Cambridge IGCSE Biology (9-1) 0970 — 2018 May/June Paper 6 · Variant 1

1 Young mammals feed on milk containing protein. Some mammals produce an enzyme called rennin. Rennin changes the protein in milk so that it can be digested by another enzyme. The action of rennin causes small lumps or clots to form in the milk. An investigation was carried out to find the effect of pH on the activity of the enzyme rennin. Step 1 Three test-tubes were labelled P, Q and R. Step 2 A syringe was used to add 5 cm3 of milk to each of these test-tubes. Step 3 A dropping pipette was used to add two drops of acid to test-tube P. Step 4 A dropping pipette was used to add two drops of distilled water to test-tube Q. Step 5 A dropping pipette was used to add two drops of alkali to test-tube R. Step 6 Another three test-tubes were labelled P1, Q1 and R1. Step 7 A clean syringe was used to add 1 cm3 of 0.1% rennin solution to each of test-tubes P1, Q1 and R1. Step 8 All six test-tubes were placed into a water-bath at 40 °C and left for three minutes. Step 9 The contents of test-tube P1 were added to test-tube P. The contents of test-tube Q1 were added to test-tube Q. The contents of test-tube R1 were added to test-tube R. Step 10 Test-tubes P, Q and R were kept in the water-bath and a stop-clock was started. Step 11 After one minute, test-tube P was removed from the water-bath. It was tipped and rotated as shown in Fig. 1.1. The appearance of the milk was observed, and the stage of clotting was decided by comparing it to the diagrams in Fig. 1.1. milk drains back small clots stick most of the milk is smoothly from to the sides of the solid and does not the sides of the test-tube pour when the test-tube test-tube is tipped stage A stage B stage C no clotting some clotting all clotted Fig. 1.1 Step 12 Test-tube P was returned to the water-bath. Step 13 Steps 11 and 12 were repeated for test-tubes Q and R. Step 14 Steps 11, 12 and 13 were repeated every minute for five minutes. The results are shown in Fig. 1.2. Test-tube P had some clotting at one minute and was all clotted at two minutes. Test-tube Q had no clotting at one, two or three minutes but some clotting at four and five minutes. Test-tube R had no clotting throughout the investigation, and remained unchanged after five minutes. Fig. 1.2 (a) Prepare a table in which to record these results. Use the information in Fig. 1.2 to complete this table. [3] (b) State a conclusion for these results. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... .............................................................................................................................................. [2] (c) (i) Suggest why, in step 8, all of the test-tubes were placed into a water-bath for three minutes before mixing the contents together in step 9. ........................................................................................................................................... ........................................................................................................................................... ...................................................................................................................................... [1] (ii) State two variables that were kept constant in this investigation. 1 ........................................................................................................................................ 2 ........................................................................................................................................ [2] (d) Identify four sources of error in this investigation. 1 ................................................................................................................................................ ................................................................................................................................................... 2 ................................................................................................................................................ ................................................................................................................................................... 3 ................................................................................................................................................ ................................................................................................................................................... 4 ................................................................................................................................................ ................................................................................................................................................... [4] (e) Identify one hazard associated with this procedure that would require the use of eye protection. ................................................................................................................................................... ................................................................................................................................................... .............................................................................................................................................. [1] (f) Clotting separates milk into a solid part and a liquid part. Describe how you could find out if there was any protein remaining in the liquid part. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... .............................................................................................................................................. [2] (g) State the name of the test that would be used to test the milk for the presence of fat. .............................................................................................................................................. [1] (h) After rennin has changed the protein in milk into a white solid, protease enzymes can be used to digest the protein. The digested protein forms a colourless liquid. A hypothesis stated: The optimum temperature for protease enzymes to digest changed milk protein is 37 °C. Describe a method that could be used to test this hypothesis. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... .............................................................................................................................................. [6] [Total: 22]

Mark scheme: 1(a) one table drawn with at least three columns and a line separating headings from data ; column / row headings with appropriate units for each heading ; five correct clot records for each of three test-tubes ; R if units given in data cells 1(b) idea that rennin or enzyme acts / forms clots quickest AW / is optimum / is best, in acid conditions ; idea that rennin or enzyme acts / forms clots more slowly AW, in water / neutral conditions ; idea that rennin or enzyme does not act / does not form clots in alkaline / basic conditions ; 2 1(c)(i) idea of equilibration / allowing time for test-tube contents to reach the same temperature / all test-tubes at the same temperature ; 1 1(c)(ii) volume / 5 cm3 / mass, of milk ; volume / 2 drops, of acid / alkali / distilled water ; volume / 1 cm3, of enzyme / rennin ; concentration / 0.1%, of enzyme / rennin ; temperature / 40 °C ; equilibration time / 3 minutes or total time / 5 minutes (for investigation) ; 2 I time unqualified Question Answer Marks Guidance 1(d) subjective nature of deciding on clotting point ; three test-tubes observed at different times (step 11) / Q and R longer in water-bath before being tested / different mixing times (step 9) ; no repeats ; insufficient time intervals / AW ; drops (of acid / water / alkali) could be of different volumes / sizes / amounts ; some contents remain in the test-tube when adding to other tube, so volumes vary ; tip and rotate not standardised ; same dropping pipette (for acid / water / alkali) could have been used, leading to contamination ; temperature of water-bath not maintained ; no control (to show that milk does not clot without rennin) ; 4 1(e) use of acid / alkali / enzyme ; 1 1(f) add biuret (reagent) (to liquid part) ; purple / mauve / lilac, colour indicates, presence of protein / is a positive result ; 2 1(g) ethanol emulsion test / add ethanol and white emulsion forms ; 1 Question Answer Marks Guidance 1(h) at least 3 stated temperature values: 37 °C and at least one above and one below ; method of maintaining temperature given ; clot / white solid / changed milk, from same sample of milk ; same pH (pH 7) of solutions ; same concentration / volume, of enzyme / (named) protease ; equilibrate temperature of enzyme and milk solutions before mixing ; time how long for samples to become colourless or shortest time has, greatest activity / is the optimum or if optimum is 37 ºC the hypothesis is correct ; repeat at least twice (and calculate the mean) ; valid safety precaution, e.g. eye protection / gloves ; AVP ; e.g. crush / blend, clotted milk sample or valid control experiment described 6 max 2 from mp 4, 5 and 6

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2 A student wanted to investigate a garden ecosystem. She counted the number of insects caught in spider webs in one small section of the garden. She found six spider webs in the small section of garden sampled. Diagrams of the spider webs are shown in Fig. 2.1. Each black dot represents one insect caught in a spider web. A B C D E F not drawn to scale Fig. 2.1 (a) (i) Use Fig. 2.1 to complete Table 2.1. Table 2.1 spider web number of insects caught in each web A B C D E F total [2] (ii) Calculate the average number of insects per web in the small section of garden, using the information in Fig. 2.1 and Table 2.1. Space for working. ........................................................... [1] (iii) The student counted the total number of spider webs in the whole garden and found that there were a total of 102 spider webs. Use this information and your answer to part 2(a)(ii) to estimate the total number of insects caught in webs in the whole garden. Space for working. ........................................................... [1] (iv) Suggest one reason why the estimated total number of insects caught in webs in the whole garden may not be accurate. ........................................................................................................................................... ........................................................................................................................................... ...................................................................................................................................... [1] (b) Fig. 2.2 is a photograph of a spider. A spider’s body has two main parts. The legs are all attached to the cephalothorax which is the upper part of the body and starts at label X on Fig. 2.2. The lower part of the body is called the abdomen and is nearest to label Y on Fig. 2.2. X Y Fig. 2.2

Mark scheme: 2(a)(i) A 6, B 3, C 4, D 6, E 8, F 9 ; (total) 36 ; 2(a)(ii) 6 ; 1 2(a)(iii) 612 ; 1 2(a)(iv) variation in insect populations in different parts of garden / unrepresentative sample / some webs difficult to see / AW; some insects eaten by spiders / or fallen off web ; some insects, too small to be visible / difficult to count ; some caught organisms may not be insects ; AVP ; 1 2(b)(i) O (outer line) single, clear, continuous lines with no shading ; S (size) occupies at least half the space provided (must be at least 75 mm in length) ; D (detail) two body parts and 2 chelicerae (on the cephalothorax) or 2 spinnerets (on abdomen) ; 8 jointed legs, attached to the body in the correct position and in the correct orientation ; L (label) line labelled abdomen ending on the abdomen ; 5 Question Answer Marks Guidance 2(b)(ii) length of XY on Fig. 2.2 given as 38.5 mm – 41 mm ; line XY shown on the candidate’s drawing in correct position ; correct calculation ; 3 max 2 if no units given 2(c)(i) A (axes) labelled with units S (scale and size) even scale bars to occupy at least half the grid in both directions ; P (plotting) all five bars plotted accurately ± half a small square bars same width (at least 1 small squares wide) gaps between bars 3 2(c)(ii) 3 : 1 ;; 2

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