Modes of Nitrogenous Excretion

Ammonia is the direct, unmodified product of deamination, and it is also the most toxic of the three nitrogenous waste forms an animal can excrete. Even a slight rise in ammonia concentration disturbs the body's pH (ammonia is basic), which in turn disrupts every enzyme-catalysed reaction and destabilises cell/plasma membranes. The only way to hold onto ammonia safely is to keep diluting it in large volumes of water — roughly 300-500 ml of water are needed to flush out just 1 gram of ammonia. Animals that have effectively unlimited access to water — aquatic invertebrates, bony fishes, and the aquatic larval stages of amphibians — can afford this, and are called ammonotelic. Because ammonia needs no chemical conversion before excretion, ammonotelism is the least energy-costly of the three strategies; ammonotelic animals release it through the general body surface (skin), gills, and kidneys. Animals with no dedicated excretory system at all, such as Protozoa, are also necessarily ammonotelic, since simple diffusion into the surrounding water is their only option.

When water is not available in unlimited quantity — as on land — animals instead convert ammonia into urea (H2N-CO-NH2) in the liver, via the ornithine (urea) cycle, discovered by Krebs and Henseleit in 1932; producing one molecule of urea costs the cell 3 ATP molecules. Urea is far less toxic than ammonia and far less soluble in water, so it can be tolerated at higher concentration in the blood and needs much less water to flush out — about 50 ml of water per gram of nitrogen excreted as urea in humans (roughly 100 times less water than ammonia would need), and several hundred times less water in animals such as the camel, kangaroo rat, or shark. This mode is called ureotelism, and it suits animals that must conserve some water without paying the very high energy cost of uricotelism — mammals, cartilaginous fishes (sharks and rays), many aquatic reptiles, and most adult amphibians are all ureotelic. Sharks additionally retain extra urea in their blood deliberately, to make their blood fluid isotonic with the surrounding seawater and so avoid losing water by osmosis. …