Homeostasis in Mammals: Urea Production

Objective

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

Amino Acid Metabolism and Urea Formation

Amino acids are the building blocks of proteins. When proteins are broken down or are not needed, the excess amino groups (-NH₂) must be removed from the amino acids. This process is called deamination.

Deamination is a complex process that occurs primarily in the liver. During deamination, the amino group is removed, leaving behind a carbon skeleton. The amino group is converted into ammonia (NH₃), which is highly toxic.

The liver converts the toxic ammonia into urea through a series of reactions known as the urea cycle. Urea is a much less toxic substance than ammonia and is water-soluble.

The Urea Cycle (Simplified Overview)

The urea cycle is a series of enzymatic reactions that take place in the liver. It involves the following key steps:

1. Ammonia (NH₃) is combined with carbon dioxide (CO₂) to form carbamoyl phosphate.
2. Carbamoyl phosphate reacts with ornithine to form citrulline.
3. Citrulline is transported out of the mitochondria into the cytoplasm.
4. Citrulline is converted to argininosuccinate.
5. Argininosuccinate is cleaved into arginine and fumarate.
6. Arginine is cleaved into urea and ornithine.
7. Ornithine is transported back into the mitochondria to continue the cycle.

Urea Excretion

The urea produced in the liver is transported to the kidneys via the bloodstream. The kidneys filter the blood and remove urea, along with other waste products, in the form of urine. This process helps to maintain water and electrolyte balance in the body.

Summary Table