Cambridge A Level Biology 9700 — 2022 May/June Paper 3 · Variant 1

1 Plant extracts contain molecules which have many uses in industry and as medicines. It is important for scientists to be able to estimate the concentration of these molecules in plant extracts. You will estimate the concentration of molecule R in a sample of plant extract U. You are provided with the materials shown in Table 1.1. Table 1.1 labelled contents hazard volume / cm3 10R 10% solution of molecule R none 100 W distilled water none 100 U unknown concentration of none 40 molecule R in a plant extract A sulfuric acid solution harmful 20 irritant K potassium manganate(VII) solution irritant 20 If any of the solutions come into contact with your skin, wash off immediately under cold water. It is recommended that you wear suitable eye protection and wear gloves to protect your hands when using A and K. (a) You will need to carry out a serial dilution of the 10% molecule R solution, 10R, to reduce the concentration by half between each successive dilution. You will need to prepare four concentrations of molecule R solution in addition to the 10% molecule R solution, 10R. After the serial dilution is completed, you will 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 molecule R solution transferred • the volume of distilled water, W, added. Under each beaker, state the concentration of molecule R solution. 0 cm3 of W .............................20 cm3 of 10.0% R, ............................. 10R ............................. ............................. 10 cm3 of 10.0% R solution to use ............................. ............................. ............................. ............................. Fig. 1.1 [3] Molecule R changes an acidified potassium manganate(VII) solution, K, from a pink colour to colourless. The lower the concentration of molecule R, the greater the volume of R needed to change the potassium manganate(VII) from pink to colourless (the end-point). Carry out step 1 to step 12. step 1 Prepare the concentrations of molecule R solution, as decided in (a)(i), in the beakers provided. step 2 Label 5 test-tubes with the concentrations of R you prepared in step 1. step 3 Put 1 cm3 of A into the test-tube labelled 10%. step 4 Put 1 cm3 of K into the same test-tube and mix well. step 5 Put the nozzle of a clean 10 cm3 syringe into the beaker containing 10% R. step 6 Pull the plunger out to the 5 cm3 mark so that 5 cm3 of 10% R enters the syringe. step 7 Put the syringe containing 10% R into the test-tube from step 4 as shown in Fig. 1.2. syringe resting on top of test-tube drop of solution 5 cm3 of solution containing molecule R containing molecule R 1 cm3 of A and 1 cm3 of K Fig. 1.2 You will be counting the number of drops needed to change K from pink to colourless (the end-point). step 8 Gently press the plunger to add one drop of solution containing molecule R. step 9 Mix well. step 10 If the mixture stays pink repeat step 8 and step 9. Count the number of drops released to reach the end-point. If the number of drops needed to reach the end-point is more than 100, record as ‘more than 100’. step 11 Record the results in (a)(ii). step 12 Repeat step 3 to step 11 for each of the concentrations of molecule R prepared in step 1. (ii) Record your results in an appropriate table. [5] (iii) State two significant sources of error when measuring the dependent variable in this investigation. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (iv) Suggest how you could improve the procedure to reduce one of the errors stated in (a)(iii). ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] step 13 Repeat step 3 to step 10 with U, the unknown concentration of molecule R. step 14 Record your result in (a)(v). (v) State the number of drops of U needed to reach the end-point .................................... [1] (vi) Complete Fig. 1.3 to show the positions of each of the percentage concentrations of molecule R solution recorded in (a)(ii). [1] 0.0% 10.0% percentage concentration of molecule R Fig. 1.3 (vii) Use your results in (a)(ii) and (a)(v) to estimate the concentration of molecule R in solution U. Show this estimate of the concentration of U on Fig. 1.3, by placing the letter U on the line in the correct position. [1] (b) A student investigated the concentration of molecule R in extracts from different plants. The student added a fixed volume of the extract to acidified potassium manganate(VII) and timed how long it took to reach the end-point. The results are shown in Table 1.2. Table 1.2 plant extract time / s B 15 C 59 D 42 E 10 F 7 (i) Plot a bar chart of the data shown in Table 1.2 on the grid in Fig. 1.4. Use a sharp pencil. Fig. 1.4 [4] (ii) Suggest which plant extract would contain the highest concentration of R. ......................................................... [1] (c) Scientists have suggested that a different molecule in the plant extract might act as an antibiotic. The scientists tested this by: • spreading bacteria over the surface of agar gel containing nutrients • putting small drops (3 μm3) of different concentrations of the plant extract into separate wells in the agar gel • incubating the agar gel for 20 hours • measuring the inhibition area (where the bacteria were not observed) for each concentration of plant extract. The results are shown in Fig. 1.5. 120 100 80 inhibition 60 area / mm2 40 20 0 0 20 40 60 80 100 concentration of solution of molecule R / μg cm–3 Fig. 1.5 (i) Use the graph in Fig. 1.5 to estimate the concentration of plant extract that results in an inhibition area of 92 mm2. concentration of plant extract = ......................................................... [1] (ii) Suggest how the molecule in the plant extract may inhibit the growth of the bacteria. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] [Total: 22]

Mark scheme: 1(a)(i) 1. correct concentrations (5.0, 2.5, 1.25, 0.625) 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 molecule R (before heading for dependent variable) and no units in body of table ; 2. heading dependent variable: number of drops ; 3. results recorded for all concentrations ; 4. number of drops recorded for the highest concentration of R is fewer than for the lowest concentration of R ; 5. results recorded as whole drops ; 5 1(a)(iii) identifies two errors, e.g. drop size varies, difficult to judge endpoint ; ; 2 1(a)(iv) any one improvement and must be linked to an error in 1(a)(iii), e.g.: 1. (drop size varies) measure the volume used instead of the number of drops ; 2. (difficult to judge endpoint) use a colorimeter ; 1 1(a)(v) records a number for U that is less than that recorded for 10 and more than that recorded for the lowest concentration ; 1 1(a)(vi) correctly labels scale bar with different concentrations of molecule R ; concentrations must match to those listed in 1(a)(ii) 1 1(a)(vii) estimates the correct concentration of R in U by placing U in the correct position on the scale bar ; 1 1(b)(i) 1. x-axis: plant extract and bars labelled B, C, D, E and F and y-axis: time / s ; 2. x-axis: even width of bars and scale on y-axis: 10 seconds to 2 cm, labelled at least each 2 cm ; 3. correct plotting of all five bars ; 4. five separate bars drawn and with horizontal and vertical lines joined precisely ; 4 1(b)(ii) plant extract F ; 1 Question Answer Marks 1(c)(i) 1 22 g cm-2 ; 1 1(c)(ii) any two from: 1. prevents formation of cell, wall / membrane ; 2. cell / bacterial, lysis or cells / bacteria, burst ; 3. inhibits, transcription / translation / protein synthesis ; 4. inhibits cell division ; 5. acts as an enzyme inhibitor ; 6. inhibits DNA, replication / synthesis ; 2

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2 J1 is a slide of a stained transverse section through a plant leaf. (a) (i) Draw a large plan diagram of the whole section on J1. Use a sharp pencil. Use one ruled label line and label to identify the epidermis. [5] (ii) The leaf section on J1 is from a plant that lives at high altitudes in very cold conditions. In the winter the ground is often frozen and plants are unable to take up water. Suggest one observable feature of the leaf section on J1 which enables it to survive in these conditions. ........................................................................................................................................... ..................................................................................................................................... [1] (iii) Observe the cells in the epidermis of the section on J1. Select a line of four adjacent cells that make up this tissue. Each cell must touch at least one of the other cells. • Make a large drawing of this line of four cells. • Use one ruled label line and label to identify the cell wall of one cell. [5] (b) Fig. 2.1 is a photomicrograph of a stained transverse section through a different type of leaf from J1. Fig. 2.1

Mark scheme: 2(a)(i) 1. uses most of the available space and no shading ; 2. whole leaf section drawn and no cells drawn ; 3. correct shape of the leaf ; 4. draws stomata opening into an air space ; 5. label line and label to epidermis ; 5 2(a)(ii) any one from: 1. cuticle ; 2. few stomata ; 1 2(a)(iii) 1. uses most of the available space and all lines sharp and continuous ; 2. draws only four whole cells and each cell touches at least one other cell ; 3. cell wall drawn as two lines around each cell and three lines where cells touch ; 4. draws inclusions in at least two cells ; 5. label line and label to one cell wall ; 5 Question Answer Marks 2(b) 1. three correct differences any three from e.g.: feature J1 Fig. 2.1 vascular tissue smaller in proportion to the whole leaf larger in proportion to the whole leaf ; air spaces more fewer ; shape of leaf triangle elongated ; stomata fewer more ; 3 2(c) 1. measures and records the correct length of the scale bar ; 2. shows the length of scale bar divided by 950 ; 3. shows the length of line X–Y divided by the answer to mp2 ; 4. records the correct width of the leaf and units ; 4

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