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Introduction
The process of removal of non-gaseous nitrogenous wastes like ammonia, urea, uric acid etc. along with an excess of water, salts, and pigments out of the body, is called excretion. The main aim of excretion is to keep a constant internal chemical composition called homeostasis. On the basis of the nature of nitrogenous end products, there are three modes of excretion.
Excretory Organs
The organs involved in the expelling of the waste products are called excretory organs. In animal groups like protozoans (Amoeba sping (Scypha) and coelenterates (Hydra), the main con matter is ammonia. The ammonia, being highly self in water, rapidly diffuses through their general hab surface so these have no special excretory or So ammonia is mainly lost by diffusion. But vertebrates of higher group animals have special come structures for performing excretion such as Kidney, Ureter, Urethra and forms excretory system.
Types of Excretion
. Based on the nature of nitrogenous end products, there are three modes of excretion.
Ammonotelism
It is the type of excretion in which the main nitrogenous waste is ammonia. The animals with ammonotelism are called ammonotelic animals.
Occurrence Ammoniotelic excretion is found in many aquatic animal groups like protozoans e.g., Amoeba, Paramecium; sponges e.g., Scypha; coelenterates e.g., Hydra; crustaceans e.g., Palaemon;. aquatic insects; echinoderms e.g. Asterias; bony fishes e.g., Teleosts; tadpole larvae and some adult aquatic amphibians e.g., salamanders
Formation Ammonia is the basic nitrogenous catabolic product of proteins and is produced in the. liver cells by oxidative deamination of excess of amino acids in the presence of an oxidase enzyme
Ammonia is highly toxic because it has high pH, so it must either be metabolized or expelled out of body, so that its concentration remains low in the blood.
Water loss in ammonotelism:
Ammonia is highly soluble in water so a large amount of water is required to expel it out of the body. One gram of ammonia needs about 300-500 ml of water to be expelled out of the body. So ammonia excretion involves loss of a considerable amount of water. So ammonotelism is mainly found in aquatic animals. There is correlation between ammonotelism and aquatic mode of living.
Ureotelism :
It is a type of excretion in which the main nitrogenous waste is urea and animals showing ureotelism, are called ureotelic animals.
Occurrence:
It is found in those land animals which can lose. large amount of water without any ill-effect (e.g frog, which can lose water equal to one-third of its body weight in a day); and those animals which can retain considerable amounts of urea (about 2.5%) as a major osmolyte to maintain osmolarity of their blood (elasmobranchs).
It is a common method of excretion in man and all other mammals, even aquatic mammals like whales and seals; and desert mammals like camels, kangaroo rats; terrestrial and semi aquatic amphibians like toads, frogs; cartilaginous fishes (elasmobranchs) such as sharks and rays; aquatic and semi-aquatic reptiles like alligator, terrapins and turtles. In man, urea accounts for 80-90% of total nitrogenous waste products.
Formation
In most ureotelic animals, urea is mainly formed in the liver by detoxification of ammonia, by a stepped ornithine or urea cycle. But in elasmobranchs urea is formed in all the body tissues except brain and heart.
Water loss in Ureotelic excretion
Ureotelism is associated with terrestrial modes of life with a tendency to conserve water. One gm of urea needs about 50 ml of water to be expelled out.
Significance of Ureotelism over Ammonotelism 1. Urea is about 100,000 times less toxic than a constant homeostasis.
3. Uricotelism:
Uric acid is least soluble nitrogenous wa (Thousands of times less soluble in water than amm or urea) and one gm of uric acid needs only 10 water to be expelled out of body Secondly, the acid is least toxic nitrogenous waste so does not h if it remains in body tissues for long period So excess of uric acid is of greater advantage to land which have limited access to water So uricotelism s adaptation to terrestrial mode of life.
Occurrence
It is common method of excretion in birds, land and reptiles eg: lizards and snakes, insects eg cockroach and land snails eg Helix
Formation
Uric acid is formed from ammonia and purines (adenine and guanine) mainly in the liver and is a high energy dependent. In human beings, the main sources of uric acid are purines. Purines like Adenine and Guanine change into Xanthine which changes into uric acid.
Conclusion
Henceforth this unit helps us to learn about excretion. Definition and types of excretion. We also learned about different types of excretion based on their excretory products. Excretion is different from egestion, we got to know this. There are three types of animals that are divided on the basis of excretion: Ammoniotelic, Ureotelic and Uricotelic. We humans are Ureotelic animals.
