Lesson 2 of 7 / Biological molecules
Building larger molecules
How can a few small units make very different large molecules?
In this lesson: Match each large molecule to the smaller units used to synthesise it.
About 5 min
The key ideaGlucose 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 biological molecules are made by joining smaller units. Starch, glycogen and cellulose are all made from glucose. Starch stores carbohydrate in plants, glycogen stores it in animals, and cellulose forms plant cell walls.
Having the same small unit does not make these molecules identical. The glucose units are joined and arranged differently. These differences help explain why a storage molecule and a supporting cell-wall material can both be made from glucose. Detailed bond chemistry is not needed here.
Amino acids join in sequences to form polypeptides. One or more polypeptides fold to form a functional protein. Different amino acid sequences can lead to different shapes and jobs, such as an enzyme acting on a particular substrate.
Fats are assembled from glycerol and fatty acids. A typical fat molecule has one glycerol joined to three fatty acids. This is different from a long chain made by repeating a single kind of basic unit. Do not describe fat as being made from amino acids or glucose units.
Digestion later breaks large food molecules into smaller products that can be absorbed. Assimilation can then use those small molecules to build substances needed by the body. Building and breaking down molecules are connected processes, but they do not necessarily use the same enzymes.
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.
Is this true? "All large food molecules are made from glucose."
Proteins are made from amino acids. Fats are made from glycerol and fatty acids. Glucose builds starch, glycogen and cellulose.