Cambridge A Level Biology 9700 — 2024 Oct/Nov Paper 3 · Variant 5

1 The enzyme amylase catalyses the hydrolysis (breakdown) of starch to reducing sugars, as shown in Fig. 1.1. amylase starch reducing sugars Fig. 1.1 A student investigated the effect of temperature on the hydrolysis of starch by the enzyme amylase. The student: • put 2 cm3 of amylase into a test-tube • put 18 cm3 of starch solution into a different test-tube • put the test-tubes into a water-bath at 0 °C • left the test-tubes for 3 minutes • put the starch solution into the test-tube containing the amylase • left the test-tube for 2 minutes • immediately determined the relative amount of reducing sugars in the test-tube • repeated this procedure at 40 °C and at 100 °C. You are provided with three beakers, S1, S2 and S3. These three beakers contain the same products of the reaction between starch and amylase as the test-tubes prepared by the student. You will determine: • the relative amount of reducing sugar present in each beaker • the presence or absence of starch in each beaker. You are provided with the materials shown in Table 1.1. Table 1.1 volume labelled contents hazard / cm3 products of the reaction between starch and S1 none 20 amylase at 0 °C products of the reaction between starch and S2 none 20 amylase at 40 °C products of the reaction between starch and S3 none 20 amylase at 100 °C products of the reaction between starch and U none 20 amylase at an unknown temperature B Benedict’s solution irritant 30 iodine iodine solution irritant 15 If any solution comes into contact with your skin, wash off immediately with cold water. It is recommended that you wear suitable eye protection. To investigate the effect of temperature on the hydrolysis of starch by the enzyme amylase you will need to: • carry out the test for reducing sugars on S1, S2 and S3 • determine the relative amount of reducing sugar in each solution • carry out the test for starch on S1, S2, and S3 • determine the presence or absence of starch in each solution. (a) (i) Explain why the student left the test-tubes in the water-bath for 3 minutes before adding the starch solution. ........................................................................................................................................... ..................................................................................................................................... [1] To determine the relative amount of reducing sugar in each solution, the time to the first colour change will be measured using 5 cm3 of each solution. (ii) Decide how you will test each solution to show the relative amount of reducing sugar present. State the reagent you will use. reagent ............................................................... Describe how you will use the reagent to carry out the test for reducing sugars. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... State how you will determine which solution has the highest amount of reducing sugar present. ........................................................................................................................................... [2] Carry out step 1 to step 5. step 1 Label one test-tube, S1. step 2 Put 5 cm3 of S1 into the test-tube. step 3 Repeat step 1 and step 2 for S2 and S3. step 4 Carry out the test for reducing sugars as you described in (a)(ii). If the time taken to the first colour change is longer than 120 seconds then record as ‘more than 120’. step 5 Record in (a)(iv) the time taken to the first colour change for each solution. Carry out step 6 to step 11 to test for the presence of starch. step 6 Label a clean test-tube, S1. step 7 Put 5 cm3 of S1 into the test-tube. step 8 Repeat step 6 and step 7 for S2 and S3. step 9 Put 2 drops of iodine solution into each test-tube. Shake gently to mix. If any starch is present the solution will change to a blue colour. step 10 Observe the colour in each test-tube. step 11 Record in (a)(iv) the colour for each solution. (iii) State the independent variable in this investigation. ..................................................................................................................................... [1] (iv) Record your results in an appropriate table. [4] (v) Complete Table 1.2 using your results in (a)(iv). For reducing sugar content use only the words: none, low, medium or high. For presence of starch use only the words: present or absent. You may use each word once, more than once or not at all. Table 1.2 reducing sugar content presence of starch S1 ............................................. ............................................. S2 ............................................. ............................................. S3 ............................................. ............................................. [2] (vi) Explain the results for S1, S2 and S3. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [3] U contains the products of the reaction between starch and amylase solution in a water-bath at an unknown temperature. You need to: • carry out the test for reducing sugars and the test for starch on U • compare the results for U with the results for S1, S2 and S3 • estimate the temperature of the water-bath used for U. Carry out step 12 and step 13. step 12 Repeat the test for reducing sugars and the test for starch on U. step 13 Record your results in (a)(vii). (vii) Record the result of the test for reducing sugars for U. time taken to first colour change ............................................................ s Record the result of the test for starch for U. colour ............................................................... [1] (viii) Use your results in (a)(iv) and (a)(vii) to estimate the temperature of the water-bath used for U. temperature of water-bath .................................................... °C [1] (ix) Describe how the student could obtain a quantitative estimate of the concentration of reducing sugar in a solution. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [4] (b) A student investigated the effect of temperature on the action of an enzyme that digests protein. The results are shown in Table 1.3. Table 1.3 temperature rate of reaction / °C / arbitrary units 20 8 25 46 30 74 40 59 45 42 Plot a graph of the data in Table 1.3 on the grid in Fig. 1.2. Use a sharp pencil. Fig. 1.2 [4] [Total: 23]

Mark scheme: Question Answer Marks 1(a)(i) reference to the contents of the test-tubes reaching the temperature of the water-bath ; 1 1(a)(ii) 1 same volume or excess Benedict’s reagent and heated to at least 80C ; 2 2 highest amount of reducing sugar has the shortest time ; 1(a)(iii) temperature ; 1 1(a)(iv) 1 heading for independent variable: temperature / C (before heading for dependent variable) and no units in body of 4 table ; 2 heading for dependent variable: time / s (for first colour change for reducing sugar) and colour (for starch) and no units in body of table ; 3 a time (reducing sugar) for each sample ; 4 a colour (starch) for each sample ; 1(a)(v) 1 correct sequence for reducing sugar based on candidate’s results ; 2 2 correct sequence for starch based on candidate’s results ; 1(a)(vi) Any three from: 3 1 S1 – enzyme hydrolyses some starch ; 2 S2 – enzyme hydrolyses some starch ; 3 S3 – enzyme does not hydrolyse starch / enzyme is denatured ; 4 correctly compares the number of enzyme substrate complexes between two solutions ; 5 correctly compares the hydrolysis of starch between S1, S2 and S3 ; 1(a)(vii) records a time (reducing sugar) and a colour (starch) for U ; 1 1(a)(viii) correct estimate for U based on candidate’s results ; 1 1(a)(ix) Any four from: 4 1 at least five known concentrations of reducing sugar ; 2 Benedict’s test and time for first colour change for known concentrations of reducing sugar ; 3 test unknown sample of reducing sugar (with Benedict’s) ; 4 compare time for unknown sample with times for known samples of reducing sugar ; 5 draw a graph of known concentrations of reducing sugar against time (to first colour change) ; 6 read off unknown sample of reducing sugar ; 1(b) 1 x-axis: temperature / C 4 and y-axis: rate of reaction / arbitrary units ; 2 scale on x-axis: 5C to 2 cm with 20 at the origin and labelled at least every 2 cm and scale on y-axis: 20 au to 2 cm and labelled at least every 2 cm ; 3 correct plotting of all five points using dots in circles or small crosses ; 4 five plots joined with thin line passing through all points ;

More questions on Factors that affect enzyme action

2 M1 is a slide of a stained transverse section through a leaf. (a) (i) Draw a large plan diagram of the whole section on M1. Use a sharp pencil. Use one ruled label line and label to identify the palisade tissue. [5] (ii) Observe the upper epidermis of the leaf on M1 and the layer of cells beneath it. The large bulge on the mid-rib is located on the upper epidermis. Select a group of four adjacent cells. This group must include two cells from the epidermis and two cells from below 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 a chloroplast in one cell. Use a sharp pencil. [6] (b) Fig. 2.1 is a photomicrograph of a stained transverse section of a different leaf from M1. Fig. 2.1 Identify three observable differences, other than colour, between the leaf section on M1 and the leaf section in Fig. 2.1. Record these three observable differences in Table 2.1. Table 2.1 feature M1 Fig. 2.1 [3]

Mark scheme: 2(a)(i) 1 minimum size and no shading ; 5 2 draws whole leaf section and no cells drawn ; 3 draws the correct shape of the palisade tissue; 4 draws the correct shape of the vascular bundle ; 5 label line and label to palisade tissue ; 2(a)(ii) 1 minimum size and all lines sharp and continuous ; 6 2 draws two cells from the upper epidermis and two cells from below the epidermis and each cell touches at least two other cells ; 3 two lines around each cell and three lines where cells touch ; 4 draws correct shape and size of cells ; 5 draws chloroplasts in palisade cells ; 6 label line and label to one chloroplast ; 2(b) three correct differences ;;; 3 any three e.g.: feature M1 Fig. 2.1 shape of vascular bundle flat oval palisade tissue upper epidermis upper and lower epidermis bulge present absent lamina / leaf blades thin thick 2(c) 1 correct measurement of lines P1, P2, P3, P4 and P5 and units ; 3 2 shows addition of five values and division by five ; 3 shows division of mean length thickness by the magnification ;

More questions on The microscope in cell studies