To relate structure to function, state the structure, name the job it does, and link the two with “so” or “because”. The same three-part sentence works for proteins, starch and lipids.
This lesson follows comparing carbohydrates, proteins and lipids in the SPM Biology chemical composition of cells chapter.
What is the sentence pattern?
Use: “[Molecule] has [structure], so it can [job].”
For example: “An enzyme has a specific shape, so it fits only its own substrate.”
Worked example: the cooked egg
A raw egg white is clear and runny. When heated, it turns solid and white, and it does not return to liquid when cooled.
- Egg white is mostly protein. Each protein has a folded shape decided by its amino acid sequence.
- Heating breaks the weak links that hold the shape, so the protein unfolds and loses its normal shape. This is denaturation.
- The unfolded proteins tangle together and form a solid.
- Cooling does not restore the shape, so the change is permanent.
The same idea explains why a fever that is too high can harm enzymes in the body: their shapes change.
More structure and function links
| Molecule | Structure | Job it suits |
|---|---|---|
| Enzyme (protein) | Specific folded shape | Fits one substrate only |
| Starch | Many glucose units in a large molecule | Stores energy without dissolving easily |
| Fat | Glycerol joined to fatty acids | Stores a large amount of energy; insulates |
The mistake that costs marks
The common slip is to say that heat “kills” a protein or breaks it into amino acids. A protein is a molecule, not a living thing, and heat changes its shape without breaking the amino acid chain.
| Wrong | Right | |
|---|---|---|
| Effect of heat | The protein dies | The protein is denatured and loses its shape |
| What changes | Amino acids are separated | The folded shape changes |
| Result | No reason given | It can no longer do its job |
Check yourself
Explain why an enzyme stops working when it is heated to a very high temperature.
Answer
An enzyme is a protein with a specific shape that fits its substrate.
At a very high temperature the shape changes, which is denaturation. The substrate no longer fits, so the enzyme cannot work.
What to study next
Move on to interpreting food-test results conceptually. Practise data questions with the biological data interpretation trainer, and log slips in the mistake log and paper-error review.
If you would like a teacher to coach your explanation sentences, see online one-to-one Biology tuition.