A student investigated how pH affects the activity of a protease enzyme. The protease digests the protein in a cloudy milk suspension, so the suspension slo...
Question 1 Report
A student investigated how pH affects the activity of a protease enzyme. The protease digests the protein in a cloudy milk suspension, so the suspension slowly turns clear. The student set up five tubes in a rack, each holding the same volume of cloudy milk buffered at a different pH, as shown in Fig. 6.1. The temperature was kept constant. She added the same volume of protease to each tube, started a stop-clock, and recorded the time for each tube to turn clear. Her results, and the calculated rate of reaction, are shown in Table 6.1. The rate was worked out using rate = 1000 / time.
| pH | 4 | 6 | 8 | 10 | 12 |
|---|---|---|---|---|---|
| Time to turn clear / s | 200 | 80 | 45 | 90 | 210 |
| Rate / units | 5.0 | 12.5 | 11.1 | 4.8 |
(a) Measure, from Table 6.1, the time taken for the milk to turn clear at pH 8. [1]
(b) Record the pH at which the protease worked fastest. [1]
(c) Complete Table 6.1 by calculating the rate of reaction at pH 8. Show your working. [2]
(d) Describe how the appearance of the milk suspension changes as the protein is digested. [1]
(e) Explain, in terms of the enzyme, why the reaction is very slow at pH 4 and at pH 12. [2]
(f) State two variables that must be kept constant. [2]
(g) Suggest why a buffer solution was added to each tube. [1]
Answer Details
This tests enzyme activity against pH, using clearing time and a rate calculation.
(a) The time to turn clear at pH 8 is 45 s. [1]
(b) The protease worked fastest at pH 8 (shortest time / highest rate). [1]
(c) Rate at pH 8:
\( \text{rate}=\dfrac{1000}{45}=\mathbf{22.2\ units} \); working [1], answer [1] [2].
(Rate uses 1 divided by time, so a shorter clearing time gives a higher rate.)
(d) As the protein is digested, the cloudy suspension gradually becomes clear (transparent) [1].
(e) At pH 4 and pH 12 the reaction is very slow because at these extreme pH values the enzyme is denatured [1]; the shape of the active site changes so the protein substrate no longer fits [1] [2], and little digestion occurs.
(f) Any two controlled variables: temperature; volume or concentration of protease; volume or concentration of milk suspension. [2]
(g) A buffer solution is added to keep the pH of each tube constant during the reaction [1], so that pH is the only variable being tested.
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