Biomolecules: Structure, Function and Quantitative Tests
The biomolecules domain of the Basque PAU Biology exam tests your ability to connect molecular structure with biological function and to solve simple quantitative problems. Questions often compare water, carbohydrates, lipids, proteins, enzymes and nucleic acids, or ask you to interpret a chemical test or a table.
Water and small molecules
- Water is polar; hydrogen bonds give cohesion, adhesion, a high specific heat, a high latent heat of vaporisation and solvent power.
- Osmosis depends on water potential, not on solute concentration alone.
Carbohydrates and lipids
- Monosaccharides are aldoses or ketoses; disaccharides form by glycosidic bonds; starch and glycogen are α-glucose polymers; cellulose is a β-1,4 glucose polymer.
- Reducing sugars reduce Benedict's reagent; sucrose is non-reducing.
- Saponifiable lipids include triglycerides and phospholipids; unsaturated fatty acids contain C=C bonds and generally have lower melting points.
Proteins and enzymes
- Amino acids have amino and carboxyl groups; peptide bonds form by condensation.
- Primary structure is sequence; secondary is α-helix or β-sheet; tertiary is three-dimensional folding; quaternary is subunit assembly.
- Denaturation disrupts non-covalent bonds but not the primary sequence.
- Enzymes lower activation energy; Km is the substrate concentration at half Vmax; competitive inhibitors raise Km, while non-competitive inhibitors lower Vmax.
Nucleic acids
- A nucleotide is a pentose plus phosphate plus nitrogenous base; DNA is antiparallel and double-stranded with A=T and G≡C; RNA is usually single-stranded.
Worked example
A polypeptide has 152 amino acids. How many peptide bonds does it contain, and how many water molecules are released during synthesis? Each peptide bond joins two amino acids and releases one water. For n amino acids in one chain, peptide bonds = n − 1 = 151, and water molecules released = 151. If the protein had two chains, the count would be 152 − 2 = 150. The trap is assuming one bond per amino acid.
Traps
- Sucrose is non-reducing even though it contains glucose and fructose.
- Starch and cellulose both contain glucose but differ in linkage and branching.
- Saturation questions refer to fatty acid chains, not glycerol.
- In DNA, if A = 30%, then T = 30%, so G + C = 40% and G = 20%.