Biology 9700/35 — October/November 2015

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

questions

marks

120

minutes

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

Q1Analysis, Conclusions and EvaluationManipulation, Measurement and ObservationPresentation of Data and ObservationsFree sample

A number of plant tissues are coloured because the cells contain chemicals called anthocyanins. These anthocyanins change colour according to the pH and the colour changes are reversible.

You are provided with red cabbage which contains anthocyanins and can be used to make a pH indicator solution.

You are required to:

prepare a pH indicator solution from red cabbage
investigate the effect of pH (the independent variable) on the colour of the indicator solution
identify the pH of the solutions, S1 and S2.

You are provided with:

Table 1.1

labelled	contents	hazard	volume / $\text{cm}^3$
pH 3	solution of pH 3	irritant	30
pH 4	solution of pH 4	irritant	30
pH 6	solution of pH 6	irritant	30
S1	solution of unknown pH	irritant	30
S2	solution of unknown pH	irritant	30
W	distilled water	none	30

labelled	contents	hazard	quantity
P	red cabbage	—	pieces of leaves

It is recommended that you wear safety glasses/goggles.

When carrying out a practical procedure, the hazards of the use of all the apparatus and solutions need to be considered. Then the level of risk needs to be assessed as low or medium or high.

(a)

State the hazard with the greatest level of risk when using the solutions in Table 1.1.
State the level of risk of using the solutions: low or medium or high.

hazard ______

level of risk ______

(b)

Proceed as follows:

Cut the red cabbage leaves into small pieces and place into the water in beaker A.
Heat the water in beaker A and boil for 5 minutes.
Turn off the Bunsen burner and leave the water in beaker A to cool for 5 minutes to produce a coloured liquid.
This is your pH indicator solution.

While you are waiting continue with Question 1.

After 5 minutes, pour approximately $100\ \text{cm}^3$ of this pH indicator solution into a container. Most of the cabbage leaves should remain in beaker A.

You are now going to investigate the effect of pH on the colour of the pH indicator solution.

You will need to decide how to standardise the volume of pH indicator solution required to observe the colour change.

So that you can observe the colour change clearly, you need to decide the volume of pH indicator solution to add.

Put $4\ \text{cm}^3$ of the pH 6 solution into a test-tube.
Put $1\ \text{cm}^3$ of pH indicator solution into the same test-tube and mix the contents.
Add further $1\ \text{cm}^3$ volumes, up to $5\ \text{cm}^3$ , in order to decide which volume to use to observe the colour change clearly.
Record in (b)(ii) the final colour of the pH indicator solution at pH 6.

(i)

State the volume of pH indicator solution you will use to observe the colour change clearly and describe how you will standardise the volume of the pH indicator solutions to carry out step 9.

volume = ______ $\text{cm}^3$

description ______

(ii)

Using the information in step 5 and (b)(i), find the colour of the pH indicator solution with pH 3, pH 4, W, S1 and S2. Record all your observations in (b)(ii).

Prepare the space below and record all your observations.

(iii)

Fig. 1.1

Fig. 1.1 shows the position of pH 7 on a scale.
Using your results in (b)(ii) complete Fig. 1.1 to show:

the positions of pH 3, pH 4 and pH 6 on the scale
the positions of W, S1 and S2 on the scale.

(c)

Like red cabbage, beetroot cells also contain anthocyanins.

A student investigated the effect of temperature on the cell surface membranes of beetroot.

Beetroot was cut into $1\ \text{cm}^3$ cubes.
One cube of beetroot was put into each of five test-tubes containing water which had been heated to $25^{\circ}\text{C}$ or $30^{\circ}\text{C}$ or $35^{\circ}\text{C}$ or $45^{\circ}\text{C}$ or $50^{\circ}\text{C}$ .
All other variables were standardised.
The cubes were removed after 15 minutes.
The absorbance of light by the coloured liquid left in the test-tubes was measured using a colorimeter.

The absorbance of light by pure water is $0.000$ arbitrary units.

The results of the student's investigation are shown in Table 1.2.

Table 1.2

temperature / $^{\circ}\text{C}$	absorbance of light by the coloured liquid / arbitrary units
25	0.080
30	0.155
35	0.225
45	0.395
50	0.580

You are required to use a sharp pencil for graphs.

(i)

Plot a graph of the data shown in Table 1.2.

graph_grid_1

(ii)

Use your graph to estimate the absorbance of light at $40^{\circ}\text{C}$ .
Show on your graph how you estimated the absorbance of light.

absorbance = ______

(iii)

Explain the effect of temperature on the permeability of cell surface membranes in beetroot.

(iv)

The student decided to investigate the effect of alcohol on the cell surface membranes of beetroot cubes at $20^{\circ}\text{C}$ .

Describe how the temperature will be standardised in this investigation.

(v)

Describe how the independent variable will be investigated.

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