IGCSE Biology Paper-4: Specimen Questions with Answers 230 - 230 of 279


Shows part of the nitrogen cycle.

Protein Synthesis

Question 230 (3 of 6 Based on Passage)


Write in Short

Short Answer▾

State the names of processes P and Q in Fig.

process P

process Q


P – Denitrification


The conversion of nitrates and nitrites in soil into atmospheric is called denitrification, which is done by denitrifying bacteria, e. g. , Micrococcus denitrificans and Bacillus denitrificans, Pseudomonas & Thiobacillus.

Denitrification occurs in four steps:

  • Nitrates are reduced to nitrites by the enzyme nitrate reductase. The nitrites are reduced to ammonia by nitrite reductase. The ammonia is so formed is enzymatically incorporated in amino acids.
  • Denitrification does not occur to any significant degree in well aerated soils with moderate number of nitrates and organic matter. It occurs in water logged anaerobic soils with high organic matter content.

Q - Nitrification


The conversion of in soil into nitrates and nitrites is called nitrification. It is done by nitrifying bacteria, e. g. , Nitrosomonas, Nitrosococcus (convert into nitrites) and Nitrobacter (convert nitrites into nitrates) .

  • Nitrifying bacteria are chemoautotrophs and are benefitted by utilizing energy released in oxidation, which is used in chemosynthesis. At soil temperature in alkaline soils and with sufficient moisture and aeration the activity of ammonifying and nitrifying bacteria is found to be maximum.
  • The nitrate thus formed is absorbed by plants and is transported to the leaves. In leaves, it is reduced to form ammonia that finally forms the amine group of amino acids.

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