Q.With regards to toxicity and the need for dilution in water, how different are ureotelic and uricotelic excretions? Give examples of animals that use these types of excretion?

The ornithine (urea) cycle in the liver converts ammonia, a toxic by-product of protein metabolism, into urea, which is then excreted by the kidneys.

Deamination of excess amino acids in the liver produces free ammonia, which is highly toxic even at low concentrations. To detoxify it, liver cells run the ornithine cycle (also called the urea cycle): ammonia combines with carbon dioxide to form carbamoyl phosphate, which then reacts with the amino acid ornithine to form citrulline; citrulline is converted to argininosuccinate and then to arginine; finally, the enzyme arginase splits arginine into urea and regenerates ornithine, so the cycle can run again. The urea formed is fa …

The excretory organ of nematodes is the renette (rennette) cell — a single large gland cell or H-shaped duct system, not nephridia or Malpighian tubules.

Different invertebrate groups have evolved distinct excretory structures:

• Flame cells (protonephridia) are found in flatworms (Platyhelminthes).
• Malpighian tubules are found in insects and other terrestrial arthropods.
• Kidneys are the excretory organs of vertebrates, not invertebrates like nematodes. …

The ornithine cycle (urea cycle) operates in the liver and its end product is urea, the main nitrogenous excretory product of humans (ureotelic animals).

When amino acids are broken down (deamination) in the liver, the amino group is released as highly toxic ammonia (NH3). Since ammonia cannot be tolerated in high concentration in the blood, the liver converts it into a much less toxic, water-soluble compound called urea, through a cyclical series of enzyme-catalysed reactions known as the ornithine cycle (or urea cycle), first described by Hans Krebs and Kurt Henseleit. In this cycle, ammonia combines with carbon dioxide and the amino acid ornithine in a series of steps (via citrulline and arginine) to regenerate ornithine and release urea as the end product. The urea formed is carried by blood to the kidneys and excreted in urine. This is why humans, along with many other terrestrial vertebrates, are called ureotelic animals. Carbon dioxide is a raw material consumed in the cycle (not the end product), …