Cambridge A Level Biology 9700 — 2023 May/June Paper 3 · Variant 2

1 Grapes are fruit that contain high concentrations of soluble sugars such as sucrose, fructose and glucose. The proportions of these sugars change as the grapes mature. You will determine the concentration of reducing sugars in a sample of grape extract by using known concentrations of reducing sugar. You are provided with the materials shown in Table 1.1. Table 1.1 labelled contents hazard volume / cm3 G grape extract none 20 W distilled water none 100 B Benedict’s solution harmful irritant 40 8.0% reducing sugar R none 40 solution If any solution 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 carry out a serial dilution of the 8.0% reducing sugar solution, R, to reduce the concentration by half between each successive dilution. You will need to prepare four concentrations of reducing sugar in addition to 8.0% reducing sugar solution, R. After the serial dilution is completed, you will need to have 10 cm3 of each concentration available to use. (a) (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 reducing sugar solution transferred • the volume of distilled water, W, added. Under each beaker, state the concentration of reducing sugar solution. 0 cm3 of W 20 cm3 of 8.0% reducing sugar solution, R 10 cm3 of 8.0% reducing sugar solution to use ........................ ........................ ........................ Fig. 1.1 [3] Carry out step 1 to step 16. step 1 Set up a water-bath and heat to boiling ready for step 6 and step 14. step 2 Prepare the concentrations of reducing sugar solution as shown in Fig 1.1. step 3 Label test-tubes with the concentrations shown in Fig. 1.1. step 4 Put 2 cm3 of the 8.0% reducing sugar solution into the appropriately labelled test-tube. step 5 Put 2 cm3 of Benedict’s solution, B, into the same test-tube. Shake gently to mix. step 6 Put this test-tube in the boiling water-bath. Start timing. step 7 Measure the time taken to the first appearance of a colour change in the test-tube. If there is no colour change after 120 seconds, stop timing and record as ‘more than 120’. step 8 Record the result from step 7 in 1(a)(ii). step 9 Remove the test-tube from the water-bath. Put the test-tube in the test-tube rack. step 10 Repeat step 4 to step 9 with the remaining concentrations of reducing sugar solution. (ii) Record your results in an appropriate table for the known concentrations of reducing sugar. [5] (iii) Identify one source of error in step 7. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (iv) To determine the concentration of reducing sugar in grape extract G, you will need to test a sample of the extract. State the volume of grape extract G that you will use to test for reducing sugars. volume = ....................................... [1] step 11 Label a test-tube G. step 12 Transfer the volume of grape extract G that you stated in (a)(iv) into test-tube G. step 13 Put 2 cm3 of Benedict’s solution, B, into the same test-tube. Shake gently to mix. step 14 Put this test-tube in the boiling water-bath. Start timing. step 15 Measure the time taken to the first appearance of a colour change in the test-tube. If there is no colour change after 120 seconds, stop timing and record as ‘more than 120’. step 16 Record the result from step 15 in (a)(v). (v) Record the time taken for the first colour change in test-tube G. time taken = ......................................................... [1] (vi) The concentration of reducing sugars in G can be estimated from a graph of your results. Draw a graph of the results you recorded in (a)(ii) on the grid in Fig. 1.2, using a line of best fit. The axes have been labelled for you. Use a sharp pencil. time to first colour change / s percentage concentration of reducing sugars Fig. 1.2 [2] (vii) Use your graph to estimate the percentage concentration of reducing sugars in G. Show on your graph how you determined your answer. percentage concentration of reducing sugars in G ................................................................ [2] (viii) Suggest how you would modify this investigation to obtain a more accurate estimate for the concentration of reducing sugars in sample G. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (b) The concentration of reducing sugars in grapes changes as the grapes age (get older). Table 1.2 shows the concentration of reducing sugars for grapes of different ages. Table 1.2 age of grapes / days percentage concentration of reducing sugars 14 1.1 28 1.9 42 2.6 56 3.9 70 7.5 84 11.3 (i) Plot a graph of the data shown in Table 1.2 on the grid in Fig. 1.3. 12 10 8 6 4 2 0 0 20 40 60 80 100 Fig. 1.3 [2] (ii) Use your estimate from (a)(vii) and your graph in (b)(i) to estimate the age of the grapes that were used to make grape extract G. age of grapes ................................................ days [1] (c) Grapes contain starch as well as reducing sugars. In a study, the concentration of amylase in grapes was measured as the grapes aged. The results of the study are shown in Fig. 1.4. 0.35 0.30 0.25 concentration of 0.20 amylase / arbitrary units 0.15 0.10 0.05 0.00 14 28 42 56 70 84 age of grapes / days Fig. 1.4 Use the data in Fig. 1.3 and Fig. 1.4 to suggest a possible explanation for the change in the concentration of reducing sugars in grapes as they age. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [3] [Total 23]

Mark scheme: 1(a)(i) 1 correct concentrations (4.0, 2.0, 1.0, 0.5) and % at least once ; 2 shows transfer of 10 (cm3) to each beaker from the previous beaker ; 3 shows 10 (cm3) of water added to each beaker ; 3 1(a)(ii) 1 heading for independent variable: percentage concentration of reducing sugar (before heading for dependent variable) and no units in body of table ; 2 heading for dependent variable: time and seconds and no units in body of table ; 3 results for all concentrations ; 4 time for the highest concentration of reducing sugar less than time for the lowest concentration of reducing sugar ; 5 results recorded in whole seconds ; 5 1(a)(iii) difficult to judge the first colour change ; 1 1(a)(iv) 2 cm3 ; 1 1(a)(v) records a time for G between that recorded for 4.0% and 1.0% and seconds ; 1 1(a)(vi) 1 scale on x-axis: 2% to 2 cm, labelled at least every 2 cm and scale on y-axis: according to candidate results ; 2 points joined with a thin ruled line of best fit ; 2 1(a)(vii) 1 shows interpolation of result for G on the graph ; 2 correct estimate from the graph ; 2 1(a)(viii) any two suggestions: 1 more concentrations around the value for the estimate in (a)(vii) ; 2 repeat and find the mean ; 3 use a white card behind the test-tube ; 2 1(b)(i) 1 x-axis: age of grapes and days and y-axis: percentage concentration of reducing sugars ; 2 correct plotting of all six points using small crosses or dots in circles ; 2 Question Answer Marks 1(b)(ii) correct age estimated from graph ; 1 1(c) 1 the concentration of amylase and reducing sugar increase as grapes age ; 2 amylase hydrolyses starch into reducing sugars ; 3 more amylase leads to an increase in reducing sugar ; 3

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2 K1 is a slide of a stained transverse section through a plant root. (a) (i) Draw a large plan diagram of the region of the root on K1 indicated by the shaded region in Fig. 2.1. Use a sharp pencil. draw this region Fig. 2.1 Use one ruled label line and label to identify the endodermis. [5] (ii) Observe the xylem vessel elements in the root on K1. Select a group of four adjacent xylem vessel elements. Each xylem vessel element must touch at least two 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 cell wall of one xylem vessel element. [5] (b) Fig. 2.2 shows a photomicrograph of a transverse section through a different root. A B scale bar 500 μm Fig. 2.2 (i) Identify three observable differences, other than colour, between the root section on K1 and the root section in Fig. 2.2. Record these observable differences in Table 2.1. Table 2.1 feature K1 Fig. 2.2 1 2 3 [3]

Mark scheme: 2(a)(i) 1 minimum size ; 2 correct section of the root drawn and no cells drawn ; 3 draws the correct number of tissues ; 4 correct shape of the section ; 5 label line and label to xylem ; 2(a)(ii) 1 minimum size and all lines sharp and continuous and no shading ; 2 draws only four whole xylem vessel elements and each xylem vessel element touches at least two other xylem vessel elements ; 3 two lines around each xylem vessel element and three lines where xylem vessel elements touch ; 4 correct shape of xylem vessel elements ; 5 label line and label to one cell wall ; 5 2(b)(i) three correct differences ; any three from e.g.: feature K1 Fig. 2.2 shape outline wavy circular ; number of xylem vessels fewer more ; relative size of vascular tissue small large ; cortex wider narrower ; 3 Question Answer Marks 2(b)(ii) 1 correct measurement of scale bar and line A–B and units ; 2 measurement of line A–B multiplied by 500 and divided by measurement of scale bar ; 3 correct answer and units ; 3 2(b)(iii) correct answer recorded to two significant figures ; 1

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