Biological molecules
Carbohydrates and lipids
Monosaccharides (glucose, fructose, galactose, ribose) join by condensation to form glycosidic bonds, releasing water; hydrolysis breaks them. Disaccharides: maltose (glucose + glucose), sucrose (glucose + fructose), lactose (glucose + galactose). α-glucose polymers store energy: amylose (unbranched, 1,4, helical, compact), amylopectin and glycogen (1,4 and 1,6 branches, many ends for rapid hydrolysis); insoluble, so no osmotic effect. β-glucose forms cellulose: straight chains, alternate glucose flipped, linked by hydrogen bonds into microfibrils: high tensile strength. Triglycerides = glycerol + 3 fatty acids (ester bonds): energy store, insulation. Phospholipids have a hydrophilic phosphate head and hydrophobic tails: membranes. Tests: Benedict's (reducing sugars), acid hydrolysis then Benedict's (non-reducing), iodine (starch), ethanol emulsion (lipids).
≈ 50 minProteins and water
Amino acids (H₂N–CHR–COOH) join by condensation into peptide bonds. Primary structure: amino acid sequence. Secondary: α-helix or β-pleated sheet held by hydrogen bonds between C=O and N–H. Tertiary: further folding held by hydrogen, ionic and disulfide bonds and hydrophobic interactions between R groups. Quaternary: more than one polypeptide (and maybe a prosthetic group). Globular proteins (e.g. haemoglobin: 4 chains, haem with Fe²⁺, hydrophilic outside) are soluble and metabolic; fibrous proteins (e.g. collagen: three chains in a triple helix, glycine every third residue, cross-links) are insoluble and structural. Biuret test: purple. Water is polar; hydrogen bonding gives it high specific heat capacity, high latent heat of vaporisation, cohesion and solvent properties.
≈ 50 min