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 h