Biology 9700/32 — May/June 2017

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, catalyses the reaction of urea with water to form ammonium carbonate.

Aqueous ammonium carbonate produces ammonium ions forming an alkaline solution. This solution causes red litmus paper to change to blue.

The end-point of the reaction is when all of the surface of the red litmus paper is blue.

Molecule S may affect the activity of the enzyme, E.

You are required to:

prepare a serial dilution of a 0.3% solution of S
prepare a control to show that the results for this investigation are due to the effects of S
record the time taken to reach the end-point for each of the concentrations of S and the control.

You are provided with:

labelled	contents	hazard	volume/ $\text{cm}^3$
S	0.3% solution of S	none	25
W	distilled water	none	100
U	10% urea solution	none	100
E	enzyme solution	harmful irritant	30

labelled	contents	hazard	quantity
R	red litmus paper	none	2 strips

If E comes into contact with your skin, wash it off immediately under cold water.

It is recommended that you wear suitable eye protection.

(a)

You are required to prepare a serial dilution of the 0.3% solution of S which reduces the concentration by a factor of 10 between each successive dilution.

You will need to prepare $9\,\text{cm}^3$ of each concentration.

Fig. 1.1 shows the first two beakers you will use to make your serial dilution.

(i)

Complete Fig. 1.1 by drawing as many extra beakers as you need for your serial dilution.

For each beaker:

state, under the beaker, the volume and concentration of the solution available for use in the investigation
use one arrow, with a label above the beaker, to show the volume and concentration of the solution of S added to prepare the concentration
use another arrow, with a label above the beaker, to show the volume of W added to prepare the concentration.

Fig. 1.1

(ii)

Proceed as follows:

Prepare the concentrations of molecule S, as decided in (a)(i), in the containers provided.
Note: syringe labelled S should be used for solutions of S only.
You are required to set up a control to show that the results for this investigation are due to the effects of molecule S. The control uses water, W, instead of molecule S.
Label one test-tube W for the control and label as many test-tubes as you require for all the concentrations of molecule S prepared in step 1.
Put one piece of red litmus paper, approximately $0.5\,\text{cm}$ in length, into the bottom of each test-tube.
Put $10\,\text{cm}^3$ of U into each test-tube.
Put $1\,\text{cm}^3$ of W into the test-tube labelled W.
The reaction will start when E is added in step 6.
Put $2\,\text{cm}^3$ of E into the test-tube labelled W. Shake gently to mix.
Start timing and record the time taken for all of the surface of the red litmus paper to change to blue (the end-point).
Shake the test-tube during this time to mix the contents.
If the piece of red litmus paper does not reach the end-point after 240 seconds (4 minutes), stop the experiment and record 'more than 240'.
Record your result in (a)(ii).
You are now required to investigate the effect of different concentrations of molecule S on the activity of the enzyme, E.
Put $1\,\text{cm}^3$ of the lowest concentration of molecule S into the appropriately labelled test-tube. Shake gently to mix.
Repeat step 6 and step 7 with the appropriately labelled test-tube.
Repeat with all the other concentrations of molecule S. Record your results in (a)(ii).

Prepare the space below and record your results for:

W, the control
all the concentrations of molecule S.

(iii)

Using your knowledge of enzymes, suggest how molecule S may have reduced the activity of the enzyme.

(iv)

This procedure could be used to estimate the concentration of molecule S in an unknown solution, X.

You are provided with:

labelled	contents	hazard	volume/ $\text{cm}^3$
X	solution with unknown concentration of molecule S	none	20

Repeat step 3 to step 7, using $1\,\text{cm}^3$ of solution X instead of $1\,\text{cm}^3$ of W. Record the time taken to reach the end-point in (a)(iv).

State the time taken to reach the end-point for solution X.

time taken = ______ $\text{s}$

(v)

Use your results in (a)(ii) and (a)(iv) to estimate the concentration of molecule S in solution X.

(vi)

Describe one significant source of error in this investigation.

(vii)

Describe how you could use this procedure to produce a more accurate estimate of the concentration of molecule S in solution X than the one given in (a)(v).

(b)

Scientists studied the effect of mercury entering water where fish and other organisms were living. It was thought that the mercury may have been taken up by these organisms.

The scientists investigated the mercury concentration in tissue samples from five different organisms living in the contaminated water. The concentration of mercury was measured in parts per million (ppm).

The results are shown in Table 1.1.

Table 1.1

organism	mercury concentration in tissue samples / ppm
china fish (F)	24.0
clam (C)	20.0
crab (B)	35.5
oyster (O)	5.5
red mullet (R)	10.5

(i)

Use a sharp pencil for charts.

Plot a chart of the data shown in Table 1.1.

The bars should be separated for each type of organism.

Chart Grid

(ii)

A scientist wanted to compare the mercury concentration in the contaminated water with the mean mercury concentration of the five organisms.

Calculate the mean mercury concentration in the five organisms.

answer = ______ $\text{ppm}$

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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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