5.1.4B2. Biological Molecules and Enzymes
Enzymes and Temperature — Combined Science 0653 - Biology Revision Notes & Topical Questions
5 min read 6 parts Lesson 6 of 6 Unit quiz
Part 1 of 4Shape controls function (Extended)
Enzymes are proteins
- Enzymes are proteins Each enzyme has a specific three-dimensional shape that is held in place by bonds between parts of the protein chain.
The active site
- The active site The part of the enzyme where the substrate binds is called the active site.
- Its shape is complementary to the substrate, so only the correct substrate fits.
- If the substrate cannot fit, the reaction cannot happen.
Optimum temperature
- Optimum temperature Each enzyme works fastest at its optimum temperature.
- For human enzymes this is about 37 °C.
Part 2 of 4Below the optimum
1Temperature rises
- Temperature rises From around 0 °C towards the optimum, enzyme and substrate molecules gain kinetic energy.
2Faster movement
- Faster movement The molecules move more quickly and collide more often.
3More successful collisions
- More successful collisions More substrate molecules enter active sites each second.
4Rate increases
- Rate increases The rate of reaction rises to a maximum at the optimum temperature.
Part 3 of 4Above the optimum
1Temperature too high
- Temperature too high The extra energy makes the enzyme molecule vibrate strongly.
2Bonds break
- Bonds break The bonds holding the enzyme in shape are broken.
3Active site changes shape
- Active site changes shape The substrate can no longer fit into the active site.
4Denaturation
- Denaturation The enzyme stops working.
- Denaturation is irreversible: lowering the temperature does not restore the shape.
Part 4 of 4Reading the temperature graph
| Part of the graph | What is happening | Explanation |
|---|---|---|
| Low temperature, rising line | Activity slowly increases | More kinetic energy gives more frequent collisions |
| Peak | Maximum rate at the optimum | Active sites are the correct shape and collisions are frequent |
| Steep fall after the peak | Activity drops rapidly | Enzymes are being denatured |
| Zero activity at high temperature | Reaction stops | All the enzymes are denatured |
Key terms
| Term | Meaning |
|---|---|
| Active site | The region of an enzyme where the substrate binds and the reaction takes place. |
| Substrate | The molecule that an enzyme acts upon. |
| Optimum temperature | The temperature at which an enzyme has the highest activity. |
| Denaturation | A permanent change in the shape of an enzyme, caused by breaking the bonds that hold it together, so that it no longer works. |
| Kinetic energy | The energy of movement of a particle. |
Avoid these mistakes
1
Saying enzymes are 'killed' by heat — enzymes are not alive; they are denatured.
2
Stating that low temperatures denature enzymes; they only slow them down because the molecules move more slowly, and the enzyme can work again when warmed.
3
Writing that the enzyme melts or the substrate changes shape; it is the shape of the active site that changes.
4
Forgetting to say that denaturation is irreversible.
5
Explaining the rise in rate without mentioning collisions between enzyme and substrate.
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