AQA A-Level Biology: Monomers and Polymers
A clear revision guide to monomers and polymers for AQA A-Level Biology: condensation and hydrolysis reactions, the bonds each group of molecules forms, and why the same two reactions build them all.
Almost every large molecule in this topic is built the same way, and broken down the same way. This guide covers what monomers and polymers are, the two reactions that join and separate them, and the bond each group of molecules forms. It is short, and it is worth learning first, because the rest of Topic 1 keeps using it.
Monomers and polymers
A monomer is a small molecule that can be joined to others of the same kind. A polymer is the long chain that results. Three of the four groups of molecules in this topic follow that pattern.
| Polymer | Built from | Monomer |
|---|---|---|
| Polysaccharide | Monosaccharides | Glucose, fructose, galactose |
| Polypeptide | Amino acids | Twenty different amino acids |
| Nucleic acid (DNA and RNA) | Nucleotides | DNA and RNA nucleotides |
Lipids are the exception. Triglycerides and phospholipids are not polymers, because they are not long chains of one repeating unit. They are still built by the same reaction, which is why they belong in this topic at all, and it is a common exam question to be asked why they are not polymers.
The phrase of the same kind in the definition of a monomer is worth holding onto, because it is what rules lipids out. A triglyceride is built from two different things, one glycerol and three fatty acids, and it stops at four units rather than continuing indefinitely. A polysaccharide is one monosaccharide repeated, which is what makes it a polymer.
Condensation and hydrolysis
Two reactions do all the work.
- A condensation reaction joins two molecules, forms a chemical bond between them, and releases one molecule of water.
- A hydrolysis reaction breaks that bond by adding one molecule of water, separating the two molecules again.
One water molecule is released for every bond formed, which gives you a way of answering a whole family of questions. Joining n monomers into one chain needs n − 1 bonds, so it releases n − 1 molecules of water. Ten amino acids joined into one polypeptide release nine molecules of water, not ten. Hydrolysing that polypeptide back to single amino acids uses nine.
Because water is a reactant in both reactions, it is described as a metabolite, which is one of the reasons the specification asks about water at all.
Telling the two reactions apart in an unlabelled diagram is a routine exam task, and there is a reliable way to do it. Find the water. If a molecule of water is being released alongside a newly formed bond, it is condensation. If a water molecule is being used up and a bond is breaking, it is hydrolysis. The name helps too: hydrolysis is literally splitting by water.
Hydrolysis is not a laboratory curiosity. It is what digestion is: every digestive enzyme you meet later catalyses the hydrolysis of a polymer into its monomers, which is why amylase, protease and lipase all need water to work.
The bond each group forms
The reaction is always condensation. What changes is the name of the bond it forms, and the examiner expects the right one.
| Group | Monomer | Bond formed |
|---|---|---|
| Carbohydrates | Monosaccharides | Glycosidic bond |
| Proteins | Amino acids | Peptide bond |
| Nucleic acids | Nucleotides | Phosphodiester bond |
| Lipids (not polymers) | Glycerol and fatty acids | Ester bond |
Learn that table in both directions. Questions are as likely to give you the bond and ask for the molecule as the other way round.
How this fits together
Every group of molecules in this topic now follows a pattern you already know: a monomer, a condensation reaction, a named bond, and hydrolysis to reverse it. The first group to see it in is carbohydrates, where the difference between two almost identical monomers decides what the polymer can do.