Chapter 14 — Biomolecules Exercise Solutions
Biomolecules is scoring when classification and examples are memorised clearly. Use tables to organise the information.
Key Concepts Quick Recap
| Biomolecule | Monomer | Bond | Function |
|---|---|---|---|
| Carbohydrates | Monosaccharides | Glycosidic bond | Energy source, cell walls, DNA backbone |
| Proteins | Amino acids | Peptide bond (amide) | Enzymes, structure, transport, immunity |
| Nucleic acids (DNA/RNA) | Nucleotides | Phosphodiester bond | Genetic information, protein synthesis |
| Lipids | Fatty acids + glycerol | Ester bond | Energy storage, membranes, hormones |
| Vitamins | Small organic molecules | — | Coenzymes, essential metabolic functions |
Carbohydrate Classification
| Type | Definition | Examples |
|---|---|---|
| Monosaccharide | Cannot be hydrolysed; single sugar unit | Glucose, fructose, galactose, ribose |
| Disaccharide | 2 monosaccharides; hydrolysed to 2 monomers | Sucrose, maltose, lactose |
| Oligosaccharide | 3–10 monosaccharides | Raffinose (3 units) |
| Polysaccharide | 100s–1000s of units | Starch, cellulose, glycogen |
Important Vitamin Table
| Vitamin | Chemical Name | Deficiency Disease | Solubility |
|---|---|---|---|
| A | Retinol | Night blindness, xerophthalmia | Fat |
| B₁ | Thiamine | Beriberi | Water |
| B₂ | Riboflavin | Cheilosis | Water |
| B₃ | Niacin | Pellagra | Water |
| B₁₂ | Cyanocobalamin | Pernicious anaemia | Water |
| C | Ascorbic acid | Scurvy | Water |
| D | Calciferol | Rickets (children), Osteomalacia (adults) | Fat |
| E | Tocopherol | Infertility, haemolysis | Fat |
| K | Phylloquinone | Impaired blood clotting | Fat |
I. Choose the Best Answer (MCQs)
1. Glucose is classified as: (a) Disaccharide (b) Ketohexose (c) Aldohexose (d) Aldopentose
2. The peptide bond in proteins is: (a) −NH−CO− (amide bond) (b) −O−CO− (ester bond) (c) −S−S− (disulphide) (d) −O−O− (peroxide)
3. Which of the following is a non-reducing sugar? (a) Glucose (b) Maltose (c) Sucrose (d) Lactose
4. The base present in DNA but NOT in RNA is: (a) Adenine (b) Guanine (c) Cytosine (d) Thymine
5. Which of the following is a fat-soluble vitamin? (a) Vitamin C (b) Vitamin B₁₂ (c) Vitamin D (d) Vitamin B₂
6. Enzymes are: (a) Inorganic catalysts (b) Lipid-based catalysts (c) Protein-based biological catalysts (d) Nucleic acid catalysts
7. The secondary structure of protein is maintained by: (a) Peptide bonds (b) Disulphide bridges (c) Hydrogen bonds (d) van der Waals forces
II. Short Answer Questions
Q1. Classify carbohydrates with examples. What are reducing sugars?
Q2. Describe the four levels of protein structure.
Q3. Compare DNA and RNA.
Q4. Write the functions of proteins with examples.
Worked Examples
Example 1 — Classifying carbohydrates
Monosaccharide: Glucose (C₆H₁₂O₆) — aldohexose
Fructose (C₆H₁₂O₆) — ketohexose
Ribose (C₅H₁₀O₅) — aldopentose (backbone of RNA)
Deoxyribose (backbone of DNA)
Disaccharide: Sucrose = Glucose + Fructose (non-reducing)
Maltose = Glucose + Glucose (reducing)
Lactose = Glucose + Galactose (reducing; present in milk)
Polysaccharide: Starch — storage in plants; α-glucose units
Cellulose — structural; β-glucose units (not digested by humans)
Glycogen — storage in animals (liver, muscle)
Example 2 — Reducing vs non-reducing sugars
Test: Fehling's solution (gives brick-red Cu₂O ppt) or Tollens' reagent (silver mirror)
Reducing sugars (positive test):
Glucose, fructose, maltose, lactose ✓
Non-reducing sugar (negative test):
Sucrose — both anomeric carbons engaged in glycosidic bond; no free −CHO ✗
After hydrolysis: sucrose → glucose + fructose → both are reducing
Invert sugar = mixture of glucose + fructose obtained by hydrolysis of sucrose
Example 3 — DNA bases and complementary base pairing
DNA double helix — Chargaff's rules:
%A = %T and %G = %C
Base pairs:
A (Adenine) pairs with T (Thymine) → 2 hydrogen bonds
G (Guanine) pairs with C (Cytosine) → 3 hydrogen bonds
If one strand of DNA is: 5'−ATGCCG−3'
Complementary strand: 3'−TACGGC−5'
RNA bases replace T with U:
mRNA transcribed: 5'−AUGCCG−3'
Example 4 — Vitamin deficiency diseases
Memorise this table:
A → Night blindness (inability to see in dim light)
B₁ → Beriberi (nervous system damage)
B₂ → Cheilosis (cracks at corner of mouth)
B₃ → Pellagra (3D: Dermatitis, Diarrhoea, Dementia)
B₁₂ → Pernicious anaemia (impaired red blood cell production)
C → Scurvy (bleeding gums, poor wound healing)
D → Rickets in children, Osteomalacia in adults (bone softening)
E → Haemolysis, reproductive problems
K → Impaired blood clotting (haemorrhage)
Example 5 — Enzyme specificity
Lock-and-key model (Emil Fischer, 1890):
- The active site of an enzyme has a specific shape (the "lock").
- Only a substrate with complementary shape (the "key") can bind.
- Enzyme–substrate complex forms → reaction occurs → products released → enzyme recycled.
Induced fit model (Koshland):
- Active site is flexible and adjusts its shape when substrate approaches.
- More accurate for most enzymes.
Examples of enzyme specificity:
Urease: only hydrolyses urea → NH₃ + CO₂
Amylase: only hydrolyses starch (α-1,4 glycosidic bonds)
Pepsin: cleaves peptide bonds in acidic stomach conditions only
Answer Pattern
1. Define the biomolecule. 2. State its classification. 3. Give one specific function. 4. Give one example with a real context.
Common Mistakes
- Confusing reducing and non-reducing sugars — sucrose is non-reducing.
- Mixing DNA and RNA bases — thymine is only in DNA; uracil only in RNA.
- Saying all vitamins are water-soluble — A, D, E, K are fat-soluble.
- Vague vitamin functions — always mention the deficiency disease.
- Confusing secondary structure (H-bonds) with primary structure (peptide bonds).
Exam Checklist
- Biomolecule classification is correct.
- Examples are valid (not generic).
- Vitamin deficiency disease is paired correctly.
- DNA/RNA comparison covers bases, sugar, and strand structure.
- Protein structure levels are described in order (1° → 4°).
Frequently Asked Questions
Yes. It is based on definitions, classification, structure, function, and examples. Memorise the key tables (carbohydrate types, vitamin deficiency diseases, DNA vs RNA).