Define Ammonotelism, Ureotelism, Uricotelism & Guanotelism organism.
Ammonotelic organism excretes soluble ammonia as a result of deamination. Ammonia is highly toxic to tissues and extremely soluble in water. Hence, it can be excreted as such only when there is sufficient water for its rapid removal from the body in the form of a dilute. solution.
A proportion of 0.5 L of water is required per 1g of nitrogen to maintain ammonia levels in the excretory fluid below the level in body fluids, otherwise toxicity may result. Ammonotelic animals include mainly aquatic animals, amphibians, protozoans, crustaceans, platyhelminths, cnidarians, poriferans, echinoderms, and other aquatic invertebrates, among others.
Animals that excrete excess nitrogen in the form of urea are ureotelic. Terrestrial animals cannot use water freely for excretion (because of less availability in the environment), so ammonia is converted into a less toxic and easily soluble product, urea. In mammals and semi-terrestrial adult amphibians, urea is a major nitrogenous excretory product, therefore these animals are called ureotelic.
When the urea is synthesized in the liver of ureotelic vertebrates by a metabolic pathway from one molecule of carbon dioxide and two of ammonia (NH3), then the whole process is called ornithine-urea cycle. This process is discovered by Kreb and Hensleit in 1932. This metabolic cyclic pathway involves five enzyme-catalised reactions explained below:
- In the first step Ammonia, carbon dioxide and ATP are converted into carbamoylphosphate in the presence of the enzyme carbamoylphosphate synthetase.
- Carbamoylphosphate and ornithine combine to form citrulline in the presence of enzyme Ornithine carbamoyl-transferase.
- Citrulline is then used in the presence of ATP and aspartic acid far the synthesis of argininosuccinic acid by the enzyme argininosuccinate synthetase.
- Arginine and fumararic acid are obtained from the Argininosuccinic acid and the reaction is catalyzed by argininosuccinate lyase.
- In the final step, arginine splits into urea and ornithine in the presence of the enzyme arginase and the cycle is completed.
In the process of the detoxification of ammonia, three molecules of ATP are consumed. This ornithine-urea cycle is associated with tricarboxylic acid cycle (Kreb cycle) and glutamate dehydrogenase reaction.
Animals that excrete a major portion of nitrogenous waste in the form of semi-solid or solid uric acid are called uricotelic animals (e.g. birds, lizards, snakes, terrestrial insects, snails.) In these animals ammonia is converted into less toxic, relatively insoluble uric acid, which can be excreted with a relatively small amount of water. Inosinic acid metabolic pathway is responsible for the synthesis of uric acid from ammonia in these animals. This pathway was first diseovered by Buchanan and his coworkers in the 1950’s in pigeon liver.
Many insects store excretory products in their body instead of eliminating them. The phenomenon is called storage excretion. It saves the expenditure of water during excretion. Since uric acid is non-toxic and highly insoluble, it can be retained in the body for a long time without any ill-effect. Cockroaches are adapted to this technique and they store uric acid in fat body and cuticle. In turn the stored uric acid provides a nitrogen deposits for the mobilization at the times of nitrogen deficiency.
Arachnids (e.g. spiders, scorpions, etc.) excreted mostly guanine, which is a nitrogen base and hence are said to be guanotelic. Like uric acid, guanine is a purine relatively non-toxic and insoluble in water. Thus, it is also excreted in solid form. It is an adaptation to life in arid habitats i.e. dry habitat without moisture. In spiders and scorpions, the inosinic acid pathway terminates in guanine formation.