urea

Examples of urea in the following topics:

• Nitrogenous Waste in Terrestrial Animals: The Urea Cycle

  • Mammals, including humans, are the primary producers of urea.
  • 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 urea cycle utilizes five intermediate steps, catalyzed by five different enzymes, to convert ammonia to urea.
  • The urea cycle converts ammonia to urea in five steps that include the catalyzation of five different enzymes.

• Nitrogenous Waste in Birds and Reptiles: Uric Acid

  • Birds and reptiles have evolved the ability to convert toxic ammonia into uric acid or guanine rather than urea.
  • The animals must detoxify ammonia by converting it into a relatively-nontoxic form such as urea or uric acid .
  • The production of uric acid involves a complex metabolic pathway that is energetically costly in comparison to processing of other nitrogenous wastes such as urea (from the urea cycle) or ammonia; however, it has the advantages of reducing water loss and, hence, reducing the need for water.
  • Uric acid is also less toxic than ammonia or urea.
  • These include (a) ammonia, (b) urea, and (c) uric acid.

• Osmoregulators and Osmoconformers

  • However, the blood of sharks contains urea and trimethylamine oxide (TMAO).
  • The shark's blood electrolyte composition is not similar to that of seawater, but maintains isotonicity with seawater by storing urea at high concentrations.
  • Sharks are "ureotelic" animals that secrete urea to maintain osmotic balance.
  • TMAO stabilizes proteins in the presence of high urea levels, preventing the disruption of peptide bonds that would otherwise occur at such high levels of urea.

• Connecting Proteins to Glucose Metabolism

  • When the amino group is removed from an amino acid, it is converted into ammonia through the urea cycle.
  • In mammals, the liver synthesizes urea from two ammonia molecules and a carbon dioxide molecule.
  • Thus, urea is the principal waste product in mammals produced from the nitrogen originating in amino acids; it leaves the body in urine.

• The Role of Blood in the Body

• Malpighian Tubules of Insects

  • Metabolic wastes, such as urea and amino acids, freely diffuse into the tubules, while ions are transported through active pump mechanisms.

• Temperate Grasslands

  • These invertebrates, along with symbiotic fungi, extend the root systems, break apart hard soil, enrich it with urea and other natural fertilizers, trap minerals and water, and promote growth.