NJC 2023 SH2 H2 Biology Paper 4 Mark Scheme

© NJC 20239744/04/SH2/H2 Biology/Preliminary Examination[Turn over H NATIONAL JUNIOR COLLEGE, SINGAPORE Senior High 2 Preliminary Examination Higher 2 CANDIDATE NAME BIOLOGY CLASS 2bi2_____ REGISTRATION NUMBER Biology Paper 4 Practical 9744/04 17 August 2023 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 CM43 CM44 For Examiner’s Use 1 19 2 17 3 19 Total 55

2 © NJC 20239744/04/SH2/H2 Biology/Preliminary Examination This document consists of 18 printed pages and 2 blank pages.

3 © NJC 20239744/04/SH2/H2 Biology/Preliminary Examination[Turn over Answer all questions. 1 Baker’s yeast, Saccharomyces cerevisiae, converts sugars to ethanol and carbon dioxide under anaerobic conditions. You will investigate the effects of different concentrations of ethanol on the rate of respiration in yeast. (a) Describe and explain the expected effect of increasing ethanol concentration on the rate of respiration in yeast. As the concentration of ethanol increases, the rate of respiration decreases; Ethanol is organic and thus dissolves and disrupts phospholipid bilayer; This destroys the integrity of membrane essential for respiration, namely the inner mitochondrial membrane; Membrane proteins such as electron carrier and ATP synthase on the inner mitochondrial membrane become non-functional; Enzymes are also denatured by the high ethanol concentration by the disruption of hydrogen bonding or hydrophobic interactions; High concentration of ethanol inhibits anaerobic respiration / leads to end product inhibition thus there is a decrease in CO2 released; R: Change in pH, decrease in frequency of effective collision (ethanol is not within the cell) [4] You will set up the apparatus as shown in Fig. 1.1. Fig. 1.1 rubber tubing yeast, sucrose, and water and/or ethanol

4 © NJC 20239744/04/SH2/H2 Biology/Preliminary Examination You are provided with: ● 5.0 g dried yeast Y, in a container labelled Y ● 10.0 g sucrose S, in a container labelled S ● 120 cm3 5.0 % ethanol E1, in a container labelled E1 ● 120 cm3 10.0 % ethanol E2, in a container labelled E2 ● 200 cm3 distilled water W, in a beaker labelled W E1 and E2 contain ethanol, which is harmful and flammable. Suitable eye protection should be worn. The lid on the plastic vial should be kept on, when not in use. Read steps 1-11. Proceed as follows. 1 Set up a water bath and maintain it at about 37 °C. 2 Measure 100 cm3 of W and pour it into a conical flask. 3 Weigh 1.0 g of Y and 2.0 g of S and add them to the conical flask. Shake to mix the content. 4 Incubate the conical flask in the water bath for 5 minutes. 5 Fill the burette with water. Invert the burette and clamp it on a retort stand, as shown in Fig 1.1. 6 Attach the rubber bung (with the rubber tubing) to the conical flask. 7 Insert the rubber tubing through the open end of the burette. Ensure that the apparatus is set up as shown in Fig. 1.1. 8 After 5 minutes of incubation, note the initial volume of water in the burette. 9 Measure the volume of carbon dioxide given off for 8 minutes by the amount of water displaced. 10 Repeat steps 1-9 using 100 cm3 of E1. 11 Repeat steps 1-9 using 100 cm3 of E2.

5 © NJC 20239744/04/SH2/H2 Biology/Preliminary Examination[Turn over (b) Record your results in an appropriate table. Concentration of ethanol / % Volume of carbon dioxide given off in 8 minutes / cm3 H – Headings with units; P – Precision of data (Vol. of CO2 at 1 or 2 d.p.); T – Trend of data (Decrease in CO2 with increase in ethanol concentration); 0 10.0 5 2.7 10 0.1 [3] (c) Use the grid provided to display your results. G – Graph: Dot-to-dot plot (R: Histogram); A – Axes with labels and units; P – Points are all plotted accurately; S – Scale of the graph should at least 2/3 of space provided; Examiner’s comments: A number of candidates did not recognise that this experiment is to investigate the effects of differe nt concentrations of ethanol on the rate of respiration in yeast. As such, water should be represented as 0% with respect to the concentration of ethanol. The graph is not a histogram depicting categorical or discrete data. A few candidates drew best fit line / curve instead. There are also some students who lost marks on scale, plots or axes with units.

6 © NJC 20239744/04/SH2/H2 Biology/Preliminary Examination (d) Without quoting numerical values, explain how this experiment allows you to monitor respiration in yeast. In link reaction and Kreb’s cycle of aerobic respiration / alcoholic fermentation of anaerobic respiration, CO2 is released; CO2 produced from the conical flask mixture is passed through the rubber tubing and collected in the burette; The increase in CO2 within the burette leads to water being displaced, thereby allowing monitoring of respiration over time; Therefore, increase in volume of CO 2 produced indicates an increase in rate of respiration; [3] (e) Describe two main sources of error in the procedure and discuss how they may reduce the confidence in your results recorded in (b). The water bath temperature is difficult to maintain at 37°C; As respiration is an enzyme-catalysed reaction and enzyme is sensitive to temperature, the varying temperature may affect the rate of reaction. The rate of reaction is not constant and thus the final volume of CO2 collection is not accurate; The release of CO2 is measured by the displacement of water in the burette, however, this is not accurate as some CO2 may dissolve in water; Thus, the reading taken is not an accurate reading of the actual volume of gas produced; AVP (precision of scale, wind affecting reading of scale) R: Under-estimation of CO2 produced due to incubation time; Air bubbles have different volumes; measuring cylinder is not a precise instrument; human errors such as uneven mixing of sucrose and yeast in solution or escape of CO 2 from the conical flask with rubber bung; Examiner’s c omments: When describing effects of temperature affecting rate of reaction, some wrote about increase in temperature which is not possible as there is only heat loss to the environment. Some candidates also included improvement which is not a requirement of the question. In addition, human error should be avoided. [4] (f) Suggest a negative control for this experiment to prove the action of yeast in converting sugars to ethanol and carbon dioxide. Replace Y with boiled yeast where the enzymes are denatured. R: Use of distilled water, powder / sand / particles [1] [Total: 19]

7 © NJC 20239744/04/SH2/H2 Biology/Preliminary Examination[Turn over 2 Fig. 2.1 shows Elodea canadensis (Canadian pondweed) which is an aquatic plant commonly used in aquarium tanks to help control algae and keep the water clear. Fig. 2.1 Fig. 2.2 shows how Elodea form dense mats on the surface of water in an aquarium tank. Fig. 2.2 Studies