DP Biology (last assessment 2024)

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Question 23M.3.HL.TZ1.19

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Date May 2023 Marks available [Maximum mark: 6] Reference code 23M.3.HL.TZ1.19
Level HL Paper 3 Time zone TZ1
Command term Explain Question number 19 Adapted from N/A
19.
[Maximum mark: 6]
23M.3.HL.TZ1.19

Explain the role of bacteria in the nitrogen cycle.

[6]

Markscheme
  1. (saprophytic) bacteria decompose the dead remains of plants/animals;
  2. convert nitrogenous compounds/wastes to ammonia/NH3;
  3. nitrifying bacteria/Nitrobacter/Nitrosomonas (in the soil) convert ammonia to nitrites/nitrates;
  4. nitrogen fixation is the conversion of atmospheric nitrogen to ammonia;
  5. Rhizobium have a mutualistic relationship with the plants;
  6. (Rhizobium) bacteria in root nodules of legumes/peas/clover/beans fix nitrogen;
  7. free-living bacteria/Azotobacter in the soil fix nitrogen;
  8. nitrates absorbed/assimilated/used by plants;
  9. denitrifying bacteria (in the soil) convert nitrates to nitrogen gas;
Examiners report

The stronger candidates were able to score four or more of the six marks available here. However, there were a large number who had no idea of the role of bacteria in the nitrogen cycle and a score of zero was very common. Some knew that there were different types of bacteria but confused nitrogen fixation, nitrification and denitrification.