DP Biology (last assessment 2024)

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Question 23M.3.SL.TZ2.12

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Date May 2023 Marks available [Maximum mark: 7] Reference code 23M.3.SL.TZ2.12
Level SL Paper 3 Time zone TZ2
Command term Describe, State, Suggest Question number 12 Adapted from N/A
12.
[Maximum mark: 7]
23M.3.SL.TZ2.12
(a)

Biofilms cause many environmental problems. State one of these problems.

[1]

Markscheme

clogging/erosion of pipes/ transfer of microorganisms in ballast water / contamination of surfaces in food production / any other valid problem;

Examiners report

Many candidates did not understand the focus of the questions on environmental problems caused by biofilms and produced vague answers.

Samples were collected from water, sediment and biofilm scraped from rock surfaces at two sites along the Weihe River in China. For each sample, the concentration of Escherichia coli, a fecal coliform bacterium, was determined as the number of colony-forming units (CFU L-1) from each sample.

[Source: Li, Q. and Zhang, Q., 2020. Prevalence and pollution characteristics of antibiotic resistant
genes in one high anthropogenically-impacted river. PLoS ONE 15(4) [e-journal]
Available at: https://doi.org/10.1371/journal. pone.0231128
[Accessed 1 September 2020]. Open access.]

(b.i)

Suggest a reason for the difference in concentration of E. coli between samples from the town and upstream.

[1]

Markscheme
  1. release of (untreated) sewage in the town;
  2. increased fecal discharge from the town;
(b.ii)

Suggest a reason for the higher concentration of E. coli in biofilms in the town samples.

[1]

Markscheme
  1. biofilms are made by (cooperative) aggregates of bacteria;
  2. bacteria need a surface/rocks for aggregation/attachment;
  3. more suitable habitats for biofilm formation in the town;
  4. lower current flow favours biofilm formation;
(c)

Describe how biofilms result in resistance to antimicrobial agents.

[4]

Markscheme
  1. biofilms produce extracellular polymeric substances/EPS;
  2. bacteria in the biofilm are densely populated;
  3. quorum sensing regulates the behavior of the entire bacterial population (with signal molecules/expression of certain genes)
    OR
    quorum sensing can regulate antibiotic resistance;
  4. biofilms can increase the survival rate/decrease the growth/metabolic rate of bacteria;
  5. (extreme) outer environment/stress can promote greater resistance
    OR
  6. (extreme) outer environment/stress reduce growth efficiency;
    higher chances of resistant plasmid being transferred;
Examiners report

Most candidates scoring marks here mentioned quorum sensing and extracellular polymeric matrix as mechanisms to resist antibiotics in biofilms.