Monomers and polymers
AQA
A-Level
Life on Earth is incredibly diverse, yet the underlying biochemical basis is fundamentally similar across all organisms. 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.
18
Objectives
17
Exam Tips
28
Pitfalls
30
Key Terms
30
Mark Points
Subtopics in this area
The variety of life, both past and present, is extensive, but the biochemical basis of life is similar for all living things.
Monomers are the smaller units from which larger molecules are made.
Polymers are molecules made from a large number of monomers joined together.
Monosaccharides, amino acids and nucleotides are examples of monomers.
A condensation reaction joins two molecules together with the formation of a chemical bond and involves the elimination of a molecule of water.
A hydrolysis reaction breaks a chemical bond between two molecules and involves the use of a water molecule.
Monomers and polymers Revision Guide
Learning Objectives
What you need to know and understand
- 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.
- 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.
Marking Points
Key points examiners look for in your answers
- 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
Expert advice for maximising your marks
- 💡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
Pitfalls to avoid in your exam answers
- 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