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Question 23M.3.SL.TZ2.16

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Date May 2023 Marks available [Maximum mark: 6] Reference code 23M.3.SL.TZ2.16
Level SL Paper 3 Time zone TZ2
Command term Explain, Predict, Suggest Question number 16 Adapted from N/A
16.
[Maximum mark: 6]
23M.3.SL.TZ2.16

The diagram shows the levels of PCBs, which are hydrophobic industrial waste pollutants, in a marine food web.

[Source: Data from Barry C. Kelly et al., Food Web-Specific Biomagnification of Persistent Organic Pollutants.
Science 317, 236–239 (2007). DOI:10.1126/science.1138275.]

(a.i)

Suggest a reason that bivalves are classified between trophic levels 2 and 3 in this food web.

[1]

Markscheme
  1. they can belong to more than one trophic level;
  2. they eat both algae and zooplankton;
  3. they feed on organisms in level 2 (so are level 3) but are eaten by organisms in level 3 (so are level 2);
(a.ii)

Predict what could happen to the food web if polar bears were to become extinct.

[1]

Markscheme
  1. prey of the polar bears could increase in population (as the predator has disappeared);
  2. increase in prey/lower level predators could cause a drop in population of organisms at lower trophic levels;

Accept relevant examples.

Examiners report

Referring to the food chain was well answered though some candidates assumed the answers be linked to the level of PCB in the diagram.

(b)

Explain how the levels of PCBs increase so greatly over the trophic levels.

[4]

Markscheme
  1. as producers/algae take in nutrients, they also collect/absorb PCBs;
  2. consumers can take in PCBs directly from the water;
  3. PCBs are not digested/metabolized;
  4. PCBs build up in cells/tissues / bioaccumulate;
  5. PCBs pass on to higher level consumers (when they feed)
    OR
    organisms in the higher trophic levels accumulate more;
  6. At each trophic level there is biomagnification;

Accept examples from the diagram.

Examiners report

Explaining how PCB levels increase in the food chain also provided some good responses with the idea that PCBs pass on to higher level consumers and biomagnification being common responses. There is some confusion between bioaccumulation and biomagnification. Few achieved marks for responding that the producers take in the PCB from water or that PCBs cannot be digested or excreted.