VJC 2023 H2 9744 P4 Answers (Sharing)

1 Victoria Junior College Biology Department 2023 H2 Biology Prelim Paper 4 Suggested Answer Marking abbreviations: A: Accept, R: Reject, BOD: benefit of doubt, AW: alternative wording, AVP: Any valid point, NAQ: not answering question, ECF: error carried forward Question 1 (a) (i) State the volume of W that you added to the test-tube in step 16. [1] • volume recorded between 10.0 and 20.0 cm3; (ii) Record your results in an appropriate table. [4] Example: Sampling time / min Colour of bromothymol blue indicator 0 blue 1 blue 2 blue-green 3 blue-green 4 green 5 green 6 green-yellow 7 green-yellow 8 yellow 9 yellow 1. I ndependent variable on leftmost column; 2. Correct headings and no units in body of table; Column 1 heading – Accept time; Column 2 heading – Accept colour of B/ (B and W) mixture/ sample; Reject colour (observed) 3. Record using only colours from Fig. 1.1, at least 4; Reject dark green, light green 4. Trend: Colour for each minute for 10 minutes or until end-point reached for two consecutive times from left to right of colours from Fig.1.1 and time 0 is blue; (iii) This investigation used colour to indicate the concentration of carbon dioxide in the sample which is subjective. Describe two improvements to this investigation that would increase the accuracy of the results other than the use of colorimeter/ colour chart. [2] Any two from: • use smaller intervals of time e.g., 30 s to determine the end-point more accurately; • use a pH meter to determine the pH of the solution due to release of carbon dioxide which dissolves in the surrounding water; • use a carbon dioxide probe to determine the concentration of carbon dioxide released; • use set volume e.g., 0.5 cm 3 of B and/ or W instead of using drops (with a 1 cm3 micropipette or syringe);

2 (iv) A student hypothesised that glucose diffuses into the surrounding of the dialysis tube. You are provided a vial labelled R which contains the remaining contents of the boiling tube, collected after 20 minutes of the investigation. Describe and carry out an appropriate test to determine whether the student’s hypothesis can be accepted. Suggest how this would impact your results. [4] • Add 2 cm3 of sample R and 2 cm 3/ equal volume of Benedict’s solution into a boiling tube and mix well; • Put the boiling tube into a boiling water-bath for 3 minutes; • *Contents changed/ turned from a clear blue solution to a cloudy, blue mixture with suspended reddish-brown precipitate; • showing the presence of (minute quantity of) reducing sugar, thus accept/ do not reject the student’s hypothesis; • Since glucose diffuses into the surrounding of the dialysis tube, with time, less glucose is available for yeast to respire to produce carbon dioxide; • The results obtained is an underestimate of the actual carbon dioxide production from the mixture of yeast and glucose / more time is required for end-point to be reached; OR • S ince only minute quantity of glucose is present after 20 minutes, most of the glucose is available as a respiratory substrate for yeast during the duration of the experiment; • there is not much impact on the results obtained; * Without this observation, maximum is 3 marks. (b) (i) State the independent variable in this investigation. [1] • temperature; The results from the investigation at 35 °C are shown in Table 1.2. (ii) Plot a graph of the data shown in Table 1.2 on the grid in Fig. 1.4. Draw a line of best-fit. [4] 1. x- axis: time / min and y-axis: carbon dioxide production/ arbitrary unit; 2. scale on x-axis: 10 min to 2 cm, labelled every 2 cm and scale on y-axis: 1.00 arbitrary unit to 2 cm, labelled every 2 cm; 3. correct plotting of all six points using small crosses or dots in circles; 4. best-fit line drawn with equal number of points on both sides of line; (iii) Explain the trend shown by the graph in Fig. 1.4. [3] 1. C arbon dioxide production increases at a decreasing rate from 0 arbitrary unit at 0 min, reaching 3.25 arbitrary unit at 66 min; 2. I nitial increase of carbon dioxide with time is due to high concentration of substrates resulting in effective collision of enzymes with substrates, forming enzyme-substrate complexes and hence the by-product of respiration, carbon dioxide; 3. W ith increasing time, substrates are used up/ become limiting and carbon dioxide production plateaus off;

3 (iv) Use the graph in Fig. 1.4 to find the initial rate of carbon dioxide production when the yeast was incubated at 35°C. [1] Initial rate of carbon dioxide production = gradient of line drawn = 2.00 15 = 0.133 arbitrary unit / min; Question 2 (a) (i) Describe how you will make up the various chloroplast concentrations using the chloroplast suspension C and the sucrose solution S. [2] Using the 5/ 10 cm 3 syringes provided, add the stated volume of chloroplast suspension and sucrose solution and mix well: Concentration of chloroplast suspension/ % Volume of stock chloroplast suspension to used/cm3 Volume of sucrose used/ cm3 100 10.0 0.0 80 8.0 2.0 60 6.0 4.0 40 4.0 6.0 20 2.0 8.0 1m – for correct concentration (in %) and use of syringes; 1m – for correct volume used; (ii) Suggest why a dilute sucrose solution is used instead of distilled water in the above preparation? [1] • To ensure that the chloroplast remain intact/ maintain the chloroplast integrity/ does not burst;

4 (iii) Tabulate your data in the space provided. Process your data to obtain the rate of photosynthesis. Add your processed data to the table. [3] Concentration of chloroplast suspension/ % Time taken for the indicator to decolorised/ s Rate of photosynthesis/ s-1 100 10 10.0 80 25 4.0 60 43 2.3 40 166 0.6 20 More than 300 Less than 0.3/ - Rate = 100/t 1. [I] Independent variable at most left column, correct headings and units for the 3 columns 2. [T] Correct trend, raw data is to the nearest whole number in seconds 3. [R] Correct processed data, rate should be calculated using 100/t or 1000/t or presented in standard form in 1 decimal place/whole number (iv) With reference to your results, explain the effect of increasing chloroplast suspension concentration on the rate of photosynthesis. [3] 1. When chloroplast suspension increases, the rate of photosynthesis increases. [QV from the results table]; 2. In the presence of light, light dependent reaction can occur resulting in electron transfer/electron transport/flow of electrons; 3. With increasing chloroplast suspension concentration, there is more electrons flow and get accepted by the indicator/reduced the indicator, resulting in faster decolorisation; (b) (i) In the space below, sketch the graphs to show how the light compensation point of species A and B can be determined. 1. Correct axes of graph, showing both light absorbance as y axis and light intensity as x axis 2. Showing the correct light compensation point of both species A and B (B having a lower light compensation point)

5 (ii) Describe the method you will carry out to determine the light compensation point of the two species of phytoplankton A and B. You do not have to plan for repeats. [5] 1. Place 10cm3 of hydrogen carbonate indicator in 0% ND filter specimen tube and place the tube 20cm from the bench lamp 2. Add 20 alginate beads of species A into the specimen tube and turn on the lamp 3. After incubating for 10min, switch off the bench lamp and decant the solution into a cuvette 4. Blank the colorimeter with hydrogen carbonate indicator (red in colour)/distilled water 5. Using the colorimeter, measure the absorbance value of the decanted solution 6. Repeat step 1-5 with 100% ND filter specimen tube and this serves as a control to show that the colour change is due to the presence of light. 7. Repeat step 1-6 with different ND filter specim