Cambridge IGCSE Biology (9-1) 0970 — 2020 Oct/Nov Paper 5 · Variant 2

1 You are going to investigate the effect of temperature on the rate of rising of bread dough. Read all the instructions but DO NOT CARRY THEM OUT until you have drawn a table for your results in the space provided in 1(a)(i). Step 1 Label one plastic cup C and a second plastic cup W. Step 2 Put 30 cm3 of water into the measuring cylinder. Step 3 Pour approximately 10 cm3 of water from the measuring cylinder into the beaker labelled dough and stir the contents for 20 seconds using the metal spatula. Repeat this step until all the water has been added. Step 4 Stir the mixture for one minute to form a dough. Step 5 Use the spatula to scrape the dough from the beaker onto the white tile. Step 6 Use the spatula to divide the dough into two equal parts. Put on the gloves provided and roll each piece of dough in your hands to form a ball. Step 7 Place one ball of dough into each of the plastic cups, C and W. Gently push each ball of dough to form a flattened layer at the bottom of the cup. Step 8 Mark the maximum height of the dough on the outside of each cup with the marker pen. Measure the height of this line from the base of each cup using a ruler, as shown in Fig. 1.1. Record these measurements in your table in 1(a)(i). plastic cup mark at the maximum height of the dough dough measure the distance between the top of the dough and the base of the cup Fig. 1.1 Step 9 Put 100 cm3 of water into the empty beaker labelled cool water-bath. Step 10 Raise your hand when you are ready for warm water to be added to the beaker labelled warm water-bath. Step 11 Place cup C into the cool water-bath and cup W into the warm water-bath. Step 12 Start the stop-clock and leave the cups in the water-baths for 15 minutes. Continue with the other questions during this time. Step 13 After 15 minutes remove the cups from the water-baths. Step 14 Mark the maximum height of the dough on each cup with the marker pen. Measure the distance of this line from the base of each cup. Record these measurements in your table in 1(a)(i). (a) (i) Prepare a table to record your results. [4] (ii) Calculate the change in the height of the dough in cup C and in cup W between step 8 and step 14. C ................................................................ W ................................................................ [1] (iii) State a conclusion for these results. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (iv) Identify the independent variable (the variable that was changed) in this investigation. ..................................................................................................................................... [1] (v) Identify the dependent variable (the variable that was measured) in this investigation. ..................................................................................................................................... [1] (b) (i) Identify one possible source of error in step 6 and describe how the method could be improved to avoid this error. error ................................................................................................................................... ........................................................................................................................................... improvement ..................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... [2] (ii) Explain why a more valid conclusion can be made by comparing the change in heights rather than the final heights of the dough. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (c) State the test that could be used to show that starch was present in a sample of bread dough. Give a positive test result. test ............................................................................................................................................ positive result ............................................................................................................................ [2] (d) The dough mixture in step 3 contained 50 g of flour, 3 g of sugar and 2 g of yeast. Carbon dioxide gas is produced when yeast respires. The gas causes the dough to rise. (i) State the name of an indicator which could be used to show that the gas is carbon dioxide and give the positive test result for this indicator. indicator ............................................................................................................................. positive test result ............................................................................................................. [2] (ii) Plan an investigation to determine the effect of the mass of sugar on the volume of carbon dioxide produced by a yeast suspension. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [6] [Total: 21]

Mark scheme: 1(a)(i) table with a line between headings and data and a minimum of two columns or rows ; correct headings for column / rows given ; record of four heights ; correct trend ; e.g. for cup W, final height greater than initial height and final height greater than final height in cup C check supervisor’s report 1(a)(ii) correct calculation of increase in height for C and W ; 1 1(a)(iii) a conclusion consistent with the candidate’s results ; 1 expect: at high(er) temperatures, the dough rises more / more carbon dioxide is produced / AW / ora 1(a)(iv) temperature ; 1 1(a)(v) (maximum) height (of dough) / distance from bottom of cup to top of dough ; 1 1(b)(i) error the dough was not accurately divided into two / AW ; improvement measure the mass / use the same mass / use a balance / scale(s) / AW (to ensure both dough sections have equal mass) ; 2 1(b)(ii) idea of the height of the dough at the beginning was not identical in both cups ; 1 1(c) add iodine (solution / reagent) ; blue-black (colour) ; 2 Question Answer Marks Guidance 1(d)(i) hydrogencarbonate (indicator) ; yellow / orange (colour) ; or limewater ; cloudy / white / milky / chalky ; 2 Question Answer Marks Guidance 1(d)(ii) independent variable: 1 at least two different masses or amounts of sugar used / masses stated ; given method: 2 putting dough, into same sized cups / containers ; 3 measuring height / volume of dough ; new method: 4 described method of maintaining temperature ; 5 a detail of using the yeast suspension ; 6 measurement method ; e.g. gas syringe / delivery tube and water displacement / delivery tube and counting bubbles / measuring height of foam formed / bubbling gas through (named) indicator dependent variable: 7 measure volume of gas (carbon dioxide) produced in set time / measure time to produce set volume of gas / measure time for positive indicator result ; variables kept constant: 8 and 9 any two from: ;; temperature for all samples species / age / source, of yeast type of sugar / sugar from same sample mass of yeast / concentration of yeast (suspension) / volume of yeast (suspension) pH / use of buffer other: 10 two or more repeats / replicates ; 11 wear goggles / eye protection ; 12 AVP ; e.g. control experiments described 6

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2 (a) Fig. 2.1 is a photomicrograph showing several cells from an Elodea sp. aquatic plant. A Fig. 2.1 (i) Draw a large diagram of the cell labelled A in Fig. 2.1. [4] (ii) P Q magnification ×1200 Fig. 2.2 Measure the length of the line PQ on Fig. 2.2. length of PQ ................................................. mm Calculate the actual length of the cell using the formula and your measurement. length of line PQ magnification = actual length of cell Include the unit. Space for working. ................................................................ [3] (b) A student investigated the effect of light intensity on the rate of photosynthesis in Elodea sp. They changed the light intensity by placing a light source at different distances from the plant. The student counted the number of bubbles of oxygen produced in three minutes. The results of the investigation are shown in Table 2.1. Table 2.1 distance of the light number of bubbles rate of bubble production source from the plant / cm produced in three minutes / bubbles per minute 80 312 104 100 312 104 120 309 140 264 88 160 162 54 180 96 32 200 57 19 (i) Calculate the rate of bubble production when the light source was 120 cm from the plant. ........................... bubbles per minute [1]

Mark scheme: 2(a)(i) single clear outline and no shading ; cell at least 10 cm from left to right ; details: two clusters of chloroplasts (minimum of three in left cluster and four in right cluster) ; two ‘protrusions’ in cell wall at cell junctions top and bottom ; 4 2(a)(ii) (length of PQ) = 94 ± 1 (mm) ; 0.078 / 0.08 mm ;; 3 MP1 correct measurements MP2 correct calculation MP3 correct unit 2(b)(i) 103 (bubbles per minute) ; 1 2(b)(ii) axes labelled with units ; even scale and with plots using at least half the grid in one direction ; six points plotted accurately + half a small square ; suitable line drawn ; 4 2(b)(iii) 43 ± 1 (bubbles per minute) ; 1 A candidate’s line at 170 cm ± ½ small square 2(b)(iv) as plant moves further away from the light: ora number of bubbles remains constant and then decreases ; change in rate, after 100–139 cm / after the first two readings (either distance or bubbles) / after the first three readings / AW ; 2 2(b)(v) idea of plant equilibrating to new conditions / AW ; 1 2(b)(vi) 39 (%) ;;; 3 MP1 selection of correct data MP2 correct calculation MP3 correct rounding to a whole number

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