Cambridge IGCSE Biology (9-1) 0970 — 2023 May/June Paper 6 · Variant 1

1 A student investigated the effect of temperature on the rate of respiration in yeast cells. When yeast cells respire they release carbon dioxide gas. The student used this method: Step 1 Fill a syringe with 15 cm3 of yeast suspension. Step 2 Gently lower the syringe into the measuring cylinder, as shown in Fig. 1.1. Step 3 Fill the measuring cylinder with hot water. Ensure that the water level in the measuring cylinder is above the syringe nozzle. measuring cylinder bubble of carbon dioxide gas water syringe nozzle yeast suspension Fig. 1.1 Step 4 Measure the temperature of the water in the measuring cylinder. Step 5 Start the stop-clock and wait for two minutes. Step 6 After two minutes, reset the stop-clock to zero. Step 7 Start the stop-clock again and count the number of bubbles produced by the yeast suspension in three minutes. Step 8 Pour the hot water in the measuring cylinder into the waste container. The syringe containing the yeast suspension should remain in the measuring cylinder. Step 9 Fill the measuring cylinder with cold water. Ensure that the water level in the measuring cylinder is above the syringe nozzle. Step 10 Repeat steps 4 to 7. Fig. 1.2 shows the thermometer during step 4 and step 10. 50 50 40 40 30 30 20 20 10 10 0 0 °C °C hot water cold water Fig. 1.2 Fig. 1.3 shows the number of bubbles the student counted in three minutes in step 7 and step 10. hot water – 54 bubbles cold water – 12 bubbles Fig. 1.3 (a) (i) Prepare a table for the results and record the temperatures shown in Fig. 1.2 and the results shown in Fig. 1.3. [3] (ii) State a conclusion for the results. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (iii) Using the results, calculate the rate of bubble production in bubbles per minute for the yeast suspension in hot water and in cold water. rate of bubble production in hot water ............................... bubbles per minute rate of bubble production in cold water ............................... bubbles per minute [1] (iv) State the independent variable in this investigation. ........................................................................................................................................... ..................................................................................................................................... [1] (v) State two variables that were kept constant in this investigation. 1 ........................................................................................................................................ ........................................................................................................................................... 2 ........................................................................................................................................ ........................................................................................................................................... [2] (vi) Suggest why the yeast suspension was left for two minutes in step 5 before starting to count the number of bubbles. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (vii) Suggest why counting bubbles is not the most accurate method of determining the rate of respiration in yeast. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (b) Measuring the volume of a gas is more accurate than counting bubbles. Using a gas syringe is one method of collecting a volume of gas. Fig. 1.4 shows part of the apparatus that can be used to measure the volume of a gas by a different method. Complete the diagram in Fig. 1.4 by drawing and labelling the two pieces of apparatus that are missing. bung yeast suspension container of water Fig. 1.4 [2] (c) Yeast can respire reducing sugars. Describe the method you would use to test a substance for the presence of reducing sugars. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [2] (d) Bread is made from flour, water and yeast which are mixed to form a dough. Fig. 1.5 shows a person making bread. dough Fig. 1.5 The carbon dioxide gas produced by yeast causes the volume of the dough to increase. Sodium chloride (salt) is often added to dough when making bread. The sodium chloride affects the rate at which the yeast respire. Plan an investigation to determine the effect of the mass of sodium chloride on the volume of dough. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [6] [Total: 20]

Mark scheme: 1(a)(i) 1 table drawn with minimum of two columns and a header line ; 2 appropriate column / row headings in the correct positions, with units for temperature ; temperature °C and number of bubbles (in 3 minutes) 3 recording of two correct temperatures (42 and 18) and two correct numbers of bubbles (54 and 12) ; 1(a)(ii) the higher the temperature the greater the rate of respiration / ora ; 1 1(a)(iii) (hot) 18 (bubbles per minute) and (cold) 4 (bubbles per minute) ; 1 1(a)(iv) temperature ; 1 1(a)(v) any two from: volume of yeast ; concentration of yeast ; type / batch / sample, of yeast ; time for counting bubbles ; time for, equilibration / waiting / AW ; (total) time, yeast (suspension) / syringe, was in the water / for the experiment ; 2 1(a)(vi) to allow the temperature of the yeast to become the same as the water / idea of equilibration/AW; 1 1(a)(vii) bubbles are different, sizes / volumes / AW ; idea that easy to miscount ; 1 1(b) delivery tube drawn and labelled ; measuring cylinder inverted in water drawn and labelled ; 2 1(c) add Benedict’s (solution / reagent) ; heat / warm ; 2 Question Answer Marks Guidance 1(d) independent variable: 1 different masses of, sodium chloride / salt ; detail of method: 2 using a, balance / scale, to measure mass of salt / (named) ingredients / dough ; 3 mixing / kneading ; 4 ref to measuring change in height / volume / circumference / size OR time taken to reach a set height / volume / circumference / size ; 5 method of measuring height / volume of dough ; 6, 7 and 8 variables kept constant, max 3 from ;;;  same initial, volume / height / size / mass / amount, of dough  type of flour / type of dough / batch of dough  mass / amount, of flour  type of yeast  volume / amount / mass / concentration, of yeast  volume / amount / mass, of water  type of salt  time, left to rise / of the reaction  temperature  diameter of container (for measuring height) 9 two or more repeats ; 6

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2 Fig. 2.1 is a photograph of a cross-section of a root from a carrot plant, Daucus carota. P Q magnification ×6 Fig. 2.1 (a) (i) Draw a large diagram of the carrot root cross-section that shows the layers visible in Fig. 2.1. [4] (ii) Line PQ on Fig. 2.1 represents the diameter of the carrot root cross-section. Measure the length of line PQ on Fig. 2.1. length of PQ ............................................... mm Calculate the actual diameter of the carrot root cross-section using the formula and your measurement. length of line PQ in Fig. 2.1 magnification = actual diameter of the carrot root cross-section Give your answer to one decimal place. Space for working. ................................................................... mm [3] (b) A student investigated the effect of the concentration of a salt solution on the mass of carrot cubes. The student used this method: • Carrots were cut into cubes. Each side of the cube was 1 cm in length. • The initial mass of each carrot cube was measured and recorded. • Each carrot cube was put into a different concentration of salt solution. • The carrot cubes were left in the salt solutions for one hour. • After one hour, the carrot cubes were removed from the salt solution and dried with a paper towel. • The final mass of each carrot cube was measured and recorded. (i) State the dependent variable in the investigation described in 2(b). ........................................................................................................................................... ..................................................................................................................................... [1] (ii) State two variables that were kept constant in this investigation. 1 ........................................................................................................................................ 2 ........................................................................................................................................ [2] (iii) Explain why it was important to dry the carrot cubes before measuring the final mass. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] The results of the investigation are shown in Table 2.1. Table 2.1 concentration of salt initial mass of final mass of change in mass / g solution / mol per dm3 carrot cube / g carrot cube / g 0.0 0.97 1.04 0.07 0.2 0.98 0.99 0.01 0.4 0.96 0.90 –0.06 0.6 0.98 0.86 –0.12 0.8 0.99 0.84 –0.15 1.0 0.95 0.79 –0.16 1.2 0.96 0.80 –0.16

Mark scheme: 2(a)(i) Outline – single clear line ; Size – width (PQ) at least 109 mm ; Detail 1: 3 or more layers ; Detail 2: white line layer with a point at approximately 9 o’clock position ; 4 2(a)(ii) 109  1 (mm) ; 18.0 (for measurement of 108 mm) / 18.2 (for measurement of 109 mm) / 18.3 (for measurement of 110 mm) ;; 3 MP1 correct measurement MP2 correct calculation of actual size MP3 calculation correctly rounded to 1 decimal place ecf for MP2 and MP3 from wrong measurement / calculation 2(b)(i) mass of, carrot / cubes ; 1 2(b)(ii) any two from: initial, size / surface area / volume of carrot ; soaking time / AW ; drying method ; type of, plant tissue / carrot ; 2 2(b)(iii) idea that, extra solution / water / AW, will affect the, mass / AW ; 1 2(b)(iv) axes labelled with units ; suitable linear scale and data occupies at least half the grid in both directions ; six points plotted accurately  half a small square ; suitable line drawn ; 4 2(b)(v) answer consistent with the line on the candidate’s graph ; intercept clearly indicated on the graph ; 2 2(b)(vi) (–)6.25(%) ;; 2 MP1 dividing by 0.96 MP2 correct answer 2(b)(vii) to identify anomalous results ; 1

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