The enzyme, E , hydrolyses (breaks down) the protein in milk. A mixture of milk and E changes from cloudy to clear as a result of the hydrolysis of protein. The end-point is reached when the mixture is clear. You will need to: investigate the activity of enzyme E on milk, at a range of temperatures from $70^\circ\text{C}$ to $30^\circ\text{C}$ record the time taken to reach the end-point for each temperature. You are provided with the materials shown in Table 1.1. Table 1.1…

Biology 9700/35 — October/November 2018

Cambridge AS Level · Advanced Practical Skills · worked solutions for every part, with the mark scheme

questions

marks

120

minutes

Topics Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation · Use of the Light Microscope

Q1Manipulation, Measurement and ObservationPresentation of Data and ObservationsAnalysis, Conclusions and EvaluationFree sample

The enzyme, E, hydrolyses (breaks down) the protein in milk.

A mixture of milk and E changes from cloudy to clear as a result of the hydrolysis of protein.

The end-point is reached when the mixture is clear.

You will need to:

investigate the activity of enzyme E on milk, at a range of temperatures from $70^\circ\text{C}$ to $30^\circ\text{C}$
record the time taken to reach the end-point for each temperature.

You are provided with the materials shown in Table 1.1.

Table 1.1

labelled	contents	hazard	volume / $\text{cm}^3$
E	0.5% enzyme solution	irritant	20
M	milk	none	50

If E comes into contact with your skin, wash off immediately under cold water.
It is recommended that you wear suitable eye protection.

You will test the activity of the enzyme E, at $70^\circ\text{C}$ and at $30^\circ\text{C}$ .

(a)

(i)

State three other temperatures that you will use to show the effect of temperature on the activity of E.

(ii)

Read step 1 to step 10 before proceeding.

Set up a water-bath and heat to $70^\circ\text{C}$ .
Put $1\,\text{cm}^3$ of E into each of five test-tubes.
Put one of the test-tubes into the water-bath and leave for 2 minutes.

While you are waiting continue with other parts of Question 1.

After 2 minutes, stir M and put $3\,\text{cm}^3$ of M into the test-tube in the water-bath. Mix gently using a glass rod.
Immediately start timing.
Record in (a)(ii) the time taken for the mixture to become clear (the end-point). You may find it easier to judge the end-point if the black card is held behind the mixture.
If the end-point is not reached by 180 seconds, record 'more than 180'.
Adjust the temperature of the water-bath so that it is at the highest temperature stated in (a)(i).
Repeat step 3 to step 6.
Adjust the temperature of the water-bath and repeat step 3 to step 6 with the other temperatures stated in (a)(i).
Adjust the temperature of the water-bath to $30^\circ\text{C}$ and repeat step 3 to step 6.

Record your results in an appropriate table.

(iii)

A student carried out the same procedure at $80^\circ\text{C}$ and recorded the time taken to reach the end-point as 75 seconds.

Calculate the rate of enzyme activity at $80^\circ\text{C}$ .

Show your working.

rate of enzyme activity = ______ $\text{s}^{-1}$

(iv)

The student identified two significant sources of error in the investigation as shown in Table 1.2.

Complete Table 1.2 to suggest how to make an improvement to reduce each of the sources of error the student identified.

Table 1.2

significant source of error	how to make an improvement
difficult to judge when the end-point is reached
difficult to maintain the temperature of the water-bath

(v)

Suggest how the student could set up an appropriate control for this investigation.

(b)

A scientist carried out an experiment to investigate the effect of pH on the activity of a different enzyme that hydrolyses the protein in milk.

Buffer solutions were used to change the pH and the investigation was carried out at the optimum temperature for this enzyme. All other variables were kept constant.

The results are shown in Table 1.3.

Table 1.3

pH	activity of enzyme / arbitrary units
1.5	78
2.0	95
3.0	91
4.5	36
6.0	13

(i)

State the independent variable in this investigation.

(ii)

Plot a graph of the data shown in Table 1.3 on the grid in Fig. 1.1.

Use a sharp pencil for drawing graphs.

Fig. 1.1

(iii)

With reference to Fig. 1.1 describe the effect of pH on the activity of the enzyme.

(iv)

Explain why the activity of the enzyme at pH 6 is different from the activity at pH 3.

(v)

The scientist decided that the optimum pH for this enzyme had not been identified accurately.

Suggest how the experiment could be modified to find a more accurate value for the optimum pH.

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Q2Use of the Light Microscope · Analysis, Conclusions and Evaluation · Manipulation, Measurement and Observation · Presentation of Data and Observations18M

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