How to Use This Page
Solve each Samacheer Kalvi exercise question on your own first, then compare your answer with the solutions below. Respiration covers glycolysis, Krebs cycle, ETC, fermentation, RQ, and the amphibolic pathway — focus on understanding each stage and tracking ATP yield.
Key Concepts (Quick Recap)
Overview of Aerobic Respiration
Glucose (C₆H₁₂O₆)
│
├── Glycolysis (Cytoplasm)
│ Glucose → 2 Pyruvate + 2 ATP (net) + 2 NADH
│
├── Pyruvate Oxidation (Mitochondrial matrix)
│ 2 Pyruvate → 2 Acetyl CoA + 2 NADH + 2 CO₂
│
├── Krebs Cycle (Mitochondrial matrix)
│ 2 Acetyl CoA → 4 CO₂ + 6 NADH + 2 FADH₂ + 2 ATP
│
└── Electron Transport Chain (Inner mitochondrial membrane)
NADH + FADH₂ → H₂O + ~32–34 ATP
Total: ~36–38 ATP per glucose molecule
Glycolysis (EMP Pathway)
| Feature | Details |
|---|
| Location | Cytoplasm |
| Substrate | Glucose (6C) |
| Products | 2 Pyruvate (3C) + 2 ATP (net) + 2 NADH |
| Oxygen required | No (anaerobic) |
| Enzymes | 10 enzymes (hexokinase, phosphofructokinase, pyruvate kinase are key) |
| Phases | Investment phase (uses 2 ATP) + Payoff phase (produces 4 ATP + 2 NADH) |
Key Steps: 1. Glucose (6C) → Glucose-6-phosphate (hexokinase, uses 1 ATP) 2. Glucose-6-phosphate → Fructose-6-phosphate (phosphoglucoisomerase) 3. Fructose-6-phosphate → Fructose-1,6-bisphosphate (phosphofructokinase, uses 1 ATP) 4. Fructose-1,6-bisphosphate → 2 × Glyceraldehyde-3-phosphate (aldolase) 5. 2 × G3P → 2 × 1,3-bisphosphoglycerate (produces 2 NADH) 6. 2 × 1,3-BPG → 2 × 3-PGA (produces 2 ATP) 7. 2 × 3-PGA → 2 × Pyruvate (produces 2 ATP)
Net: Glucose → 2 Pyruvate + 2 ATP + 2 NADH
Pyruvate Oxidation (Link Reaction)
| Feature | Details |
|---|
| Location | Mitochondrial matrix |
| Reaction | 2 Pyruvate (3C) + 2 CoA + 2 NAD⁺ → 2 Acetyl CoA (2C) + 2 NADH + 2 CO₂ |
| Enzyme | Pyruvate dehydrogenase complex |
| Note | This links glycolysis to the Krebs cycle; CO₂ is released here |
Krebs Cycle (TCA Cycle / Citric Acid Cycle)
| Feature | Details |
|---|
| Location | Mitochondrial matrix |
| Substrate | Acetyl CoA (2C) |
| Products per Acetyl CoA | 2 CO₂ + 3 NADH + 1 FADH₂ + 1 ATP |
| Products per glucose (2 Acetyl CoA) | 4 CO₂ + 6 NADH + 2 FADH₂ + 2 ATP |
Key Steps (per Acetyl CoA): 1. Acetyl CoA (2C) + OAA (4C) → Citrate (6C) 2. Citrate → Isocitrate (6C) 3. Isocitrate → α-ketoglutarate (5C) + CO₂ + NADH 4. α-ketoglutarate → Succinyl CoA (4C) + CO₂ + NADH 5. Succinyl CoA → Succinate (4C) + ATP (substrate-level phosphorylation) 6. Succinate → Fumarate (4C) + FADH₂ 7. Fumarate → Malate (4C) 8. Malate → OAA (4C) + NADH
Note: The cycle starts and ends with OAA. One turn produces 1 ATP, 3 NADH, 1 FADH₂, and 2 CO₂.
Electron Transport Chain (ETC) and Oxidative Phosphorylation
| Feature | Details |
|---|
| Location | Inner mitochondrial membrane |
| Process | Electrons from NADH and FADH₂ pass through protein complexes |
| Complexes | Complex I (NADH dehydrogenase) → Complex II (Succinate dehydrogenase) → Complex III (Cytochrome bc1) → Complex IV (Cytochrome c oxidase) |
| Final electron acceptor | O₂ (oxygen) → H₂O |
| ATP produced | ~2.5–3 ATP per NADH; ~1.5–2 ATP per FADH₂ |
Chemiosmosis:
- Energy from electron transport pumps H⁺ from matrix to intermembrane space
- Creates a proton gradient (higher H⁺ in intermembrane space)
- H⁺ flows back through ATP synthase (F₀-F₁ complex) → ADP + Pi → ATP
ATP Yield per Glucose Molecule
| Stage | ATP (direct) | NADH | FADH₂ | Total ATP |
|---|
| Glycolysis | 2 (net) | 2 | 0 | 2 + (2 × 2.5) = 7 |
| Pyruvate oxidation | 0 | 2 | 0 | (2 × 2.5) = 5 |
| Krebs cycle | 2 | 6 | 2 | 2 + (6 × 2.5) + (2 × 1.5) = 20 |
| Total | 4 | 10 | 2 | ~36–38 |
Anaerobic Respiration (Fermentation)
| Feature | Alcoholic Fermentation | Lactic Acid Fermentation |
|---|
| Organism | Yeast (Saccharomyces) | Bacteria, animal muscle cells |
| Substrate | Pyruvate | Pyruvate |
| Products | Ethanol + CO₂ | Lactic acid |
| Enzyme | Pyruvate decarboxylase → alcohol dehydrogenase | Lactate dehydrogenase |
| ATP | 2 ATP (from glycolysis only) | 2 ATP (from glycolysis only) |
| Reaction | Pyruvate → Acetaldehyde + CO₂ → Ethanol | Pyruvate → Lactate |
Respiratory Quotient (RQ)
RQ = Volume of CO₂ evolved / Volume of O₂ consumed
| Substrate | RQ Value | Explanation |
|---|
| Carbohydrates | 1.0 | CO₂ = O₂ (e.g., C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O) |
| Fats | < 1.0 (~0.7) | More O₂ needed than CO₂ released |
| Proteins | ~0.8 | More O₂ needed |
| Organic acids | > 1.0 | More CO₂ released than O₂ consumed |
| Anaerobic respiration | Infinity (∞) | CO₂ released but no O₂ consumed |
Amphibolic Pathway
- Definition: A pathway that serves both catabolic (breakdown) and anabolic (synthesis) functions
- Krebs cycle is amphibolic:
- Catabolic: Breaks down Acetyl CoA to CO₂ and H₂O, releasing energy
- Anabolic: Provides intermediates for synthesis of amino acids, fatty acids, and other molecules
- Examples:
- OAA → Aspartate (amino acid synthesis)
- α-ketoglutarate → Glutamate (amino acid synthesis)
- Acetyl CoA → Fatty acid synthesis
- Succinyl CoA → Porphyrin (haem synthesis)
I. Choose the Best Answer (MCQs)
Q1. Glycolysis occurs in the (a) Mitochondrial matrix (b) Cytoplasm (c) Inner mitochondrial membrane (d) Chloroplast
Answer(b) Cytoplasm — Glycolysis occurs in the cytoplasm and does not require oxygen or mitochondria. The Krebs cycle and ETC occur in the mitochondria.
Q2. The net ATP produced per glucose molecule in glycolysis is (a) 1 (b) 2 (c) 4 (d) 36
Answer(b) 2 — Glycolysis produces 4 ATP but uses 2 ATP (in the investment phase), so the net ATP is 2 per glucose molecule. Additionally, 2 NADH are produced (which can yield more ATP in the ETC).
Q3. The Krebs cycle occurs in the (a) Cytoplasm (b) Mitochondrial matrix (c) Inner mitochondrial membrane (d) Nucleus
Answer(b) Mitochondrial matrix — The Krebs cycle (TCA cycle) occurs in the mitochondrial matrix. The electron transport chain occurs on the inner mitochondrial membrane.
Q4. The final electron acceptor in the ETC is (a) NAD⁺ (b) FAD (c) Oxygen (d) Cytochrome c
Answer(c) Oxygen — Oxygen is the final electron acceptor in the electron transport chain. It accepts electrons and combines with H⁺ to form water (H₂O). Without oxygen, the ETC cannot function.
Q5. The RQ for carbohydrates is (a) Less than 1 (b) Greater than 1 (c) Equal to 1 (d) Zero
Answer(c) Equal to 1 — For carbohydrates, RQ = 1 because the volume of CO₂ evolved equals the volume of O₂ consumed. For glucose: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. RQ = 6CO₂/6O₂ = 1.
Q6. Alcoholic fermentation produces (a) Lactic acid (b) Ethanol and CO₂ (c) Ethanol and H₂O (d) Acetic acid
Answer(b) Ethanol and CO₂ — In alcoholic fermentation (by yeast), pyruvate is converted to acetaldehyde (releasing CO₂) and then to ethanol. The overall products are ethanol and CO₂.
Q7. The amphibolic pathway among the following is (a) Glycolysis (b) Krebs cycle (c) ETC (d) Fermentation
Answer(b) Krebs cycle — The Krebs cycle is an amphibolic pathway because it serves both catabolic (breaking down Acetyl CoA for energy) and anabolic (providing intermediates for synthesis of amino acids, fatty acids, etc.) functions.
II. Fill in the Blanks
1. Glycolysis occurs in the cytoplasm of the cell. 2. The net ATP produced per glucose molecule in glycolysis is 2. 3. The Krebs cycle occurs in the mitochondrial matrix. 4. The final electron acceptor in the ETC is oxygen (O₂). 5. Glycolysis converts glucose into 2 pyruvate molecules. 6. Pyruvate is converted to acetyl CoA in the mitochondrial matrix. 7. The enzyme that converts pyruvate to acetaldehyde in alcoholic fermentation is pyruvate decarboxylase. 8. The RQ for carbohydrates is 1. 9. The RQ for fats is less than 1 (approximately 0.7). 10. During anaerobic respiration, the net ATP yield per glucose is 2. 11. The link between glycolysis and the Krebs cycle is acetyl CoA. 12. ATP is produced in the ETC by the process of chemiosmosis (oxidative phosphorylation). 13. The F₀-F₁ complex is also known as ATP synthase. 14. The Krebs cycle is an amphibolic pathway (both catabolic and anabolic). 15. The total ATP yield from aerobic respiration of one glucose molecule is approximately 36–38.
III. True or False (with corrections for false statements)
Q1. Glycolysis requires oxygen.
AnswerFalse — Correction: Glycolysis does NOT require oxygen. It is an anaerobic process that occurs in the cytoplasm. It is common to both aerobic and anaerobic respiration.
Q2. The Krebs cycle is an amphibolic pathway.
AnswerTrue — The Krebs cycle is amphibolic because it serves both catabolic (breaking down Acetyl CoA for energy) and anabolic (providing intermediates for biosynthesis) functions.
Q3. The RQ for fats is greater than 1.
AnswerFalse — Correction: The RQ for fats is LESS THAN 1 (approximately 0.7) because more O₂ is consumed than CO₂ released. Fats require more oxygen for oxidation. RQ greater than 1 is for organic acids.
Q4. Anaerobic respiration produces more ATP than aerobic respiration.
AnswerFalse — Correction: Anaerobic respiration produces only 2 ATP (from glycolysis) per glucose molecule, while aerobic respiration produces approximately 36–38 ATP. Aerobic respiration is far more efficient.
Q5. Oxygen is the final electron acceptor in the ETC.
AnswerTrue — Oxygen is the final electron acceptor in the electron transport chain. It accepts electrons from Complex IV and combines with H⁺ to form water (H₂O).
IV. Match the Following
| Column A | Column B | Answer |
|---|
| 1. Glycolysis | (a) Mitochondrial matrix | (d) Cytoplasm, 2 pyruvate |
| 2. Krebs cycle | (b) Inner mitochondrial membrane | (a) Mitochondrial matrix, 2 CO₂ |
| 3. ETC | (c) Produces ethanol and CO₂ | (b) Inner mitochondrial membrane |
| 4. Alcoholic fermentation | (d) Cytoplasm, 2 pyruvate | (c) Cytoplasm, ethanol + CO₂ |
| 5. RQ for carbohydrates | (e) ~0.7 | (f) 1.0 |
| 6. RQ for fats | (f) 1.0 | (e) ~0.7 |
V. Answer Briefly (2-mark questions)
Q1. Differentiate between aerobic and anaerobic respiration.
Answer
| Feature | Aerobic Respiration | Anaerobic Respiration | |---|---|---| | Oxygen | Required | Not required | | Location | Cytoplasm + Mitochondria | Cytoplasm only | | Complete breakdown | Yes (glucose → CO₂ + H₂O) | No (glucose → ethanol/ lactate) | | ATP yield | 36–38 ATP | 2 ATP | | Final electron acceptor | O₂ | Organic molecule (acetaldehyde, pyruvate) | | Products | CO₂ + H₂O | Ethanol + CO₂ (or lactic acid) |
Q2. What is the respiratory quotient (RQ)? Give its value for carbohydrates and fats.
Answer
RQ (Respiratory Quotient) = Volume of CO₂ evolved / Volume of O₂ consumed - Carbohydrates: RQ = 1.0 (equal CO₂ and O₂) - Fats: RQ ≈ 0.7 (more O₂ consumed than CO₂ released) - Proteins: RQ ≈ 0.8 - Organic acids: RQ > 1.0 (more CO₂ released than O₂ consumed) - Anaerobic respiration: RQ = ∞ (CO₂ released but no O₂ consumed)
Q3. What is glycolysis? Where does it occur?
Answer
Glycolysis (also called the EMP pathway, after Embden, Meyerhof, and Parnas) is the first stage of respiration in which one molecule of glucose (6C) is broken down into two molecules of pyruvate (3C). - Location: Cytoplasm - Oxygen required: No (anaerobic) - Net products per glucose: 2 Pyruvate + 2 ATP (net) + 2 NADH - Key enzymes: Hexokinase, Phosphofructokinase (PFK), Pyruvate kinase
Q4. What is the significance of the Krebs cycle?
Answer
The Krebs cycle (TCA cycle, citric acid cycle) is significant because: 1. It is the central pathway of aerobic respiration — completes the oxidation of Acetyl CoA 2. Produces reduced co-enzymes (NADH, FADH₂) that drive ATP synthesis in the ETC 3. Produces 2 ATP directly per glucose (substrate-level phosphorylation) 4. It is an amphibolic pathway — provides intermediates for biosynthesis (amino acids, fatty acids) 5. Produces CO₂ as a waste product 6. Provides carbon skeletons for various biosynthetic pathways
VI. Answer in Detail (5-mark questions)
Q1. Describe the process of glycolysis in detail.
Answer
Glycolysis (EMP pathway) is the first stage of respiration, occurring in the cytoplasm. It converts one glucose molecule (6C) into two pyruvate molecules (3C each). It does not require oxygen. Phase 1: Investment Phase (Energy Investment) 1. Glucose → Glucose-6-phosphate: Hexokinase catalyses the phosphorylation of glucose using 1 ATP 2. Glucose-6-phosphate → Fructose-6-phosphate: Phosphoglucoisomerase 3. Fructose-6-phosphate → Fructose-1,6-bisphosphate: Phosphofructokinase (PFK) adds a second phosphate using 1 ATP. This is the committed step of glycolysis. 4. Fructose-1,6-bisphosphate → 2 × Glyceraldehyde-3-phosphate (G3P): Aldolase splits the 6C molecule into two 3C molecules (G3P and DHAP; DHAP is converted to G3P) Investment Phase uses 2 ATP per glucose Phase 2: Payoff Phase (Energy Production) For each G3P (x2, since there are 2 G3P per glucose): 5. G3P → 1,3-bisphosphoglycerate (1,3-BPG): Glyceraldehyde-3-phosphate dehydrogenase produces NADH 6. 1,3-BPG → 3-phosphoglycerate (3-PGA): Phosphoglycerate kinase produces 1 ATP (substrate-level phosphorylation) 7. 3-PGA → 2-phosphoglycerate (2-PGA): Phosphoglycerate mutase 8. 2-PGA → Phosphoenolpyruvate (PEP): Enolase; releases H₂O 9. PEP → Pyruvate: Pyruvate kinase produces 1 ATP (substrate-level phosphorylation) Payoff Phase produces 4 ATP + 2 NADH per glucose Net yield per glucose molecule: - 2 Pyruvate - 2 ATP (4 produced - 2 used) - 2 NADH
Q2. Describe the Krebs cycle (TCA cycle).
Answer
The Krebs cycle (also called the TCA cycle or citric acid cycle) occurs in the mitochondrial matrix. It was discovered by Hans Krebs (1937). It oxidises Acetyl CoA to CO₂ and produces NADH, FADH₂, and ATP. Link reaction (Pyruvate oxidation): - Pyruvate (3C) + CoA + NAD⁺ → Acetyl CoA (2C) + NADH + CO₂ - This occurs in the mitochondrial matrix - Enzyme: Pyruvate dehydrogenase complex Steps of the Krebs Cycle (per Acetyl CoA): 1. Condensation: Acetyl CoA (2C) + Oxaloacetate (OAA, 4C) → Citrate (6C) - Enzyme: Citrate synthase 2. Isomerisation: Citrate (6C) → Isocitrate (6C) - Enzyme: Aconitase 3. Oxidative decarboxylation I: Isocitrate (6C) → α-ketoglutarate (5C) + CO₂ + NADH - Enzyme: Isocitrate dehydrogenase 4. Oxidative decarboxylation II: α-ketoglutarate (5C) → Succinyl CoA (4C) + CO₂ + NADH - Enzyme: α-ketoglutarate dehydrogenase 5. Substrate-level phosphorylation: Succinyl CoA (4C) → Succinate (4C) + ATP - Enzyme: Succinyl CoA synthetase - This is the only ATP produced directly in the Krebs cycle (substrate-level phosphorylation) 6. Oxidation: Succinate (4C) → Fumarate (4C) + FADH₂ - Enzyme: Succinate dehydrogenase - This is the only FADH₂ produced in the cycle 7. Hydration: Fumarate (4C) → Malate (4C) - Enzyme: Fumarase 8. Oxidation: Malate (4C) → Oxaloacetate (OAA, 4C) + NADH - Enzyme: Malate dehydrogenase Products per Acetyl CoA (one turn): - 3 NADH - 1 FADH₂ - 1 ATP - 2 CO₂ Products per glucose (two turns): - 6 NADH - 2 FADH₂ - 2 ATP - 4 CO₂
Q3. Describe the electron transport chain (ETC) and oxidative phosphorylation.
Answer
The electron transport chain (ETC) and oxidative phosphorylation occur on the inner mitochondrial membrane. They produce the majority of ATP in aerobic respiration. Electron Transport Chain: Electrons from NADH and FADH₂ are passed through a series of protein complexes embedded in the inner mitochondrial membrane: Complex I (NADH dehydrogenase): - Receives electrons from NADH - Transfers electrons to Ubiquinone (Coenzyme Q) - Pumps H⁺ from matrix to intermembrane space Complex II (Succinate dehydrogenase): - Receives electrons from FADH₂ (from succinate in Krebs cycle) - Transfers electrons to Ubiquinone - Does NOT pump H⁺ Ubiquinone (Coenzyme Q): - Mobile electron carrier that shuttles electrons from Complex I and II to Complex III Complex III (Cytochrome bc₁ complex): - Receives electrons from Ubiquinone - Transfers electrons to Cytochrome c - Pumps H⁺ from matrix to intermembrane space Cytochrome c: - Mobile electron carrier between Complex III and IV Complex IV (Cytochrome c oxidase): - Receives electrons from Cytochrome c - Transfers electrons to the final electron acceptor — O₂ - O₂ + 4e⁻ + 4H⁺ → 2H₂O - Pumps H⁺ from matrix to intermembrane space Oxidative Phosphorylation (Chemiosmosis): The H⁺ ions pumped by Complexes I, III, and IV create a proton gradient across the inner mitochondrial membrane: - Higher H⁺ concentration in the intermembrane space - Lower H⁺ concentration in the matrix This proton gradient (proton motive force) drives H⁺ ions back into the matrix through ATP synthase (F₀-F₁ complex): - F₀ = transmembrane channel for H⁺ - F₁ = catalytic head that synthesises ATP - As H⁺ flows through F₀, the F₁ head rotates and catalyses: ADP + Pi → ATP ATP yield: - Each NADH produces approximately 2.5–3 ATP - Each FADH₂ produces approximately 1.5–2 ATP (enters at Complex II, bypasses Complex I) Total ATP per glucose: - Glycolysis: 2 ATP + 2 NADH → ~7 ATP - Pyruvate oxidation: 2 NADH → ~5 ATP - Krebs cycle: 2 ATP + 6 NADH + 2 FADH₂ → ~20 ATP - Total: ~36–38 ATP (varies depending on shuttle systems)
Common Mistakes to Avoid
- Saying glycolysis requires oxygen: Glycolysis does NOT require oxygen. It is an anaerobic process in the cytoplasm.
- Confusing ATP yield in aerobic vs anaerobic respiration: Aerobic = ~36-38 ATP per glucose. Anaerobic = 2 ATP (from glycolysis only).
- Confusing NADH and FADH₂ ATP yields: NADH produces ~2.5-3 ATP. FADH₂ produces ~1.5-2 ATP. FADH₂ enters at Complex II, bypassing Complex I.
- Forgetting the link reaction: Pyruvate → Acetyl CoA is a separate step between glycolysis and the Krebs cycle. It produces 2 NADH and 2 CO₂ per glucose.
- Confusing RQ values: Carbohydrates RQ = 1. Fats RQ < 1 (approx 0.7). Organic acids RQ > 1. Anaerobic RQ = infinity.
- Saying the Krebs cycle only breaks down molecules: The Krebs cycle is AMPHIBOLIC — it both breaks down (catabolic) and provides building blocks for synthesis (anabolic).
- Confusing substrate-level phosphorylation and oxidative phosphorylation: Substrate-level = direct ATP production (glycolysis, Krebs cycle). Oxidative = ATP production through ETC and chemiosmosis.
Exam Checklist
- [ ] Can you write the overall equation for aerobic respiration?
- [ ] Can you describe glycolysis with its steps and net products?
- [ ] Can you describe the Krebs cycle with products per turn and per glucose?
- [ ] Can you explain the ETC and oxidative phosphorylation?
- [ ] Can you calculate the total ATP yield per glucose molecule?
- [ ] Can you differentiate aerobic and anaerobic respiration?
- [ ] Can you describe alcoholic and lactic acid fermentation?
- [ ] Can you define and calculate RQ for different substrates?
- [ ] Can you explain why the Krebs cycle is an amphibolic pathway?
- [ ] Can you explain chemiosmosis and ATP synthase?
