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3.1 Communicable diseases

3.1 Communicable diseases

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Question 5

Name one chemical barrier the human body uses to defend itself against pathogens.

A student investigated the antimicrobial effect of tea tree oil at various concentrations on the survival of Staphylococcus aureus bacteria.

This is the method used:

  1. Prepare four test tubes each with 15 cm3 of nutrient broth.
  2. Add tea tree oil to adjust the concentrations to 0%, 2%, 4%, and 8%.
  3. Add 1.5 cm3 of a Staphylococcus aureus bacterial culture to each test tube.
  4. Take a 0.15 cm3 sample from each test tube immediately (0 hours) and record the number of live bacteria.
  5. Incubate the test tubes at 37∘C37^\circ\text{C}37∘C for 16 hours.
  6. Repeat step 4.

Table 1 shows some of the results.

Time in hoursNumber of live bacteria at tea tree oil concentration
0%2%4%8%
0480480480485
16400Y9615
a.

Name one chemical barrier the human body uses to defend itself against pathogens.

[1]
b.

What fraction of the bacteria present at 0 hours for 4% tea tree oil concentration survived for 16 hours? Give your answer in its simplest form.

[2]
c.

How many more bacteria were killed at 8% concentration than at 0% concentration in 16 hours? Complete the following steps:

  • Calculate the number of bacteria killed at 8%
  • Calculate the number of bacteria killed at 0%
  • Calculate how many more bacteria were killed at 8% than at 0%
[3]
d.

A student calculated value Y in Table 1 to be 248. Suggest how the student calculated this value.

[2]
Markscheme

3.1 Communicable diseases Questions

  1. GCSE
  2. /Biology
  3. /3.1 Communicable diseases

25 exam-style questions on AQA GCSE Biology 3.1 Communicable diseases, covering 3.1.1 Communicable (infectious) diseases, 3.1.2 Viral diseases, 3.1.3 Bacterial diseases, 3.1.4 Fungal diseases, 3.1.5 Protist diseases, 3.1.6 Human defence systems, 3.1.7 Vaccination, 3.1.8 Antibiotics and painkillers, and 3.1.9 Discovery and development of drugs. Each one has a worked solution and a mark scheme showing where the marks go.

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