — Fermentation: When oxygen is unavailable, pyruvate is converted to either:

• Alcoholic fermentation: Pyruvate → Ethanol + CO₂ (in yeast, many plants) Pyruvate decarboxylase (requires TPP) → Acetaldehyde + CO₂ Alcohol dehydrogenase: Acetaldehyde + NADH → Ethanol + NAD⁺

• Lactic acid fermentation: Pyruvate → Lactate (in animal muscles, some bacteria) Lactate dehydrogenase: Pyruvate + NADH → Lactate + NAD⁺

⚡ Oxygen debt: During heavy exercise, muscles produce lactate. After exercise, when oxygen becomes available, lactate is converted back to pyruvate in the liver via the Cori cycle and further metabolised. This requires extra oxygen consumption — the “debt” repaid.

Cori Cycle: Lactate from muscles → transported to liver → converted to glucose via gluconeogenesis → glucose returns to muscles.

Enzyme Regulation of Respiration:

Enzyme	Regulatory Factor
Hexokinase	Inhibited by glucose-6-phosphate (product feedback)
PFK-1	Inhibited by ATP, citrate; activated by AMP, fructose-2,6-bisphosphate
Pyruvate kinase	Inhibited by ATP, alanine; activated by fructose-1,6-bisphosphate
Citrate synthase	Inhibited by ATP, NADH, succinyl CoA
Isocitrate dehydrogenase	Activated by ADP, inhibited by ATP, NADH

Beta-oxidation of Fatty Acids (alternative respiration substrate): Fats are broken down to fatty acids + glycerol. Fatty acids undergo β-oxidation in mitochondria: 1 fatty acid (e.g., palmitic acid, C₁₆) → 8 Acetyl CoA Each round of β-oxidation produces: 1 NADH + 1 FADH₂ + 1 Acetyl CoA Total ATP from complete oxidation of 1 palmitic acid: ~106 ATP (much more than glucose!)

This explains why fats yield more energy per gram than carbohydrates.

Photorespiration (C3 plants): In hot, dry conditions, when stomata close and O₂ concentration rises relative to CO₂: Rubisco (normally a carboxylase) acts as an oxygenase → produces 3-carbon (3-PG) + 2-carbon (glycolate) instead of 2 × 3-PG. Glycolate is then metabolised via the photorespiratory pathway (C2 cycle) → releases CO₂ → net loss of energy. C4 plants (e.g., maize, sugarcane) avoid this by having spatial separation of initial CO₂ fixation (in mesophyll) and Rubisco activity (in bundle sheath).

NEET Pattern Analysis: Respiration is a high-frequency topic (2–3 questions per year). Key areas: ATP yield calculations, differences between aerobic/anaerobic, glycolysis enzyme names and step numbers, Krebs cycle products, and the electron transport chain. Photorespiration and C4 pathway are also tested, especially the anatomical differences between C3 and C4 plants.

⚡ NEET 2023 Qn: How many ATP are produced from 1 molecule of glucose during aerobic respiration in a plant cell? Answer: 36 ATP (not 38 — plant cells exclude the 2 ATP used in the preparatory phase of glycolysis from the count since those are cytoplasmic).