Key ideas
Biological macromolecules are polymers built from small monomers by condensation reactions. Knowing which monomer, which bond and which test belongs to each class answers most questions, and a few counting rules answer the rest.
- Polysaccharides come from monosaccharides (glycosidic bonds), proteins from amino acids (peptide bonds), nucleic acids from nucleotides (phosphodiester bonds) and triglycerides from glycerol and three fatty acids (ester bonds).
- Joining n monomers into a chain by condensation releases n − 1 molecules of water. Hydrolysis adds water to break the bonds again.
- Counting rules: k kinds of monomers give kⁿ different chains of length n, and A–T pairs have 2 hydrogen bonds while G–C pairs have 3.
- Benedict’s test is positive for reducing sugars (glucose, fructose, maltose, lactose) but not for sucrose, starch or cellulose.
Key equations
peptides of length n from k kinds = kⁿmass ≈ 110n + 18 DaH-bonds = 2 × (A–T) + 3 × (G–C)saturated fatty acid CₙH₂ₙO₂water released = n − 1
Worked example
A DNA piece with 10 A–T pairs and 10 G–C pairs has 2 × 10 + 3 × 10 = 50 hydrogen bonds. Joining 10 amino acids into a peptide releases 9 water molecules, and a chain of 4 nucleotides can be arranged in 4⁴ = 256 ways. A C₁₈ fatty acid with two C=C bonds has 36 − 4 = 32 hydrogens, as in linoleic acid, C₁₈H₃₂O₂.
Common mistakes
- Counting n water molecules instead of n − 1 for a chain of n monomers.
- Expecting sucrose to give a positive Benedict’s test. It has no free anomeric carbon.
- Saying that a double bond adds hydrogens. Each C=C bond removes two hydrogens from the fully saturated chain.
Practice
Test yourself with practice questions on this topic: Biological macromolecules