The excess of amino acid in the liver is converted into ammonia by a process called deamination. In some animals detoxification of ammonia within the liver takes place and less toxic urea and uric acid are formed.
Based on the nature of nitrogenous excretory end product, animals show three modes of excretion.
I) Ammonotelism : Phenomenon of formation of excretory product in the form of ammonia is called ammonotelism. Organisms which excrete ammonia as main excretory product are ammonotelic. Ammonia is highly soluble in water, highly toxic. It can excreted by simple diffusion. So, its concentration in the body is kept very low. It is harmful to surrounding tissues. So, it cannot be retained in the body for long time. There it has to be eliminated out as soon as it is formed.
For elimination of ammonia, large quantity of water is required. About 300 to 500 ml of water is required for elimination of one gm of ammonia. Due to this reason ammonotelism is seen in aquatic invertebrates, bony fishes, tadpole larva of frog. In these animals, ammonia diffuses out through skin, gills and kidneys.
II) Ureotelism : Elimination of nitrogen wastes in the forms of urea is ureotelism. Certain animals cannot get sufficient amount of water to excrete ammonia. As ammonia is very toxic, it must be converted to less toxic form.
In liver, in these animals, ammonia combines with carbon-di-oxide to form urea. Urea formation requires expenditure of energy. It is formed in liver by ornithine cycle. Urea can be stored for sometime and can be excreted at a lower rate. Urea is soluble in water and is stored in dissolved form called urine. Elimination of urea requires moderate amount of water i.e. 50 ml of water for one gm of urea. Ureotelic animals are terrestrial animals like frog, turtles, toads, mammals, marine fishes.
Uricotelism : Elimination of nitrogen waste in the form of uric acid. Synthesis of uric acid from ammonia requires more energy. It takes place in liver by ionosinic pathway. So, it is advantageous for desert and terrestrial animals as it is less toxic, harmless and insoluble in water. Uric acid is eliminated in the form of solid pallets or thick paste which requires negligible amount of water for its elimination i.e. about 10ml of water for elimination of one gm uric acid. So, these animals can conserve water by uricotelism.
Animals like land snails, reptiles, birds, terrestrial insects, lizards, snakes which cannot afford to lose water. In human and other mammals, small quantity to uric acid is formed in the body by breakdown of purine and pyrimidine nitrogen bases of nucleic acid. In some persons, due to defective metabolism, excess uric acid gets deposited in joints of bones which causes painful arthritis called gout.
Guanotelism : Arachnids (spiders and scorpions, penguins) excrete mostly guanine and hence are called guanotelic.