Summary
A liver pathway that converts toxic ammonia into urea for excretion.
In depth
Occurs partly in mitochondria and cytosol of liver cells. Ammonia and CO2 form carbamoyl phosphate, then citrulline, argininosuccinate, arginine and urea; costs 4 ATP equivalents. Defects cause hyperammonaemia with brain damage.
It occurs mainly in the liver and clears nitrogen from amino acid breakdown. Excreted by kidneys. Ammonia toxicity damages the brain.
Remember: urea is less toxic than ammonia and needs less water to excrete than ammonia.
The urea cycle, discovered by Hans Krebs and Kurt Henseleit in 1932, converts toxic ammonia from amino acid breakdown into urea in the liver. It starts in mitochondria, where carbamoyl phosphate synthetase combines ammonia, CO2 and ATP, then proceeds in the cytoplasm through citrulline, argininosuccinate and arginine, which arginase cleaves to release urea and regenerate ornithine.
Each turn disposes of two nitrogen atoms (one from ammonia and one from aspartate) and costs about 4 ATP equivalents. Urea travels in blood to the kidneys for excretion in urine. Defects cause hyperammonaemia, harming the brain.
Example: liver failure causes confusion from raised ammonia.
Key idea: ureotelic animals save water compared with ammonia excretion.
Frequently asked questions
What is Urea Cycle?
A liver pathway that converts toxic ammonia into urea for excretion.
Which area of biology covers Urea Cycle, and how advanced is it?
Urea Cycle is one of the 53 topics in Metabolism & Bioenergetics. It is rated Advanced, so it suits learners who want specialist depth. Related topics in this area include Pentose Phosphate Pathway, Protein Catabolism, Beta-Oxidation.
Can you give an example of Urea Cycle?
Liver failure raises blood ammonia, causing confusion.
What should I learn before Urea Cycle?
Work through the Beginner topics in Metabolism & Bioenergetics first, such as Metabolism, Anabolism and Catabolism, ATP, then return to Urea Cycle. Advanced topics assume the Intermediate vocabulary.
What topics are related to Urea Cycle?
The closest topics to Urea Cycle are Pentose Phosphate Pathway, Protein Catabolism, Beta-Oxidation, Gluconeogenesis. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Urea Cycle?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Urea Cycle. To practise the whole of Metabolism & Bioenergetics, use the Metabolism & Bioenergetics quiz.
How is Urea Cycle different from Pentose Phosphate Pathway?
Urea Cycle: A liver pathway that converts toxic ammonia into urea for excretion. Pentose Phosphate Pathway: Produces NADPH and ribose sugars for biosynthesis and antioxidant defence. Read both side by side and note what each one explains.
How should I study Urea Cycle step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Pentose Phosphate Pathway and explain the difference aloud. Finally, take the Metabolism & Bioenergetics quiz and come back to this page for any question you miss.