DP Biology (last assessment 2024)

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Question 23M.2.HL.TZ1.3

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Date May 2023 Marks available [Maximum mark: 6] Reference code 23M.2.HL.TZ1.3
Level HL Paper 2 Time zone TZ1
Command term Explain, Outline Question number 3 Adapted from N/A
3.
[Maximum mark: 6]
23M.2.HL.TZ1.3

Antibiotics such as penicillin are secreted by fungi and other microorganisms in soil. By secreting them, a microorganism can inhibit the growth of a competitor.

In research published in 2014, nearly 3000 antibiotic resistance genes were discovered in soil microorganisms, giving resistance to 18 different antibiotics. The types of antibiotic resistance gene varied between soil types.

(a)

Explain how natural selection could increase the prevalence of an antibiotic resistance gene in a species of soil bacterium.

[3]

Markscheme
  1. if antibiotic is in the environment/soil there is selection (pressure);
  2. bacteria without resistance (gene) die / converse;
  3. bacteria with resistance (gene) reproduce
    OR
    bacteria exchange/obtain resistance genes using plasmids/by conjugation;
  4. offspring inherit (the gene for) resistance/resistance passed on (to offspring);

Do not award marks for general statements about natural selection – answers must refer to antibiotic resistance.

Examiners report

This question discriminated very effectively, with well-prepared candidates giving clear explanations of how natural selection can increase antibiotic resistance.

(b.i)

There are viruses in soils that are pathogens of animals. Outline a reason for antibiotics in soil not eliminating these viruses.

[1]

Markscheme
  1. viruses lack metabolism/plasma membranes/cell walls / viruses are not livingorganisms;
  2. antibiotics kill/target bacteria/prokaryotes but not viruses;
  3. viral pathogens of animals use animal metabolism (which is not affected by antibiotics);

 

Examiners report

This was also a discriminating question with about half of the candidates giving an acceptable reason for antibiotics not eliminating viruses.

(b.ii)

Explain the reasons for antibiotics secreted into soil not harming insects or other animals in the soil.

[2]

Markscheme
  1. antibiotics (only) affect/kill/block processes in bacteria/prokaryotes / do not affect eukaryotes;
  2. metabolism/protein synthesis/ribosomes/cell walls not targeted in insects/animals/eukaryotes;
  3. no cell walls in animals (so antibiotics cannot attack cell walls);
  4. antibiotics are secreted (by microbes/fungi) to prevent competition (with other microbes);

 

Examiners report

Weaker candidates tended to struggle with this question. This was shown by statements about antibiotics not being 'designed' to target insects, or about animals developing resistance to antibiotics. There was confusion between antibiotics, antibodies and antigens. Some answers were repeats of (b)(i) and stated only that antibiotics affect bacteria. The best answers made the point that insects and other animals are eukaryotic and lack the features that antibiotics target, such as a cell wall.

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