Monomers and polymers — AQA A-Level Biology
Test yourself on Monomers and polymers with AQA A-Level practice questions.
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Monomers and polymers explained
Life on Earth is incredibly diverse, yet the underlying biochemical basis is fundamentally similar across all organisms.
Read the full explanation
Every living thing is constructed from four main groups of carbon-based organic molecules: carbohydrates, lipids, proteins, and nucleic acids. Organic molecules contain carbon bonded to hydrogen. Carbon's ability to form four strong covalent bonds allows it to build complex chains and rings. The same monomers—monosaccharides, amino acids, and nucleotides—are used universally. Furthermore, the same fundamental reactions, condensation and hydrolysis, assemble and dismantle these polymers. This shared biochemistry, including the universal use of DNA, RNA, and ATP, provides strong evidence for evolution and a common ancestor.
Monomers are the smaller units from which larger molecules are made.
A monomer is the small, repeating unit from which a much larger molecule is built. The word only means anything in relation to the polymer it forms, so a full definition has two halves: the smaller units, and the larger molecule they are joined into. Monomers join by condensation, each join forming one named chemical bond and releasing one molecule of water, and the polymer is taken apart again by hydrolysis. Glucose is the monomer of starch, glycogen and cellulose; an amino acid is the monomer of a polypeptide; a nucleotide is the monomer of DNA and RNA. The monomers in one polymer need not be identical - a polypeptide is built from twenty different amino acids - but they are all joined by the same type of bond. Not every large biological molecule is a polymer: triglycerides are built from glycerol and fatty acids, but these are not monomers because no repeating chain is formed.
Polymers are molecules made from a large number of monomers joined together.
A polymer is a large molecule made of many monomers joined in a chain by condensation reactions, one chemical bond and one released water molecule per join. Starch, glycogen and cellulose are polymers of glucose; polypeptides are polymers of amino acids; DNA and RNA are polynucleotides. What gives a polymer its properties is not only which monomer it uses but how the chain is arranged. Cellulose and chitin chains are straight and unbranched, because alternate beta monomers are flipped through 180 degrees so their hydroxyl groups can line up, and hydrogen bonds then form between neighbouring chains. Amylopectin and glycogen carry 1,6 branch points, while amylose coils into a helix. When a question supplies a diagram of an unfamiliar polymer, read the monomer, the bond and the chain shape directly from the figure rather than recalling a memorised example.
Monosaccharides, amino acids and nucleotides are examples of monomers.
Three monomers cover nearly every polymer in the specification. Monosaccharides such as glucose join by glycosidic bonds to make disaccharides and polysaccharides. Amino acids join by peptide bonds to make dipeptides and polypeptides. Nucleotides join by phosphodiester bonds to make the polynucleotides DNA and RNA. Be precise about a nucleotide: it is three components joined together - a pentose sugar, which is deoxyribose in DNA and ribose in RNA, a phosphate group, and a nitrogen-containing organic base - not simply the base. Each monomer has a general structure you may have to recognise or draw: a hexose monosaccharide has the formula C₆H₁₂O₆; an amino acid carries an amine group, a carboxyl group, a hydrogen atom and an R group on the same central carbon. All three are joined by condensation, releasing one water molecule per bond, and all three are released again by hydrolysis.
A condensation reaction joins two molecules together with the formation of a chemical bond and involves the elimination of a molecule of water.
A condensation reaction joins two molecules, forms a new chemical bond between them and eliminates one molecule of water. The water comes from the groups being joined: a hydroxyl group from one molecule and a hydrogen atom from the other. The bond formed depends on the molecules involved - a glycosidic bond between two monosaccharides, a peptide bond between two amino acids, an ester bond between glycerol and a fatty acid, a phosphodiester bond between two nucleotides - so an answer that only says 'a bond forms' usually loses the mark. Count carefully, because one water molecule leaves per bond formed: joining n amino acids into one polypeptide forms n-1 peptide bonds and releases n-1 water molecules, and building a single triglyceride releases three.
A hydrolysis reaction breaks a chemical bond between two molecules and involves the use of a water molecule.
Hydrolysis breaks a chemical bond between two molecules using a molecule of water: the water splits and its hydroxyl group and hydrogen atom are added to the two products. It is the exact reverse of condensation and uses one water molecule per bond broken. Digestion is the obvious example - peptide bonds in a polypeptide, glycosidic bonds in maltose or starch, and ester bonds in a triglyceride are all hydrolysed - but hydrolysis also happens continuously inside cells, when glycogen is mobilised or when ATP is hydrolysed to ADP and inorganic phosphate. Enzymes catalyse it. Enzyme names are not all derived from the bond acted on: endopeptidases and exopeptidases are named for their site of action (internal or terminal peptide bonds), dipeptidases for their substrate (dipeptides), and maltase for its substrate (maltose).
Your focus
- Name the four groups of organic molecule found in all living organisms, and give the monomer from which carbohydrates, proteins and nucleic acids are built.
- Explain why carbon is the element at the centre of biological molecules, referring to its ability to form four covalent bonds and long chains.
- Suggest why comparing DNA base sequences or amino acid sequences between two species gives better evidence of relatedness than comparing their appearance.
Show all 18 objectives
- Define a monomer in one sentence that names both the small unit and the larger molecule it builds.
- Identify the repeating monomer and the bond between monomers from a diagram of an unfamiliar polymer such as chitin.
- Explain why glycerol and fatty acids are not monomers even though a triglyceride is a large molecule.
- Describe how a polymer is built from monomers, naming the reaction, the bond formed and the molecule released.
- Use a diagram of an unfamiliar polymer to describe three structural features, referring to the monomer, the bond and the shape of the chain.
- Explain why alternate beta-glucose molecules must be rotated through 180 degrees before a glycosidic bond can form.
- Name the three monomers in the specification and state the polymer and the bond associated with each.
- Describe the three components of a DNA nucleotide and say how this differs from an RNA nucleotide.
- Explain why a triglyceride is not described as a polymer, using the meaning of monomer in your answer.
- Describe a condensation reaction in terms of the two molecules joined, the bond formed and the water molecule eliminated.
- Calculate the number of bonds formed and water molecules released when a stated number of monomers is joined into one polymer.
- Name the bond formed by condensation for each of monosaccharides, amino acids, nucleotides and glycerol with fatty acids.
- Describe hydrolysis in terms of the bond broken, the water molecule used and the products formed.
- Explain why yeast supplied with maltose releases carbon dioxide more slowly than yeast supplied with glucose.
- Distinguish between the action of endopeptidases, exopeptidases and dipeptidases on a polypeptide chain.
Monomers and polymers exam tips
Marking Points
- State that all living organisms share the same fundamental groups of carbon-based organic molecules: carbohydrates, lipids, proteins, and nucleic acids.
- Identify that shared monomers, such as monosaccharides, amino acids, and nucleotides, are common to all life forms.
- Explain that the universal use of DNA, essentially the same genetic code, and ATP provides biochemical evidence for a common ancestor.
- Describe how comparing DNA base sequences, mRNA base sequences, or amino acid sequences can be used to determine evolutionary relationships between diverse species.
- Recognise that organic molecules are carbon-based compounds containing carbon-hydrogen bonds, distinguishing them from inorganic carbon compounds like carbon dioxide.
- Define a monomer as a smaller or repeating unit from which a larger molecule is made, naming both the small unit and the larger molecule in one sentence
- Identify the repeating monomer from a diagram of an unfamiliar polymer, for example N-acetylglucosamine in chitin or beta-glucose in cellulose
- Name the bond that joins the monomers in a named example, such as a glycosidic bond between glucose units or a peptide bond between amino acids
- State that monomers are joined by condensation, releasing water, and separated by hydrolysis, which uses water
- Explain why glycerol and fatty acids are not monomers even though a triglyceride is a large molecule, because no repeating chain is formed
- State that a polymer is many monomers joined together, formed by condensation, with one water molecule released per bond formed
- Name the bond between the monomers in a named example, such as glycosidic bonds in a polysaccharide or peptide bonds in a polypeptide
- Describe the chain as straight, linear or unbranched where the diagram shows it, and identify 1,4 linkages in the backbone
- Note that alternate monomers are flipped, rotated or inverted through 180 degrees, as in cellulose and chitin, allowing hydrogen bonds to form between chains
- Identify 1,6 linkages only where branching is shown, and explain that branching produces a more compact molecule with more free ends for rapid hydrolysis
- Award credit for naming monosaccharides, amino acids and nucleotides as monomers.
- Award credit for stating that DNA is a polymer of nucleotides, accepting polynucleotide.
- Award credit for describing a nucleotide as deoxyribose, a phosphate group and an organic or nitrogen-containing base.
- Award credit for naming the bond each monomer forms: glycosidic, peptide or phosphodiester.
- Award credit for naming the polymer each monomer builds.
- Award credit for stating that two molecules are joined and a named chemical bond is formed.
- Award credit for stating that a molecule of water is released, eliminated or removed.
- Award credit for naming the correct bond for the molecules given, such as peptide, glycosidic or ester.
- Award credit for stating that one water molecule is released per bond formed, so n monomers release n-1 water molecules.
- Award credit for identifying condensation as the reaction that builds polymers, and hydrolysis as its reverse.
- one mark for stating that a water molecule is added or used to break the bond
- one mark for naming the bond hydrolysed, such as peptide bond or glycosidic bond
- one mark for naming the products released, for example amino acids from a dipeptide or glucose from maltose
- one mark for stating that the reaction is catalysed by a named enzyme, such as a peptidase or maltase
- one mark for explaining a delay or slower rate because the substrate must be hydrolysed before it can be used
Examiner Tips
- 💡When evaluating evolutionary relationships, always specify the exact biochemical evidence being compared, such as the 'DNA base sequence' or 'amino acid sequence', rather than just writing 'DNA'.
- 💡The concept of a shared biochemical basis is an excellent linking theme for synoptic essays, connecting biological molecules to classification, evolution, and the genetic code.
- 💡Read the monomer off the diagram you are given - a question about an unfamiliar polymer like chitin is testing whether you can spot the repeating unit, not whether you have memorised it.
- 💡Pair every monomer with its polymer and bond when you revise: glucose-polysaccharide-glycosidic, amino acid-polypeptide-peptide, nucleotide-polynucleotide-phosphodiester.
- 💡In a definition question write both halves of the idea in one sentence: the small repeating unit and the larger molecule it builds.
- 💡When a question says 'use Figure 1', every point you make must be visible in the figure - the flipped monomer and the straight chain are there to be seen.
- 💡Check the branching before you mention 1,6 bonds; if the drawn chain is straight, a 1,6 bond is a marked error.
- 💡Structure means monomer, bond, chain shape and chain interactions - not the elements the molecule contains.
- 💡Learn the three parts of a nucleotide as a set phrase - pentose sugar, phosphate group, organic base - because the mark is usually for all three.
- 💡Match monomer to polymer to bond in a three-column table when revising; questions test the links, not the lists.
- 💡If a question asks for examples of monomers, give the three named in the specification rather than a specific molecule such as glucose alone.
- 💡Write condensation answers as a three-part sentence: what joins, which bond forms, and that one water molecule is released.
- 💡If a question gives a number of monomers, expect to calculate bonds and water molecules as one less than that number.
- 💡Name the bond every time, even when the question does not ask for it - it is often the difference between one mark and two.
- 💡Always include the water molecule; 'breaking the bond' with no reference to water does not describe hydrolysis.
- 💡When a question compares maltose and glucose, the marks are for identifying maltose as a disaccharide and for the time taken to hydrolyse the glycosidic bond.
- 💡Learn endopeptidase, exopeptidase and dipeptidase as a sequence, and describe where each acts on the chain.
Common Mistakes
- Assuming all carbon-containing compounds are organic; correct by remembering that simple carbon compounds like carbon dioxide (CO2) and carbonates are inorganic because they lack carbon-hydrogen bonds.
- Misinterpreting a 'similar biochemical basis' to mean organisms have identical molecules; correct by understanding that while the classes of molecules (e.g., proteins) and monomers (e.g., amino acids) are shared, the specific sequences (e.g., amino acid sequences) differ between species.
- Stating species are related solely because of similar physical appearances when asked for biochemical evidence; correct by specifically referencing shared molecules like DNA, RNA, or proteins.
- Defining a monomer as just a small molecule, with no reference to the larger molecule it builds; a full definition needs both halves
- Calling a disaccharide such as maltose a monomer because it is smaller than starch; it is a dimer, not a monomer
- Naming glycerol or a fatty acid as a monomer of a triglyceride, when lipids are not polymers
- Saying the monomers of DNA are bases rather than nucleotides; the monomer is the nucleotide, which contains a base, sugar and phosphate
- Answering with the elements present, carbon, hydrogen and oxygen, when asked to identify a repeating unit
- Offering that both polymers contain carbon, hydrogen and oxygen as a structural similarity, which is ignored when the question asks about structure
- Stating that a straight, unbranched polymer such as cellulose or chitin contains 1,6 bonds, which is rejected; 1,6 bonds occur only at branch points
- Saying both molecules are polysaccharides when the question has already told you that, so it gains nothing
- Calling a triglyceride a polymer of fatty acids; triglycerides are not polymers because no repeating chain is formed
- Describing a polymer as 'lots of molecules stuck together' without naming the bond or the condensation reaction
- describing a nucleotide as a base only, leaving out the pentose sugar and phosphate group; correction: a nucleotide has three parts - pentose sugar, phosphate group and organic base
- writing that DNA is a polymer of bases rather than of nucleotides; correction: DNA is a polynucleotide, a polymer of nucleotides
- giving ribose as the sugar in a DNA nucleotide; correction: DNA contains deoxyribose, RNA contains ribose
- naming glycerol and fatty acids as the monomers of lipids; correction: glycerol and fatty acids are components of triglycerides, which are not polymers
- writing 'sugar' without specifying deoxyribose or ribose when the question names DNA or RNA; correction: state the specific pentose sugar
- saying water is added or used, which describes hydrolysis rather than condensation; correction: condensation eliminates water, hydrolysis adds it
- writing that 'hydrogen and oxygen are removed' instead of one molecule of water; correction: state that one water molecule is eliminated
- describing the reaction without naming the bond formed; correction: name the specific bond, such as peptide or glycosidic
- stating that one water molecule is released when three fatty acids join a glycerol, instead of three; correction: one water molecule is released per bond, so three bonds release three water molecules
- assuming condensation only happens in digestion or only outside cells; correction: condensation builds polymers in cells, while hydrolysis breaks them down in digestion
- writing that hydrolysis releases water, which reverses the definition; correction: hydrolysis uses water, condensation releases it
- saying the molecule is 'broken down' without naming the bond or mentioning water; correction: always name the bond and state that water is used
- stating that exopeptidases hydrolyse internal peptide bonds, when they act on the terminal bonds; correction: endopeptidases act internally, exopeptidases at the ends
- forgetting that a disaccharide must be hydrolysed before it can enter respiration, and answering that maltose is simply 'harder to respire'; correction: maltose is a disaccharide that must first be hydrolysed to glucose
- confusing hydrolysis with denaturation, which changes the folding of a protein rather than breaking peptide bonds; correction: denaturation disrupts bonds maintaining tertiary structure, hydrolysis breaks peptide bonds