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1.1 Cell structure

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Question 24

An outdoor activity centre is choosing between two antiseptic liquids, R and S, for its first-aid kits. A technician compares the antiseptics in a laboratory.

The technician used nutrient agar plates that were sterilised before being inoculated aseptically with one species of bacterium.

On each plate, the technician:

  • added the same volume of antiseptic R and antiseptic S to separate sterile paper discs
  • added sterile water to a third sterile paper disc
  • placed the three discs well apart on the agar
  • incubated the plate at 25 °C for 48 hours
  • measured the diameter of the inhibition zone around each disc.

The table shows the results.

PlateDiameter of inhibition zone around R in mmDiameter of inhibition zone around S in mm
11814
22115
31813
Mean14
a.

State the purpose of the paper disc containing sterile water.

[1]
b.

State one feature of the paper discs, other than sterility, that should be kept the same.

[1]
c.

Calculate the mean diameter of the inhibition zone around antiseptic R.

[2]
d.

Explain which antiseptic was more effective against this species of bacterium.

[2]
e.

Give one reason why the technician used three agar plates rather than one.

[1]
Markscheme

1.1 Cell structure Questions

  1. GCSE
  2. /Biology
  3. /1.1 Cell structure

282 exam-style questions on AQA GCSE Biology 1.1 Cell structure, covering 1.1.1 Eukaryotes and prokaryotes, 1.1.2 Animal and plant cells, 1.1.3 Cell specialisation, 1.1.4 Cell differentiation, 1.1.5 Microscopy, and 1.1.6 Culturing microorganisms. Each one has a worked solution and a mark scheme showing where the marks go.

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