Types of Nitrogenous Excretion

Types of Nitrogenous Excretion

In different animals the types of nitrogenous waste vary as per their mode of life.

Ammonotelism :

Ammonia formed during deamination of proteins, is most toxic hence, needs to be eliminated immediately.

Being highly soluble in water, it diffuses and dissolves in surrounding water, hence in aquatic animals e.g. protozoan, sponges, coelenterates, molluscs, some fresh water fishes, aquatic

amphibians (salamanders), etc, this is the common process.

Ureotelism :

NH3 is converted into urea (a less toxic) which is eliminated in solution form, hence requires plenty of water.

Ex. mammals, amphibians (frogs or toads) cartilaginous fishes, aquatic reptiles like crocodiles, turtles and terrapins.

Lung fishes, Xenopus (toad) are ammonotelic in water but ureotelic in mud during summer.

Tadpole of frog / toads are ammonotelic but adult is ureotelic.

Sharks retain urea to raise its body concentration to minimise the water loss.

Uricotelism :

NH3 is converted into uric acid (least toxic), which can be retained (storage excretion) in the body for even longer time without any harmful effect.

It’s also a means for conservation of water since it is eliminated in solid form and doesn’t require water, hence useful for the animals with less traffic of water in their body.

Ex. insects, reptiles, birds, land snails, land crustaceans. White material in the droppings of birds, lizards consist of insoluble crystals of urates. In this form it is commercially obtained

from birds droppings (guano).

Aminotelism :

Few molluscs (Unio, Limnaea) and echinoderms excrete excess amino acids directly.

Other nitrogenous wastes :

(i) Trimethylamine Oxide (TMO)

Marine teleosts synthesise TMO from NH3 and retain to raise the concentration of body fluid to make it isotonic to sea water. NH3 is first methylated and then oxidised to form TMO.

(ii) Guanine

Spiders, some amphibians, reptiles and birds convert NH3 into guanine, an insoluble substance hence require no water for its excretion.

(iii) Allantoin

Many mammals excrete allantoin as product of purines and pyrimidines.

In man purine is converted into uric acid while pyrimidine to alanine or aminoisobutyric acid.

(iv) Hippuric acid

In some mammals benzoic acid (formed by fat metabolism or in food) is highly toxic hence is changed into hippuric acid by combining with glycine.

(v) Ornithuric acid

In birds benzoic acid is converted into Ornithuric acid.

(vi) Creatinine

Excess amount of creatine during dystrophy of muscle is generally excreted as creatinine.