Chapter 11 — Respiration in Plants
Key Concepts Quick Recap
| Term | Definition |
|---|
| Respiration | Oxidative breakdown of organic molecules to release energy (ATP) |
| Aerobic respiration | Uses O₂; complete oxidation; CO₂ + H₂O + 38 ATP per glucose |
| Anaerobic respiration | Without O₂; incomplete oxidation; fermentation |
| Glycolysis | Glucose (6C) → 2 Pyruvate (3C); in cytoplasm; 2 net ATP |
| Krebs cycle (TCA cycle) | Pyruvate → CO₂ + NADH + FADH₂; in mitochondrial matrix |
| ETC (Oxidative phosphorylation) | NADH and FADH₂ → ATP via electron transport chain; inner mitochondrial membrane |
| Fermentation | Anaerobic breakdown: glucose → ethanol + CO₂ (yeast) or lactate (bacteria) |
| RQ | Respiratory Quotient = CO₂ released / O₂ consumed |
| Gluconeogenesis | Synthesis of glucose from non-carbohydrate substrates |
I. Choose the Best Answer (MCQs)
1. Glycolysis occurs in: (a) Mitochondrial matrix (b) Cytoplasm (c) Thylakoid membrane (d) Nucleus
Answer(b) Cytoplasm Glycolysis (Embden-Meyerhof-Parnas pathway) occurs in the cytoplasm (cytosol). It does not require mitochondria and occurs in both aerobic and anaerobic conditions. It converts 1 glucose (6C) → 2 pyruvate (3C), producing 2 net ATP and 2 NADH.
2. The net ATP produced during glycolysis from one molecule of glucose is: (a) 4 (b) 2 (c) 38 (d) 36
Answer(b) 2 Glycolysis: 4 ATP produced, 2 ATP consumed (activation) = 2 net ATP. Also produces 2 NADH. The 2 NADH can be used in ETC to produce more ATP (but in cytoplasm, needs shuttle to enter mitochondria).
3. The Krebs cycle takes place in: (a) Cytoplasm (b) Nucleus (c) Mitochondrial matrix (d) Chloroplast
Answer(c) Mitochondrial matrix The Krebs (TCA/citric acid) cycle enzymes are in the mitochondrial matrix. Pyruvate from glycolysis enters the mitochondria, is converted to Acetyl-CoA (by pyruvate decarboxylation), then enters the Krebs cycle.
4. The respiratory quotient (RQ) for carbohydrates is: (a) 0.7 (b) > 1 (c) 1 (d) 0
Answer(c) 1 For carbohydrates: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O RQ = 6CO₂/6O₂ = 1.0 For fats: RQ ≈ 0.7 (need more O₂ to oxidise). For organic acids (malic acid): RQ > 1 (produce more CO₂ than O₂ consumed).
5. In fermentation by yeast, the end products are: (a) Lactic acid + CO₂ (b) Ethanol + CO₂ (c) Pyruvate + H₂O (d) Acetic acid + H₂
Answer(b) Ethanol + CO₂ Yeast fermentation: Glucose → Pyruvate (glycolysis) → Acetaldehyde → Ethanol + CO₂. Enzyme: pyruvate decarboxylase (pyruvate → acetaldehyde + CO₂), then alcohol dehydrogenase (acetaldehyde + NADH → ethanol + NAD⁺).
6. Which coenzyme accepts electrons during Krebs cycle? (a) ATP (b) FAD and NAD⁺ (c) NADP⁺ (d) CoA
Answer(b) FAD and NAD⁺ In the Krebs cycle, NAD⁺ is reduced to NADH (at 3 steps) and FAD is reduced to FADH₂ (at 1 step) per turn. These carry electrons to the ETC in the inner mitochondrial membrane for oxidative phosphorylation.
7. Cyanide is a respiratory poison because it: (a) Inhibits glycolysis (b) Inhibits cytochrome oxidase (Complex IV), blocking ETC (c) Prevents glucose entry into cells (d) Inhibits the Krebs cycle
Answer(b) Inhibits cytochrome oxidase (Complex IV) Cyanide (CN⁻) binds irreversibly to cytochrome oxidase (Complex IV = terminal electron acceptor in ETC) → blocks electron transfer to O₂ → ETC stops → no more ATP synthesis → cell death. Even glycolysis continues, but cells cannot produce enough ATP.
II. Short Answer Questions
Q1. Describe glycolysis with all the key steps.
Answer
Glycolysis (in cytoplasm): 1 Glucose (6C) → 2 Pyruvate (3C)
Phase 1 — Energy investment (2 ATP used): 1. Glucose + ATP → Glucose-6-phosphate (hexokinase) 2. Glucose-6-phosphate → Fructose-6-phosphate (phosphoglucoisomerase) 3. Fructose-6-phosphate + ATP → Fructose-1,6-bisphosphate (phosphofructokinase — rate-limiting step) 4. Fructose-1,6-bisphosphate → 2 × Glyceraldehyde-3-phosphate (G3P)
Phase 2 — Energy recovery (4 ATP + 2 NADH produced): 5. 2 G3P + 2NAD⁺ → 2 × 1,3-BPG + 2 NADH (G3P dehydrogenase) 6. 2 × 1,3-BPG + 2 ADP → 2 × 3-PGA + 2 ATP (phosphoglycerate kinase) 7. 2 × 3-PGA → 2 × 2-PGA (phosphoglycerate mutase) 8. 2 × 2-PGA → 2 × PEP + 2H₂O (enolase) 9. 2 × PEP + 2 ADP → 2 Pyruvate + 2 ATP (pyruvate kinase)
Net yield: 2 ATP + 2 NADH (per glucose)
Q2. Describe the Krebs cycle.
Answer
The Krebs (TCA/citric acid) cycle occurs in the mitochondrial matrix. Discovered by Hans Krebs (1937).
Entry: Pyruvate (3C) → Acetyl-CoA (2C) + CO₂ + NADH [pyruvate decarboxylation]
One turn of Krebs cycle (per Acetyl-CoA):
1. Acetyl-CoA (2C) + Oxaloacetate (4C) → Citrate (6C) [citrate synthase] 2. Citrate → Isocitrate 3. Isocitrate → α-ketoglutarate (5C) + CO₂ + NADH 4. α-ketoglutarate → Succinyl-CoA (4C) + CO₂ + NADH 5. Succinyl-CoA → Succinate + GTP (= ATP) 6. Succinate → Fumarate + FADH₂ 7. Fumarate → Malate 8. Malate → Oxaloacetate + NADH
Per Acetyl-CoA: 2 CO₂, 3 NADH, 1 FADH₂, 1 GTP Per glucose (2 Acetyl-CoA): 4 CO₂, 6 NADH, 2 FADH₂, 2 GTP
Q3. Calculate the total ATP yield from aerobic respiration of one glucose.
Answer
| Stage | ATP | NADH → ATP | FADH₂ → ATP |
|---|
| Glycolysis | 2 net ATP | 2 NADH × 2.5 = 5 ATP | — |
| Pyruvate decarboxylation | — | 2 NADH × 2.5 = 5 ATP | — |
| Krebs cycle (×2) | 2 GTP | 6 NADH × 2.5 = 15 ATP | 2 FADH₂ × 1.5 = 3 ATP |
| Total | ~38 ATP | | |
(Older textbooks use 36-38 ATP; modern P/O ratio: NADH = 2.5 ATP, FADH₂ = 1.5 ATP → 30 ATP from ETC + 4 substrate-level + 4 from glycolysis cytoplasmic NADH shuttle = ~30-38 depending on shuttle)
Common Mistakes
- Saying glycolysis occurs in mitochondria — it occurs in CYTOPLASM.
- Forgetting Krebs cycle per glucose requires 2 turns (2 pyruvate per glucose).
- Confusing NADH (from Krebs cycle) and NADPH (from photosynthesis light reaction).
- RQ for fats: 0.7 (not 1); RQ for organic acids: > 1 (not 1).
Exam Checklist
- Glycolysis: cytoplasm; glucose → 2 pyruvate; 2 net ATP + 2 NADH.
- Krebs cycle: mitochondrial matrix; 2 CO₂ + 3 NADH + 1 FADH₂ + 1 GTP per acetyl-CoA.
- Total ATP: ~38 per glucose (substrate + oxidative phosphorylation).
- RQ: carbohydrates = 1; fats = 0.7; organic acids > 1.
- Fermentation: yeast → ethanol + CO₂; bacteria → lactic acid.
- Cyanide inhibits Complex IV (cytochrome oxidase).
