DP Biology (last assessment 2024)

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Question 23M.3.HL.TZ2.20

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Date May 2023 Marks available [Maximum mark: 6] Reference code 23M.3.HL.TZ2.20
Level HL Paper 3 Time zone TZ2
Command term Outline, State, Suggest Question number 20 Adapted from N/A
20.
[Maximum mark: 6]
23M.3.HL.TZ2.20

The availability of nitrogen can affect the rate of an ecosystem’s key processes.

(a)

Outline the effect of nitrogen compounds from agricultural land leaching into a lake.

[3]

Markscheme
  1. excess nutrients/eutrophication;
  2. excessive growth of algae/algal blooms;
  3. blocking of sun prevents photosynthesis;
  4. increased decomposition of algae/organisms;
  5. increased biochemical oxygen demand/BOD;
  6. other organisms/fish die due to lack of oxygen;
Examiners report

Most candidates provided partial answers as they all seem to have basic knowledge of eutrophication and algal blooms, but most do not seem to understand about BOD and the process that leads to this situation.

Nitrogen content was measured in leaves of insectivorous California pitcher plants (Darlingtonia californica) at five sites with different elevations above sea level along the California–Oregon border, USA. The bar chart shows the median of all measurements at each site.

[Source: Reproduced from Ellison, A.M. and Farnsworth, E.J., 2005. The Cost of Carnivory for Darlingtonia californica
(Sarraceniaceae): Evidence From Relationships Among Leaf Traits. American Journal of Botany 92(7), pp. 1085–1093,
with permission from Wiley.]

(b)

State the relationship between elevation and nitrogen concentration in leaves of pitcher plants.

[1]

Markscheme

the higher the elevation, the greater the nitrogen content
OR
positive relationship;

Examiners report

Most candidates could correctly state the relationship between elevation and nitrogen leaf content.

(c)

Suggest two reasons, other than elevation, for the differences in nitrogen concentration in leaves of pitcher plants.

[2]

Markscheme
  1. pitcher plants consume more insects (at different sampling sites);
  2. larger insects contain more nitrogen;
  3. more nitrogen concentration in soil
    OR
    more nitrifying bacteria in the soil
    OR
    water-logged soil has less nitrogen;
  4. use of N/NPK fertilizers
    OR
    leaching of N in different sites;
Examiners report

It was much more difficult to suggest two reasons for differences in nitrogen concentration, although differences in soil concentration, presence of nitrifying bacteria and use of fertilizers were mentioned.

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