1.9 Required practical: antibiotics and antiseptics on bacteria
7 exam questions worth 17 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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A control disc soaked in sterile water is placed on the agar plate.
What is its purpose?
Choose one answer.
Correct Not quite: the answer is To show that the disc itself does not stop the bacteria growing
Mark scheme
B (To show that the disc itself does not stop the bacteria growing)
Model answer
B: To show that the disc itself does not stop the bacteria growing. If no clear zone forms round the control disc, the disc and water do not affect the bacteria.
A student investigates three antibiotics using agar plates and paper discs.
Identify the independent variable, the dependent variable and one control variable.
Mark scheme
- independent variable: the antibiotic / type of antibiotic (1)
- dependent variable: the size / diameter / area of the clear zone (1)
- control variable, any one from: same species of bacteria / same size of disc / same volume of antibiotic on the disc / same agar thickness / same temperature / same incubation time (1)
Model answer
The independent variable is the antibiotic. The dependent variable is the diameter (or area) of the clear zone. A control variable is the volume of antibiotic on each disc.
The clear zone around an antibiotic disc has a diameter of 16 mm.
Calculate the area of the clear zone. Use π = 3.14.
Mark scheme
- radius = 8 (mm) (1)
- area = 3.14 × 82 (1)
- = 201 (mm2) (1) (allow 200.96)
Model answer
radius = 16 ÷ 2 = 8 mm. area = πr2 = 3.14 × 82 = 201 mm2.
A student tested three antibiotics.
| Antibiotic | Diameter of clear zone (mm) |
|---|---|
| P | 12 |
| Q | 20 |
| R | 8 |
Which antibiotic was the most effective? Give a reason for your answer.
Mark scheme
- Q (1)
- it has the largest clear zone / killed the most bacteria (1)
Model answer
Antibiotic Q was the most effective, because it produced the largest clear zone, so it stopped the most bacteria growing.
Give two safety precautions the student should take in this investigation.
Mark scheme
Any two from:
- use bacteria that are not harmful to humans (1)
- keep the lid taped on / do not open the dish after incubation (1)
- disinfect the bench and wash hands (1)
- dispose of the plates safely / autoclave them (1)
Model answer
Use bacteria that are not harmful to humans, and keep the lid taped on so the dish is never opened after incubation.
Suggest two ways of making the results of this investigation more reliable.
Mark scheme
Any two from:
- repeat the investigation / use more plates (1)
- calculate a mean for each antibiotic (1)
- identify and ignore anomalous results (1)
Model answer
Repeat the investigation several times and calculate a mean clear zone for each antibiotic.
Describe how a student could use agar plates to compare the effectiveness of three different antiseptics.
Mark scheme
Any four from:
- spread bacteria over the agar plate using aseptic technique (1)
- place discs soaked in each antiseptic on the plate (1)
- include a control disc soaked in sterile water (1)
- tape the lid on and incubate at 25 °C for 24 to 48 hours (1)
- measure the diameter of the clear zone around each disc (1)
- the antiseptic with the largest clear zone is the most effective (1)
Model answer
Spread bacteria evenly over a sterile agar plate using aseptic technique. Place discs soaked in each antiseptic on the plate, along with a control disc soaked in sterile water. Tape the lid on and incubate at 25 °C for 24 to 48 hours. Then measure the diameter of the clear zone around each disc. The antiseptic with the largest clear zone is the most effective.
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