Biomolecules
Biomolecules tests the structure and function of carbohydrates, proteins, enzymes, vitamins, nucleic acids and hormones, with an emphasis on classification and simple tests.
Carbohydrates
Monosaccharides cannot be hydrolysed (glucose, fructose); disaccharides give two units (sucrose, maltose, lactose); polysaccharides give many (starch, cellulose, glycogen). Reducing sugars have a free anomeric –CHO or –CO– group and reduce Tollens/Fehling reagents. Sucrose is non-reducing because its glycosidic bond joins the anomeric carbons; hydrolysis gives glucose and fructose (invert sugar).
Proteins and enzymes
Amino acids join by peptide bonds; primary structure is sequence, secondary is α-helix/β-sheet, tertiary is 3D folding, quaternary is subunit assembly. Denaturation disrupts secondary and tertiary structure without breaking peptide bonds. Enzymes are highly specific biocatalysts; activity depends on pH and temperature and they are not consumed.
Nucleic acids, vitamins and hormones
DNA contains deoxyribose and thymine; RNA contains ribose and uracil. The double helix is held by hydrogen bonds between complementary bases (A–T, G–C). Vitamins A, D, E, K are fat-soluble; B and C are water-soluble. Hormones are chemical messengers; insulin is a protein hormone.
Worked example
Sucrose gives a negative Tollens test, but after boiling with dilute acid and neutralising, the hydrolysate gives a positive test. Reason: hydrolysis releases glucose and fructose, both reducing sugars, so the anomeric carbon becomes free.
Mark-losing traps
Calling sucrose reducing, confusing starch (blue with iodine) with cellulose, saying denaturation breaks peptide bonds, mixing DNA and RNA bases, and assuming all vitamins are water-soluble are common errors.