Cambridge A Level Biology 9700 — 2018 Oct/Nov Paper 3 · Variant 6

1 The emulsion test is used to detect the presence of lipids in a sample. To carry out the emulsion test, ethanol is added to the sample. If lipids are present they will dissolve in the ethanol. When water is then added, the lipids in the ethanol will form droplets in the water. The droplets in the water form an emulsion. The emulsion appears as cloudiness in the layer towards the bottom of the test-tube, as shown in Fig. 1.1. emulsion Fig. 1.1 (a) A student investigated the effect of the concentration of ethanol on the cloudiness of the emulsion during the emulsion test. The student suggested the following hypothesis: lowering the concentration of ethanol below 100% will have no effect on the cloudiness of the emulsion. You will need to investigate this hypothesis by: • diluting 100% ethanol, A, to provide further concentrations of ethanol • carrying out the emulsion test using these concentrations of ethanol • recording the cloudiness of the emulsion formed. You are provided with the materials shown in Table 1.1. Table 1.1 labelled contents hazard volume / cm3 A 100% ethanol flammable 60 L vegetable oil (lipid) flammable 40 W distilled water none 60 If A comes into contact with your skin, wash off immediately under cold water. It is recommended that you wear suitable eye protection. You will need to use simple (proportional) dilution of the ethanol, A, to make further concentrations of ethanol between 100% and 75%, by reducing each successive dilution by 5%. You need to prepare 10 cm3 of each concentration. Table 1.2 shows how to prepare the highest concentration (100%) and the lowest concentration (75%). (i) Complete Table 1.2 to show how you will prepare further concentrations of ethanol. Table 1.2 volume of A volume of W percentage concentration / cm3 / cm3 of ethanol 10.0 0.0 100 7.5 2.5 75 [2] Read step 1 to step 13 before carrying out this investigation. 1. Prepare the concentrations of ethanol as shown in Table 1.2, in the beakers provided. 2. Label one test-tube as a control. 3. Put 2 cm3 of L into this test-tube. 4. Put 4 cm3 of W into the same test-tube. Shake the test-tube gently for 20 seconds. The control test-tube will be needed for step 10. 5. Put 2 cm3 of L into all the other test-tubes. 6. Put 2 cm3 of 100% ethanol into one of the test-tubes containing L. Shake the test-tube gently for 20 seconds. 7. Put 2 cm3 of W into the same test-tube. Shake the test-tube gently for 20 seconds. 8. Repeat step 6 to step 7 with each of the other concentrations of ethanol as prepared in step 1. 9. Leave all the test-tubes for approximately 1 minute so that the layers become visible. 10. Observe the cloudiness of the emulsion, as shown in Fig. 1.1, in each test-tube compared with the control. The control will not be cloudy. Use the key in Fig. 1.2 and record the results in (a)(ii). You may find it useful to observe the cloudiness of the emulsion with a piece of black card behind it. key ++++++ most cloudy +++++ ++++ +++ ++ + least cloudy 0 not cloudy Fig. 1.2 11. Tip the contents of each test-tube, except the control, into the container labelled For waste. 12. Rinse these test-tubes twice using a syringe and the water in the container labelled For washing. 13. Repeat step 5 to step 10. (ii) Record your results in an appropriate table, including the mean result for each concentration of ethanol. [5] (iii) State the independent variable in the investigation you have just carried out. ...................................................................................................................................... [1] (iv) Identify one significant source of error when measuring the dependent variable and suggest one improvement to reduce this source of error. source of error ................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... improvement ..................................................................................................................... ........................................................................................................................................... [2] (v) The student’s hypothesis stated that: lowering the concentration of ethanol below 100% will have no effect on the cloudiness of the emulsion. State whether you support or reject this hypothesis. Explain how your results provide evidence for this decision. support or reject ................................................................................................................ explanation ......................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... [3] (vi) In the test-tubes the layer above the emulsion consists of undissolved lipids. Suggest how the student could find the volume of ethanol required to dissolve all the lipid in L. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... .......................................................................................................................................[2] Question 1 continues on page 8 (b) Some foods contain two types of fat, unsaturated fat (U) and saturated fat (S). The proportions of these two types of fat vary according to the food. A scientist carried out some tests to determine the mass of these two types of fat found in three food samples, X, Y and Z. The total mass of each food sample was 100 g. The results are shown in Table 1.3. Table 1.3 type of mass of fat in 100 g of food / g fat U S food sample X 10.75 23.50 Y 1.50 1.25 Z 8.50 6.50 (i) Calculate the percentage of the total fat in food sample X that is unsaturated. Show all the steps in your working and use appropriate units. [3] (ii) Draw a bar chart of the data shown in Table 1.3 on the grid in Fig. 1.3. The bars should be separated for each food sample. Use a sharp pencil for drawing bar charts. Fig. 1.3 [4] [Total: 22]

Mark scheme: 1(a)(i) mp 1 completes Table 1.2 by stating three further concentrations of ethanol ; mp 2 volumes of A and W add up to ten for each concentration ; 2 1(a)(ii) mp 1 records results using heading percentage concentration of ethanol ; mp 2 uses heading cloudiness ; mp 3 records results for 5 concentrations of ethanol, using key ; mp 4 results show expected pattern, it is cloudiest for the highest concentration of ethanol ; mp 5 shows mean result for each concentration of ethanol ; 5 1(a)(iii) mp 1 states independent variable as concentration of ethanol ; 1 1(a)(iv) mp 1 identifies a significant source of error such as difficulty of judging cloudiness ; mp 2 suggests an improvement such as using a colorimeter ; 2 1(a)(v) mp 1 statement supporting or rejecting hypothesis according to candidate’s results ; mp 2 refers to the results of two concentrations of ethanol ; mp 3 refers to the results of two concentrations with supporting data ; 3 1(a)(vi) mp 1 reference to how to measure the volume of ethanol such as using a burette ; mp 2 suggestion of when to stop adding ethanol, such as the disappearance of the layer above the emulsion ; 2 1(b)(i) mp 1 shows 10.75 divided by 34.25 (addition of 10.75 and 23.5) multiplied by 100 ; mp 2 correct answer ; mp 3 final answer to the correct degree of accuracy ; 3 Question Answer Marks 1(b)(ii) mp 1 x-axis: labelled food sample, y-axis: labelled mass of fat in 100 g of food / g ; mp 2 even width of bars, scale on y-axis is 5.0 to 2 cm labelled each 2 cm ; mp 3 correct plotting of bars in the order of the table ; mp 4 6 bars drawn, with separation between food samples and all bars drawn with vertical lines meeting horizontal lines exactly, labelled (X, Y, Z, U, S) ; 4

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2 (a) N1 is a slide of a stained transverse section through a leaf-like structure. You are not expected to be familiar with this specimen. Use a sharp pencil for drawing. (i) Select a field of view so that you can observe the different tissues shown by the shaded area in Fig. 2.1. draw this section Fig. 2.1 Draw a large plan diagram from the selected field of view which has: • part of the epidermis • the outline of two vascular bundles • any other observable tissues. No details of the internal tissues of the vascular bundles are required. Use one ruled label line and the label C to identify the tissue containing chloroplasts. You are expected to draw the correct shape and proportions of the different tissues. [5] (ii) Observe the largest vascular bundle on N1. Select one group of four adjacent, touching xylem vessel elements. Each xylem vessel element must touch at least one of the other xylem vessel elements. Make a large drawing of this group of four xylem vessel elements. Use one ruled label line and label to identify the lumen of one xylem vessel element. You are expected to draw the correct shape and proportions of the different cells. [5] (iii) On your drawing in (a)(ii), use one ruled label line and the label T to identify an observable feature which enables water to be transported through the xylem vessel element. Next to the letter T explain how this feature enables the xylem vessel element to carry out its function. [2] (b) Fig. 2.2 is a photomicrograph of a stained transverse section through the trachea of a mammal. P1P1 P3P3 P2P2 Fig. 2.2 (i) On Fig. 2.2, use one ruled label line and the label Q to identify an observable feature which prevents the tube from collapsing. [1] (ii) Determine the simplest whole number ratio of the length of P3 (the air space) to the total length of P1 and P2. Show all the steps in your working and use appropriate units. ratio ............................................................[5] [Total: 18]

Mark scheme: 2(a)(i) mp 1 drawing of a minimum size with minimum number of tissue layers and no cells drawn ; mp 2 draws only 2 vascular bundles ; mp 3 draws one closed end ; mp 4 draws two caps on a vascular bundle ; mp 5 uses label line and label C to identify tissue containing chloroplasts ; 5 2(a)(ii) mp 1 drawing of a minimum size and drawn with thin, continuous lines ; mp 2 draws only 4 touching xylem vessel elements ; mp 3 two lines drawn around each xylem vessel element ; mp 4 at least one xylem vessel element with angular sides ; mp 5 uses label line and label to identify the lumen ; 5 2(a)(iii) mp 1 uses label T to identify xylem wall or lumen ; mp 2 explanation of how xylem wall or lumen enables water to be transported through element, such as the wall preventing vessel collapsing or the lumen having no contents so less resistance to flow of water ; 2 2(b)(i) mp 1 uses label line and the label Q to identify the area of cartilage as the feature preventing tube from collapsing ; 1 Question Answer Marks 2(b)(ii) mp 1 uses same units to measure P1, P2 and P3 ; mp 2 collects measurements within range for P1 and P2 ; mp 3 collects measurements within range for P3 ; mp 4 shows answer as larger whole number to smaller whole number ; mp 5 answer to lowest common denominator ; 5

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