Biological molecules, at a glance

Alpha and beta glucose differ at carbon 1; the orientation of their glycosidic bonds changes the polymer built.

• Alpha/beta differ at carbon 1.
• Condensation forms a glycosidic bond and releases water.
• Hydrolysis consumes water to break the bond.

Keep in mind: They have the same composition; the arrangement around carbon 1 differs.

Alpha-glucose polymers store glucose compactly; beta-1,4 cellulose chains form strong microfibrils.

• Amylose: unbranched alpha-1,4 coil.
• Amylopectin and glycogen: alpha-1,6 branches.
• Cellulose: beta-1,4 straight chains and hydrogen-bonded microfibrils.

Keep in mind: Cellulose is unbranched; its beta-1,4 chains are strengthened by numerous interchain hydrogen bonds.

Triglycerides have three fatty acids and are largely hydrophobic; phospholipids have a polar head and two non-polar tails.

• Ester bond: glycerol to fatty acid.
• Triglyceride: three fatty-acid tails.
• Phospholipid: hydrophilic head and hydrophobic tails.

Keep in mind: Its tails are non-polar, but its polar head interacts with water; this contrast drives bilayer formation.

Amino-acid sequence constrains folding; different interactions stabilise secondary, tertiary and quaternary structures.

• Primary: sequence.
• Secondary: backbone hydrogen bonding.
• Tertiary: one chain; quaternary: multiple polypeptide subunits.

Keep in mind: A protein with one polypeptide may have no quaternary structure.

Denaturation changes higher-level structure and function; it usually leaves the primary peptide sequence intact.

• High heat can disrupt folding.
• pH affects charge and interactions.
• Primary structure often survives denaturation.

Keep in mind: Moderate cooling usually lowers collision frequency and rate without permanently disrupting the protein.

Haemoglobin is four folded chains, each holding a haem group whose Fe2+ binds one oxygen molecule reversibly. Hydrophilic R groups on the outside make it soluble.

• Four subunits: two alpha-globin and two beta-globin chains (quaternary structure).
• Each subunit has one haem group with Fe2+, which binds one O2 reversibly. Up to four O2 per molecule.
• Hydrophobic R groups inside, hydrophilic R groups outside, so it is soluble.
• Cooperative binding: one O2 binding makes the next easier to load.

Keep in mind: Oxygen binds to the Fe2+ ion in each haem group. The polypeptide chains hold the haem in place and pass the shape change between subunits.

Three chains wind into a triple helix (tropocollagen) held by hydrogen bonds. Staggered molecules joined by covalent cross-links form fibrils and fibres with high tensile strength.

• Tropocollagen: three polypeptide chains wound into a triple helix.
• Every third amino acid is glycine. Its small R group lets the chains pack tightly at the centre.
• Hydrogen bonds between N-H and C=O of neighbouring chains hold the triple helix together.
• Covalent cross-links between lysine residues join staggered tropocollagen molecules into fibrils, then fibres.

Keep in mind: Peptide bonds join amino acids within each chain. Three chains form each tropocollagen molecule, held by hydrogen bonds, and covalent cross-links join neighbouring molecules.