Biological Molecules
Carbohydrates, lipids, proteins, and biochemical tests
Carbohydrates: Made of C, H, O. **Monosaccharides** (glucose, fructose) → **disaccharides** (maltose, sucrose, lactose) via condensation → **polysaccharides** (starch, glycogen, cellulose). Starch = amylose (helical) + amylopectin (branched). Cellulose has β-glucose with hydrogen bonds between chains → strong cell walls.
Lipids: Triglycerides = glycerol + 3 fatty acids (ester bonds). **Saturated** = no C=C double bonds (solid fats). **Unsaturated** = has C=C (liquid oils). Phospholipids have hydrophilic head + hydrophobic tails → form cell membranes.
Proteins: Chains of amino acids joined by **peptide bonds** (condensation). Primary structure = amino acid sequence. Secondary = α-helix or β-pleated sheet (H-bonds). Tertiary = 3D folding (H-bonds, disulfide bridges, ionic bonds, hydrophobic interactions). Quaternary = multiple polypeptide chains (e.g., haemoglobin).
Biochemical tests: Benedict's test (reducing sugars → brick red). Iodine test (starch → blue-black). Biuret test (protein → purple). Emulsion test (lipids → white emulsion).
Key Points to Remember
- 1Carbs: mono → di → polysaccharides via condensation
- 2Lipids: triglycerides = glycerol + 3 fatty acids
- 3Protein structure: primary → secondary → tertiary → quaternary
- 4Benedict's (sugars), Iodine (starch), Biuret (protein)
Pakistan Example
Biryani Biochemistry — Every Macromolecule on One Plate
A plate of Karachi biryani contains all biological molecules: rice is rich in starch (polysaccharide), chicken provides protein (amino acid chains with quaternary structures in haemoglobin), ghee contains saturated triglycerides, and the yoghurt raita has lactose (disaccharide). A perfect A Level revision meal.