A monomer is a small unit, and a polymer is a long molecule made by joining many monomers. Exam questions ask you to name the units that make up carbohydrates, proteins and lipids, so you must match each large molecule to its correct building blocks.
This lesson is part of biological molecules and enzymes. You will need it again when you study digestion and enzymes.
How do monomers and polymers fit together?
Think of a train. Each carriage is a monomer, and the whole train is the polymer. Adding carriages builds the train, and removing them breaks it up again.
Digestion does the breaking up: enzymes split polymers back into monomers small enough to be absorbed into the blood.
Which large molecule is built from what?
| Large molecule | Built from | Notes |
|---|---|---|
| Starch | Many glucose units | Energy store in plants |
| Glycogen | Many glucose units | Energy store in animals and fungi |
| Cellulose | Many glucose units | Strengthens plant cell walls |
| Protein | Many amino acids | Sequence of amino acids sets the shape |
| Fat or oil (lipid) | 1 glycerol and 3 fatty acids | Not a chain of identical units |
Three different carbohydrates share one monomer. That is why all three give glucose when fully broken down.
Worked example
A food label lists starch, protein and fat. A student writes: “Starch is made of amino acids, protein is made of glucose, and fat is a polymer of fatty acids.” Correct the statement.
Step 1, starch: it is a carbohydrate, so its monomer is glucose. The student used the protein monomer.
Step 2, protein: its monomers are amino acids. The student swapped the two.
Step 3, fat: it is made from glycerol and fatty acids. It is not described as a polymer of one repeating unit.
Corrected: starch is a polymer of glucose, protein is a polymer of amino acids, and a fat is made from glycerol and three fatty acids.
The mistake to watch for
A common error is to call the whole large molecule the monomer, for example “starch is a monomer”. The clue is the word “large”. If a molecule is a long chain, it is the polymer.
A second error is to treat glucose and starch as the same substance because both are carbohydrates. Glucose is one unit. Starch is thousands of them joined together.
A quick test: ask “is it the unit or the chain?” before you write the word.
Check yourself
1. Name the monomer of glycogen.
Show answer
Glucose.
2. A protein molecule is broken down completely during digestion. Name the smaller molecules produced.
Show answer
Amino acids.
3. Explain why cellulose and starch are both polymers of glucose but have different jobs.
Show answer
Both are made of many glucose units, but the units are arranged and joined differently. Starch is an energy store in plants. Cellulose forms strong fibres in plant cell walls.
Where this leads next
Next, see how enzymes act on these molecules in explaining enzyme specificity using a model. You can also revisit the food tests in connecting an observation with a limited inference.
A class diagram is one thing, but being asked to name the units aloud exposes the gaps fast. Our teachers do this in online one-to-one Biology tuition.