Biomolecules
Biomolecules
Questions cover carbohydrates, proteins, enzymes, vitamins and nucleic acids, with emphasis on structure and simple chemical tests.
Core rules
- Glucose is an aldohexose; fructose is a ketohexose. Both give the same osazone.
- Sucrose is non-reducing because the glycosidic bond links the anomeric carbons; it hydrolyses to glucose and fructose.
- Starch gives a blue colour with iodine; cellulose is a β-1,4 polymer and is not digested by humans.
- Amino acids exist as zwitterions near neutral pH; the isoelectric point is the pH of zero net charge.
- Peptide bonds join amino acids; primary, secondary, tertiary and quaternary structures describe folding levels.
- DNA contains deoxyribose and A/T/G/C in a double helix; RNA contains ribose and A/U/G/C.
Worked example Why is sucrose non-reducing while maltose is reducing? In sucrose, the glycosidic link joins C-1 of glucose to C-2 of fructose, so neither anomeric carbon is free to open to an aldehyde. In maltose, one anomeric carbon remains free, so it can reduce Tollens' or Fehling's reagent.
Traps
- Saying all carbohydrates are reducing sugars.
- Confusing a nucleoside (base + sugar) with a nucleotide (base + sugar + phosphate).
- Forgetting that denaturation destroys secondary and tertiary structure, not the primary sequence.