4.7
Overview of Nitrogen Metabolism
Nitrogen is a very important element for life because it is a major constituent of proteins and nucleic acids. It is a macronutrient, and in nature, i…
Atmospheric nitrogen is the largest source of nitrogen. However, most living organisms cannot directly take up atmospheric nitrogen because of the strong triple bond between the nitrogen atoms, which makes it relatively unreactive.
Therefore nitrogen-fixing bacteria present in the soil convert atmospheric nitrogen into ammonia, a biologically useful form.
Ammonia can be taken up by plant cells, where it combines with glutamate to produce glutamine, which is further converted into other amino acids and nucleotides.
Later, animals consume these plants and use the amino acids to synthesize their own proteins and nucleotides.
Since animals cannot store large pools of amino acids, the ingested amino acids are regularly broken down via the urea cycle into urea.
Urea is then excreted through urine, which eventually goes back into the soil.
Later, the soil bacteria with urease activity degrade the excreted urea into ammonia. Finally, the nitrifying and denitrifying bacteria convert ammonia into gaseous nitrogen, returning it to the atmosphere.
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Q1: Why can't most organisms directly use atmospheric nitrogen?
Atmospheric nitrogen exists as N2 gas with a strong triple bond between nitrogen atoms, making it relatively unreactive and unavailable to most living organisms. Nitrogen-fixing bacteria in soil convert this gaseous nitrogen into ammonia, a biologically useful form that plants and animals can utilize for protein and nucleotide synthesis.
Q2: How do plants incorporate nitrogen into amino acids?
Plants take up ammonia from soil and combine it with glutamate to produce glutamine through a metabolic process. Glutamine is then further converted into other amino acids and nucleotides, which form the building blocks for plant proteins and genetic material. This process is part of the broader overview of metabolism.
Q3: What is the urea cycle and why is it important for animals?
The urea cycle is a series of reactions that converts excess ammonia, which is highly toxic to cells, into non-toxic urea. Since animals cannot store large pools of amino acids, they regularly break down ingested amino acids and use the urea cycle to safely eliminate excess nitrogen through urine excretion.
Q4: How does ammonia return to the soil after animal excretion?
Animals excrete urea through urine into the soil, where bacteria with urease enzyme activity degrade the urea back into ammonia. This ammonia can then be taken up again by plants, completing a key recycling step in the nitrogen cycle and enabling continuous nutrient availability.
Q5: What is nitrification and which bacteria carry out this process?
Nitrification is the progressive oxidation of ammonium ions by soil bacteria into nitrite and then nitrate. Bacteria belonging to genera Nitrosomas, Nitrobacter, and Nitrospira perform this process, converting ammonium into more oxidized nitrogen forms that are readily available for plant uptake and utilization.
Q6: How do denitrifying bacteria complete the nitrogen cycle?
Denitrifying bacteria, certain archaea, and some fungi convert nitrates back into gaseous nitrogen, returning it to the atmosphere and completing the nitrogen cycle. This process transforms soil nitrogen back into atmospheric N2 gas, allowing the cycle to begin anew with nitrogen fixation.
Q7: What happens to nitrogen when plants and animals die?
When organisms die, soil microbes degrade their protein content to produce amino acids. Nitrifying microbes then convert these amino acids into nitrite and nitrate, which can be reused by living plants or further processed by denitrifying bacteria to return nitrogen to the atmosphere.