Chapter 1 — Carbohydrates
Key Concepts Quick Recap
| Type | Carbon atoms | Examples | Function |
|---|
| Triose | 3 | Glyceraldehyde, DHAP | Glycolysis intermediates |
| Pentose | 5 | Ribose, Deoxyribose | RNA and DNA backbone |
| Hexose | 6 | Glucose, Fructose, Galactose | Energy (primary fuel) |
| Disaccharides | — | Maltose, Sucrose, Lactose | Transport, sweetening |
| Polysaccharides | — | Starch, Glycogen, Cellulose | Storage and structural |
I. Choose the Best Answer (MCQs)
1. The empirical formula of carbohydrates is: (a) CₙH₂ₙOₙ (b) (CH₂O)ₙ (c) CₙH₂ₙ₊₂Oₙ (d) (CHOH)ₙ
Answer(b) (CH₂O)ₙ Carbohydrates = hydrates of carbon: general formula (CH₂O)ₙ. Glucose: C₆H₁₂O₆ = (CH₂O)₆.
2. Glucose and fructose are: (a) Epimers (b) Enantiomers (c) Structural isomers (d) Diastereomers
Answer(c) Structural isomers Glucose (aldohexose) and fructose (ketohexose) have the same molecular formula (C₆H₁₂O₆) but different structural arrangement (glucose has aldehyde group; fructose has ketone group).
3. The linkage between glucose units in cellulose is: (a) α-1,4 glycosidic (b) β-1,4 glycosidic (c) α-1,6 glycosidic (d) β-1,6 glycosidic
Answer(b) β-1,4 glycosidic Cellulose: linear polymer of glucose with β-1,4 glycosidic bonds. This makes cellulose structurally rigid (hydrogen bonds between chains → microfibrils). Humans lack cellulase → cannot digest cellulose.
4. Which polysaccharide is the storage form of glucose in animals? (a) Starch (b) Cellulose (c) Glycogen (d) Chitin
Answer(c) Glycogen Glycogen: stored mainly in liver (blood glucose regulation) and muscle (local energy). Highly branched (α-1,4 and α-1,6 linkages). Called "animal starch."
5. Lactose is composed of: (a) Glucose + Glucose (b) Glucose + Fructose (c) Glucose + Galactose (d) Fructose + Galactose
Answer(c) Glucose + Galactose Lactose (milk sugar): β-galactose + glucose (β-1,4 glycosidic bond). Digested by lactase enzyme. Lactose intolerance: deficiency of lactase → undigested lactose fermented by gut bacteria → bloating, diarrhoea.
6. Benedict's test detects: (a) Polysaccharides (b) Reducing sugars (c) Amino acids (d) Fats
Answer(b) Reducing sugars Benedict's reagent (copper sulphate + sodium carbonate + sodium citrate): reducing sugars reduce Cu²⁺ to Cu⁺ → brick-red precipitate. Reducing sugars: glucose, fructose, galactose, maltose, lactose. Sucrose is NOT a reducing sugar.
7. Which of the following is a non-reducing sugar? (a) Glucose (b) Fructose (c) Sucrose (d) Maltose
Answer(c) Sucrose Sucrose: glucose + fructose joined by 1,2-glycosidic bond at the anomeric carbon of both → no free anomeric -OH → cannot reduce Cu²⁺ → non-reducing sugar.
II. Short Answer Questions
Q1. Compare starch, glycogen, and cellulose.
Answer
| Feature | Starch | Glycogen | Cellulose |
|---|
| Source | Plants | Animals | Plants |
| Location | Seeds, tubers | Liver, muscle | Cell wall |
| Function | Storage | Storage | Structural |
| Linkage | α-1,4 (amylose); α-1,6 (branch in amylopectin) | α-1,4 and α-1,6 (highly branched) | β-1,4 (linear) |
| Branching | Amylose: none; Amylopectin: moderate | High | None |
| Digestibility | Yes (amylase) | Yes | No (humans lack cellulase) |
| Iodine test | Blue-black (starch) | Reddish-brown | No colour change |
Q2. What is the significance of the glycosidic bond?
Answer
A glycosidic bond is a covalent bond formed between the anomeric carbon (C1) of one monosaccharide and a hydroxyl group of another, with loss of water (condensation reaction).
Significance:
- Links monosaccharides to form disaccharides and polysaccharides.
- Type of bond (α or β) determines properties: α-1,4 → digestible (starch); β-1,4 → indigestible (cellulose).
- Can be hydrolysed by specific enzymes: amylase (α-bonds), cellulase (β-bonds).
Exam Checklist
- Monosaccharides: glucose (aldose), fructose (ketose) — C₆H₁₂O₆; ribose — C₅H₁₀O₅.
- Disaccharides: maltose (glu+glu), sucrose (glu+fru), lactose (glu+gal).
- Polysaccharides: starch (plant, α-1,4/α-1,6), glycogen (animal, highly branched), cellulose (β-1,4).
- Benedict's test: brick-red precipitate with reducing sugars.
- Sucrose = non-reducing sugar.
- Cellulose: humans cannot digest (no cellulase); dietary fibre.
