Cambridge A Level Biology 9700 — 2018 Oct/Nov Paper 3 · Variant 1

1 Yeast cells contain enzymes which catalyse the breakdown of glucose to produce ethanol and carbon dioxide. This process is used to make beer. After the beer has been made, the yeast is no longer needed and is removed. The yeast cells are removed by allowing the cells to sink slowly to the bottom of the container, forming a sediment. This process is called sedimentation. You will 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. A B yeast cell suspension at start after 10 minutes 0 minutes Fig. 1.1 [2] (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 1.1 labelled contents hazard volume / cm3 Y yeast cell suspension none 50 B3 buffer pH3 none 25 B4 buffer pH4 none 25 B5 buffer pH5 none 25 B6 buffer pH6 none 25 BU buffer (pH unknown) none 25 C calcium chloride solution irritant 25 If any of C comes into contact with your skin, wash off immediately under cold water. It is recommended that you wear suitable eye protection. Read step 1 to step 9 before proceeding. 1. Put 7 cm3 of B3 into a test-tube. 2. Put 1 cm3 of C into the same test-tube. 3. Stir Y in the beaker, then put 7 cm3 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. [5] (v) Complete Table 1.2 by recording your results for BU. Table 1.2 time / minutes 10 height of sediment / mm 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 1.3 time number of yeast cells / hours / arbitrary units cm–3 0 1180 10 720 20 250 25 190 30 180 (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. Fig. 1.2 [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] [Total: 21]

Mark scheme: 1(a)(i) mp 1 draws on test-tube B a line level with liquid in test tube A and a layer indicating level of sediment at bottom ; mp 2 label line and label to sediment ; 2 1(a)(ii) mp 1 describes how to count squares on graph paper scale to measure height of sediment ; 1 1(a)(iii) mp 1 states at least 4 regular time intervals ; 1 1(a)(iv) mp 1 records results using heading pH ; mp 2 uses heading height of sediment / mm ; mp 3 records heights of sediment at each time interval for each pH ; mp 4 results show expected pattern, pH 3 or pH 4 greatest height of sediment ; mp 5 results as whole numbers ; 5 1(a)(v) mp 1 records height of sediment at each time interval for BU ; mp 2 estimates pH according to candidate’s results ; 2 1(a)(vi) mp 1 suggests using at least 5 pH values ; mp 2 suggests comparison with results for known pH values ; 2 1(a)(vii) mp 1 suggests at the pH where greatest sedimentation occurred yeast cells were more attracted towards each other causing yeast to stick together and drop to bottom ; 1 1(a)(viii) mp 1 identifies a source of error such as the difficulty of determining the boundary of the sediment ; 1 Question Answer Marks 1(b)(i) mp 1 x-axis: labelled time / hours, y-axis: labelled number of yeast cells / arbitrary units cm–3 ; mp 2 scale on x-axis is 5 to 2 cm, scale on y-axis is 200 to 2 cm labelled each 2 cm ; mp 3 correct plotting of 5 points with a cross or dot in circle, to within half a square ; mp 4 five plots, joined point to point, with a thin line ; 4 1(b)(ii) mp 1 shows on graph how to find the number of yeast cells at 18 hours ; mp 2 correct answer according to candidate’s graph ; 2

More questions on Factors that affect enzyme action

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. draw this section Fig. 2.1 You are expected to draw the correct shape and proportions of the different tissues. [5] (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. [5] 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. X magnification ×13 Fig. 2.2 (b) (i) Use the magnification on Fig. 2.2 to calculate the actual length, in µm, of line X (length of a vascular bundle). Show all the steps in your working and use appropriate units. actual length of X = .................................................... µm [4]

Mark scheme: 2(a)(i) mp 1 drawing of a minimum size with a minimum number of tissue layers and no cells drawn ; mp 2 draws correct quarter of the stem ; mp 3 draws at least 4 layers of tissue ; mp 4 draws irregular outline of vascular tissue layer ; mp 5 uses label line and label to identify the xylem ; 5 2(a)(ii) mp 1 drawing of a minimum size and drawn with thin, continuous lines ; mp 2 draws only four touching cells ; mp 3 two lines drawn around each cell and one cell with angular sides ; mp 4 each cell touching at least two of the other cells ; mp 5 uses label line and label to identify the cell wall ; 5 Question Answer Marks 2(b)(i) mp 1 measures the line X within range and with units ; mp 2 shows measurement of X (mm) multiplied by 1000 ; mp 3 shows division by 13 ; mp 4 shows answer to correct degree of accuracy ; 4 2(b)(ii) mp 1 count number of eyepiece graticule units equivalent to the length of the vascular bundle and using a calibration technique to determine the actual length ; 1 2(c) mp 1 organises table into three columns with one column for feature ; mp 2, mp 3, mp 4 three observable differences between the stem on J1 and the stem in Fig. 2.2 such as the vascular bundles are joined in J1 and separate in Fig. 2.2 ;;; 4

More questions on Structure of transport tissues