IGCSE Biology Paper-4: Specimen Questions with Answers 233 - 234 of 279

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Passage

Shows part of the nitrogen cycle.

Protein Synthesis

Question 233 (6 of 6 Based on Passage)

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State two ways by which nitrogen fixation can occur.

Explanation

Nitrogen is found in essential compounds like proteins, nucleic acids, growth regulators and many vitamins. Plants compete with microbes for the limited nitrogen that is available in soil. Thus, nitrogen is a limiting nutrient for both natural and agricultural eco-systems. N2 gas of atmosphere is converted into ammonia by process of nitrogen-fixation. In nature, lightening and ultraviolet radiation provide enough energy to convert nitrogen to nitrogen oxides (NO, NO2, N2O) . Industrial combustions, forest fires, automobile exhausts and power generating stations are also the sources of atmospheric nitrogen oxides.

Question 234

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Fertilizers can cause pollution to aquatic systems. Overuse of fertilizers may cause eutrophication. Lake Udai Sagar in India is an example of an aquatic system that shows high levels of eutrophication.

Explain what happens in aquatic environments, such as Lake Udai Sagar, when eutrophication occurs.

Explanation

There is ageing of water bodies in nature which affect the aquatic ecosystem. Such natural ageing of a water body due to biological enrichment of its water, leading to depletion of species diversity, is called Eutrophication.

  • A young lake has cold and clear water supporting little life.
  • With the time, the streams draining into lake introduce the nutrients like,
  • Nitrogen and Phosphorus, which encourage the growth of aquatic fauna and flora.
  • When the lake fertility increases, the plants, animals and the micro- organisms multiply very fast and organic remains begin to be deposited at the bottom of the lake.
  • Over the centuries, the organic debris and silt pile up and the lake becomes shallower and warmer. The warm water organisms now replace the cold-water organisms.
  • The marsh plants, now, begin to fill the original lake. This later gives the way to large masses of floating plants (Bog) which finally converts the lake into land.
  • The natural ageing of a lake depends upon climate, size of lake and other factors, normally taking thousands of years.

The ageing process can however, be accelerated by the transportation of pollutants from homes, agricultural fields, and industries due to human activities. Such wastes may poison the whole population of fish and the decomposing- remains further deplete the dissolved oxygen content, and the lake can literally choke to death. This phenomenon of choking of lake is called Cultural or Accelerated Eutrophication.

Eutrophication Phosphorus

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