Modes of Excretion
கழிவுநீக்க முறைகள்
Ammonia is the most toxic waste but needs the most water to remove; uric acid is the least toxic and needs the least. Match the animal's water supply to its waste and you can answer every 'ammonotelic vs uricotelic' question.
The idea
Animals accumulate ammonia, urea, uric acid, carbon dioxide, water and ions such as Na+, K+, Cl-, phosphate and sulphate through metabolism or excess intake. Ammonia is the most toxic and needs large amounts of water for elimination; uric acid is the least toxic and can be removed with minimum loss of water. Animals that excrete ammonia are ammonotelic — many bony fishes, aquatic amphibians and aquatic insects; ammonia, being readily soluble, is excreted by diffusion across body surfaces or through gill surfaces as ammonium ions. Mammals, many terrestrial amphibians and marine fishes mainly excrete urea and are ureotelic; ammonia is converted into urea in the liver. Reptiles, birds, land snails and insects excrete nitrogenous wastes as uric acid in the form of pellet or paste with minimum loss of water and are uricotelic. Excretory structures vary: protonephridia or flame cells in Platyhelminthes (e.g. Planaria), rotifers, some annelids and Amphioxus; nephridia in annelids such as the earthworm; Malpighian tubules in most insects including cockroaches; antennal or green glands in crustaceans like prawns.
Why it matters
Protein breakdown releases nitrogen as toxic ammonia; each animal converts it to the form its water budget can afford to flush out.
NEET lens
Know the three categories with NCERT examples and the excretory organ of each phylum: flame cells — Planaria (Platyhelminthes), Taenia; nephridia — earthworm (Pheretima); Malpighian tubules — cockroach (Periplaneta); antennal/green glands — prawn; contractile vacuole — Paramoecium (osmoregulation). 'Pellet or paste' = uricotelic birds like Pavo (peacock).
Clinical significance
In liver failure the urea cycle slows, ammonia builds up in blood and affects the brain. In humans, excess uric acid in blood crystallises in joints and causes gout.
Introduction
Every animal must remove the nitrogen left over from breaking down proteins. Depending on how much water it can spare, it excretes ammonia, urea or uric acid, using very different excretory organs.
Core concept
Ammonotelic (ammonia; most toxic, most water): many bony fishes, aquatic amphibians, aquatic insects — by diffusion across body surface/gills as NH4+. Ureotelic (urea): mammals, many terrestrial amphibians, marine fishes — urea made in the liver from ammonia; some urea retained in kidney matrix for osmolarity. Uricotelic (uric acid; least toxic, least water; pellet/paste): reptiles, birds, land snails, insects. Organs: protonephridia/flame cells — Platyhelminthes (Planaria), rotifers, some annelids, Amphioxus (mainly osmoregulation); nephridia — earthworm and other annelids; Malpighian tubules — most insects including cockroach; antennal/green glands — crustaceans like prawn. Vertebrates: kidneys.
Go deeper
Flame cells in flatworms are mainly for osmoregulation and ionic balance. In marine fishes urea is retained, and in mammals some urea stays in the kidney medulla to keep it hyperosmotic.
From Class 10
Class 10 said kidneys remove nitrogenous wastes like urea. NEET adds why different animals make ammonia, urea or uric acid, and which organs they use.