2023 RI Prelim H2 Biology P4 Answers

H2 Biology ● RI Prelim 2023 PAPER 4 (a) You will need to carry out a serial dilution of the 10.00% protein solution, P, to reduce the concentration by half between each successive dilution. You will need to prepare four concentrations of protein solution in addition to the 10.00% protein solution, P. After the serial dilution is completed, you will need to have 5 cm3 of each concentration available for use. (i) In the space below, draw a table to show how you will prepare the serial dilution. [3] Final concentration of protein solution/% Concentration of protein solution used/% Volume of protein solution used/cm3 Volume of W/water used/cm3 10.00 10.00 10.0 0.0 5.00 10.00 5.0 5.0 2.50 5.00 5.0 5.0 1.25 2.50 5.0 5.0 0.63 1.25 5.0 5.0 1. C – Headings and units with correct final concentration of protein solution and concentration of protein solution used; 2. V - correct volumes of protein solution and water (W) used; 3. P - concentration recorded to 2 dp and volume recorded to 1 dp ; (ii) Record your results in an appropriate table. You may use the same symbols for more than one test-tube. [2] protein concentration/% symbol 10.00 +++++ 5.00 ++++ 2.50 +++ 1.25 ++ 0.63 + 0.00 - 1. H - correct heading with units for independent variable (protein concentration/%) correct heading for dependent variable (symbol); A: colour intensity, colour after 1 min, etc (instead of symbol) in heading

R: concentration of P in heading 2. P - records readings for all concentrations with symbols with decreasing trend; (iii) Fig. 1.1 shows a scale of protein concentrations used in this investigation. The position for 0.00% and 10.00% are shown on the scale. Complete the scale in Fig. 1.1 by showing the positions of the protein concentrations you prepared in step 1. [1] 0.00% 10.00% correct positions of protein concentrations; (iv) Use your results in (a)(ii) and (a)(iii) to estimate the protein concentration of U, X and U+X. Show your estimates on Fig. 1.1 by drawing labeled arrows (↓) at the correct positions on the scale. [2] 1. U correctly labeled at 1.25%<U<2.5% A: U=1.25 or U=2.5 R: if U is greater than 2.5 2. Both X and U+X labeled at 0%<X<0.63% and 0%<U+X<0.63% R: if X or U+X is pointing to 0 (v) Using your result in (a)(iv), estimate the concentration of protein in U over a period of 24 hours and state the possible medical condition of the patient indicated by U. Show your working in the space provided. [2] 1. Correct calculation 10.00% ➔ 1000mg 2.00% ➔ 1000/10 X 2 = 200 mg (A: range of values between 125 to 250mg) (ECF: if U in a(iv) is incorrect) 2. Correct medical condition - Urinary tract infection or kidney tubular disease (vi) Solution X was extracted from the human gastrointestinal tract. Suggest the identity of solution X and explain the basis of your suggestion. [3] 1. X is an enzyme/protease/pepsin/peptidase. (A gastric juice containing protease) 2. which digests the proteins into amino acids; 3. Thus after X was added the concentration of U decreased but it did not reach 0 due to the presence of X which is an enzyme that is protein in nature and hence a slight purple colouration was noted when the biuret test was conducted on both U+ X and X. (b) (i) Outline how a colorimeter is set up so as to obtain a correct measurement of the colour intensity of a coloured solution placed in the cuvette. [2] any two from: X U + X U (2%) 0.63% 1.25% 2.50% 5.00%

1. set wavelength to use; 2. set colorimeter (absorbance) to zero/calibrate; 3. using water (to calibrate colorimeter); (ii) State the limitation of the experiment described in part (a), that can overcome by the use of the colorimeter. [1] 1. human perception/interpretation of colour intensity is subjective and this affects the determination of the colour intensity; 2. colorimeter enable objective, numerical determination of colour intensity; R: determine colour (iii) Plot a calibration curve using the data given, on the grid in Fig. 1.3. Fig. 1.3 [4] 1. Scale : 2/3 of graph provided 2. Axes : correct with units (x-axis: protein concentration/% and y-axis: absorbance/ arbitrary units) Penalise for: No intervals markings, No 0.0 (origin) 3. Pts : all 5 points correctly plotted using X 4. Line : smooth curve/line passes all pts Penalise for extrapolation

(iv) Describe how the student could modify the previous experiment to determine the concentration of protein in sample R. Do not repeat any detail of how the standard protein solutions are prepared, the biuret test and how the colorimeter would be used. Your method should be set out in a logical way and be detailed enough to let another person follow it. [5] 1. Using a 10cm3 syringe, add 5 cm3 of distilled water to 5 cm3 of sample R; This is a 2x dilution. (compulsory point) A other dilutions e.g. 1 in 2, 1 in 4, etc (must match with pt 5) 2. Conduct biuret test on the diluted sample R from step 1; 3. Transfer the diluted sample R after the biuret’s test into a cuvette, measure the absorbance of the solution using the colorimeter; 4. Use the absorbance of the diluted sample R, read off the calibration/standard curve to find the concentration of protein; 5. Multiply the concentration of protein by the dilution factor of 2 (must match with point 1) to obtain the protein concentration in undiluted sample R; [Total: 25] 2(a) Compare the morphology of W1 and W2 in the space below. [3] Differences Point of comparison W1 W2 1 number of seed There are less seeds. There are more seeds. 2 hair on skin There are less hair on skin/shorter hair on skin. R no hair A smoother There are more hair on skin/ longer hair on skin. 3 colour of flesh The flesh is yellow. The flesh is green. 4 size of middle core The middle core is smaller. The middle core is larger. 5 shape of core/middle The shape is circular. The shape is oval. AVP colour of skin The skin yellowish brown in colour. The skin darker brown in colour. AVP shape of cross section of fruit The shape is circular. The shape is oval. Max 2 Similarities: 6 Both have black seeds/flesh/skin; 7 Both have a pale middle core. 8 Both have hair.

9 Both have brown skin. 10 Both have numerous seeds arrange around the centre core/middle of the fruit Max 2 (b) (i) Cut a 5mm thick transverse slice of specimen W1 on the cutting board. The slice should contain a full complement of seeds and should not be taken from the extreme ends of the fruit. Blot dry the cut surface with a paper towel. Cover one cut surface of the slice with bromocresol green, G. Leave it to stain for one minute. G stains proteins blue. In the space provided, draw a large, detailed diagram of the cut surface of the slice as shown by the shaded area in Fig. 2.1. You are required to use a sharp pencil for drawings. Your drawing should show details of the arrangement of different regions and their correct shapes and proportions. Indicate where the highest concentration of protein is present. [4] Fig. 2.1 1. Scale : detailed diagram of appropriate size (>3/4 space given) + no shading; 2. Proportion: Correct proportion of layers including correct proportions of core compared to the diameter of the fruit, and skin, size of the seeds; 3. Accuracy : skin, seeds, locules + correct section drawn; R if draw LS 4. Annotations : blue green core/locules indicating highest concentrations of proteins present;

(ii) You are required to estimate the total number of seeds that were present in the fruit from which W1 was taken. • Cut the 5mm slice of W1 into equal qua