2024 NJC H2 Bio P4 MS

© NJC 20249744/04/SH2 Preliminary Examination[Turn over H NATIONAL JUNIOR COLLEGE, SINGAPORE Senior High 2 Preliminary Examination Higher 2 CANDIDATE NAME MARK SCHEME BIOLOGY CLASS 2bi2_____ REGISTRATION NUMBER Biology Paper 4 Practical 9744/04 26 August 2024 2 hours 30 minutes READ THESE INSTRUCTIONS FIRST Write your name, Biology class, and registration number on all the work you hand in. Give details of the practical shift and laboratory in the boxes provided. Write in dark blue or black pen on both sides of the paper. You may use a soft pencil for any diagrams, graphs. Do not use staples, paper clips, highlighters, glue or correction fluid. 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 workings or if you do not use appropriate units. At the end of the examination, fasten all your work securely together. The number of marks is given in the brackets [ ] at the end of each question or part of question. Shift 1 2 3 Laboratory BI23 BI24 CM42 CM43 CM44 For Examiner’s Use 1 20 2 20 3 15

2 © NJC 20249744/04/SH2/H2 Biology/Preliminary Examination Total 55 This document consists of 19 printed pages and 1 blank pages.

3 © NJC 20249744/04/SH2/H2 Biology/Preliminary Examination Answer all questions. 1 Yeast can convert different respiratory substrates into carbon dioxide via cellular respiration. You will investigate the effect of different respiratory substr ates on the rate of respiration in yeast. Hydrogencarbonate indicator is a pH indicator which can be used to measure the amount of carbon dioxide produced by the respiring yeast cells. When carbon dioxide dissolves in water, carbonic acid is formed, reducing the pH and changing the colour of the indicator solution. The initial pH of the reaction mixture containing yeast and respiratory substrate may be slightly different depending on the type of respiratory substrate. This can be standardised by using sodium bicarbonate solution to adjust pH so that the starting c olour corresponds to the colour number 6 on the hydrogencarbonate indicator colour chart provided. You are provided with: ● yeast suspension, labelled Y ● alginate solution, labelled A ● calcium chloride solution, labelled C ● respiratory substrate 1, labelled S1 ● respiratory substrate 2, labelled S2 ● respiratory substrate 3, labelled S3 ● hydrogencarbonate indicator, labelled H ● sodium bicarbonate solution, labelled B ● hydrogencarbonate indicator colour chart Read steps 1–12 before starting the investigation. Proceed as follows. 1 Add about 30cm 3 of C into a 50cm3 beaker. 2 Stir Y to suspend the yeast cells. Transfer 3.0cm3 of Y into another 50cm3 beaker. 3 Add 6.0cm3 of A into the beaker containing Y, taking care not to introduce air bubbles into the mixture. Stir the resulting yeast-alginate mixture gently using a glass rod. Do not mix vigorously as this may introduce air bubbles into the mixture. 4 Remove the plunger of a 5.0cm 3 syringe. Hold the empty syringe barrel with the nozzle facing down above the beaker containing C, as shown in Fig. 1.1. 5 Pour the yeast-alginate mixture prepared from step 4 into the syringe barrel and allow the yeast-alginate mixture to drip into C, as shown in Fig. 1.1. As it drips, gently swirl the beaker to prevent the beads from aggregating with each other. The mixture will form a bead upon contact with calcium chloride.

4 © NJC 20249744/04/SH2/H2 Biology/Preliminary Examination Fig. 1.1 6 Transfer only the yeast-alginate beads into a Petri dish. Rinse the beads with distilled water. Keep the yeast-alginate beads in distilled water. Remove and discard any bead that is obviously different in size, distorted in shape, or floating. You will need to use 15 yeast-alginate beads for Question 1. Keep 25 yeast-alginate beads for Question 2. 7 Add 2.0cm3 of S1 and 0.2cm3 of H into a test tube. Mix the tube contents well. 8 Check if the colour of the mixt ure matches colour 6 on the hydr ogencarbonate indicator colour chart. If not, using a 1cm 3 Pasteur pipette, add B dropwise to the mixture until colour 6 is achieved. 9 Add five yeast-alginate beads into the test tube. You should en sure that as little distilled water is transferred along with the beads as possible. Start timing immediately. 10 At the end of three minutes, mix the tube contents well. 11 Record, in (a)(i), the final colour of the mixture, using the colour number whic h it matches with on the colour chart. 12 Repeat steps 7–11 with each of the other respiratory substrates, S2 and S3.

5 © NJC 20249744/04/SH2/H2 Biology/Preliminary Examination (a) (i) Record your results in an appropriate table. respiratory substrate final colour number of the mixture S1 1 S2 1 S3 5 Headings; (IV) respiratory substrate (DV) final colour number of the mixture Trend; S3 has the highest colour number (6 or below) Complete; 3 final colours recorded in the table [3] (ii) All three respiratory substrates, S1, S2 and S3, are carbohydrates but only one of them is a polysaccharide. Based on your results obtained in (a)(i), put a tick ( ✓) in the appropriate box to indicate the respiratory substrate that is a polysaccharide and explain your answer. S1 (sucrose) S2 (glucose) S3 ✓ (starch) explanation Compulsory: smallest decrease / change in colour number / pH in the presence of S3; OR least amount of carbon dioxide produced by the respiring yeast cells in the presence of S3;

6 © NJC 20249744/04/SH2/H2 Biology/Preliminary Examination Enzyme: Yeast lacks the enzyme to utilise the polysaccharide as a respi ratory substrate; OR The amount of yeast enzyme that can break down S3 for use in respiration is lower than the amount of yeast enzyme that can break down S1/S2 for use in respiration; OR Longer time needed for yeast enzyme to break down S3 for use in respiration; [3] (iii) Describe a suitable control for this investigation and explain its purpose. control purpose In step 2, replace the yeast suspension with equal volume (3.0cm3) of boiled and cooled yeast suspension while keeping all other experimental conditions the same; to show that the change in colour number / pH of the mixture containing the yeast cells and the respiratory substrate is due to the carbon dioxide produced by the respiring yeast cells; In step 7, replace the respiratory substrate with equal volume (2.0cm3) of distilled water while keeping all other experimental conditions the same; to show that the change in colour number / pH of the mixture containing the yeast cells and the respiratory substrate is due to the presence of the respiratory substrate; [2] ( i v ) Identify one significant source of error in this investigation and suggest an improvement to the procedure that will reduce the effect of the error. error improvement Determination of final colour of the mixture by comparison with the colour chart is subjective; Use a colorimeter / spectrophotometer / carbon dioxide sensor / pH meter / pH probe + datalogger / describe different set up + count the number of bubbles produced in one minute; Size of yeast-alginate beads may vary; Use a syringe to add known volume of the yeast suspension directly to the tube containing the respiratory substrate;

7 © NJC 20249744/04/SH2/H2 Biology/Preliminary Examination Ambient temperature may fluctuate; Use a thermostatically controlled water bath; [2] (b) Microalgae synthesise various carbohydrates to support cellular functions. Cellulose and starch are the most abundant polysaccharides found in microal