3.1.1 is the chemistry foundation of the whole of 3.1. Three monomer classes, monosaccharides, amino acids and nucleotides, join into three polymer classes by condensation, and break back to monomers by hydrolysis.
Polymers are large molecules built from many monomer units.
A monomer is a small repeating unit that acts as the building block of a larger molecule. A polymer is a large molecule built from many such units joined together. Both "large" and "many" carry mark-scheme weight. AQA names three monomer classes at this level: monosaccharides, amino acids, and nucleotides.
Write "large molecule made from many monomers". Both "large" and "many" carry the mark; either word missing loses the polymer definition mark.
The three monomer–polymer pairings.
| Monomer | Bond formed by condensation | Polymer example |
|---|---|---|
| Monosaccharide | Glycosidic bond | Polysaccharide (e.g. starch) |
| Amino acid | Peptide bond | Polypeptide or protein |
| Nucleotide | Phosphodiester bond | Polynucleotide (DNA, RNA) |
Pair the polymer with its named monomer. "Amino acid with polypeptide", "nucleotide with DNA or RNA", "α-glucose with starch". Naming the polymer or the monomer alone does not score the example mark.
Condensation joins monomers by forming a bond and releasing water.
Condensation is the universal mechanism that joins monomers into polymers. Every condensation mark has two halves: a covalent bond forms between the two monomers, and a single water molecule is released. Both halves are required for credit. The water half alone does not score.
Write "condensation reaction". Don't write
dehydrationordehydration synthesis; AQA does not credit either.
- Two monomers come into position with their reactive functional groups aligned.
- A covalent bond forms between the two monomers: a glycosidic bond between monosaccharides, a peptide bond between amino acids, a phosphodiester bond between nucleotides.
- A single water molecule is released as the bond forms.
Every condensation mark needs both halves: bond formed AND water released. Naming the water half on its own scores zero on the canonical 5-mark Describe question.
Hydrolysis reverses condensation by adding water across a bond.
Hydrolysis is the chemical reverse of condensation. A water molecule is added across the covalent bond joining two monomers in a polymer, the bond breaks, and the monomers are freed. The same two-component rule applies: bond broken and water added are both required for the mark.
Write "hydrolysis". Don't write
breaking downorsplittingwithout naming the water mechanism; the water component must be stated.
- A water molecule is added across the covalent bond joining two monomers in a polymer.
- The bond breaks.
- The two monomers are released, available for digestion, transport, or further metabolic reactions.
Every hydrolysis mark needs both halves: bond broken AND water added. The water half on its own scores zero, exactly as with condensation.
Two biological contexts come up at A-level. Digestion hydrolyses dietary polymers into absorbable monomers: starch into glucose, proteins into amino acids, nucleic acids into nucleotides. Intracellular mobilisation hydrolyses stored polymers when their monomers are needed, such as glycogen releasing glucose for respiration.
Polymer status, condensation status, and bond identity are three independent properties.
A molecule's classification under one of these properties does not predict the others. Examiner commentary on 2022 P3 Q01.2 reported that mastery on the discriminating rows fell to 13 per cent on this exact failure mode: students collapsed three independent judgements into one and lost marks across the classification table.
Three independent properties of a molecule.
| Molecule | Made by condensation? | Is a polymer? |
|---|---|---|
| Polypeptide | Yes | Yes |
| Polynucleotide (DNA, RNA) | Yes | Yes |
| Phospholipid | Yes | No |
| Disaccharide or dipeptide | Yes | No |
Don't write
ester bondas the named bond inside a polymer. Ester bonds belong to triglyceride and phospholipid synthesis only; they are not a 3.1.1 polymer bond.
Polymer examples come from three categories only: polysaccharide, polypeptide or protein, and polynucleotide. Disaccharides, dipeptides, triglycerides and phospholipids do not count.
Disaccharides (sucrose, maltose), dipeptides, triglycerides, and phospholipids are all formed by condensation but are not polymers. A polymer requires many monomer units, not two. The bond inside a triglyceride or phospholipid is an ester bond, which AQA does not accept as the named bond inside a polymer.
Two condensation events do not make a polymer. A disaccharide is two monosaccharides joined; a dipeptide is two amino acids joined. Both are condensation products; neither is a polymer.
Key terms
- polymer
- condensation
- hydrolysis
- monomer
- condensation reaction
- DNA
- glycosidic bond
- peptide bond
- phosphodiester bond