2022 TJC JC2 H2 Biology Prelim P4 ANS

TEMASEK JUNIOR COLLEGE PRELIMINARY EXAMINATION PAPER 4 ANSWER 1 Antibiotics act on bacteria by killing the bacteria or preventing their growth. Their effectiveness can be tested by setting a model. In this model, an acid represents the antibiotic solution and the blue stain in the agar block represents the bacteria. You are to investigate the effect of the antibiotic solution on ‘killing the bacteria’. This is shown by the colour of the stain changing from blue to yellow as the end-point. You are required to: • prepare different concentrations of antibiotic solution, A, using serial dilution. • record the time taken to reach the end-point (yellow) for each of the concentrations of A. • record the time taken to reach the end -point for an unknown concentration of antibiotic solution X. • use the results to estimate the concentration of antibiotic in X. You are provided with: Labelled Contents Hazard level Volume / cm3 X Unknown concentration of an antibiotic solution Irritant 30.0 A 1% antibiotic solution Irritant 50.0 W Distilled water None 100.0 B Agar block containing a blue stain None - You are advised to wear suitable eye protection. If antibiotic solution comes into contact with your skin, wash off with cold water. (a) You are required to make a serial dilution of the 1% antibiotic solution, (IV) A, which reduces the concentration by half between each successive dilution. You need to prepare 40.0 cm3 of each concentration. Note: Refer to Fig. 1.1 for the clue. Fig. 1.1 shows the first two beakers you will use to make your serial dilution. (i) Complete Fig. 1.1 by drawing as many extra beakers as you need for your serial dilution. Interpreting the meaning of the statements 1. “You will need to prepare 20.0 cm3 of (named) solution to use.” (This means the FINAL volume. Refer to Fig. 1.1.) 2. You need to prepare 40.0cm3 of each solution. Some of this will be used to make the next solution. (This means the volume is NOT the final volume.)

For each beaker: • state, under the beaker, the volume and concentration of the antibiotic solution available for use in the investigation. • use one arrow, with a label above the beaker, to show the volume and concentration of antibiotic solution added to prepare the concentration. • use another arrow, with a label above the beaker, to show the volume of W added to prepare the concentration. Fig. 1.1 [3] Mark scheme: 1. Correct concentrations of 0.5%, 0.25%, 0.125%, 0.0625% 2. Shows transfer of 20.0 cm3 of 1% to next dilution + 20.0 cm3 transferred from 2nd to 3rd beaker, and from 3rd to 4th beaker and from 4th to 5th beaker. Reject if units not provided. 3. Adds 20.0 cm3 of water to each beaker. Reject if units not provided. Max 2 if precision is incorrect. Feedback: • Most candidates did well for this question. Only a few candidates did not consider the precision of the apparatus (10mL syringe) for the volume of solution or water.

Proceed as follows: 1. Prepare the concentrations of A as shown in (a)(i). 2. Label a beaker as X and put 20.0 cm3 of X into this beaker. You will need to cut the agar block, B, into smaller pieces as shown in Fig. 1.2. To avoid staining your skin, minimise touching the agar with your bare hands. You may use the blunt forceps and paper towels to handle the agar. Fig. 1.2 3. Place the agar block, B, onto a white tile and cut into identical pieces, each 5 mm x 5 mm as shown in Fig. 1.2. You do not need to adjust the height. 4. (ii) Calculate the total surface area of one agar piece. Show all the steps in your calculation, including the appropriate units. Height of agar piece = 1 cm = 10 mm Total surface area of one agar piece = (4 x 10 mm x 5 mm) + (2 x 5 mm x 5 mm) [1] = 200 mm2 + 50 mm2 = 250 mm2 [1] [2] Mark scheme: Working shown [1 mark] Correct with appropriate units (mm2) [1 mark] Feedback: • Not all candidates realize that there were 4 identical sides (10 mm x 5 mm) and 2 identical top and base surface of the agar block (5 mm x 5 mm). • A few candidates also mixed up the units for volume (mm3) and surface area (mm2). 1cm 10 mm

5. Put one piece of agar into each beaker containing the each concentration of A prepared in step 1 and start timing. 6. Gently stir the contents of each beaker at regular intervals. 7. Record in (a)(iii) the time taken for the pieces of agar to reach the end-point. (DV) Note that the colour of the agar may change from blue to green and then to yellow If any piece of agar has not changed to yellow after 240 s, stop timing and record as ‘more than 240’. (iii) Record your results for the known concentrations of antibiotic solution. Concentration of antibiotic / % Time taken for the pieces of agar to reach the end point / s 1 0.5 0.25 0.125 0.0625 [5] Feedback: • Generally well done. • A few candidates included units in the body of the table. Units should only be in the column headings of the table. 8. Put one piece of agar into the beaker labelled X and start timing. (iv) Record the time taken for the piece of agar in X to reach the end-point. time taken …………………………... [1] Mark scheme: Acceptable range is candidate’s experimental time between 1% and 0.25% antibiotics + appropriate time in whole no. Reject if unit is not provided (v) Use your results in (a)(iii) and (a)(iv) to estimate the concentration of antibiotic solution in X. ………………………………………………………………………………………………….…… [1] Correct estimation in accordance with recorded time. Feedback: • Some candidates provided precise concentration when they could simply mention that the concentration of X is within a range. E.g. The concentration of X is between 0.5% and 1%. Mark scheme: 1. Table drawn + heading: percentage concentration of antibiotic or concentration of antibiotic / % 2. Heading: time / s 3. Records results for at least 4 concentrations 4. Correct trend of results (i.e. the highest concentration of antibiotic recorded as the shortest time for colour change) + for at least 3 concentrations 5. Precision: whole number

(vi) Identify one significant source of error in this investigation. …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………….……[1] (vii) Describe how this source of error could be minimised by the modification of the procedure. …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………….……[1] Any one Source of error Modification 1. Difficulty in judging colour change / ref to subjectivity in interpreting end-point. Either one: Allow one agar piece to reach end-point (yellow). Use this colour as a reference to decide how the end-point looks like. Use a video camera to film the progress of colour change to determine the end -point more accurately. Reject use of colorimeter because it cannot accurately measure the time required for colour change. 2. Temperature was not maintained throughout the experiment. Conduct the experiment in a thermostatically controlled water bath. 3. Difficulty in cutting to the required dimension / ensuring 5 mm x 5 mm dimension agar piece using ruler and knife. This results in agar pieces having unequal surface area. Use a cork borer to create discs of equal dimension. 4. Too few known concentrations of antibiotics were