Low temperature slows a reaction because molecules collide less often. High temperature or extreme pH stops the enzyme working because its shape changes. The first is reversible, and the second is not.
This lesson completes biological molecules and enzymes and ties together specificity and graph reading.
How do the two explanations differ?
| Feature | Cold (slow collisions) | Hot or extreme pH (denaturation) |
|---|---|---|
| Enzyme shape | Unchanged | Active site changes shape |
| What limits the rate | Fewer, lower-energy collisions | Substrate no longer fits |
| Effect when conditions return to the optimum | Rate recovers | Rate does not recover |
| Place on graph | Rising part | Falling part |
A question to ask before you write
- Is the enzyme’s shape the problem? If yes, say denatured and active site.
- Or is it collision frequency? If yes, say fewer collisions and lower kinetic energy.
- Can the rate recover? This is your evidence for which one applies.
Worked example
Equal samples of an enzyme and its substrate were treated as shown. The data are invented for practice.
| Tube | Treatment | Rate of reaction afterwards at 37 °C |
|---|---|---|
| A | Kept at 5 °C for 10 minutes, then warmed to 37 °C | 14 units per minute |
| B | Kept at 80 °C for 10 minutes, then cooled to 37 °C | 0 units per minute |
| C | Kept at 37 °C throughout | 15 units per minute |
Tube A: the rate is close to tube C. So the cold did not change the enzyme. It only slowed collisions, and warming restored them.
Tube B: the rate is zero even after cooling. The enzyme’s shape was changed by the high temperature, so the active site no longer fits the substrate and cooling cannot repair it.
Conclusion: cold slows the reaction without damaging the enzyme, while 80 °C denatures it. The small gap between A and C (1 unit per minute) does not show damage, because single readings vary.
The mistake to watch for
Students often write “the enzyme is denatured at low temperatures”. This gives the wrong reason for the slow rate in the cold.
Another common wording error is “the enzyme was killed”. An enzyme is not alive, so it can only be denatured.
The correction for a cold tube: “the enzyme is not denatured; collisions are less frequent and have less energy, so the rate is lower”.
Check yourself
1. A reaction is fast at 40 °C but almost stops at 70 °C and does not recover on cooling. Explain.
Show answer
At 70 °C the enzyme is denatured: the active site changes shape and the substrate no longer fits. Cooling does not restore the shape, so the rate does not recover.
2. A reaction at 10 °C is slower than at 30 °C. Give the reason in terms of particles.
Show answer
At 10 °C the particles move more slowly, so enzyme and substrate collide less often and with less energy. Fewer successful reactions occur per minute.
3. A student says a tube kept at 0 °C shows denaturation. What simple test would check this?
Show answer
Warm the tube to the optimum temperature. If the rate recovers, the enzyme was not denatured and the slow rate was due to fewer, lower-energy collisions.
Where this leads next
Test yourself across the whole topic in the biological molecules and enzymes practice set. The inheritance model board shows how a simple model can be used with stated assumptions, which is a useful habit for enzyme diagrams as well.
If your explanations keep mixing the two causes, it may be a matter of practising the question before the answer. Our teachers do this in online one-to-one Biology tuition.