DP Biology (last assessment 2024)

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Question 23M.3.HL.TZ1.11

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Date May 2023 Marks available [Maximum mark: 6] Reference code 23M.3.HL.TZ1.11
Level HL Paper 3 Time zone TZ1
Command term Describe, Explain, State Question number 11 Adapted from N/A
11.
[Maximum mark: 6]
23M.3.HL.TZ1.11

The image shows the life cycle of a biofilm.

[Source: Tang, L., Zhang, Y., Li, C., et al., 2022. Biological Stability of Water-Based Cutting Fluids: Progress and Application.
Chin. J. Mech. Eng. [e-journal] 35(3). https://doi.org/10.1186/s10033-021-00667-z. Open access.]

(a)

State one function of biopolymers (EPS) in the biofilm.

[1]

Markscheme

allows bacteria to stick together/to the surface/retains water/protection/structural stability;

Examiners report

This was often the only part of question 11 which candidates were able to answer correctly. This may have been because the diagram provided showed EPS and 'irreversible attachment".

(b)

Describe emergent properties in mature colonies of biofilms, other than EPS production.

[2]

Markscheme
  1. emergent properties are those in the biofilm that may not be present in individual cells/are outcomes of interactions of the elements of a system;
  2. resistance to antibiotics due to reduced cell division rates;
    OR
    resistance to antimicrobials due to physical barriers to penetration;
  3. social cooperation as cells organize to form a complex structure
    OR
    release of signaling molecules leads to gene expression to develop biofilm;
  4. cells use matrix to move, leading to (whole) colony moving;
  5. formation of channels for water flow inside a colony;

Accept other valid examples with descriptions

Examiners report

Few could describe emergent properties in mature colonies of biofilms, many simply repeating what was already given in part (a).

(c)

Explain how biofilms can be useful in the treatment of sewage.

[3]

Markscheme
  1. (trickling filter) system/bed uses support medium/rocks/ stones/rubber/plastic on which biofilm develops/ grows;
  2. sewage trickled/sprinkled/dripped/sprayed over the medium to provideaeration/oxygen;
  3. biofilms feed off/digest organic matter in the sewage to obtain energy/forgrowth;
  4. resulting sludge removed may be used as fertilizer;
Examiners report

Very few understood the role of biofilms in sewage treatment.