2025 RI H2 Bio Prelim P4 Answers

©RI 2025 Preliminary Examination 9744/04 [Turn over RAFFLES INSTITUTION 2025 Year 6 Preliminary Examination Higher 2 BIOLOGY 9744/04 Paper 4 Practical 22nd August 2025 2 hours 30 minutes Candidates answer on the Question Paper. Additional Materials: As listed in the Confidential Instructions. (a) Before you proceed, read carefully through the whole of Question 1(a). You are provided with a blue solution, labelled I, which is alkaline. I is blue because it contains an indicator. Carbon dioxide reacts with I when bubbled into it. When enough carbon dioxide reacts with I , the indicator will change from blue to yellow (even if the solution is cloudy). This is the end-point. Hydrochloric acid, H, can also turn indicator I yellow. If only a small volume of carbon dioxide is bubbled into I then the indicator will remain blue. Hydrochloric acid, H, can then be added slowly until the indicator turns from blue to yellow. (i) What is the expected trend of volume of H that needs to be added to reach end point with increased volume of carbon dioxide that has reacted with indicator, I. [1] The greater the volume of carbon dioxide that has reacted with I, the lower volume of H needed to be added to reach the end-point; You are provided with: labelled contents YA yeast cell suspension I alkaline solution (blue) H hydrochloric acid CIVICS GROUP CANDIDATE NAME INDEX NUMBER 5 S 0 3 2

2 ©RI 2025 Preliminary Examination 9744/04 For Examiner’s Use I and H are irritants. Suitable eye protection should be worn. If I or H comes into contact with your skin, wash it off immediately under tap water. You are first required to find the volume of H needed to reach the end- point, when no carbon dioxide has been bubbled into I. Proceed as follows: 1. Put 5 cm3 of I into a test-tube. 2. Use a syringe, containing 2 cm3 of H, to put drops of H into I as shown in Fig. 1.1. Mix well as you add H, until the end -point is reached. You may need to fill the syringe again. Fig 1.1 (ii) Record the volume of H needed to reach the end-point. volume of H 1.6 – 2.0 cm3 [1] records volume of H to 1d.p or 2 d.p + cm3 + accuracy Yeast cells release carbon dioxide from some of their metabolic reactions.

3 ©RI 2025 Preliminary Examination 9744/04 [Turn over For Examiner’s Use A student investigated the release of carbon dioxide from a yeast cell suspension, using the apparatus shown in Fig. 1.2. Fig 1.2 The student set up this apparatus and left it for 10 minutes. The student observed that during the 10 minutes the bubbles of carbon dioxide were not released at a constant rate. (iii) Suggest an explanation for the student’s observation on the rate of release of carbon dioxide. [1] 1. pH / temperature was not kept constant; 2. Depletion of respiratory substrate over time, hence concentration of respiratory subtrate not the same; 3. Yeast activity / numbers of yeast decreases / increases over time; 4. AVP e.g. O2 depletion (Since boiling tube is airtight, concentration of O2 will decrease as it is being used in respiration, so rate of respiration will decrease, which causes rate of CO2 to decrease as well.) You are required to investigate this observation using the apparatus set up as in Fig. 1.2. You will move the delivery tube to different test -tubes containing I, at different times during the 10 minutes. (iv) Decide the length of time you will leave the delivery tube in each test-tube containing I. State the length of time. time 2 minutes; [1] [Marker to accept: every 2 min / at 2, 4, 6, 8 + 10 minutes] (v) State one significant source of error that may occur when the delivery tube is moved from one test-tube to the next. Describe how you will reduce this error. [2] Source of error [1] 1. end of delivery tube different level in each test-tube; 2. time to transfer delivery tube different each time;

4 ©RI 2025 Preliminary Examination 9744/04 For Examiner’s Use 3. loss of CO2 from delivery tube during transfer; R : colour determination of end-point by naked eye / temp not maintained constant / lack of equilibration time for the setup; Description [1] 1. constant level: mark delivery tube or test-tube/ mtds to ensure test-tubes place at constant level; 2. time in I constant: keep clock running, record time delivery tube in I / mtds to ensure time in I is constant (e.g. pause & reset the stopwatch during trf); 3. no loss of CO2 during trf : seal end of delivery tube during transfer; Read steps 3 to 10 before proceeding: 3. Put 5 cm3 of I into a test-tube. Repeat for the number of test -tubes you need to use for your times in (a)(iv). 4. Obtain YA by placing the nozzle of a large syringe below any froth on the surface. 5. Put 20 cm3 of YA into the large test-tube. 6. Put the bung containing the delivery tube into the large test-tube. It must be airtight. 7. Put the end of the delivery tube into the first test -tube containing I. Immediately start timing. 8. After the time you decided in (a)(iv), move the delivery tube to the next test -tube containing I. 9. Repeat step 8 for all the test-tubes which you set up in step 3. 10. After removing the delivery tube from the last test-tube, repeat step 2 with any of the test tubes where there is still a blue colour. Unplug the rubber bung from the large test tube after step 10 is completed. Record your results in (vi). For any test-tubes where I is yellow (even if the solution is cloudy) record ‘0’. (vi) Prepare the space below to record your results. [4] Time / min Volume of H required to reach end- point / cm3 2 4 6 8 10 1. Organised into table all columns separated by a line; 2. Headings (top or to left of data) time / minutes (A: seconds) + volume of H to reach end-point / cm3; 3. Records all volumes less than volume recorded in (a)(ii) and cannot be 0 cm3; 4. All recorded results to 1 d.p or 2 d.p; I: processed data, number of bubbles etc. A: trend with decreasing vol or same vol or no specific trend.

5 ©RI 2025 Preliminary Examination 9744/04 [Turn over For Examiner’s Use (vii) Explain whether your results in (vi) support the student’s observation. [1] Yes, it supports the student’s observation because the volume of H across different timepoints was different / increasing / decreasing; OR No, it does not support student’s observation because volume of H across different timepoints was the same/ (relatively) consistent / did not differ significantly ( ± 0.1cm3); (b) Before you proceed, read carefully through the whole of Question 1(b). Yeast cells also contain enzymes that hydrolyse sucrose into reducing sugars, as shown in Fig. 1.3. enzymes in yeast cells sucrose reducing sugars Fig.1.3 You will investigate the activit