NITROGEN EXCRETION IN ANIMALS

NITROGEN EXCRETION IN ANIMALS

Nitrogen is a characteristic constituent of amino acids and proteins. Animals receive amino acids in diet and use them to synthesise a variety of functional nitrogenous compounds such as nucleic acids, proteins both enzymic and non-enzymic, some hormones, and neurotransmitters. The amount of amino acids obtained in diet is usually in excess of the need for the synthesis of the functional nitrogenous compounds. The excess of amino acids is either catabolised for the release of energy or is used for the synthesis of glycogen and fat. When amino acids proteins or nucleic acids are catabolised, nitrogen-containing excretory end-products are formed. These are ammonia, urea and uric acid.

i) Those that excrete mainly ammonia (NH4) as the endproduct of protein metabolism are called ammonotelic. 

ii) Those that excrete mostly urea are ureotelic.

iii) Those that form mainly uric acid are uricotelic.

iv) Those that secrete guanine are guanotelic.

• Formation of ammonia

The amino acids which enter the body of the animal through diet are catabolised by a process known as oxidative transdeamination. This process is a combination of transamination, deamination and oxidation, and is catalysed by enzymes, transaminases and dehydrogenases.

1. Ammonotelism

Ammonia diffuses through cell membranes extremely fast because of its high water solubility and small molecular size. Hence, it can be excreted as such only when there is ample water for its rapid removal from the body in the form of a dilute solution. Prompt excretion of ammonia therefore, occurs in aquatic animals, both freshwater and marine, in which there are constant water fluxes occurring between the environment and the body. Freshwater and marine invertebrates and fishes, larval and permanently aquatic amphibians excrete a major portion of their waste nitrogen as ammonia and thus called ammonotelic or ammoniotelic. The route of ammonia diffusion in these animals is skin, gills or

 kidneys.

2. Ureotelism

Terrestrial animals with restricted water availability in the environment are faced with the formidable task of water conservation. Since they cannot afford to use liberal quantities of water for excretion, ammonia is converted into a less toxic product. In mammals and semi-terrestrial adult amphibians, the major nitrogenous excretory product is urea, which is less toxic and easily soluble. These animals are therefore called ureotelic.

3. Uricotelism

In animals which inhabit extremely arid environments, ammonia is converted into uric acid. Uric acid is least toxic, relatively insoluble and is easily precipitated. Hence, it can be excreted in solid form without loss of substantial amount of water. Pulmonate snails, terrestrial insects, squamate reptiles (lizards and snakes) and birds excrete a major portion of their waste nitrogen in the form of semi-solid or solid uric acid and hence are referred to as uricotelic animals.

4. Guanotelism

Arachnids (spiders and scorpions) excrete mostly guanine and hence are said to be guanotelic. Guanine, like uric acid, is relatively non-toxic and insoluble and is excreted in solid form. It is an adaptation to life in arid habitats. It is also a purine and has the same atoms in its ring structure as in uric acid. In fact, guanine is formed as an intermediate during uric acid synthesis in uricotelic animals. In spiders and scorpions, the inosinic acid pathway terminates in guanine formation. The origin of the four nitrogen atoms of guanine is similar to that of uric acid.