Quick Summary
Respiration breaks down organic molecules to release ATP (energy). Aerobic respiration has three stages: glycolysis (cytoplasm), Krebs cycle (mitochondrial matrix), and ETC/oxidative phosphorylation (inner mitochondrial membrane). Anaerobic respiration (fermentation) occurs without oxygen. ATP yield and RQ values are the key numbers to remember.
Overview
Aerobic: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 38 ATP
Anaerobic (yeast): C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + 2 ATP
Anaerobic (muscle): C₆H₁₂O₆ → 2 lactic acid + 2 ATP
Stage 1 — Glycolysis (EMP Pathway)
Site: Cytoplasm (cytosol) Requires: O₂? NO Substrate: Glucose (6C) Products: 2 Pyruvate (3C) + 2 ATP (net) + 2 NADH
Key Steps
1. Glucose (6C) → Glucose-6-phosphate → Fructose-1,6-bisphosphate (uses 2 ATP) 2. 6C split into 2 × G3P (3C) 3. G3P oxidised → 2 pyruvate; generates 4 ATP + 2 NADH
Net yield of glycolysis: 2 ATP + 2 NADH (per glucose)
Pyruvate Oxidation (Link Reaction)
Site: Mitochondrial matrix Pyruvate (3C) + CoA → Acetyl-CoA (2C) + CO₂ + NADH (×2 per glucose → 2 Acetyl-CoA + 2CO₂ + 2 NADH)
Stage 2 — Krebs Cycle (TCA / Citric Acid Cycle)
Site: Mitochondrial matrix Per turn (1 Acetyl-CoA): 1 ATP (GTP) + 3 NADH + 1 FADH₂ + 2CO₂
Per glucose (2 turns): 2 ATP + 6 NADH + 2 FADH₂ + 4CO₂
Key Steps (simplified)
1. Acetyl-CoA (2C) + OAA (4C) → Citrate (6C) 2. Citrate → isocitrate → α-ketoglutarate (5C, 1CO₂ + 1 NADH) 3. α-ketoglutarate → succinyl-CoA (4C, 1CO₂ + 1 NADH) 4. Succinyl-CoA → Succinate (1 GTP = ATP) 5. Succinate → Fumarate → Malate (1 FADH₂, 1 NADH) 6. Malate → OAA (regenerated)
Stage 3 — Electron Transport Chain (ETC) and Oxidative Phosphorylation
Site: Inner mitochondrial membrane NADH and FADH₂ donate electrons → electron carriers (NADH dehydrogenase, cytochrome bc1, cytochrome c, cytochrome oxidase) Final electron acceptor: O₂ → H₂O
ATP synthesis: H⁺ gradient drives ATP synthase (chemiosmosis)
- 1 NADH → 2.5 ATP
- 1 FADH₂ → 1.5 ATP
Total ATP Yield Per Glucose
| Stage | ATP | NADH | FADH₂ |
|---|---|---|---|
| Glycolysis | 2 | 2 | — |
| Pyruvate oxidation | — | 2 | — |
| Krebs cycle (×2) | 2 | 6 | 2 |
| ETC (NADH × 10 × 2.5) | 25 | — | — |
| ETC (FADH₂ × 2 × 1.5) | 3 | — | — |
| Total | 32–38 |
(Traditional textbook value: 38 ATP; modern estimate: ~30–32 ATP)
Anaerobic Respiration / Fermentation
Fermentation in yeast: Pyruvate → acetaldehyde → ethanol + CO₂ (by alcohol dehydrogenase)
Lactic acid fermentation (muscle, bacteria): Pyruvate → lactic acid (by lactate dehydrogenase)
Purpose: Regenerate NAD⁺ so glycolysis can continue (no mitochondria needed)
Respiratory Quotient (RQ)
RQ = CO₂ released / O₂ consumed
| Substrate | RQ |
|---|---|
| Carbohydrates | 1.0 |
| Fats (lipids) | ~0.7 |
| Proteins | ~0.9 |
| Anaerobic | ∞ (undefined — no O₂) |
| Organic acids | > 1 |
Important Questions
One Mark
- Where does glycolysis occur in the cell?
- What is the final electron acceptor in the ETC?
- State the RQ value for fats.
- Name the enzyme that converts pyruvate to acetaldehyde in yeast.
Short Answer (3 marks)
- What is the net ATP yield from glycolysis? List the products.
- Compare aerobic and anaerobic respiration on the basis of four characters.
- What is the respiratory quotient? Calculate it for carbohydrates.
Long Answer (5 marks)
- Describe the Krebs cycle with a simplified diagram. What are the products per turn?
- Explain the electron transport chain and chemiosmosis. How is ATP synthesised?
Common Mistakes
- Glycolysis yields 4 ATP gross but only 2 ATP net (2 are consumed at the start).
- Krebs cycle produces NADH and FADH₂, not ATP directly — ATP comes from ETC later.
- The final electron acceptor in ETC is O₂ (NOT CO₂ or NADH).
- RQ for anaerobic respiration is ∞ (or infinity) because no O₂ is consumed.
Board Exam Tips
- Glycolysis steps (input/output) is a 3-mark question — know the net yield.
- Krebs cycle diagram with all 8 steps is a standard 5-mark question.
- "Compare aerobic and anaerobic respiration" is another 5-mark staple.
- NEET: RQ values, ATP yield (glycolysis + Krebs + ETC), and fermentation products appear in 2–3 questions per paper.