סקירה כללית על חילוף החומרים של חנקן

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.