Chemistry

Macromolecules

3,371 words, 10 sections

This chapter is the chemistry of very large molecules, and in the papers it has been asked 218 distinct times across 75 official papers between 2000 and 2026, on six boards. The weight is uneven: protein classification and structure alone supplies 115 of those questions, carbohydrates and lipids 69, and enzymes as biocatalysts 33. A student who masters it can name a polymer and its monomer, tell addition from condensation polymerisation, classify a protein by composition and by function, describe the four levels of protein structure, and match a sugar, a base or an enzyme to its role.

A macromolecule is a molecule large enough that its size, not its functional groups, dominates its behaviour, and the usual way to build one is to join many small units. The repeating unit is the monomer and the large molecule is the polymer, so a polymer is a monomer written many times over. Ethene polymerises to polythene, written with the repeating unit in brackets and $n$ monomers consumed:

$n\text{CH}_2=\text{CH}_2 \xrightarrow{100\ \text{atm},\ 400^\circ\text{C}} [\text{CH}_2-\text{CH}_2]_n$

The idea reaches past plastics. Starch is a polymer of $\alpha$ -D-glucose, a protein is a polymer of amino acids joined by peptide bonds, and chlorophyll, the pigment of photosynthesis, is a naturally occurring macromolecule built round a single $\text{Mg}^{2+}$ ion. The distinction that matters is origin and route: some macromolecules are natural and some synthetic, and the synthetic ones are the ones whose monomers you are expected to name.

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Introduction

1. The macromolecule and the polymer conceptA macromolecule is a molecule large enough that its size, not its functional groups, dominates its behaviour, and the usual way to build one is to join many sma