MGS Markers Report of 2022 Sec 4 Biology Prelim Paper 2 for students

Marker’s report for 2022 Prelim Paper 2 Section A 1 Fig. 1.1 shows a Stentor, a unicellular organism that lives in freshwater. Fig. 1.1 Due to the difference in the concentration of salt inside the cell and the surrounding freshwater, the Stentor absorbs water from its surrounding, then discharges it by regular contractions of the contractile vacuole. (a) Explain the process of how water enters the Stentor. Water molecules enter the cell through osmosis by passing through the partially permeable membrane of the cell membrane; The surrounding freshwater has a higher water potential than the water potential of the contractile vacuole / cytoplasm inside the cell; [2] Common errors were not stating partially permeable membrane in their answers and still using terms such as concentration gradient for movement of water. Majority were able to distinguish between which region was of a higher or lower water potential. (b) Explain the importance of controlling the water potential of the Stentor living in a freshwater environment. The Stentor needs to maintain the water potential of water in it under control OR If water potential of freshwater env. is too high, water molecules will enter the Stentor; Stentor would expand/ swell and eventually burst; [2] contractile vacuole at early stage of filling feeding funnel contractile vacuole almost ready to discharge its contents

Page 2 of 23 Methodist Girls’ School Biology Paper 2 Sec 4 Preliminary Examination 2022 Some did not read the question carefully and gave answers of Stentor gaining and losing water. Many used terms such as flaccid/ plasmolysed/turgid to describe the Stentor but it is not a plant cell. (c) The Stentor was taken from its freshwater habitat and placed in a saltwater aquarium. Explain what will happen to the Stentor in the saltwater aquarium after 2 days. Water molecules will move out of the cytoplasm of the Stentor into the saltwater + by osmosis; It would shrink / crenated (eventually dying). [2] Many gave answers such as Stentor dying without stating that it would shrink first, before proceeding to mention that it will die. [Total: 6]

Page 3 of 23 Methodist Girls’ School Biology Paper 2 Sec 4 Preliminary Examination 2022 protein- based jelly 2 Certain fruits contain biological molecules that prevent the solidification of a protein - based jelly when it is cooled. Protein-based jelly exits in liquid state at high temperature but becomes solid when it is cooled. An experiment was conducted to investigate the presence of these biological molecules in fruits. The protein -based jelly was prepared by mixing jelly powder with warm water. The mixture was poured into 5 bowls each with different contents as shown in Fig. 2.1. The bowls were left in the refrigerator to chill for 3 hours. no fruits pineapple slice banana slice crushed pineapple crushed banana Fig. 2.1 Table 2.1 shows the texture of the jelly after being chilled in the refrigerator for 3 hours. Table 2.1 Bowl texture of jelly 1 solid 2 semi-solid 3 solid 4 liquid 5 solid (a) Explain the purpose of Bowl 1. Bowl 1 is a control set up to show that the fruits contains a biological molecule / enzyme that prevented the jelly from solidifying. Most could not go beyond saying that it serves as a control setup / expt. you need to say that it shows that something in the fruits prevented the solidifying. Control = without the treatment (no fruits) Experimental = with treatment (with fruits) (b) Explain the observation for the different textures of jelly between Bowls 2 and 4. SAV Crushing the pineapple increased the surface area to volume ratio / More exposed surface area between the crushed pineapple and jelly ; R This increased the rate of diffusion of the biological molecule out of the fruit ; Bowl 1 Bowl 2 Bowl 3 Bowl 4 Bowl 5

Page 4 of 23 Methodist Girls’ School Biology Paper 2 Sec 4 Preliminary Examination 2022 Many could not relate the increased rate of diffusion of the molecules out of the crushed pineapple. They jumped from increased SA/vol ratio to the inability to solidify [2] (c) The jelly powder used in this experiment is made up of fibrous proteins in the form of long-chain polypeptides. An analysis of the proteins in Bowl 2 shows that it contains a mixture of polypeptides of 2 different lengths. 50% of the polypeptides are of t he original length, and the other 50% of polypeptides are one-third of the original length. (i) Explain the presence of 50% of polypeptides which are one -third of the original length. The enzymes / protease in pineapple digested the polypeptide. ; need to use the correct terms - digest or break down some lost the mark by not being able to identify the biological molecules as enzymes or protease (ii) Describe the analysis of the proteins in Bowl 4. Bowl 4 contains more than 50% of the shorter polypeptide / 1/3 length of original polypeptide; (Reject: polypeptides shorter than 1/3 of original length) Most said it is 100%. In fact it can be between 51 - 100%. (d) State the conclusion that can be made from the results of Bowls 1, 2 and 3 in Table 2.1. Pineapple contains an enzyme / protease that prevented the jelly from fully solidifying, while lacking in bananas Mostly correct. Some reversed the conclusion (banana contains the enzyme that enabled solidifying. Determine the aim of the experiment to get the correct conclusion. [Total: 6]

Page 5 of 23 Methodist Girls’ School Biology Paper 2 Sec 4 Preliminary Examination 2022 3 An experiment was conducted to investigate the effect of different colours of light on the rate of photosynthesis. Fig. 3.1 shows the set up for the experiment. Fig. 3.1 A healthy stalk of Hydrilla was placed in a beaker of 5% sodium hydrogen carbonate solution. A light bulb giving out red light was used, and volume of oxygen collected over five minutes was recorded. The experiment was repeated using lig ht bulbs giving out green, blue, and yellow light. All light bulbs used are of the same power. The results are presented in Table 3.1. Table 3.1 colour of light bulb volume of oxygen collected in 5 minutes / cm3 red 4.8 green 1.2 blue 4.2 yellow 2.0 (a) Write the balanced chemical equation for photosynthesis. Many did not include the conditions light 6CO2 + 12H2O → C6H12O6 + 6O2+ 6H2O chlorophyll [1] oxygen 5% sodium hydrogen carbonate solution Hydrilla red light bulb

Page 6 of 23 Methodist Girls’ School Biology Paper 2 Sec 4 Preliminary Examination 2022 (b) Algae are aquatic micro-organisms that are capable of photosynthesis. They grow quickly in a fish tank and stick to the side of the tank, making it difficult to see the fish. (i) Based on the results of the experiment, state the colour of light that would best be used to minimise the growth of algae. Green Colour [1] Almost all got this correct (ii) Explain your choice of colour. the rate of photosynthesis is the lowest / slowest leading to the slowest growth rate) at this colour / green colour Mostly correct - need to link the smallest volume of oxygen to the rate of photosynthesis [1] (c) The rate of photosynthesis is affected by the colour of light. The colour of light is a limiting factor. Using the colour of light, explain the term limiting factor. Very badly answered The rate of photosynthesis will increase or decrease when there is a change in the colour of light used.; OR A specific coloured light / (wavelength of the light)