Monomers and Polymers
SPEC 3.1.1 - BIOLOGICAL MOLECULES
Understanding the building blocks of life and how they join together
The Chemistry of Life
All life on Earth shares a common chemistry. Despite the incredible variety of living organisms, from bacteria to blue whales, they are all made from the same types of carbon-based compounds.
Key Concept
This shared chemistry provides indirect evidence for evolution - it suggests all life evolved from a common ancestor.
The cells of all living organisms contain only a few groups of carbon-based compounds that interact in similar ways:
Carbohydrates - energy and structure
Lipids - energy storage and membranes
Proteins - enzymes, structure, transport
Nucleic acids - genetic information
Monomers and Polymers
Definition
Monomer: A small, single unit molecule from which larger molecules are made.
Definition
Polymer: A large molecule made from many monomers joined together in a chain.
Memory Tip
Mono = one (like "monorail" - one rail)
Poly = many (like "polygon" - many sides)
Examples of Biological Monomers
There are three main types of biological monomers you need to know:
MonomerPolymer it FormsExample
MonosaccharidesPolysaccharidesGlucose forms starch
Amino acidsPolypeptides/Proteins20 different amino acids form proteins
NucleotidesPolynucleotides (DNA/RNA)Nucleotides form DNA
Key Concept
Monomers join together by condensation reactions and are separated by hydrolysis reactions.
Condensation Reactions
Definition
Condensation reaction: A chemical reaction that joins two molecules together with the formation of a covalent bond, releasing a molecule of water.
Condensation Reaction
Important
The covalent bond formed depends on the type of monomer:
- Monosaccharides form glycosidic bonds
- Amino acids form peptide bonds
- Nucleotides form phosphodiester bonds
Hydrolysis Reactions
Definition
Hydrolysis reaction: A chemical reaction that breaks a covalent bond between two molecules using a molecule of water.
Hydrolysis Reaction
Memory Tip
Hydro = water, Lysis = splitting
Hydrolysis literally means "splitting with water"
Key Concept
Hydrolysis is the reverse of condensation. It is used in digestion to break down large food molecules into smaller, absorbable units.
Worked Example01
Two glucose molecules join together to form maltose. What type of reaction is this? What is produced alongside maltose?
1
Two molecules are joining together - this is a condensation reaction.
2
In condensation reactions, a covalent bond forms between the monomers.
3
A molecule of water (H2O) is released as a by-product.
AnswerCondensation reaction; water is produced.
Worked Example02
A protein is broken down into its amino acids during digestion. Name the type of reaction involved and explain the role of water in this process.
1
A large molecule (polymer) is being broken down into smaller units (monomers).
2
This is a hydrolysis reaction.
3
Water is used in this reaction - it is added across the bond to break it.
AnswerHydrolysis reaction; water molecules are used to break the peptide bonds between amino acids.
Worked Example03
Starch is a polymer made of many glucose monomers. If 100 glucose molecules join to form a starch molecule, how many water molecules are released?
1
Each condensation reaction joins TWO monomers and releases ONE water molecule.
2
To join 100 monomers in a chain, we need 99 bonds (one less than the number of monomers).
3
Each bond releases one water molecule, so 99 water molecules are released.
Answer99 water molecules
Comparing the Reactions
FeatureCondensationHydrolysis
What happensMonomers join togetherPolymers break apart
BondCovalent bond formsCovalent bond breaks
WaterReleased (produced)Used (consumed)
ExampleMaking starch from glucoseDigesting starch to glucose
Where in bodyCells (synthesis)Digestive system
Exam Tip
Remember: these reactions are reversible. Condensation and hydrolysis are opposite processes.
Practice
Question 1: Sucrose is formed when glucose and fructose join together.
(a) Name the type of reaction that forms sucrose.
(b) What type of bond is formed between glucose and fructose?
(c) What molecule is released during this reaction?
(a) Condensation reaction
(b) Glycosidic bond (since both are monosaccharides/sugars)
(c) Water (H2O)
Practice
Question 2: Explain why the breakdown of starch in the digestive system requires water.
Starch is broken down by hydrolysis.
In hydrolysis, water molecules are used to break the glycosidic bonds between glucose monomers.
The water molecule is added across each bond - the H from water attaches to one monomer and the OH attaches to the other, breaking the bond and separating the monomers.
Practice
Question 3: A polypeptide contains 50 amino acids. How many peptide bonds does it contain? How many water molecules were released during its formation?
Number of peptide bonds: 49
(One bond joins each pair of adjacent amino acids. For 50 amino acids in a chain, there are 50 - 1 = 49 bonds)
Water molecules released: 49
(Each condensation reaction releases one water molecule, so 49 bonds = 49 water molecules)
Summary
Key Concepts
TermDefinition
MonomerSmall, single unit molecule (building block)
PolymerLarge molecule made of many monomers joined together
CondensationJoins monomers; forms bond; releases water
HydrolysisBreaks polymers; breaks bond; uses water
Key Exam Skills
Identify monomers and their corresponding polymers
Distinguish between condensation and hydrolysis reactions
Calculate water molecules released/used (n monomers = n-1 bonds = n-1 water molecules)
Explain how shared chemistry provides evidence for evolution
Next: The Chemistry of Life