Lesson 2 of 8 / Biological molecules
Building larger molecules
How can a few small units make very different large molecules?
I can match each large molecule to the smaller units used to synthesise it.
I can match large molecules with their smaller building units.Syllabus
Syllabus K224 / K225, Topic 3(c). Match large molecules with their smaller building units.
About 5 min
Make a guess. You are not marked.
Starch in rice and cellulose in a plant cell wall are both made of the same small unit. Which unit?
Show the answer
Glucose, joined and arranged in different ways.
Starch, glycogen and cellulose are all chains of glucose. Different joining gives them very different properties.
The key idea
Glucose units build starch, glycogen and cellulose. Amino acids build polypeptides and proteins. Glycerol and fatty acids build fats.
Synthesis means making a larger molecule from smaller units. The shapes below represent units, not atoms or their exact chemical bonds.
Explore the idea
Which units build this molecule?
Glucose. These are the smaller units needed to synthesise starch.
G = glucose; aa = amino acid. Original unit model, not a molecular structure or reaction equation. Starch and cellulose differ in how glucose units are joined; the exact bonds are not shown. Glycogen has many more branches than this small example.
Explanation
Many large molecules are made by joining small units together. Starch, glycogen and cellulose are all made of glucose. Plants store carbohydrate as starch, and animals store it as glycogen. Cellulose makes plant cell walls.
These three molecules are made from the same unit, but they are not the same. The glucose units are joined and arranged in different ways. That is why one can be a store of energy and another a strong wall material.
Amino acids join in a chain to make a polypeptide. One or more polypeptides fold up to make a protein. A different order of amino acids gives a different shape, and the shape decides the job, for example which substrate an enzyme acts on.
Fats are made of glycerol and fatty acids. A typical fat molecule has one glycerol joined to three fatty acids. Fats are not made of glucose or amino acids.
Digestion breaks large food molecules into small ones that can be absorbed. The body then uses these small molecules to build what it needs. This use is called assimilation.
Step by step
- 1
Identify the large molecule
Be specific: starch, glycogen, cellulose, a protein or a fat.
- 2
Name the smaller units
Glucose for the first three; amino acids for proteins; glycerol and fatty acids for fats.
- 3
Keep arrangement in mind
The same glucose units can form different large molecules because of how the units are joined.
Worked example
Storage versus support in a plant
A plant cell makes starch for storage and cellulose for its wall. Name the starting unit for both and explain why the products need not have the same properties.
One way to explain it
Both use glucose units. The units are joined in different arrangements, producing molecules with different structures and properties. Starch serves as a carbohydrate store, whereas cellulose forms a supporting cell wall.
Why this answer works
- Identify the shared starting unit.
- Distinguish a unit from the final molecule.
- Connect different arrangement to different roles.
Watch out for this
A student says: "All large food molecules are made from glucose." What is wrong with this?
Show the answer
Proteins are made from amino acids. Fats are made from glycerol and fatty acids. Glucose builds starch, glycogen and cellulose.
Check your understanding
Which list correctly gives the smaller units for protein and fat?
Show the answer
Protein: amino acids; fat: glycerol and fatty acids.
Keep both components when describing the basic units of a fat.
Quick recall
How can a few small units make very different large molecules?