DP Biology (first assessment 2025)

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Question 23M.2.SL.TZ2.19

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Date May 2023 Marks available [Maximum mark: 3] Reference code 23M.2.SL.TZ2.19
Level SL Paper 2 Time zone TZ2
Command term Suggest Question number 19 Adapted from N/A
19.
[Maximum mark: 3]
23M.2.SL.TZ2.19

Duckweed (Lemna gibba) is a plant that grows on the surface of water. It was grown in cultures and the percentage increase in total area covered per day was obtained. The graph shows the specific growth rates per day for duckweed measured over 22 days.

Material from: Mkandawire, M. and Dudel, E.G., Assignment of Lemna gibba L. (duckweed) bioassay for in situ
ecotoxicity assessment, published 2005, Aquatic Ecology, reproduced with permission of SNCSC.

Suggest reasons for the slowing down of duckweed population growth after day 6.

[3]

Markscheme
  1. high growth rate (before day 6) depleted nutrients;
  2. overcrowding/competition for resources;
  3. (intraspecific) competition slows growth rate;
  4. carrying capacity has been reached/stabilized;
  5. intoxication from excretion limits growth;

mp b: Accept resources examples such as CO2, nutrients, light, space, etc.

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