Chapter 10: Biomolecules
Key Topics: Carbohydrate Classification, Glucose Chemistry, Proteins & Zwitterion Structure, Nucleic Acids (DNA vs. RNA Base Pairing), and Vitamins & Enzymes.
1. Carbohydrates & Their Classification
Carbohydrates are optically active polyhydroxy aldehydes or ketones, or compounds that yield such units on hydrolysis.
| Category | Hydrolysis Behavior | Examples |
|---|---|---|
| Monosaccharides | Cannot be hydrolyzed further into simpler polyhydroxy aldehydes/ketones. | Glucose, Fructose, Ribose |
| Oligosaccharides | Yield 2 to 10 monosaccharide units on hydrolysis (e.g., Disaccharides). | Sucrose, Lactose, Maltose |
| Polysaccharides | Yield a large number of monosaccharide units on hydrolysis. | Starch, Cellulose, Glycogen |
2. Structure and Reactions of D-Glucose
Glucose ($\text{C}_6\text{H}_{12}\text{O}_6$) is an aldohexose and the most abundant carbohydrate in nature.
Action of Hydroiodic Acid (HI) - Straight Chain Proof:
$$\text{CHO}-(\text{CHOH})_4-\text{CH}_2\text{OH} \xrightarrow{\text{Prolonged heating with HI}} \text{CH}_3-(\text{CH}_2)_4-\text{CH}_3 \ (\text{n-Hexane})$$
Reaction with Hydroxylamine ($\text{NH}_2\text{OH}$) - Carbonyl Group Proof:
$$\text{CHO}-(\text{CHOH})_4-\text{CH}_2\text{OH} + \text{NH}_2\text{OH} \rightarrow \text{CH}=\text{N-OH}-(\text{CHOH})_4-\text{CH}_2\text{OH} \ (\text{Oxime})$$
Oxidation with Bromine Water ($\text{Br}_2\text{ water}$) - Aldehydic Group Proof:
$$\text{CHO}-(\text{CHOH})_4-\text{CH}_2\text{OH} \xrightarrow{\text{Br}_2\text{ water}} \text{COOH}-(\text{CHOH})_4-\text{CH}_2\text{OH} \ (\text{Gluconic Acid})$$
3. Proteins, Amino Acids & Peptide Linkage
Proteins are high molecular mass polymers of $\alpha$-amino acids linked together by peptide bonds.
Zwitterionic Structure of Amino Acids:
$$\text{H}_2\text{N-CHR-COOH} \rightleftharpoons \text{H}_3\text{N}^+-\text{CHR-COO}^- \ (\text{Zwitterion})$$
$$\text{In zwitterionic form, amino acid shows amphoteric behavior (reacts with both acids and bases).}$$
Peptide Bond Formation:
$$-\text{COOH} + \text{H}_2\text{N}- \xrightarrow{-\text{H}_2\text{O}} -\text{CO-NH}- \ (\text{Peptide Linkage})$$
4. Nucleic Acids (DNA and RNA)
Nucleic acids are long-chain polymers of nucleotides responsible for the transmission of hereditary characteristics.
| Feature | DNA (Deoxyribonucleic Acid) | RNA (Ribonucleic Acid) |
|---|---|---|
| Sugar Component | $\beta$-D-2-deoxyribose | $\beta$-D-ribose |
| Nitrogenous Bases | Adenine (A), Guanine (G), Cytosine (C), Thymine (T) | Adenine (A), Guanine (G), Cytosine (C), Uracil (U) |
| Base Pairing (Chargaff's Rule) | $A = T$ (linked by 2 H-bonds), $G \equiv C$ (linked by 3 H-bonds) | Usually single-stranded |
| Primary Function | Carries genetic information from generation to generation. | Protein synthesis in the cell. |
5. Vitamins and Enzymes
- Vitamins: Organic compounds required in small amounts for normal health and growth. Divided into Fat-soluble (Vitamins A, D, E, K) and Water-soluble (Vitamins B-complex and C).
- Enzymes: Biocatalysts produced by living cells which are proteinaceous in nature. They are highly specific and efficient in catalyzing biochemical reactions.