Biomolecules and Polymers
This domain tests the classification and reactions of carbohydrates, the structure of proteins and nucleic acids, and the classification, synthesis and properties of polymers.
Carbohydrates
Monosaccharides (glucose, fructose) are reducing because of a free anomeric hydroxyl. Sucrose is non-reducing since the glycosidic bond ties up both anomeric carbons; on hydrolysis it gives equimolar glucose and fructose, and the rotation changes from +66.5° to about −39.9° (invert sugar). Maltose and lactose reduce. Starch is a polymer of α-glucose (amylose plus amylopectin), cellulose of β-glucose and cannot be digested by humans.
Proteins and enzymes
Amino acids are zwitterionic near pH 7 and join through peptide bonds formed with loss of water. Structure levels run from primary (sequence) to secondary (α-helix, β-sheet through hydrogen bonds), tertiary and quaternary. Denaturation destroys the higher levels but keeps the primary sequence. Enzymes are highly specific biocatalysts; vitamins are fat- or water-soluble.
Nucleic acids
A nucleotide = base + sugar + phosphate. DNA has deoxyribose, thymine and a double helix with A–T and G–C hydrogen bonding; RNA has ribose, uracil and is single-stranded (mRNA, tRNA, rRNA).
Polymers
Addition polymers: polyethylene, PVC, polystyrene, PAN. Condensation polymers: nylon-6,6 from adipic acid and hexamethylenediamine, terylene from ethylene glycol and terephthalic acid, bakelite from phenol and formaldehyde. Copolymers: Buna-S (butadiene + styrene) and Buna-N (butadiene + acrylonitrile). Vulcanisation cross-links rubber chains with sulphur; natural rubber is cis-1,4-polyisoprene.
Worked example
Sucrose gives no silver mirror with Tollens reagent, yet the solution does after boiling with dilute acid. Reasoning: the reducing C1 hydroxyl of glucose and the C2 hydroxyl of fructose are locked into the glycosidic linkage, so no free aldehyde or ketone group exists. Hydrolysis releases both monosaccharides, each of which reduces Tollens reagent, so the hydrolysed solution gives a positive test.
Traps that cost marks
- Calling every polysaccharide reducing.
- Forgetting that starch and cellulose differ by α- and β-linkage, not by monomer.
- Confusing nylon-6 with nylon-6,6 (the latter is a condensation polymer).
- Treating denaturation as hydrolysis of peptide bonds.