state that urea is produced in the liver from the deamination of excess amino acids

Homeostasis in Mammals

Key Objective

State that urea is produced in the liver from the deamination of excess amino acids.

Why the Body Must Remove Excess Nitrogen

Proteins are constantly being broken down and rebuilt. Deamination of amino acids releases ammonia (NH₃), a highly toxic compound. Mammals must convert ammonia into a less toxic form for safe transport in the bloodstream and eventual excretion.

The Urea Cycle (Ornithine Cycle)

The liver carries out the urea cycle, a series of enzymatic reactions that transform ammonia into urea, which can be safely carried to the kidneys for excretion.

1. Deamination of excess amino acids produces free ammonia.
2. Ammonia combines with carbon dioxide in the mitochondria of hepatocytes to form carbamoyl phosphate (requires 2 ATP).
3. Carbamoyl phosphate enters the urea cycle, reacting with ornithine to form citrulline.
4. Citrulline is transported to the cytosol, where it combines with aspartate to form argininosuccinate.
5. Argininosuccinate is split into arginine and fumarate.
6. Arginine is hydrolysed to produce urea and regenerate ornithine, completing the cycle.

The overall stoichiometric equation can be written as:

\$\text{2 NH}3 + \text{CO}2 + 3\text{ATP} + \text{H}2\text{O} \rightarrow \text{(NH}2)2\text{CO} + 2\text{ADP} + 4\text{P}i + \text{AMP}\$

Key Points to Remember

• Urea is the primary nitrogenous waste in mammals.
• The liver is the site of urea synthesis.
• Urea is far less toxic than ammonia and is highly soluble in water.
• Urea is transported in the blood to the kidneys, where it is filtered and excreted in urine.

Comparison of Nitrogenous Waste Products in Different Animal Groups