When each amylase mixture starts with the same amount of starch, a shorter time to the same iodine endpoint indicates a faster reaction. Compare the times at different pH values while keeping temperature and starting concentrations the same.
The 2027 SEC G3 Biology K325 syllabus includes investigating how pH affects enzyme-catalysed reactions. This guide shows how to use a fixed endpoint to compare rates and how to handle a mixture that has not reached that endpoint.
- Start each mixture with the same amount of starch and amylase.
- Use the same iodine test and sampling interval.
- Interpret shorter time as faster progress to the same endpoint.
- Treat an unfinished trial as a time limit rather than an exact time.
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Define the endpoint before comparing
In this investigation, small samples are placed into iodine solution at regular intervals. Blue-black indicates detectable starch. The first sample that leaves the iodine yellow-brown marks the recorded endpoint.
The endpoint is a detection threshold. When the last blue-black reading was at 30 seconds and the first yellow-brown reading is at 40 seconds, the change occurred between those observations.
Use buffers to set pH and hold other conditions as similar as possible. Compare samples at the same time intervals because a wider sampling interval gives a less precise endpoint time.
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Worked example | Three pH values, one endpoint
A pupil tests identical amounts of starch and amylase at three pH values, keeping the temperature and total volume the same. A sample is tested every 10 seconds for up to 120 seconds. These readings are illustrative.
| pH | Recorded result | Interpretation |
|---|---|---|
| 5 | 80 s | Reached endpoint after pH 7 |
| 7 | 40 s | Shortest recorded time |
| 9 | Still blue-black at 120 s | Endpoint not reached in the observation period |
The fastest observed condition among those tested is pH 7. If asked to use 1/time as a rate index, use 1/40 = 0.025 s⁻¹ at pH 7 and 1/80 = 0.0125 s⁻¹ at pH 5.
The reciprocal compares how fast each mixture reaches the same endpoint. It is a rate index rather than a direct measurement of grams of starch broken down per second.
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Practise reporting a time limit
For pH 9, the recorded endpoint time is greater than 120 seconds if the endpoint is eventually reached. Entering 120 seconds as its exact time would overstate the rate index.
If the question asks whether pH 7 is the precise optimum, say that it was the fastest of the tested pH values. More closely spaced pH values around 7 would help refine the estimate.
If one group changes temperature as well as pH, the experiment no longer isolates the effect of pH. Match temperatures before drawing that conclusion.
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Frequently asked questions
Does a longer endpoint time mean a higher rate?
For the same starting amount and endpoint, a shorter time indicates a faster reaction.
Why do I keep the same amount of amylase?
Changing the amount of enzyme can change the rate and obscure the effect of pH.
Can I conclude that no starch was broken down at pH 9?
Starch was still detectable at 120 seconds. Some may already have been broken down.
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