AS & A Level Biology - 9700 Paper 3 2018 Winter Zone 1

You need to follow the progress of sedimentation of yeast cells in a test-tube. You will need to measure the height of the sediment at different times, for a total of 10 minutes. You will use a graph paper scale to measure the sediment.

(a) Test-tube A in Fig. 1.1 shows how the test-tube will be set up at the start (0 minutes).

(i) Decide what you would expect the contents of the test-tube to look like after 10 minutes of sedimentation.
Draw on test-tube B in Fig. 1.1:

• the layers that you expect to see after 10 minutes of sedimentation
• label lines and labels for the layers.

(ii) Describe how you will use the graph paper scale to measure the sediment. You may draw on test-tube B to help you with your answer.
.................................................................................................................................
.................................................................................................................................
................................................................................................................................. [1]

(iii) Decide how often you will take these measurements, including measuring the sediment at 10 minutes.
State the times you will use for measuring the sediment.
.................................................................................................................................
................................................................................................................................. [1]

There are molecules on the surface of yeast cells which cause the yeast cells to stick together.
The more cells that stick together, the faster the rate of sedimentation. This process may be affected by the pH of the yeast cell suspension.

You will need to:

• investigate the effect of the independent variable, pH, on the sedimentation of a yeast cell suspension
• use the results to estimate the pH of buffer BU.

You are provided with the materials shown in Table 1.1. [Table_1]

Read step 1 to step 9 before proceeding.

1. Put $7 \text{ cm}^{3}$ of B3 into a test-tube.
2. Put $1 \text{ cm}^{3}$ of C into the same test-tube.
3. Stir Y in the beaker, then put $7 \text{ cm}^{3}$ of Y into the same test-tube.
4. Repeat step 1 to step 3 for B4, B5, B6 and BU.
5. Put a bung into one of the test-tubes and invert the test-tube three times to mix the contents.
6. Repeat step 5 for all the other test-tubes.
7. Immediately start timing.
8. At your selected times, as stated in (a)(iii), measure the sediment in each test-tube, as you described in (a)(ii). Do not disturb the contents of the test-tubes.
9. Record your results for B3, B4, B5 and B6 in (a)(iv) and record the results for BU in (a)(v).

(iv) Record your results for B3, B4, B5 and B6 in an appropriate table. [1]

(v) Complete Table 1.2 by recording your results for BU. [Table_2]
Use your results in (a)(iv) and (a)(v) at 10 minutes to estimate the pH for BU.
estimated pH of BU = ........................ [2]

(vi) Suggest how to modify this procedure to obtain a more accurate estimate of the pH for BU.
.................................................................................................................................
.................................................................................................................................
................................................................................................................................. [2]

(vii) Suggest an explanation for the effect of pH on the sedimentation of yeast cells.
.................................................................................................................................
................................................................................................................................. [1]

(viii) Identify one significant source of error in this investigation. Explain why this is a source of error.
source of error .................................................................................................
explanation ...................................................................................................... [1]

(b) At the end of beer making, the yeast is no longer needed and is separated from the beer by sedimentation. The progress of sedimentation can be monitored by removing samples of beer at intervals over a period of 30 hours and counting the number of yeast cells remaining in the beer, as shown in Table 1.3. [Table_3]

(i) Plot a graph of the data shown in Table 1.3 on the grid in Fig. 1.2. Use a sharp pencil for drawing graphs. [4]

(ii) Use your graph to find the number of yeast cells at 18 hours. Show on the graph how you determined your answer. number of yeast cells ............................ arbitrary units cm$^{-3}$ [2]

You need to follow the progress of sedimentation of yeast cells in a test-tube. You will need to measure the height of the sediment at different times, for a total of 10 minutes. You will use a graph paper scale to measure the sediment.

(a) Test-tube A in Fig. 1.1 shows how the test-tube will be set up at the start (0 minutes).

(i) Decide what you would expect the contents of the test-tube to look like after 10 minutes of sedimentation.
Draw on test-tube B in Fig. 1.1:

• the layers that you expect to see after 10 minutes of sedimentation
• label lines and labels for the layers.

(ii) Describe how you will use the graph paper scale to measure the sediment. You may draw on test-tube B to help you with your answer.
.................................................................................................................................
.................................................................................................................................
................................................................................................................................. [1]

(iii) Decide how often you will take these measurements, including measuring the sediment at 10 minutes.
State the times you will use for measuring the sediment.
.................................................................................................................................
................................................................................................................................. [1]

There are molecules on the surface of yeast cells which cause the yeast cells to stick together.
The more cells that stick together, the faster the rate of sedimentation. This process may be affected by the pH of the yeast cell suspension.

You will need to:

• investigate the effect of the independent variable, pH, on the sedimentation of a yeast cell suspension
• use the results to estimate the pH of buffer BU.

You are provided with the materials shown in Table 1.1. [Table_1]

Read step 1 to step 9 before proceeding.

1. Put $7 \text{ cm}^{3}$ of B3 into a test-tube.
2. Put $1 \text{ cm}^{3}$ of C into the same test-tube.
3. Stir Y in the beaker, then put $7 \text{ cm}^{3}$ of Y into the same test-tube.
4. Repeat step 1 to step 3 for B4, B5, B6 and BU.
5. Put a bung into one of the test-tubes and invert the test-tube three times to mix the contents.
6. Repeat step 5 for all the other test-tubes.
7. Immediately start timing.
8. At your selected times, as stated in (a)(iii), measure the sediment in each test-tube, as you described in (a)(ii). Do not disturb the contents of the test-tubes.
9. Record your results for B3, B4, B5 and B6 in (a)(iv) and record the results for BU in (a)(v).

(iv) Record your results for B3, B4, B5 and B6 in an appropriate table. [1]

(v) Complete Table 1.2 by recording your results for BU. [Table_2]
Use your results in (a)(iv) and (a)(v) at 10 minutes to estimate the pH for BU.
estimated pH of BU = ........................ [2]

(vi) Suggest how to modify this procedure to obtain a more accurate estimate of the pH for BU.
.................................................................................................................................
.................................................................................................................................
................................................................................................................................. [2]

(vii) Suggest an explanation for the effect of pH on the sedimentation of yeast cells.
.................................................................................................................................
................................................................................................................................. [1]

(viii) Identify one significant source of error in this investigation. Explain why this is a source of error.
source of error .................................................................................................
explanation ...................................................................................................... [1]

(b) At the end of beer making, the yeast is no longer needed and is separated from the beer by sedimentation. The progress of sedimentation can be monitored by removing samples of beer at intervals over a period of 30 hours and counting the number of yeast cells remaining in the beer, as shown in Table 1.3. [Table_3]

(i) Plot a graph of the data shown in Table 1.3 on the grid in Fig. 1.2. Use a sharp pencil for drawing graphs. [4]

(ii) Use your graph to find the number of yeast cells at 18 hours. Show on the graph how you determined your answer. number of yeast cells ............................ arbitrary units cm$^{-3}$ [2]

J1 is a slide of a stained transverse section through a plant stem.
You are not expected to be familiar with this specimen.
(a) Observe all the different tissues in the stem on J1.
(i) Draw a large plan diagram of a quarter of the stem on J1, shown by the shaded area in Fig. 2.1.
Use one ruled label line and label to identify the xylem.

Use a sharp pencil for drawing.



You are expected to draw the correct shape and proportions of the different tissues.

(ii) Observe the tissues close to the epidermis of the stem on J1.
Near to the epidermis there are cells which are larger than any other cells in the stem.
Select one group of four adjacent, touching cells that make up this tissue.
Each cell must touch at least two of the other cells.
Make a large drawing of this group of four cells.
Use one ruled label line and label to identify the cell wall of one cell.

You are expected to draw the correct shape and proportions of the different cells.

Fig. 2.2 is a photomicrograph of a stained transverse section through the stem of a different type of plant.
You are not expected to be familiar with this specimen.


(b) (i) Use the magnification on Fig. 2.2 to calculate the actual length, in $\mu$m, of line X (length of a vascular bundle).
Show all the steps in your working and use appropriate units.

actual length of X = ................................................... $\mu$m

(ii) A student used a light microscope to observe a slide of the section shown in Fig. 2.2.
State how the student would use an eyepiece graticule scale to measure the actual length of a vascular bundle.

(c) Prepare the space below so that it is suitable for you to record the observable differences between the stem on J1 and the stem in Fig. 2.2.
Record your observations in the space you have prepared.

J1 is a slide of a stained transverse section through a plant stem.
You are not expected to be familiar with this specimen.
(a) Observe all the different tissues in the stem on J1.
(i) Draw a large plan diagram of a quarter of the stem on J1, shown by the shaded area in Fig. 2.1.
Use one ruled label line and label to identify the xylem.

Use a sharp pencil for drawing.



You are expected to draw the correct shape and proportions of the different tissues.

(ii) Observe the tissues close to the epidermis of the stem on J1.
Near to the epidermis there are cells which are larger than any other cells in the stem.
Select one group of four adjacent, touching cells that make up this tissue.
Each cell must touch at least two of the other cells.
Make a large drawing of this group of four cells.
Use one ruled label line and label to identify the cell wall of one cell.

You are expected to draw the correct shape and proportions of the different cells.

Fig. 2.2 is a photomicrograph of a stained transverse section through the stem of a different type of plant.
You are not expected to be familiar with this specimen.


(b) (i) Use the magnification on Fig. 2.2 to calculate the actual length, in $\mu$m, of line X (length of a vascular bundle).
Show all the steps in your working and use appropriate units.

actual length of X = ................................................... $\mu$m

(ii) A student used a light microscope to observe a slide of the section shown in Fig. 2.2.
State how the student would use an eyepiece graticule scale to measure the actual length of a vascular bundle.

(c) Prepare the space below so that it is suitable for you to record the observable differences between the stem on J1 and the stem in Fig. 2.2.
Record your observations in the space you have prepared.