Biology » The Excretory System » Nitrogenous Wastes

Nitrogenous Waste in Terrestrial Animals: the Urea Cycle

Nitrogenous Waste in Terrestrial Animals: The Urea Cycle

The urea cycle is the primary mechanism by which mammals convert ammonia to urea. Urea is made in the liver and excreted in urine. The overall chemical reaction by which ammonia is converted to urea is 2 NH3 (ammonia) + CO2 + 3 ATP + H2O → H2N-CO-NH2 (urea) + 2 ADP + 4 Pi + AMP.

The urea cycle utilizes five intermediate steps, catalyzed by five different enzymes, to convert ammonia to urea, as shown in the figure below. The amino acid L-ornithine gets converted into different intermediates before being regenerated at the end of the urea cycle. Hence, the urea cycle is also referred to as the ornithine cycle. The enzyme ornithine transcarbamylase catalyzes a key step in the urea cycle and its deficiency can lead to accumulation of toxic levels of ammonia in the body. The first two reactions occur in the mitochondria and the last three reactions occur in the cytosol. Urea concentration in the blood, called blood urea nitrogen or BUN, is used as an indicator of kidney function.

Nitrogenous Waste in Terrestrial Animals: the Urea Cycle

The urea cycle converts ammonia to urea.

Evolution Connection: Excretion of Nitrogenous Waste

The theory of evolution proposes that life started in an aquatic environment. It is not surprising to see that biochemical pathways like the urea cycle evolved to adapt to a changing environment when terrestrial life forms evolved. Arid conditions probably led to the evolution of the uric acid pathway as a means of conserving water.

Do you want to suggest a correction or an addition to this content? Leave Contribution

[Attributions and Licenses]

Test Your Knowledge | Examination Past Questions

Which of the following is not a function of the liver in mammals?


A) Storage of some vitamins

B) Conversion of glycogen into glucose

C) Deamination of excess nitrogenous substances

D) Filtration of urea from the blood.

Exam Body: West African Examinations Council (WAEC)

View Past Question

This is a lesson from the tutorial, The Excretory System and you are encouraged to log in or register, so that you can track your progress.

Log In

Do NOT follow this link or you will be banned from the site!