Cambridge IGCSE Biology (9-1) 0970 — 2020 May/June Paper 6 · Variant 1

1 Sugars are a source of energy in the diet. A student wanted to estimate the concentration of glucose in an energy drink (D). Step 1 Three test-tubes were labelled A, B and C. Step 2 Three different concentrations of glucose solution were prepared as shown in Table 1.1. (a) (i) Complete Table 1.1 by calculating and writing in the volume of 4% glucose solution used in test-tube B. Table 1.1 volume of 4% glucose solution volume of distilled water percentage concentration test-tube / cm3 / cm3 of glucose solution A 10.0 0.0 4 B 5.0 2 C 2.5 7.5 1 [1] Step 3 5 cm3 of Benedict’s solution was added to each of the glucose solutions in test-tubes A, B and C. Each test-tube was gently shaken for 3 seconds to mix the contents. Step 4 5 cm3 of Benedict’s solution was added to test-tube D which contained 10 cm3 of energy drink D. The test-tube was gently shaken for 3 seconds to mix the contents. Step 5 Hot water was put into a beaker that was used as a water-bath. Step 6 The temperature of the hot water in the water-bath (initial temperature) was measured. Step 7 Test-tube A was put into the water-bath and a stop-clock was started. Step 8 Test-tube A was observed and the stop-clock was stopped when the contents of the test-tube first changed colour. Step 9 Test-tube A was removed from the water-bath. Step 10 Steps 7, 8 and 9 were repeated with test-tubes B, C and D. Step 11 The final temperature of the water in the water-bath was measured at the end of the investigation. (ii) Fig. 1.1 shows the appearance of the thermometer in step 6 and in step 11. °C °C initial temperature final temperature step 6 step 11 Fig. 1.1 Record the temperatures shown in Fig. 1.1 in Table 1.2. Table 1.2 step 6 initial temperature / °C step 11 final temperature / °C [1] (iii) Fig. 1.2 shows the stop-clock readings for each test-tube in step 8. minutes:seconds minutes:seconds minutes:seconds minutes:seconds 00:36 01:27 02:45 01:30 test-tube A test-tube B test-tube C test-tube D Fig. 1.2 Convert the times shown in Fig. 1.2 to seconds. Prepare a table and record these times in your table. [3] (iv) Estimate the concentration of glucose in energy drink D using the information in Table 1.1 and the results in your table in 1(a)(iii). .......................................................% [1] (v) Explain how you estimated the concentration of glucose in energy drink D in 1(a)(iv). ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (b) Fig. 1.1 and Table 1.2 may indicate that there is a source of error in this investigation. (i) Identify the possible source of error and suggest one way the method could be improved to reduce this error. source of error ................................................................................................................... ........................................................................................................................................... improvement ..................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... [2] (ii) Explain how the error identified in 1(b)(i) could affect the results and the estimation of the concentration of glucose in energy drink D. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (c) (i) State two variables that were kept constant in the investigation in 1(a). 1 ........................................................................................................................................ 2 ........................................................................................................................................ [2] (ii) State one reason why the student should wear eye protection when carrying out this investigation. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (d) Barley grains are used in the food industry as a source of sugars. Barley grains contain starch. Starch can be broken down into sugars by enzymes. Students made a starch suspension from barley grains to investigate the breakdown of starch by enzymes. The starch suspension contained 100 g of starch. They mixed the starch suspension with enzymes and recorded the mass of starch remaining in the mixture at different times. The results are shown in Table 1.3. Table 1.3 time / minutes mass of starch remaining / g 0 100 5 58 10 40 20 32 40 28 (i) Plot a line graph on the grid of the data in Table 1.3. [4] (ii) Describe the trend shown in your graph. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (iii) Estimate the mass of starch remaining at 15 minutes. Show on your graph how you obtained your answer. ............................................................. g [2] (iv) Calculate the rate of starch breakdown in the first five minutes using the information in Table 1.3 or your graph. ..................................... g per minute [1] (e) Plan an investigation to determine the effect of temperature on the breakdown of starch by enzymes. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [6] [Total: 27]

Mark scheme: 1(a)(i) 5.0 ; 1 1(a)(ii) (initial temperature) 80 and (final temperature) 60 ; 1 1(a)(iii) table drawn with two columns and a header row ; column headings with unit: test-tube and time (taken for colour change) / s ; four correct times recorded in seconds (A:36, B:87, C:165, D:90) ; 3 1(a)(iv) 2(%); 1 1(a)(v) idea of comparing the time for D with A, B and C / B was the closest time to D / AW ; 1 1(b)(i) error: temperature has decreased ; improvement: maintain temperature by, using a thermostatically controlled water-bath / Bunsen burner / adding hot water throughout the investigation (and monitoring) temperature / insulate beaker / AW ; 2 1(b)(ii) times are slower as temperature decreases / unable to identify the concentration of glucose in D with certainty / may underestimate the concentration ; 1 1(c)(i) any two from: volume of glucose solution ; volume of Benedict’s solution ; concentration of Benedict’s solution ; time for mixing of test-tube contents ; 2 1(c)(ii) heating liquid (that might spill out of test-tube) / use of Benedict’s solution / AW ; 1 1(d)(i) axes correctly labelled with units: time / minutes and mass of starch remaining / g ; scale evenly spaced and occupying half of the grid in both directions ; five correct plots ± half a small square ; line joining all five plot points ; 4 1(d)(ii) mass of starch remaining decreases and levels off / AW ; 1 1(d)(iii) mass correct from candidate’s graph ; graph marked at interpolation to show where reading taken ; 2 Question Answer Marks 1(d)(iv) (42 ÷ 5 =) 8.4 (g per minute) ; 1 1(e) method: 1&2 use of Benedict’s reagent to measure reducing sugar produced, at set time / at regular intervals ; ref. to use of hot water-bath / use of (thermostatically controlled) water-bath to maintain temperatures ; OR use of iodine solution to determine starch remaining ; ref. to drops on spotting tile at regular intervals ; independent variable: 3 ref. to using at least two different temperatures ; variables kept constant – max two from mp 4-8: 4 same type of enzyme ; 5 concentration of starch suspension ; 6 concentration of enzyme ; 7 same pH ; 8 incubating enzymes and starch suspension for same time ; dependent variable: 9 concentration of reducing sugar produced / time taken for drops and iodine solution to remain brown ; 10 ref. to at least two further replications / repeat investigation two more times ; 11 ref. to relevant safety precaution ; 6

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2 (a) Human reproduction involves a male gamete (sperm) and a female gamete (ovum). Fig. 2.1 is a photomicrograph of a single ovum and many sperm cells during fertilisation. Line AB represents the diameter of the ovum. Line CD represents the length of one sperm cell. C A D B Fig. 2.1 (i) Measure the lengths of lines AB and CD on Fig. 2.1. Include the unit. length of line AB ................................ length of line CD ................................ [1] (ii) The actual diameter of the ovum is 0.10 mm. Calculate the magnification of the ovum in Fig. 2.1 using the formula: length of line AB on Fig. 2.1 magnification = actual diameter of ovum .......................................................... [1] (iii) Calculate the actual length of the sperm cell using the magnification you calculated in 2(a)(ii) and the formula: length of line CD on Fig. 2.1 magnification = actual length of sperm cell Give your answer to two decimal places. ......................................................... mm [3] (b) Fig. 2.2 is a different photomicrograph of an ovum and one sperm cell. Fig. 2.2 (i) Make a large drawing of the ovum and the sperm cell shown in Fig. 2.2. Label the ovum on your drawing. [5]

Mark scheme: 2(a)(i) 1 2(a)(ii) (x)700 / 710 / 690 ; 1 2(a)(iii) 0.03 mm ;;; 3 Question Answer Marks 2(b)(i) outline: has clear single lines without shading ; size: occupies at least half the space available ; details: correct proportions sperm and ovum ; shape of sperm – round / oval head and long tail ; ovum labelled with label line touching the ovum ; 5 2(b)(ii) any three from: ovum is larger than the sperm ; ovum is round, sperm long and thin ; ovum has rough surface, sperm is smooth ; ovum has a uniform shape, sperm is wider at one end ; 3

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