Biology 9700/33 — October/November 2015

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

questions

marks

120

minutes

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

Q122MManipulation, Measurement and ObservationPresentation of Data and ObservationsAnalysis, Conclusions and EvaluationFree sample

Yeast cells contain an enzyme, catalase, which catalyses the hydrolysis (breakdown) of hydrogen peroxide into oxygen and water with the transfer of heat to the surroundings. Sodium chloride solution can affect this reaction in yeast cells.

The effect of sodium chloride solutions on yeast cells can be measured by the increase in temperature.

You are required to:

prepare different concentrations of the sodium chloride solution, S
investigate the effect of different concentrations of sodium chloride solution (independent variable) on the yeast cells by finding the highest increase in temperature.

You are provided with:

labelled	contents	hazard	volume / $\text{cm}^3$
S	10% sodium chloride solution	none	60
W	distilled water	none	60
Y	yeast cell suspension	none	60
H	hydrogen peroxide solution	harmful irritant	60

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 the 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 apparatus and solutions and state the level of risk of the procedure: low or medium or high.

hazard ______

level of risk ______

(b)

You are required to change the concentration of the sodium chloride solution.

Table 1.1 shows how to make up two of the sodium chloride concentrations you will use.

Table 1.1

volume of 10% sodium chloride solution S / $\text{cm}^3$	volume of distilled water W / $\text{cm}^3$	percentage concentration of sodium chloride solution S
0	10	0
10	0	10

During the investigation you will mix $10\ \text{cm}^3$ of each concentration of sodium chloride solution with $10\ \text{cm}^3$ of yeast cell suspension to make a mixture, SY.

This will result in the concentration of the sodium chloride solution in SY being halved as shown in Table 1.2.

Table 1.2

volume of 10% sodium chloride solution S / $\text{cm}^3$	volume of distilled water W / $\text{cm}^3$	percentage concentration of sodium chloride solution S	percentage concentration of sodium chloride solution in SY
0	10	0	0
10	0	10	5

(i)

Decide which concentrations of sodium chloride solution to prepare:

using simple dilution
using 10% sodium chloride solution, S.

Complete the table to show how you will prepare these concentrations.

volume of 10% sodium chloride solution S / $\text{cm}^3$	volume of distilled water W / $\text{cm}^3$	percentage concentration of sodium chloride solution S	percentage concentration of sodium chloride solution in SY
0	10	0	0



10	0	10	5

(ii)

Read step 1 to step 13 before proceeding.

To help you answer (b)(ii), you should consider how to record the temperatures (raw results) and then process your raw results to find the temperature rise.

Proceed as follows:

Prepare the concentrations of S as stated in (b)(i) in the beakers provided.
Stir the yeast cell suspension, Y.
Use a clean syringe to collect $10\ \text{cm}^3$ of Y from below the froth.
Put $10\ \text{cm}^3$ of Y into each of the beakers used in step 1 to make a mixture, SY in each beaker.
Stir well.
Leave for at least 5 minutes.

While you are waiting continue with Question 1.

Label test-tubes with the concentration of sodium chloride solution in the mixture SY you prepared in step 4.
Put $5\ \text{cm}^3$ of H into each of these test-tubes.
Record the temperature of H in each test-tube.
After leaving SY for at least 5 minutes (step 6), put $1\ \text{cm}^3$ of the mixture SY with 0% sodium chloride solution into the appropriate test-tube. Stir well.
Time for 2 minutes and record the highest temperature of the contents of the test-tube during the 2 minutes.
Repeat step 10 and step 11 using the mixture of SY with the next lowest concentration of sodium chloride solution.
Repeat step 12 with the remaining concentrations.

Prepare the space below to record your raw results and your processed results.

(iii)

Explain why it is necessary to collect a result for the 0% sodium chloride solution and yeast mixture, SY.

(iv)

State the value of the smallest division on the scale of your thermometer.

smallest division = ______ $^{\circ}\text{C}$

State the actual error in measuring an increase in temperature of $6\ ^{\circ}\text{C}$ using this thermometer.

actual error $6\ ^{\circ}\text{C} \pm$ ______

(v)

The actual error of using the thermometer is the same for each measurement. The trend in the results is not affected and it is not a significant source of error.

Due to the procedure, the highest temperature recorded may be inaccurate (not close to the true temperature).

Suggest how step 9 and step 11 (on page 4) introduced an error which caused the highest temperature recorded to be inaccurate.

(vi)

This procedure investigated the effect of the concentration of sodium chloride solution on the activity of catalase in yeast cells.

To modify this procedure for investigating another variable, the independent variable (concentration of sodium chloride) would need to be standardised.

Describe how the independent variable (concentration of sodium chloride solution) will be standardised.

Consider how you would modify this procedure to investigate the effect of the concentration of hydrogen peroxide on the activity of the catalase in yeast cells.

Describe how the independent variable (concentration of hydrogen peroxide) will be investigated.

(c)

In a similar investigation, a student investigated how changing the concentration of copper sulfate solution (independent variable) affected the hydrolysis of hydrogen peroxide.

The release of oxygen was measured by counting the bubbles when the oxygen was released through a delivery tube into a test-tube of water.

The results are shown in Table 1.3.

Table 1.3

percentage concentration of copper sulfate solution	rate of catalase activity / number of bubbles released $\text{min}^{-1}$
0.3000	0
0.1500	1
0.0750	2
0.0375	37
0.0000	39

You are required to use a sharp pencil for graphs.

(i)

Plot a graph of the data shown in Table 1.3.

Fig. 1.3_graph_grid

(ii)

Explain the relationship between the concentration of copper sulfate solution and the activity of the catalase.

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