Monomers and polymersAQA A-Level Biology: Revision notes
Section 1
Monomers and polymers
Monomers are the smaller units from which larger molecules are made. A polymer is a molecule made from a large number of monomers joined together in a chain.
Three groups of monomers are in the specification: monosaccharides, amino acids and nucleotides. Each one forms a different type of polymer.
Section 2
Three examples of monomers
- Monosaccharides (for example glucose) join to form polysaccharides such as starch, glycogen and cellulose.
- Amino acids join to form polypeptides, which make up proteins.
- Nucleotides join to form polynucleotides, which are DNA and RNA.
The small molecules are few in number, but the polymers can be built in many sequences and lengths, which gives huge variety.
Learn the pairs as monomer to polymer: monosaccharide to polysaccharide, amino acid to polypeptide, nucleotide to polynucleotide.
Section 3
Condensation reactions
A condensation reaction joins two molecules together with the formation of a chemical bond and involves the elimination of a molecule of water.
The bond has a different name depending on the monomers: glycosidic bond (monosaccharides), peptide bond (amino acids), and an ester bond when glycerol joins a fatty acid. Condensation is repeated many times to build a polymer.
Do not say water is 'used' or 'added' in a condensation reaction. Water is eliminated (released).
Section 4
Hydrolysis reactions
A hydrolysis reaction breaks a chemical bond between two molecules and uses a molecule of water. It is the reverse of condensation.
Digestion is hydrolysis: enzymes break the bonds in polymers such as starch and protein so that the small monomers can be absorbed. Cells also use hydrolysis to release stored glucose from glycogen.
Section 5
Worked example: counting the water
Joining n monomers into one unbranched chain makes n − 1 bonds, so n − 1 water molecules are eliminated. Breaking the chain fully uses n − 1 water molecules.
Example: a tripeptide from glycine (75), alanine (89) and serine (105). Two bonds form, so 2 × 18 = 36 is lost. Relative formula mass = (75 + 89 + 105) − 36 = 233.
Section 6
A common biochemical basis for life
The biochemical basis of life is similar for all living things. Bacteria, plants and animals all use the same types of monomer (for example the same twenty amino acids) and the same types of reaction, condensation and hydrolysis, to build and break down polymers.
Differences between species come from the sequence of monomers, not from using different building blocks. This is why one organism can digest another and reuse the amino acids and sugars.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Monomers and polymers
- A student is revising how cells build the large molecules they need. She lists glucose, glycine and a nucleotide containing adenine as small molecules, and starch, a protein and DNA as large molecules made from them.Name the type of reaction that joins glycine molecules to form a polypeptide, and state what is released each time two glycine molecules join.2 marks
- A protease enzyme is added to a solution containing a polypeptide of 120 amino acids. After incubation the solution contains only free amino acids.Explain why water is needed for the protease to break down the polypeptide.2 marks
- Three amino acids, glycine, alanine and serine, are joined in that order to make a tripeptide. The relative formula masses are: glycine 75, alanine 89, serine 105 and water 18.Describe how the three amino acids are joined to form the tripeptide.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).