NEET Biology Notes Excretory Products And their elimination Modes of Excretion
Modes of Excretion
The process of removal of nitrogenous wastes from the body is called excretion. The organs of excretion are called excretory organs. There are different types of excretory products or waste materials in different animals.
There are three modes of excretion
Ammonotelic excretion is found in aquatic animals like protozoans (e.g. Amoeba and Paramecium), sponges (e.g. Sycon), coelenterates (e.g. Hydra), aquatic arthropods (e.g. prawn), most aquatic molluscs (e.g. Pila), bony fishes (e.g. Labeo) and frog’s tadpole. Ammonia as a waste product is most toxic because of high pH and high solubility in water, so large amount of water is required for its excretion. It is lost through general body surface into surrounding water.
The main nitrogenous waste is urea. Ureotelic excretion is found in those animals, which can take in water and can retain considerable amounts of urea in their blood. It is a common method of excretion in human, whales, seals, camels, kangaroo, toads, frogs, sharks, etc. Urea formation in liver cells of mammals was studied by Krebs and Henseleit. Urea is formed in liver by detoxification of ammonia through ornithine cycle. Urea is transported in the blood by plasma.
A person, who takes large amount of protein in his diet will excrete more urea. A person having no food, water will have more urea in his blood. Less water is required for excretion of urea.
The rpain nitrogenous waste is crystals of uric acid. It is common in birds, land reptiles (snakes and lizards), insects, snails, etc. Uric acid is least soluble in water and is less toxic. Excreted in solid crystal forms. Formed from ammonia in the liver of uriotelic animals and Malpighian tubules in insects.