2025 NYJC H2 Bio 9744 P4 QP

9744 / H2 Biology / 04 NANYANG JUNIOR COLLEGE PRELIMINARY EXAMINATIONS Higher 2 CANDIDATE NAME CLASS BIOLOGY 9744/04 Paper 4 Practical 28 August 2025 Candidates answer on the Question Paper Additional Materials: As listed in the Confidential Instructions 2 hour 30 minutes READ THESE INSTRUCTIONS FIRST Write your name and CT on all the work you hand in. Give details of the practical shift and laboratory, where appropriate in the boxes provided. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, highlighters, glue or correction fluid. DO NOT WRITE IN ANY BARCODES. Answer all questions in the spaces provided on the Question Paper. The use of an approved scientific calculator is expected, where appropriate. You may lose marks if you do not show your working or if you do not use appropriate units. Shift Laboratory At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use 1 2 Total This document consists of 20 printed pages. [Turn over H

9744 / H2 Biology / 04 2 Answer all the questions. 1 Yeast cells contain enzymes that break down sugars to release energy. In these reactions carbon dioxide is released. Some of this carbon dioxide dissolves in water forming carbonic acid. The progress of these enzyme -catalysed reactions can be followed by measuring the time taken for an indicator to change colour as carbonic acid is formed. You will investigate the effect of changing the concentration of yeast suspension on the time taken for the indicator to change colour. You are provided with the materials shown in Table 1.1. Table 1.1 labelled contents hazard volume/ cm3 Y 10% yeast suspension none 100 H hydrogencarbonate indicator solution none 50 W distilled water none 100 Note that it is normal for Y to froth over time and the froth may overflow from the container. If any solution comes into contact with your skin, wash off immediately under cold water. It is recommended that you wear suitable eye protection. Read step 1 to step 4. (a) (i) Using Table 1.1 and step 1 to step 4, assess the risk of this procedure as low, medium or high. Give one reason for your answer. risk reason [1] The hydrogencarbonate indicator solution, H, will change colour from red-brown to yellow as carbon dioxide passes through it. To help you identify the end-point (yellow) carry out step 1 to step 4. 1 Put 5cm3 of H into a test-tube. 2 Put a drinking straw into this test-tube. 3 Gently breathe out through the straw so that you are blowing air bubbles through H. 4 Stop blowing when the colour changes to yellow. This is the end-point.

9744 / H2 Biology / 04 3 You will need to use this test-tube for colour comparison in step 14. You will use proportional dilution to make different concentrations of yeast suspension, Y. You will need to prepare 20cm3 of each concentration of yeast suspension. (ii) Complete Table 1.2 to show how you will prepare the concentrations of yeast suspension you will use. Table 1.2 percentage concentration of yeast suspension volume of Y / cm3 volume of W / cm3 10.0 20.0 0.0 [2] Carry out step 5 to step 18. 5 Use a glass rod to stir the yeast suspension, Y. Carefully decant the yeast suspension into a clean 250cm3 beaker, leaving the froth behind. 6 Prepare the concentrations of yeast suspension as decided in (a)(ii) and shown in Table 1.2, using the small beakers provided. 7 Label a small test-tube for each of the concentrations of yeast suspension you have prepared in step 6. 8 Put 5cm3 of H into each labelled test-tube. 9 Using the container and beakers labelled water-bath, hot water and cold water, set up and maintain a water-bath with water between 35°C and 40°C.

9744 / H2 Biology / 04 4 Fig. 1.1 shows the apparatus you will set up for the investigation. Fig. 1.1 10 Use a glass rod to stir the 10% yeast suspension, then p our the prepared 20cm3 of the 10% yeast suspension into the boiling tube. 11 Put the boiling tube in the water-bath. 12 Put the bung in the boiling tube, and make sure it covers the mouth of the boiling tube tightly. Ensure the end of the delivery tube is in H, as shown in Fig. 1.1. 13 Start timing. 14 Stop timing when the end-point is reached. The test-tube from step 4 can be used to confirm the end-point. Record the result in (a)(iii). If the time taken is more than 240 seconds, record the result as ‘more than 240’. 15 Put the contents of the boiling tube into the container labelled For waste. 16 Rinse the boiling tube. You may use water from the beaker labelled For washing. 17 Put another test-tube of H, prepared in step 7, into the apparatus shown in Fig. 1.1. 18 Repeat step 10 to step 17 for the other concentrations of yeast suspension you prepared in step 6. boiling tube

9744 / H2 Biology / 04 5 (iii) Record your results in an appropriate table. [4] (iv) Describe two significant sources of error in this investigation. [2]

9744 / H2 Biology / 04 6 (b) Baker’s yeast, Saccharomyces cerevisiae, has two metabolic pathways to produce ATP, aerobic respiration and fermentation. Oxygen is not used in fermentation. These pathways are summarised in Fig. 1.2. Fig. 1.2 Some students were researching details of the respiration of yeast. In their research, the students found information about the Crabtree effect. This effect occurs when yeast is kept in high concentrations of glucose. The yeast gains sufficient ATP from substrate-level phosphorylation during glycolysis. Yeast does not carry out the Krebs cycle or oxidative phosphorylation and therefore does not use oxygen. The students wanted to investigate the effect of temperature on the activity of yeast and decided to use yeast solutions in which the yeast obtained all its ATP by fermentation. • The students made a suspension of yeast from 1g of dried yeast and 25cm 3 of water. • The suspension was left for 2 hours at 20°C. • After 2 hours, 25cm 3 of glucose solution was added to the yeast suspension and the mixture was stirred. The students measured the volume of carbon dioxide produced by yeast fermentation.

9744 / H2 Biology / 04 7 Plan an investigation to find the temperature at which yeast carries out fermentation at its maximum rate. You can assume that you have access to all standard laboratory equipment. Your plan should: • have a clear and helpful structure such that the method you use is able to be repeated by anyone reading it • be illustrated by relevant diagrams, if necessary • identify the independent variable and dependent variable • identify the variables you will need to control • use the correct technical and scientific terms • indicate how the results will be recorded and analysed. You do not need to include details of how to make the yeast suspension or reference to any safety measures in your plan. space for diagram

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9744 / H2 Biology / 04 10 (c) A student carried out an investigation into the effect of temperature on the activity of the enzymes in Y. The student immobilised the yeast cells in alginate beads which were then dropped into a solution of the substrate at different temperatures. As carbon dioxide collected in the beads , they rose to the surface of the solution of the substrate as shown in Fig. 1.3. Fig. 1.3 The stud