In an investigation into the effect of pH on amylase, a group of learners added a different buffer solution to each of five test tubes so that the pH ranged...
Question 1 Report
In an investigation into the effect of pH on amylase, a group of learners added a different buffer solution to each of five test tubes so that the pH ranged from 4 to 8. To every tube they added the same volume of amylase and the same volume of starch. Using the iodine test they timed how long the starch took to disappear and worked out the rate of reaction. The mean rates for the class are drawn in Fig. 1.1, and Table 1.1 records some of the values.
| pH | 4 | 5 | 6 | 7 | 8 |
|---|---|---|---|---|---|
| Time for starch to clear / s | 330 | 140 | 70 | 50 | 170 |
| Rate / 1000 รท time | 3.0 | 7.1 | 20.0 | 5.9 |
Fig. 1.2 shows the buffered tubes of amylase and starch and the iodine test carried out on the dimple tile.
(a) Measure the height of the bar for pH 7 in Fig. 1.1. [1]
(b) Measure the height of the bar for pH 5 and use the scale to state the rate it represents. [1]
(c) Complete Table 1.1 by calculating the rate at pH 6. Show your working. [2]
(d) State the optimum pH for this amylase. [1]
(e) Describe how increasing the pH from 4 to 8 affects the rate of reaction. [3]
(f) Explain why the activity of amylase is very low at pH 4. [2]
(g) The learners repeated each pH three times and took a mean. Explain why. [2]
(h) Identify the independent variable and state how it was set in each tube. [2]
(i) Suggest two variables that were kept constant. [2]
(j) Draw, on the grid provided, the shape of the line you would expect if rate were plotted against pH. [2]
(k) Name the sugar produced when amylase breaks down starch. [2]
Answer Details
This task tests reading a bar chart and table, calculating a rate, and explaining how pH affects an enzyme.
(a) Measure the height of the pH 7 bar with a ruler; award for a value consistent with it being the tallest bar (it represents the highest rate, 20). [1]
(b) Reading the pH 5 bar against the scale gives a rate of 7.1; award for a correct read and use of the scale. [1]
(c) Rate = 1000 / time, so at pH 6: \(1000 / 70 = 14.3\). Working [1], answer [1]. Completed row:
| pH | 4 | 5 | 6 | 7 | 8 |
|---|---|---|---|---|---|
| Rate / 1000 divided by time | 3.0 | 7.1 | 14.3 | 20.0 | 5.9 |
[2]
(d) The optimum pH is the one giving the highest rate, pH 7. [1]
(e) As the pH is raised from 4 the rate increases [1], reaching a maximum at pH 7 [1], then it decreases from pH 7 to pH 8 [1]. [3]
(f) At pH 4 the strongly acidic conditions denature the enzyme, changing the shape of its active site [1]; so the starch substrate can no longer fit and bind, and almost no reaction occurs, giving a very low rate [1]. [2]
(g) Repeats let any anomalous result be spotted and left out [1]; taking a mean of the repeats gives a more reliable value by reducing the effect of random error [1]. [2]
(h) The independent variable is pH [1]; it was set by adding a different buffer solution to each tube [1]. [2]
(i) Any two variables kept constant: the volume or concentration of amylase; the volume or concentration of starch; the temperature; the size of the iodine drop used for testing. [2]
(j) The expected line rises to a single peak at pH 7 [1] and falls away on both sides, giving a bell shape [1]. [2]
(k) Amylase breaks starch down into the sugar maltose (accept reducing sugar). [2]
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