Cambridge A Level Biology 9700 — 2024 May/June Paper 3 · Variant 3

1 Yeast cells contain enzymes that catalyse the breakdown of sugars, such as glucose and sucrose, to produce ethanol and carbon dioxide. As the sugars are broken down, the yeast cells in a suspension will sink slowly to the bottom of the container, forming a sediment with a clear liquid above. This process is called sedimentation. You will investigate the effect of different concentrations of ethanol on the sedimentation of yeast cells. (a) Test-tube R in Fig. 1.1 shows how a test-tube will be set up at the start of the investigation. (i) Decide what you expect the contents of the test-tube to look like after sedimentation. On test-tube S in Fig. 1.1, draw: • lines to show the layers that you expect to see after sedimentation • a label line and label to identify the sediment. R S yeast cell suspension at the start after sedimentation Fig. 1.1 [2] You are provided with the materials shown in Table 1.1. Table 1.1 volume labelled contents hazard / cm3 flammable E 100% ethanol 50 harmful A calcium chloride solution irritant 15 W distilled water none 150 Y dried yeast none If any solution comes into contact with your skin, wash off immediately with cold water. You should wear suitable eye protection. You will investigate the effect of different concentrations of ethanol on the sedimentation of yeast cells. You will need to: • prepare different concentrations of ethanol • measure the height of the yeast sediment in each concentration of ethanol every four minutes. You will use proportional dilution to make different concentrations of ethanol. You will prepare 10 cm3 of each concentration, using E and W. Table 1.2 shows how to prepare two of the concentrations you will use. Decide which other concentrations of ethanol you will use. (ii) Complete Table 1.2 to show how you will prepare the concentrations of ethanol you will use. Table 1.2 percentage volume of E volume of W concentration of / cm3 / cm3 ethanol 100 10.0 0.0 60 6.0 4.0 [2] Carry out step 1 to step 15. step 1 In the beakers provided, prepare the concentrations of ethanol as shown in Table 1.2. step 2 Label one test-tube with the label C. step 3 Label the other test-tubes with the concentrations of ethanol prepared in step 1. step 4 Put 1 cm3 of A into each of the test-tubes labelled in step 2 and step 3. step 5 Put 7 cm3 of W into the test-tube labelled C. step 6 Put 7 cm3 of the 100% ethanol, E, into the test-tube labelled 100%. step 7 Repeat step 6 for each of the other concentrations you prepared in step 1. step 8 Put 100 cm3 of W into the beaker labelled Y and stir until all of the dried yeast forms a suspension. step 9 Stir the yeast cell suspension and put 7 cm3 of the yeast cell suspension into the test-tube labelled 100%. step 10 Repeat step 9 for each of the other labelled test-tubes, including C. step 11 Put a clean bung into one of the test-tubes and invert the test-tube to mix the contents. step 12 Repeat step 11 for all the other test-tubes. step 13 Start timing and immediately measure the height of the sediment in each test-tube. These are the results at the start (0 minutes). Record your results in (a)(iii). step 14 After 4 minutes, measure the height of the sediment in each test-tube. Record your results in (a)(iii). step 15 Repeat step 14 so that results are recorded every 4 minutes until the final results are recorded at 20 minutes. (iii) Record your results in an appropriate table. [5] (iv) Using your results in (a)(iii), state one conclusion that can be made about the effect of the concentration of ethanol on the sedimentation of yeast cells. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (v) A student suggested the hypothesis: ethanol is needed for sedimentation of yeast cells to occur. Using your results in (a)(iii), state whether you support or reject this hypothesis. Explain how your results provide evidence for this decision. support or reject ............................................... explanation ........................................................................................................................ ........................................................................................................................................... ........................................................................................................................................... [1] (vi) Using the results at 20 minutes, state the concentration, or concentrations, of ethanol that caused the most sedimentation. ........................................................................................................................................... Suggest how you could modify this procedure to obtain a more accurate estimate of the concentration that causes the most sedimentation. modification ....................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... [3] (vii) Identify the main source of error in the investigation into the effect of different concentrations of ethanol on the sedimentation of yeast cells. ........................................................................................................................................... ..................................................................................................................................... [1] (viii) Another possible source of error, when carrying out step 6 of the investigation, is shown in Table 1.3. Complete Table 1.3 by stating whether the type of error is systematic or random and the effect the error may have on the trend seen in the results. Table 1.3 systematic error source of error or effect on the trend random error the 7 cm3 mark on the syringe used to measure the volume of ethanol actually measured 7.1 cm3 [1] (b) In fermentation, the action of yeast converts some of the carbohydrates in plants to ethanol and carbon dioxide. A scientist investigated the production of ethanol during the fermentation of carbohydrates from different sources. The investigation was carried out at 30 °C and at pH 5, using the yeast Saccharomyces cerevisiae. All other variables were kept constant. The results are shown in Table 1.4. Table 1.4 percentage ethanol source of produced per 100 g of carbohydrate carbohydrate molasses 3.80 oranges 1.05 grapes 2.65 beetroot 3.35 rice 4.70 (i) Draw a bar chart of the data in Table 1.4 on the grid in Fig. 1.2. Use a sharp pencil. Fig. 1.2 [4] The investigation was repeated using a different yeast, Schizosaccharomyces pombe. All the variables were kept the same as in the first investigation. The percentage of ethanol produced using Schizosaccharomyces pombe was found to be lower with all of the sources of carbohydrate. The results are shown in Table 1.5. Table 1.5 percentage ethanol produced per 100 g of carbohydrate source of carbohydrate Saccharomyces Schizosaccharomyces cerevisiae pombe molasses 3.80 3.68 oranges 1.05 1.00 grapes 2.65 2.57 beetroot 3.35 3.29 rice 4.70 4.34 (ii) Suggest why the percentage of ethanol produced by Schizosaccharomyces pombe is lower than the percentage of ethanol produced by Saccharomyces cerevisiae for all sources of carbohydrate. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] [Total: 22]

Mark scheme: 1(a)(i) 1 one line drawn on test-tube S at the same height as the line at the top of test-tube R ; 2 another line drawn on test-tube S with the label ‘sediment’ ; 2 1(a)(ii) 1 states three additional concentrations of ethanol ; 2 correct volumes of ethanol and water for each concentration stated ; 2 1(a)(iii) 1 heading for independent variable: percentage concentration of ethanol ; 2 heading for dependent variables: height of sediment / mm ; 3 records a height for each concentration stated in (a)(ii) and height in test-tube C, for every 4 minutes ; 4 records height of sediment at 20 minutes ; 5 all heights recorded as whole millimetres ; 5 1(a)(iv) states a conclusion with reference to the concentration of ethanol and height of sedimentation ; 1 1(a)(v) states whether supports or rejects the hypothesis and explains how results provide evidence ; 1 1(a)(vi) 1 from candidate’s results states the concentration of ethanol that caused the most sedimentation at 20 minutes ; modifications 2 use concentrations of ethanol either side of concentration stated ; 3 use narrower intervals close to the concentration that caused the most sedimentation ; 3 1(a)(vii) identifies a source of error, e.g. the test-tubes were not left for the same length of time ; 1 1(a)(viii) systematic and no effect on the trend ; 1 1(b)(i) 1 label on x-axis: source of carbohydrate and each bar labelled with a substrate and label on y-axis: percentage ethanol produced per 100 g of carbohydrate ; 2 scale on x-axis: bars of the same width and evenly spaced and scale on y-axis: 1 to 2 cm and labelled at least every 2 cm ; 3 correct plotting of all five bars ; 4 five bars plotted with straight horizontal lines and vertical lines join exactly ; 4 Question Answer Marks 1(b)(ii) any two from : 1 the optimum pH for S. pombe not the same as for S. cerevisiae ; 2 the optimum temperature for S. pombe not the same as for S. cerevisiae ; 3 S. pombe was unable to ferment all the sources of carbohydrate ; 2

More questions on Respiration

2 L1 is a slide of a stained transverse section through a plant stem. (a) (i) Draw a large plan diagram of the whole section on L1. Use a sharp pencil. Use one ruled label line and label to identify the cortex. [5] (ii) Observe the cells in the cortex on the section of the stem on L1. Select a group of four adjacent cortex cells. Each cell must touch at least two other cells. • Make a large drawing of this group of four cortex cells. • Use one ruled label line and label to identify the cell wall of one cortex cell. [5] (b) Fig. 2.1 is a photomicrograph of a stained transverse section through a different plant. Fig. 2.1 Identify three observable features, other than colour, that are different between the section on L1 and the section in Fig. 2.1. Record the differences between these three observable features in Table 2.1. Table 2.1 feature L1 Fig. 2.1

Mark scheme: 2(a)(i) 1 uses most of the available space and no shading ; 2 draws the correct shape of the whole section and no cells included ; 3 draws a correct number of tissue layers ; 4 draws the epidermis as two lines ; 5 label line and label to the cortex ; 5 2(a)(ii) 1 uses most of the available space and lines continuous, thin and sharp ; 2 draws only four cortex cells and each cell touches at least two other cells ; 3 two lines around each cell and three lines where cells touch ; 4 detailed shapes of the cortex cells ; 5 label line and label to the cell wall ; 5 2(b) records only observable differences ; three correct differences between L1 and Fig. 2.1 ;;; e.g. feature L1 Fig. 2.1 shape of section oval wavy number of tissue layers more fewer position of vascular bundles arranged in a circle scattered size of vascular bundles all the same size some large and some small 4 Question Answer Marks 2(c) 1 takes at least two measurements and uses appropriate units ; 2 shows addition of least two measurements and division by the number of measurements ; 3 shows the mean measurement divided by the magnification ( 40) ; 4 correct answer and uses appropriate units ; 4

More questions on Cells as the basic units of living organisms