Biomolecules
1. Introduction
Biomolecules are organic compounds essential for life. This chapter covers carbohydrates, proteins, vitamins, and nucleic acids — the building blocks of living organisms.
2. Carbohydrates
Optically active polyhydroxy aldehydes or ketones. General formula Cₓ(H₂O)ᵧ.
2.1 Classification
Monosaccharides: Simplest, cannot be hydrolysed. (Glucose, Fructose) Oligosaccharides: 2-10 monosaccharide units. (Sucrose, Maltose, Lactose) Polysaccharides: Many monosaccharide units. (Starch, Cellulose, Glycogen)
2.2 Glucose (C₆H₁₂O₆)
Open chain structure (Fischer projection): Aldohexose with -CHO and five -OH groups.
Key reactions:
- With HCN: Cyanohydrin formation.
- With Br₂ water: Gluconic acid (confirms -CHO is present).
- With NH₂OH: Oxime formation.
- Acetylation: Pentaacetate (confirms five -OH groups).
- Reduction: Sorbitol.
Ring structure: Glucose exists as pyranose (6-membered ring). α and β anomers.
2.3 Fructose (C₆H₁₂O₆)
Ketohexose. Reducing sugar. Ring structure: furanose (5-membered ring).
2.4 Sucrose (C₁₂H₂₂O₁₁)
Glucose + Fructose (α-1,2 glycosidic bond). Non-reducing sugar. Hydrolysed by invertase.
2.5 Polysaccharides
Starch: Amylose (20%, linear) + Amylopectin (80%, branched). Storage in plants. Cellulose: β-1,4 glycosidic linkage. Structural component of plant cell walls. Not digestible by humans.
3. Proteins
3.1 Amino Acids
Building blocks of proteins. General formula R-CH(NH₂)-COOH.
Zwitterion: In neutral solution, amino acids exist as dipolar ions (NH₃⁺-CH(R)-COO⁻).
Amino acids are amphoteric (act as both acid and base). Isoelectric point: pH at which the amino acid has no net charge.
3.2 Peptide Bond
-CO-NH- bond formed between the carboxyl of one amino acid and the amino of another, with elimination of water.
Primary structure: Linear sequence of amino acids. Secondary structure: α-helix or β-pleated sheet (H-bonding). Tertiary structure: 3D folding (disulphide bonds, ionic bonds, H-bonds). Quaternary structure: Multiple polypeptide chains (e.g., haemoglobin).
3.3 Denaturation
Loss of biological activity due to disruption of secondary, tertiary, and quaternary structures. Example: Boiling an egg (albumin coagulates).
4. Vitamins
| Vitamin | Solubility | Source | Deficiency Disease |
|---|---|---|---|
| A | Fat | Carrots, milk | Night blindness |
| B₁ | Water | Cereals | Beriberi |
| B₂ | Water | Milk, eggs | Cheilosis |
| B₆ | Water | Meat, bananas | Anaemia |
| B₁₂ | Water | Meat, eggs | Pernicious anaemia |
| C | Water | Citrus fruits | Scurvy |
| D | Fat | Sunlight, fish | Rickets |
| E | Fat | Nuts, seeds | Sterility |
| K | Fat | Green vegetables | Bleeding |
5. Nucleic Acids
DNA: Deoxyribonucleic acid. Double helix (Watson-Crick). Stores genetic information. RNA: Ribonucleic acid. Single stranded. Involved in protein synthesis.
Components: Sugar (deoxyribose/ribose), phosphate, nitrogenous bases (A, G, C, T/U).
6. Worked Problems
Problem 1: Glucose reacts with Br₂ water but not with Schiff's reagent. Explain. Solution: Glucose exists predominantly in cyclic hemiacetal form. Br₂ water slowly oxidises the open chain form. Schiff's test requires free -CHO.
Problem 2: How many peptide bonds are in a tripeptide? Solution: A tripeptide has 3 amino acids and 2 peptide bonds.
7. Common Mistakes
'Students often confuse DNA and RNA bases: DNA has A, G, C, T. RNA has A, G, C, U (no T, has U instead).'
8. ISC Exam Focus
| Topic | Theory Marks | Practical Marks |
|---|---|---|
| Carbohydrates | 4 | 2 |
| Proteins and amino acids | 4 | 2 |
| Vitamins | 2 | 1 |
| Nucleic acids | 2 | 1 |
9. Self-Test Questions
- Write the open chain structure of D-glucose and explain any three reactions confirming its functional groups.
- What are essential and non-essential amino acids? Give two examples of each.
- Explain the secondary structure of proteins with the α-helix model.
- Differentiate between DNA and RNA in structure and function.
- Why is sucrose a non-reducing sugar while maltose is a reducing sugar?
