An enzyme’s rate curve has three parts: a rise, a peak at the optimum, and a fall. Each part has a different reason, and you earn marks by giving the right reason for the right section.
This lesson follows explaining enzyme action in metabolism and enzymes.
What is the three-part frame?
Treat a temperature curve as three sections and give each its own cause and effect.
- Rising section: a higher temperature gives particles more kinetic energy, so there are more collisions between enzyme and substrate per second. More enzyme-substrate complexes form, and the rate rises.
- Peak: at the optimum temperature the rate is highest, because collisions are frequent and the enzyme still has its correct shape.
- Falling section: above the optimum, bonds holding the enzyme’s shape break. The active site changes shape and the substrate no longer fits, so the enzyme is denatured.
Between the optimum and the zero point, part of the enzyme population is already denatured, so the rate falls steeply, not gently.
Worked example: a temperature table
An original table gives the amount of product formed in 5 minutes for an enzyme at five temperatures.
| Temperature (°C) | Product formed (cm³) |
|---|---|
| 10 | 2 |
| 20 | 5 |
| 30 | 9 |
| 40 | 12 |
| 50 | 3 |
Question: Describe and explain the results between 40 °C and 50 °C.
Start with the data. The product falls from 12 cm³ to 3 cm³, a decrease of 9 cm³ (12 − 3).
Then the reason. Above the optimum, which is near 40 °C, the heat breaks bonds holding the enzyme’s shape. The active site changes, so the substrate cannot fit, and enzyme-substrate complexes form less often.
A complete answer quotes at least one pair of numbers and uses the word denatured.
How does the pH curve differ?
The pH curve is also bell-shaped, and the reason on both sides is the same: the shape of the active site changes. Extreme pH disrupts the bonds that hold the protein’s shape, whether the pH is too high or too low.
Different enzymes have different optimum pH values. Pepsin in the stomach works best in acid conditions, while amylase in the mouth works best near neutral.
Here is an original pH question. An enzyme has a rate of 8 units at pH 7, 3 units at pH 5 and 2 units at pH 9. The rate is highest at pH 7, so pH 7 is the optimum.
At pH 5 and pH 9, the shape of the active site is altered, so the substrate fits less well and the rate is lower. The drop from 8 to 3 is 5 units, and from 8 to 2 is 6 units.
The mistake that costs marks
The common error is to explain the fall above the optimum with “the enzyme is killed”. Enzymes are proteins, not living things. Write “denatured” and describe the change in shape.
Another slip is to say that low temperature denatures the enzyme. At low temperature the enzyme is inactive, but its shape is intact. The rate is low because collisions are few and slow.
| Condition | Wrong | Right |
|---|---|---|
| Very low temperature | “Enzyme is denatured” | “Few collisions; enzyme is inactive but not denatured” |
| High temperature | “Enzyme dies” | “Enzyme is denatured; the active site changes shape” |
| Extreme pH | “Substrate is destroyed” | “The active site changes shape; the substrate no longer fits” |
Check yourself
An enzyme works at 35 °C. A student heats a sample to 70 °C for 5 minutes, then cools it back to 35 °C. The enzyme does not work. Explain.
Answer
At 70 °C the heat broke bonds holding the enzyme’s shape, so the active site changed and the enzyme was denatured. Denaturation is a permanent change.
Cooling to 35 °C does not restore the original shape, so the substrate still cannot fit and no reaction occurs.
What to study next
The next lesson, distinguishing limiting factors, adds enzyme and substrate concentration to these curves. Practise with more tables in the graph evidence and fair-comparison lab.
If you want a teacher to listen to your curve explanations and tighten the causes, see online one-to-one Biology tuition or the one-hour trial class (from RM50).