Chapter 6 — Metabolism of Carbohydrates
Key Concepts Quick Recap
| Pathway | Location | Reactant | Key Products |
|---|
| Glycolysis | Cytoplasm | Glucose | 2 Pyruvate + 2 ATP (net) + 2 NADH |
| Pyruvate decarboxylation | Mitochondrial matrix | Pyruvate | Acetyl-CoA + CO₂ + NADH |
| Krebs cycle (TCA) | Mitochondrial matrix | Acetyl-CoA | CO₂ + GTP + NADH + FADH₂ |
| ETC (Oxidative phosphorylation) | Inner mitochondrial membrane | NADH, FADH₂ | ATP + H₂O |
| Gluconeogenesis | Liver, kidney | Lactate, amino acids, glycerol | Glucose |
| Glycogenesis | Liver, muscle | Glucose-6-phosphate | Glycogen |
| Glycogenolysis | Liver, muscle | Glycogen | Glucose-1-phosphate → Glucose |
I. Choose the Best Answer (MCQs)
1. Glycolysis takes place in the: (a) Mitochondria (b) Nucleus (c) Cytoplasm (d) Endoplasmic reticulum
Answer(c) Cytoplasm Glycolysis (Embden-Meyerhof-Parnas pathway): 10 enzymatic steps; all occur in cytoplasm. Glucose → 2 pyruvate. Net yield: 2 ATP + 2 NADH + 2 pyruvate.
2. The end product of glycolysis under aerobic conditions is: (a) Lactic acid (b) Ethanol (c) Pyruvate (d) Acetyl-CoA
Answer(c) Pyruvate Glycolysis produces pyruvate. Aerobic: pyruvate → acetyl-CoA (mitochondria) → Krebs cycle. Anaerobic (yeast): pyruvate → ethanol + CO₂. Anaerobic (muscle): pyruvate → lactate.
3. The Krebs cycle occurs in the: (a) Cytoplasm (b) Mitochondrial matrix (c) Nucleus (d) Lysosome
Answer(b) Mitochondrial matrix Krebs cycle (TCA/citric acid cycle): 8 enzymatic reactions in mitochondrial matrix. Per acetyl-CoA: 2CO₂ + 3NADH + 1FADH₂ + 1GTP produced.
4. How many NADH molecules are produced per glucose in glycolysis? (a) 1 (b) 2 (c) 4 (d) 6
Answer(b) 2 Glycolysis: 2 NADH produced (one at each glyceraldehyde-3-phosphate dehydrogenase step). Per glucose: 2 pyruvate → 2 × 1 NADH = 2 NADH total.
5. The key enzyme that converts pyruvate to acetyl-CoA is: (a) Pyruvate kinase (b) Pyruvate dehydrogenase complex (c) Citrate synthase (d) Isocitrate dehydrogenase
Answer(b) Pyruvate dehydrogenase complex Pyruvate dehydrogenase complex (PDC): multi-enzyme complex in mitochondrial matrix. Catalyses: Pyruvate + CoA + NAD⁺ → Acetyl-CoA + CO₂ + NADH (irreversible step). Requires thiamine (B₁), lipoic acid, FAD, NAD, CoA.
6. Gluconeogenesis is the synthesis of glucose from: (a) Glycogen only (b) Non-carbohydrate precursors (lactate, amino acids, glycerol) (c) Fatty acids (d) Sucrose
Answer(b) Non-carbohydrate precursors (lactate, amino acids, glycerol) Gluconeogenesis: produces glucose from lactate (Cori cycle), glucogenic amino acids (most amino acids), glycerol (from fat breakdown). Occurs mainly in liver and kidney. NOT from fatty acids (acetyl-CoA cannot be converted to pyruvate in animals — no net gluconeogenesis from fats).
7. The final electron acceptor in the mitochondrial electron transport chain is: (a) NAD⁺ (b) FAD (c) Oxygen (O₂) (d) Carbon dioxide
Answer(c) Oxygen (O₂) In aerobic respiration, O₂ is the final electron acceptor. At Complex IV (cytochrome c oxidase): 4e⁻ + 4H⁺ + O₂ → 2H₂O. Cyanide and CO inhibit Complex IV → block ETC → ATP synthesis stops → death.
II. Short Answer Questions
Q1. Describe the net products of glycolysis.
Answer
Glycolysis: Glucose (6C) → 2 Pyruvate (3C each)
Input: 1 Glucose + 2 ATP + 2 NAD⁺ + 2 Pi
Output: 2 Pyruvate + 4 ATP + 2 NADH + 2 H₂O
Net gain: 2 ATP + 2 NADH
Key regulatory enzymes: hexokinase (step 1), phosphofructokinase-1 (PFK-1, step 3 — rate-limiting), pyruvate kinase (step 10).
Q2. What is the Cori cycle?
Answer
The Cori cycle (lactic acid cycle) connects muscle glycolysis with liver gluconeogenesis:
1. Muscle (active): O₂ insufficient → anaerobic glycolysis → glucose → 2 lactate + 2 ATP (for quick energy).
2. Lactate transport: Lactate released into blood → transported to liver.
3. Liver: Lactate → pyruvate (lactate dehydrogenase) → gluconeogenesis → glucose.
4. Glucose: Released into blood → taken up by muscle.
Energy balance: Muscle gains 2 ATP; liver uses 6 ATP for gluconeogenesis. Net: 4 ATP consumed (liver subsidises muscle energy).
Named after Carl and Gerty Cori (Nobel Prize 1947).
Exam Checklist
- Glycolysis: cytoplasm; glucose → 2 pyruvate; net 2 ATP + 2 NADH.
- Pyruvate → Acetyl-CoA: PDC, mitochondrial matrix; produces 1 NADH + CO₂.
- Krebs cycle: mitochondrial matrix; per acetyl-CoA: 2CO₂ + 3NADH + 1FADH₂ + 1GTP.
- ETC: inner mitochondrial membrane; O₂ is final acceptor; NADH → 2.5 ATP; FADH₂ → 1.5 ATP.
- Total per glucose: ~30-32 ATP (modern) or 36-38 (old).
- Gluconeogenesis: liver/kidney; from lactate, amino acids, glycerol (NOT from fatty acids).
- Cori cycle: muscle lactate → liver → glucose → back to muscle.
