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: • Mo