Quick Summary
Biomolecules are the organic molecules that form the basis of life. The four main classes — carbohydrates, proteins, lipids, and nucleic acids — have distinct structures and functions. Enzymes deserve special attention as they control nearly all metabolic reactions. This chapter is high-yield for NEET with 3–5 questions.
Carbohydrates
Elements: C, H, O (ratio H:O usually 2:1)
| Class | Description | Examples |
|---|---|---|
| Monosaccharides | Simplest; cannot be hydrolysed | Glucose (6C), Fructose (6C), Ribose (5C) |
| Disaccharides | 2 monosaccharides + glycosidic bond | Sucrose (glucose + fructose), Maltose (glucose + glucose), Lactose (glucose + galactose) |
| Polysaccharides | Many monosaccharides | Starch (storage in plants), Glycogen (storage in animals), Cellulose (structural in plants), Chitin (fungal cell wall, insect exoskeleton) |
Functions: Energy (glucose), storage (starch, glycogen), structure (cellulose), component of nucleic acids (ribose, deoxyribose).
Proteins
Elements: C, H, O, N (and S in some) Monomer: Amino acids (20 types) — connected by peptide bonds
Levels of Protein Structure
| Level | Description |
|---|---|
| Primary | Linear sequence of amino acids |
| Secondary | α-helix or β-pleated sheet (H-bonds) |
| Tertiary | 3D folding of polypeptide (disulfide bonds, H-bonds, hydrophobic) |
| Quaternary | Multiple polypeptide subunits (e.g., haemoglobin — 4 subunits) |
Functions: Enzymes (catalysis), structural (collagen, keratin), transport (haemoglobin), hormones (insulin), antibodies, receptors.
Lipids
Elements: C, H, O (much less O than carbohydrates)
| Type | Description | Example |
|---|---|---|
| Fats/Oils | Glycerol + 3 fatty acids (triglyceride) | Butter (fat), olive oil |
| Phospholipids | Glycerol + 2 fatty acids + phosphate head | Cell membrane component |
| Sterols | Ring structure | Cholesterol, sex hormones |
| Waxes | Long-chain fatty acid + alcohol | Cuticle of leaves |
Functions: Energy storage (2× carbohydrates per gram), cell membrane structure, hormones, insulation, protection.
Nucleic Acids
Monomer: Nucleotide = Nitrogenous base + Pentose sugar + Phosphate group
| Feature | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose | Ribose |
| Bases | A, T, G, C | A, U, G, C |
| Strands | Double | Single |
| Location | Nucleus (mainly) | Nucleus + Cytoplasm |
| Function | Stores genetic info | Protein synthesis |
Base pairing in DNA: A=T (2 H-bonds), G≡C (3 H-bonds)
Types of RNA: mRNA (messenger), tRNA (transfer), rRNA (ribosomal)
Enzymes
Definition: Biological catalysts — proteins that speed up biochemical reactions without being consumed.
Properties of Enzymes
| Property | Detail |
|---|---|
| Specificity | One enzyme catalyses one reaction (lock-and-key model) |
| Catalytic efficiency | Can convert millions of substrate molecules per minute |
| Not consumed | Recovered unchanged after reaction |
| Optimum pH | Most work at pH 6–8; pepsin works at pH 2 |
| Optimum temperature | Most at 35–40°C; denatured above 60°C |
| Cofactors | Some need non-protein helpers (coenzyme, metal ions) |
Mechanism
- Substrate binds to active site → enzyme-substrate complex → products released → enzyme free
Lock and Key model (Fischer): Rigid active site fits only specific substrate. Induced fit model (Koshland): Active site changes shape slightly to fit substrate.
Enzyme Inhibition
| Type | Mechanism | Example |
|---|---|---|
| Competitive | Inhibitor resembles substrate, binds active site | Malonate inhibits succinate dehydrogenase |
| Non-competitive | Inhibitor binds elsewhere (allosteric site), changes active site shape | Heavy metals |
Important Questions
One Mark
- Name the monomer of proteins.
- Which base in RNA replaces thymine of DNA?
- What type of bond links amino acids in a protein?
- Define an enzyme.
Short Answer (3 marks)
- Compare DNA and RNA on the basis of sugar, bases, strands, and function.
- Describe the four levels of protein structure with examples.
- What are the properties of enzymes?
Long Answer (5 marks)
- Describe the structure of DNA double helix. Who proposed it and when?
- Explain enzyme inhibition with the difference between competitive and non-competitive inhibition.
Common Mistakes
- Sucrose is a non-reducing sugar; glucose and fructose are reducing sugars — Sucrose has no free aldehyde/ketone group.
- DNA base pairing: A with T (not U) in DNA; A with U only in RNA.
- Cellulose is NOT digested by humans (no cellulase enzyme) — forms dietary fibre.
- Enzymes lower activation energy — they do NOT change the free energy of reactants or products.
Board Exam Tips
- DNA vs RNA comparison (5 characters) is a standard 3-mark question.
- Enzyme properties and the lock-and-key model appear as 5-mark questions.
- Protein structure levels (primary to quaternary) with examples are regularly tested.
- NEET: Biomolecules contribute 3–5 questions — carbohydrates, enzymes, and nucleic acids are most common.