Lesson focus: Temperature changes enzyme reaction rateHow fast something happens, such as how much changes each second or each minute. by altering collision energy and, at high values, the enzyme’s three-dimensional structure.
Plan and interpret an amylase investigation while controlling pH, concentration, volume and endpointThe clear sign you agree in advance that tells you when to stop the timer or the test, such as a cross disappearing from view. judgement.
Build on earlier learning
Bacteria and enzymes in digestionThe previous lesson established enzyme specificity. This practical investigates how environmental conditions change enzyme activity.
Practical
Temperature and amylase
- Mix starch and amylase at a set temperature in a water bath.
- Every 30 seconds add a drop of the mixture to iodine on a spotting tile.
- Record the time when iodine stays orange-brown (all starch digested).
- Repeat at other temperatures.
Record: A table and graph of temperature against time to digest the starch.
Safety: Wear eye protection; iodine stains skin.
Enzyme activity and temperature
Warming speeds up collisions between enzyme and substrate, until the active site changes shape and the enzyme denatures.
Enzyme Substrate Product
Controls
You will be successful when you can…
- Explain the effect of temperature on enzyme rate.
- Plan an amylase–starch investigation.
- Keep pH and reagent quantities the same as control variables.
- Calculate rate and interpret anomalies.
Words that carry the science
- denaturation
- A change to protein structure that alters the active site and reduces function.
- optimum
- The condition at which the measured rate is greatest for the stated system.
- rate
- Change in a measured quantity divided by time.
- buffer
- A solution that resists changes in pH.
- endpoint
- A defined observation used to decide that a process has reached a stated stage.
- water bath
- A container of water used to keep samples near a chosen temperature.
Measure enzyme rate with a defensible endpoint
01Warming increases successful interactions at first
At low temperature, enzyme and substrate molecules move more slowly, so successful collisions occur less often. Warming generally increases kinetic energy and reaction rateHow fast something happens, such as how much changes each second or each minute. up to an optimumThe best condition, where something works fastest, such as the best temperature for an enzyme. for the stated enzyme and conditions.
The optimum is not a universal fixed temperature for every enzyme. It depends on organism, enzyme form, pH, substrate and measurement method.
02High temperature disrupts protein structure
Above the optimum, bonds maintaining the enzyme’s three-dimensional structure are disrupted. The active site changes and fewer substrate molecules bind productively. This is denaturationWhen high temperature or extreme pH changes the shape of an enzyme's active site, so the substrate no longer fits and the enzyme stops working..
Cooling a strongly denatured enzyme does not necessarily restore the original folding, so the high-temperature loss can be irreversible.
03The iodine endpoint estimates starch disappearance
Mix equal volumes and concentrations of amylase and starch equilibrated at a chosen temperature and pH. At regular intervals, place a drop into iodine on a spotting tile. The endpointThe clear sign you agree in advance that tells you when to stop the timer or the test, such as a cross disappearing from view. occurs when iodine remains orange-brown, indicating starch is no longer detected.
Use rate = 1/time as a simple reciprocal measure. Keep pH with a bufferA solution that keeps the pH steady when small amounts of acid or alkali are added., standardise drop size and repeat temperatures. A colorimeter or automated sensor can reduce subjective endpoint judgement.
Calculate a reciprocal rate
At 30 °C, starch is no longer detected after 80 s. Calculate the simple rate measure.
- Use the defined rate measure: rate = 1 ÷ time.
- Substitute 80 s: rate = 1 ÷ 80.
- Calculate 0.0125.
- Attach the reciprocal-time unit: s⁻¹.
How to make the reasoning visible: The value compares trials consistently but does not give a concentration change per second.
Interpret an amylase data set
Times are illustrative means collected at the same pH and concentrations.
| Temperature / °C | Mean endpoint time / s | Rate 1/time / s⁻¹ | Interpretation |
|---|---|---|---|
| 10 | 240 | 0.0042 | Slow |
| 20 | 135 | 0.0074 | Faster |
| 30 | 80 | 0.0125 | Faster |
| 40 | 52 | 0.0192 | Fastest measured |
| 50 | 150 | 0.0067 | Rate falls |
| 60 | No endpoint by 300 | Below 0.0033 | Very little detected activity |
Describe the pattern and identify the measured optimum.
Why should the method equilibrate starch and enzyme before mixing?
Check your understanding
Answer from memory, then check against the suggested answer.
What happens to rate below the optimum as temperature rises?
What is denaturation?
Why use a buffer?
A student concludes 40 °C is the exact optimum after testing only 30, 40 and 50 °C once. Evaluate.
Explain why reciprocal time is useful but biologically limited.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A student investigates the effect of temperature on amylase. The pH is not controlled, there is one trial at each temperature, and the end point is judged by eye.
Explain three weaknesses of this investigation.
(3)Suggest three improvements.
(3)Total for this question: 6 marks
Command word: For evaluateA command word: weigh up the good points and bad points, then give your overall judgement., connect every limitation to the conclusion and then give a matching improvement.