Lesson 6 of 7 / Biological molecules
Temperature and enzyme activity
Why does warming help only up to a point?
In this lesson: Explain temperature effects and compare rates using a controlled enzyme investigation.
About 6 min
The key ideaWarming initially increases productive collisions. Above the optimum, denaturation changes the active site and reduces the rate. Cooling usually slows activity without denaturing the enzyme.
Reaction rate is how quickly substrate is used or product is formed. For a fixed amount of reaction, a shorter completion time means a higher average rate.
Explore the idea
Shorter time, higher relative rate
30 degrees C: 60 s. Relative rate = 1/60 = 0.0167 per second. The same endpoint takes half as long as at 20 degrees C, so the relative rate is twice as large.
Original practice data, not published measurements. All mixtures use the same starting amounts, pH and endpoint. Dotted lines guide the eye; intermediate temperatures were not measured.
Explanation
At low temperature, enzyme and substrate molecules move more slowly. They collide less frequently, so fewer enzyme-substrate complexes form per unit time. Increasing temperature initially increases movement and the frequency of productive collisions, raising the reaction rate.
An enzyme has an optimum temperature at which its activity is greatest under the conditions tested. Beyond it, heat disrupts the protein structure. The active site changes shape, so the substrate no longer fits as well. The enzyme is denatured and the rate falls. The optimum is not the same for every enzyme.
Cold usually reduces activity without permanently changing the active site. Warming a cold enzyme appropriately can restore a higher rate. Heating a denatured enzyme and then cooling it does not usually restore its original function. Avoid saying that an enzyme is alive and can die.
To investigate temperature with amylase and starch, use water baths to bring enzyme and substrate separately to the chosen temperature before mixing. Keep pH, concentrations and volumes constant. At regular intervals, test a small drop of reaction mixture with iodine on a spotting tile. The endpoint is the first test where starch is no longer detected.
Use the same sampling interval and endpoint rule for every temperature, repeat each condition and compare means. If the same amount of starch is digested, 1/time is a relative measure of rate. An endpoint time is less precise when sampling intervals are long; shorter regular intervals improve the estimate.
Step by step
- 1
Control the comparison
Change temperature only. Keep pH and the amounts and concentrations of enzyme and substrate constant.
- 2
Measure the same endpoint
Use the same iodine sampling method to identify when starch is no longer detected.
- 3
Explain both sides of the pattern
Rising rate: more productive collisions. Falling rate at high temperature: denaturation and altered active sites.
Worked example
A shorter time means a faster reaction
Equal starch mixtures reach the same endpoint in 120 s at 20 degrees C and 60 s at 30 degrees C. How do the relative rates compare?
One way to explain it
The relative rates are 1/120 and 1/60 per second. The reaction at 30 degrees C is twice as fast by this measure because the same endpoint is reached in half the time. This comparison does not by itself identify the optimum temperature.
Why this answer works
- Use the same amount of reaction and endpoint.
- Compare reciprocals, rather than treating a larger time as a larger rate.
- Do not infer an optimum from only two temperatures.
Is this true? "An enzyme becomes denatured whenever it works slowly."
Low temperature can slow collisions while leaving the active site intact. A low rate alone does not establish denaturation; the cause and conditions matter.