Cambridge A Level Biology 9700 — 2019 May/June Paper 3 · Variant 2

1 Fusicoccin is a chemical that increases the sucrose concentration in phloem at the source. In this investigation you will be provided with a sample, U, which has been taken from the phloem of a plant. You will need to: • prepare proportional dilutions of a 10% sucrose solution • carry out the test for non-reducing sugars on each concentration of sucrose solution • carry out the test for non-reducing sugars on the unknown concentration of sucrose, U • estimate the concentration of sucrose in U. You are provided with the materials shown in Table 1.1. Table 1.1 labelled contents hazard volume / cm3 S 10% sucrose solution none 50 Benedict’s Benedict’s solution harmful 30 W distilled water none 50 H dilute hydrochloric acid irritant 20 A 10 g sodium hydrogencarbonate powder none – U solution of unknown sucrose concentration none 10 If any of the materials come 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: • make a proportional dilution of 10% sucrose solution, S, which reduces the concentration by 2% between each successive dilution • prepare 5 cm3 of each concentration of sucrose solution. (i) Complete Table 1.2 to show how you will prepare these concentrations. Table 1.2 percentage concentration of volume of S / cm3 volume of W / cm3 sucrose solution 10.0 5.0 0.0 [2] Carry out the test for non-reducing sugars on the concentrations of sucrose using step 1 to step 17. 1. Set up a water-bath and heat to boiling, ready for step 7 and step 14. 2. Prepare the concentrations of sucrose solution as shown in Table 1.2, using the beakers provided. 3. Label test-tubes with the concentrations of sucrose prepared in step 2. 4. Put 2 cm3 of each concentration of sucrose into the appropriately labelled test-tubes. 5. Label another test-tube U and put 2 cm3 of U into this test-tube. 6. Put 2 cm3 of H into all the test-tubes. Shake gently to mix. 7. Put all the test-tubes into the boiling water-bath (set up in step 1). Leave the test-tubes for 2 minutes. 8. After 2 minutes, remove all the test-tubes from the water-bath and put them into the beaker of water labelled For cooling. You will need the boiling water-bath again for step 14. 9. Leave the test-tubes in the beaker for 3 minutes. 10. After 3 minutes, put a small amount of A into each test-tube. The mixture will fizz and rise up the test-tube. 11. Continue to put a small amount of A into each test-tube until there is no more fizzing and there is a small amount of A in the bottom of the test-tube. 12. Put 4 cm3 of Benedict’s solution into the test-tube containing the highest concentration of sucrose solution. 13. Shake the test-tube gently to mix. 14. Put this test-tube in the boiling water-bath. Start timing. 15. Measure the time taken for 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’. 16. Record the result from step 15 in (a)(ii). 17. Remove the test-tube from the water-bath. Put the test-tube in the test-tube rack. 18. Repeat step 12 to step 17 with the remaining concentrations of sucrose solution. (ii) Record your results in an appropriate table for the known concentrations of sucrose solution. [5] 19. Repeat step 12 to step 15 with U and record the result from step 15 in (a)(iii). (iii) State the result for U. .......................................................... [1] A plant that has been treated with fusicoccin has a phloem sucrose concentration of between 5.5% and 7.5%. (iv) State whether U is taken from the phloem of a plant treated with fusicoccin. Give a reason for your answer. answer ................................................................................................................................ reason ................................................................................................................................ ..................................................................................................................................... [1] (v) Suggest how you could change the independent variable to have more confidence in your answer to (iv). ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] An investigation was carried out to show the effect of fusicoccin on two plants, F and G. • Plant F was treated with 10 cm3 of fusicoccin solution. • Plant G was treated with 10 cm3 of water instead of fusicoccin solution. • Plant F and plant G were grown in moist soil and in identical conditions. (vi) State why plant G was treated with 10 cm3 of water. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (b) A scientist investigated the effect of treating a plant with fusicoccin. Samples were taken from the phloem every 35 minutes for a total of 175 minutes. The concentration of sucrose in each sample was measured. These raw results were then used to calculate the rate of mass flow. The results are shown in Table 1.3. Table 1.3 rate of mass flow time / min / cm3 min–1 35 6.00 70 10.25 105 15.00 140 19.50 175 23.75 (i) Plot a graph of the data in Table 1.3 on the grid in Fig. 1.1. Use a sharp pencil for drawing graphs. Fig. 1.1 (ii) Use your graph to find the time when the rate of mass flow was 20 cm3 min–1. Show on the graph how you determined your answer. time = ......................................................... [2] (iii) Before treating the plant with fusicoccin, the scientist had obtained a rate of mass flow of 1.5 cm3min–1. Describe the effect of fusicoccin on the rate of mass flow shown in Fig. 1.1. ..................................................................................................................................... [1] (iv) Suggest how fusicoccin increases the loading of sucrose into phloem sieve tubes. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] [Total: 21]

Mark scheme: 1(a)(i) 1. four stated percentage concentrations of sucrose solution, 8.0 / 8%, 6.0 / 6%, 4.0 / 4% and 2.0 / 2% ; 2. correct volumes of S and W to make up the four stated concentrations ; 2 1(a)(ii) 1. heading for percentage concentration of sucrose ; 2. heading for time / seconds ; 3. records results for at least four concentrations of sucrose ; 4. shows correct trend ; 5. records whole seconds ; 5 1(a)(iii) states the result for U + units ; 1 1(a)(iv) states whether U is taken from the phloem of a plant treated with fusicoccin + reference to the concentration of sucrose in U ; 1 1(a)(v) 1. makes a range of concentrations of sucrose with narrower intervals ; 2. uses proportional dilution ; 2 1(a)(vi) G was acting as a control ; 1 1(b)(i) 1. x-axis labelled as time / min + y-axis labelled as rate of mass flow / cm3 min–1 ; 2. scale on x-axis is 50 to 2 cm, labelled at least every 2 cm + scale on y-axis is 5 to 2 cm, labelled at least every 2 cm ; 3. five points plotted accurately with a small cross or a small dot in a circle ; 4. five points connected plot to plot or with a line of best fit ; 4 1(b)(ii) 1. 20 cm3 min–1 ; 2. shows on graph from y-axis to x-axis ; 2 Question Answer Marks 1(b)(iii) increase the rate of mass flow ; 1 1(b)(iv) any two from: 1. ref. to transport of sucrose via plasmodesmata ; 2. ref. to fusicoccin binding to receptors of companion cell ; 3. ref. to active transport of hydrogen ions protons ; 4. ref. to cotransporters (sucrose and hydrogen ions) ; 2

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2 M1 is a slide of a stained transverse section through a plant organ. You are not expected to be familiar with this specimen. (a) Select a field of view so that you can observe: • the epidermis • at least three vascular bundles. Use a sharp pencil for drawings. You are expected to draw the correct shape and proportions of the different tissues. (i) Draw a large plan diagram from the selected field of view which has: • part of the epidermis • only three vascular bundles • any other observable tissues. Use one ruled label line and label to identify the epidermis. [5] (ii) Observe the epidermis of the organ on M1. Select two epidermal cells and two adjacent, touching cells on the layer below the epidermis. Each cell must touch at least two 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 cell. [5] (b) Fig. 2.1 is a photomicrograph of a stained transverse section through an organ from a different species. You are not expected to be familiar with this specimen. Z magnification ×20 Fig. 2.1 (i) Identify the organ shown in Fig. 2.1. Describe one observable feature that supports your identification. name of organ .................................................................................................................... feature ................................................................................................................................ ..................................................................................................................................... [1] (ii) Use the magnification and the line Z on Fig. 2.1 to calculate the actual length of the vascular bundle. Show all the steps in your working and use appropriate units. actual length of the vascular bundle = ............................................................[5]

Mark scheme: 2(a)(i) M1 is TS Bryony stem 1. minimum size + at least two layers of tissue ; 2. draws part of stem + only three vascular bundles ; 3. correct proportion of vascular tissue to the width of the plant organ ; 4. draws at least three layers of tissue ; 5. label line and label to identify the epidermis ; 5 2(a)(ii) 1. minimum cell size + lines thin and continuous ; 2. draws only four cells + each cell touching at least two others ; 3. draws two epidermal cells + two adjacent cells on the layer below the epidermis ; 4. two lines drawn around each cell, three lines where cells touch ; 5. label line and label to identify the cell wall ; 5 2(b)(i) organ identified as a stem + one observable feature supporting identification e.g. vascular bundles located around outside / near the epidermis ; 1 2(b)(ii) 1 uses line Z to measure the length of the vascular bundle within range ; 2 show conversion from mm to µm (×1000) ; 3 shows division by 20 ; 4 decides to give answer in µm or mm ; 5 answer stated to the appropriate degree of accuracy ; 5 Question Answer Marks 2(b)(iii) graticule or stage micrometer ; 1 2(c) records two observable differences between M1 and Fig. 2.1 ; ; e.g. M1 has fewer vascular bundles than Fig. 2.1 M1 has scattered vascular tissue while in Fig. 2.1 it is located towards the epidermis 2

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