Cambridge A Level Biology 9700 — 2025 May/June Paper 3 · Variant 1

1 Agar that has been stained with a blue indicator can be used to investigate diffusion. Hydrochloric acid diffuses into blue-stained agar changing the colour of the agar from blue to yellow. You will investigate the effect of different concentrations of hydrochloric acid on the distance the acid diffuses into the agar (diffusion distance). You are provided with the materials shown in Table 1.1. Table 1.1 labelled contents hazard volume / cm3 H 2.0 mol dm–3 hydrochloric acid irritant 30 U unknown concentration of hydrochloric acid irritant 10 W distilled water none 50 6 test-tubes containing blue agar none – 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: • prepare different concentrations of hydrochloric acid • measure the diffusion distance of each concentration of hydrochloric acid after 10 minutes and after 20 minutes • use your results to estimate the concentration of hydrochloric acid in U. You will need to carry out a serial dilution of the 2.0 mol dm–3 hydrochloric acid, H, to reduce the concentration by a factor of ten between each successive dilution. You will need to prepare four concentrations of hydrochloric acid in addition to 2.0 mol dm–3 hydrochloric acid, H. After the serial dilution is completed, you will need to have 9 cm3 of each concentration available to use. (a) (i) Complete Fig. 1.1 to show how you will prepare your serial dilution. Each beaker should have: • a labelled arrow to show the volume of hydrochloric acid transferred • a labelled arrow to show the volume of distilled water, W, added • a label under each beaker to show the concentration of the hydrochloric acid. 0 cm3 of distilled water, W 10 cm3 of 2.0 mol dm–3 hydrochloric acid, H 2.0 mol dm–3 hydrochloric acid Fig. 1.1 [3] Carry out step 1 to step 10. step 1 Prepare the concentrations of hydrochloric acid as shown in Fig. 1.1. step 2 Label one test-tube containing blue agar with the label U. step 3 Label the other test-tubes containing blue agar with the concentrations of hydrochloric acid prepared in step 1. step 4 Put 1 cm3 of 2.0 mol dm–3 hydrochloric acid, H, into the appropriately labelled test-tube. step 5 Put 1 cm3 of each of the other concentrations of hydrochloric acid, as prepared in step 1, into the appropriately labelled test-tube. step 6 Put 1 cm3 of the unknown concentration of hydrochloric acid, U, into the test-tube labelled U. In step 7 and step 9 you will need to wait for 10 minutes. While you are waiting, use your time to continue with other parts of Question 1. step 7 Start timing and wait for 10 minutes. step 8 After the 10 minutes, measure the depth of yellow agar (diffusion distance), shown in Fig. 1.2, for each concentration of hydrochloric acid and for U. Record your results in (a)(ii). hydrochloric acid diffusion distance yellow agar blue agar Fig. 1.2 step 9 Continue timing and wait for a further 10 minutes (20 minutes in total). step 10 After the 10 minutes, measure the depth of yellow agar (diffusion distance) for each concentration of hydrochloric acid and U. Record your results in (a)(ii). (ii) Record your results in an appropriate table. [6] (iii) Explain how the concentration of hydrochloric acid affects the diffusion distance. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (iv) Suggest one source of error in the procedure described in step 8 and step 10 of this investigation. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (v) Use your results at 20 minutes to estimate the concentration of hydrochloric acid in U. concentration of hydrochloric acid in U = ...........................................mol dm–3 [1] (vi) Suggest two improvements to the procedure that would give you a more accurate value for your estimated concentration of hydrochloric acid in U. 1 ........................................................................................................................................ ........................................................................................................................................... ........................................................................................................................................... 2 ........................................................................................................................................ ........................................................................................................................................... ........................................................................................................................................... [2] (vii) For the 2.0 mol dm–3 hydrochloric acid, calculate the rate of diffusion over 20 minutes. Show your working and use appropriate units. rate of diffusion = ............................................................... [2] (viii) One hypothesis for this investigation is: the rate of diffusion decreases over time. Show, with a tick (✓) in the appropriate box, whether your results support or do not support this hypothesis. results support hypothesis results do not support hypothesis Give one reason for your answer. reason ................................................................................................................................ ........................................................................................................................................... ..................................................................................................................................... [1] Question 1 continues on page 8. (b) A student investigated the effect of temperature on cell membranes by measuring the diffusion of pigment from beetroot cells. The student immersed discs of beetroot in water at different temperatures for 10 minutes. A colorimeter was used to determine how much pigment had diffused into the water by measuring the percentage transmission of light through each sample of water. The results are shown in Table 1.2. Table 1.2 temperature / °C percentage transmission of light 10 86 30 84 45 70 60 23 75 14 (i) Plot a graph of the data shown in Table 1.2 on the grid in Fig. 1.3. Use a sharp pencil. Fig. 1.3 [4] (ii) Temperature affects the permeability of the cell membrane. State one reason for the change in permeability between 45 °C and 60 °C. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] [Total: 22]

Mark scheme: Question Answer Marks 1(a)(i) 1 four correct concentrations (0.2, 0.02, 0.002, 0.0002) and mol dm–3 at least once ; 3 2 shows transfer of 1 (cm3) to each beaker from the previous beaker ; 3 shows 9 (cm3) of water added to each beaker ; 1(a)(ii) 1 heading for independent variable: concentration of hydrochloric acid / mol dm–3 (before heading for dependent variable) 6 and no units in body of table ; 2 heading for dependent variable: diffusion distance after 10 and 20 minutes / mm and no units in body of table ; 3 a measurement at 10 and 20 minutes for each concentration ; 4 (at 20 minutes) diffusion distance recorded for the highest concentration of hydrochloric acid longer than for the lowest concentration of hydrochloric acid and the diffusion distance for each concentration is equal to or longer than the diffusion distance recorded at 10 minutes ; 5 records diffusion distance in whole millimetres ; 6 diffusion distance recorded for U between the diffusion distance recorded for 0.2 and 0.02 mol dm–3 ; 1(a)(iii) the higher the concentration the steeper the concentration gradient (so) the greater the diffusion distance ; 1 1(a)(iv) difficult to judge where to measure the diffusion distance / AW ; 1 1(a)(v) correct estimate for U based on candidate’s results ; 1 1(a)(vi) 1 stated concentrations both sides of the estimate for U ; 2 any one from: 2 more concentrations with narrower intervals ; 3 repeat and calculate a mean ; 4 do each concentration separately ; 1(a)(vii) 1 shows measurement at 20 minutes for 2.0 mol dm–3 divided by 20 ; 2 2 correct answer and units ; 1(a)(viii) any one from: 1 results support: 1 diffusion distance is greater in the first 10 minutes than in the second 10 minutes ; results do not support: 2 diffusion distance in the first 10 minutes is less than or equal to the diffusion distance in the second 10 minutes ; 1(b)(i) 1 x-axis: temperature / C 4 and y-axis: percentage transmission of light ; 2 x-axis: 20 C to 2 cm and labelled at least each 2 cm and scale on y-axis: 20% to 2 cm and labelled at least each 2 cm ; 3 correct plotting of all five points using small dots in circles or crosses ; 4 five plots joined with a thin line passing through all points ; 1(b)(ii) any one from: 1 1 reference to effect on proteins e.g. denatured ; 2 reference to effect on phospholipids / fatty acids e.g. increased fluidity ; 3 cell membrane e.g. increased fluidity ;

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2 J1 is a slide of a stained transverse section through a leaf. (a) (i) Draw a large plan diagram of the region of the leaf section on J1 indicated by the shaded area in Fig. 2.1. Use a sharp pencil. draw this region Fig. 2.1 Use one ruled label line and label to identify the epidermis. [5] (ii) The upper epidermis of the leaf on J1 has a thicker cuticle than the lower epidermis. Observe the upper epidermis and the layer of cells beneath it. Select a group of four adjacent cells. This group must include two cells from the upper epidermis and two cells from the layer beneath the epidermis. 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 epidermal cell. [5] (b) Fig. 2.2 is a photomicrograph of a stained transverse section of a leaf from a different plant from J1. Fig. 2.2 (i) Identify three observable differences, other than colour, between the section on J1 and the section in Fig. 2.2. Record these three observable differences in Table 2.1. Table 2.1 feature J1 Fig. 2.2

Mark scheme: 2(a)(i) 1 appropriate size and no shading ; 5 2 draws correct section of the leaf ; 3 draws at least three vascular bundles in a line and no cells drawn ; 4 draws a circle (of tissue) around each vascular bundle ; 5 label line and label to epidermis ; 2(a)(ii) 1 appropriate size and lines are sharp and continuous ; 5 2 draws only two cells from the epidermis and two cells from the layer beneath the epidermis and each cell touches at least two other cells ; 3 draws two lines around each cell and three lines where cells touch ; 4 draws correct shape and arrangement of cells ; 5 label line and label to the cell wall of one epidermal cell ; 2(b)(i) any three (correct differences) from: 3 feature J1 Fig. 2.2 shape elongated circular ; vascular bundles in a line In the centre ; stomata many fewer ; air spaces many fewer ; 2(b)(ii) cuticle / thick epidermis / few stomata and reduces transpiration ; 1 2(c) 1 correct measurement of the width of the whole leaf section (X–Y) and units ; 4 2 correct measurement of the width of the central region (A–B) and units ; 3 shows the width of the central region divided by the width of the whole leaf section and multiplied by 100 ; 4 correct answer ;

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