Cambridge A Level Biology 9700 — 2021 Oct/Nov Paper 3 · Variant 3

1 During the manufacture of a fruit juice, an unwanted colour can sometimes appear in the juice. An enzyme can be used to remove this colour. You will carry out an investigation to determine the concentration of enzyme that is most effective at removing the colour in mock fruit juice, J. Solution J is not real fruit juice, so is not safe to drink. You are provided with the materials shown in Table 1.1. Table 1.1 labelled contents hazard volume / cm3 J mock fruit juice harmful 60 E 2.0% enzyme solution harmful irritant 25 W distilled water none 100 If any solution comes into contact with your skin, wash off immediately under cold water. It is recommended that you wear suitable eye protection. (a) You will need to carry out a serial dilution of the 2.0% enzyme solution, E, to reduce the concentration by half between each successive dilution. You will need to prepare four concentrations of enzyme solution in addition to the 2.0% enzyme solution, E. After the serial dilution is completed you need to have 10 cm3 of each concentration available to use. (i) Complete Fig. 1.1 to show how you will prepare your serial dilution. Fig. 1.1 shows the first two beakers you will use to make your serial dilution. You will need to draw three additional beakers. For each beaker add labelled arrows to show: • the volume of enzyme solution transferred. • the volume of distilled water, W, added. Under each beaker, state the concentration of enzyme solution. 0 cm3 of W ............................ 20 cm3 of 2.0 % enzyme ............................ solution, E ............................ ............................ 10 cm3 of 2.0 % enzyme solution to use ............................ ............................ ............................ ............................ Fig. 1.1 [3] Carry out steps 1 to 10. 1. Prepare the concentrations of enzyme solution, as decided in (a)(i), in the beakers provided. 2. Label the test-tubes with the concentrations you prepared in step 1. 3. Put 5 cm3 of J into each test-tube. 4. Using the beakers labelled hot water and cold water, set up a water-bath with water at approximately 40 °C. Maintain the water-bath at approximately 40 °C during step 5 to step 8. 5. Put the test-tubes from step 3 into the water-bath. Leave the test-tubes for 3 minutes. 6. Put 5 cm3 of the 2.0% enzyme solution into the appropriately labelled test-tube. Shake gently to mix. 7. Repeat step 6 with the other concentrations of enzyme solution you prepared in step 1. 8. Start timing and leave the test-tubes in the water-bath for 10 minutes. While you are waiting carry on with Question 1. 9. After 10 minutes (step 8) remove the test-tubes from the water-bath. Observe the colour of the solution in each test-tube. To see the colour more clearly, it may help to hold a piece of white paper behind the test-tube. You may see the same colour in more than one test-tube. 10. Record your results in (a)(ii) using the symbols shown in Table 1.2. Table 1.2 intensity of colour symbol dark blue +++++ ++++ decreasing intensity of blue +++ colour ++ no colour + (ii) Record your results in an appropriate table. You may use the same symbols for more than one test-tube. [5] (iii) Using your results in (a)(ii), state which concentration of enzyme removed the colour most effectively. ..................................................................................................................................... [1] (iv) Using your knowledge of enzymes, explain the trend in your results. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (v) State one variable, other than temperature, that needs to be controlled in this investigation. ..................................................................................................................................... [1] (vi) The procedure used in this investigation has several sources of error. Table 1.3 shows one of these sources of error. Complete Table 1.3 by: • stating two other sources of error • describing an improvement to the procedure for each of the three sources of error. Table 1.3 source of error how to improve the procedure the test-tube with 2.0% enzyme was left for longer than the other test-tubes [5] (b) Grapes are a type of fruit that can be eaten freshly picked or dried. Table 1.4 shows the sugar content of fresh grapes and dried grapes. Table 1.4 sugar content / g per 100 g of grapes type of sugar fresh dried glucose 6.5 27.0 fructose 7.5 29.5 sucrose 0.5 1.0 (i) Plot a bar chart of the data in Table 1.4 on the grid in Fig. 1.2. Use a sharp pencil for drawing graphs. Fig. 1.2 [4] (ii) The concentration of all sugars in dried grapes is higher than in fresh grapes. Calculate the percentage increase in the concentration of glucose in dried grapes. Show your working. answer = ......................................................% [2] (iii) Suggest why the glucose concentration is higher in the dried grapes than in the fresh grapes. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] [Total: 23]

Mark scheme: 1(a)(i) 1 correct concentrations (1.0, 0.5, 0.25, 0.125) and % ; 2 shows transfer of 10 cm3 to each beaker from the previous beaker ; 3 shows addition of 10 cm3 of water to each beaker ; 3 1(a)(ii) 1 heading for independent variable: percentage concentration enzyme ; 2 heading for dependent variable: intensity of colour ; 3 records readings for all enzyme concentrations ; 4 correct trend ; 5 records results as symbols ; 5 1(a)(iii) concentration of enzyme that matches results in (a)(ii) which removed the colour most effectively ; 1 1(a)(iv) more enzyme substrate complexes form at high concentration of enzyme ; 1 1(a)(v) pH ; 1 1(a)(vi) source of error any two from: 1 temperature difficult to control ; 2 enzyme not equilibrated ; 3 colour difficult to judge ; 4 mixing of solutions different for each concentration ; improvements any three from: 1 carry out tests individually ; 2 use thermostatically controlled water-bath ; 3 equilibrate the enzyme ; 4 use colorimeter ; 5 shake for a set time ; 5 Question Answer Marks 1(b)(i) 1 x-axis: type of sugar and y-axis: sugar content / g per 100 g of grapes and the labels for glucose, fructose, sucrose, fresh and dried ; 2 x-axis: even width of bars and scale on y-axis: 5 to 2 cm, labelled at least every 2 cm ; 3 correct plotting of six bars ; 4 vertical and horizontal lines drawn with a ruler and joining up precisely ; 4 1(b)(ii) shows subtraction of 6.5 from 27, shows division by 6.5 and shows multiplication by 100 ; correct answer ; 2 1(b)(iii) dried grapes have less water ; 1

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2 K1 is a slide of a stained transverse section through a plant stem. (a) Set up the microscope so that you can observe the section on K1. Observe the different tissues in the area on K1 shown by the shaded region in Fig. 2.1. shaded region Fig. 2.1 Use a sharp pencil for drawing. (i) Draw a large plan diagram of the area of the section on K1 shown by the shaded region in Fig. 2.1. Your drawing should show the correct shapes and proportions of the different tissues. Use one ruled label line and label to identify the epidermis. [5] (ii) Observe the vascular tissue of the section of the stem on K1. Select one large xylem vessel element and three adjacent smaller cells. Each smaller cell must touch the large xylem vessel element and at least one of the other smaller cells. • Make a large drawing of this group of four cells. • Use one ruled label line and label to identify a cell wall of one cell. [5] (b) Fig. 2.2 is a photomicrograph of a stained transverse section through a root of a different type of plant. Fig. 2.2 Identify the observable differences between the section on K1 and the section in Fig. 2.2. Record the observable differences in Table 2.1. Table 2.1 feature K1 Fig. 2.2

Mark scheme: 2(a)(i) 1 minimum size and no shading ; 2 no cells and draws a quarter of the stem ; 3 shows details of outer layer ; 4 draws at least 5 layers of tissue ; 5 label line and label to epidermis ; 5 2(a)(ii) 1 minimum size and all lines continuous and thin ; 2 draws only four cells and each cell touches the large xylem vessel and at least one of the other cells ; 3 two lines drawn around each cell and three lines where cells touch ; 4 draws correct shape of cells ; 5 label line and label to one cell wall ; 5 2(b) features only observable differences ; three correct differences e.g. feature Fig. 2.3 Fig. 2.4 epidermis not intact intact ; location of vascular tissue nearer the epidermis central ; central tissue parenchyma cells present not vascular tissue vascular tissue present ; 4 2(c) 1 states 78 eyepiece graticule divisions across actual diameter of section ; 2 shows number of eyepiece graticule divisions multiplied by 67 ; 3 correct answer and units (μm or mm) ; 3

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