2021 CGSS Bio Prelim P3 Answers

Page 1 of 4 Cedar Girls’ Secondary School PRELIM/EYE 2021 Answer Scheme and Markers’ Comments SUBJECT: BIOLOGY LEVEL: S4/IP4 Answers Marks 1 (a) (i) Fill large test-tube with water up to mark; Volume of water measured using measuring cylinder or syringe; 1 1 (ii) 36.0 1 (b) Diffusion; 1 (c) Small intestine/duodenum/ileum; 1 (d) solution / test tube time / s G4 / T4 G8 / T8 G12 / T12 G16 / T16 Headings – IV: solution OR test tube; DV: time / s (units included only in headings); Organisation – 1 column per variable, IV on left, DV on right; Recording – records times for G4, G8, G12 and G16 + recorded to the nearest 0.01s; 1 1 1 (e) As time increases from G4 to G16, rate of movement of glucose solution from syringe into the water in the boiling tube decreases; Time taken for first colour change to appear increases with experiment duration; Concentration gradient for glucose becomes less steep with every transfer of syringe at the end of each 2- minute interval; Rate of diffusion of glucose decreases with increasing time intervals; 1 1 1 1 max. 3m

Page 2 of 4 (f) Volume of water / glucose solution; To allow for fair comparison between the test tubes; To prove that any difference in results is caused by the difference in glucose concentration in the syringe among the 2-minute time intervals; 1 1 1 max. 2m (g) To ensure there is direct contact between the glucose solution and water so as to allow for diffusion to occur; This prevents an underestimation of the time measured / prevents time measured from being higher than expected; 1 1 (h) (i) Titles – x-axis: time / min + y-axis: rate of movement of glucose solution / au Appropriate size/scale – at least half of grid + scale is suitable (x-axis: 5 min represented by 2 cm; y-axis: 0.05 au represented by 2cm) Points – all correctly and clearly plotted; Graph – smooth best-fit straight curve + no extrapolation 1 1 1 1 (ii) shows on graph how answer is determined (line drawn at t = 5 min to find corresponding rate on y-axis); correct answer for the rate of movement of glucose solution at 5 minutes from candidate’s graph; 1 1 2 (a) petri dish colour intensity after 20 minutes A 1 control 2 or 3 1 1 *Control should show a darker colour intensity; (b) Yes; When hydrochloric acid is used to treat the apple slices, colour intensity is a lighter shade of brown compared to control; 1 1

Page 3 of 4 This is because the extreme low pH of hydrochloric acid denatures the enzyme polyphenol oxidase which is responsible for the oxidation reaction that results in the browning of the apple; 1 (c) To allow for comparison of setups with/without solution OR at different pH values to verify any change to the apple is due to the effect of pH on the oxidation reaction occurring; for a fair experiment; 1 1 max. 1m (d) Oxygen (from the air) is needed for the reaction to occur; 1 (e) 1. [IV - ID] ref to using at least three temperatures (values clearly stated); 2. [IV – how to vary] method described to maintain temperature(s) – e.g. use of thermostat / water bath to allow enzyme & substrate separately to reach the temperatures being investigated 3. [DV - ID] Measure time taken for brown colour to appear OR colour intensity value reached within a set duration of 20 min 4. [DV – how to measure] by using a stopwatch / checking in at fixed time intervals (e.g. every 5 min) OR by observing after a fixed duration of 20 min 5. [CV – ID] controlled variables any one from: same concentration of enzyme / same concentration of substrate / same volume of enzyme / same volume of substrate / constant OR optimum pH; 6. [Ensuring reliability] repeat (at least) twice; 7. [Answering experiment aim] ref to substrate at optimum temperature turning brown first or having the highest colour intensity value; 1 1 1 1 1 1

Page 4 of 4 1 max. 5m (f) (i) Size – at least 2/3 of space provided Lines – clean unbroken lines; no shading Accuracy – resembles specimen + cut surface (longitudinal section) shown Proportion – in terms of size and numbers 1 1 1 1 (ii) line drawn in widest position across drawing + correct measurement recorded to nearest whole number; Working – Length of drawing (mm) / length of actual specimen (mm); Answer – ‘x’ symbol shown + calculated correctly + recorded to nearest whole number 1 1 1 End of Markers’ Report