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Animals must keep the water and salt content of their body fluids within limits, a task called osmoregulation, and they must get rid of the nitrogen released when they break down proteins and nucleic acids. Marine fish, freshwater fish and land animals face opposite water problems and solve them in different ways. The nitrogen is excreted as ammonia, urea or uric acid, and each animal's choice reflects a trade-off among toxicity, water use and energy cost. These two tasks are linked because excreting nitrogen requires water, and both are handled by excretory organs such as nephridia, Malpighian tubules and kidneys.
Set up three columns, for a marine bony fish, a freshwater bony fish and a desert mammal, and have students predict which way water moves by osmosis and what each animal must do about it. Then match the three nitrogenous wastes to animals and habitats, and ask why a bird embryo inside a shelled egg could not safely store ammonia.
Every animal cell sits in a fluid whose salt and water content must stay within narrow limits. Animals also produce nitrogen waste whenever they break down amino acids and nucleic acids. Both problems are solved by excretory systems, and the solutions differ widely with habitat.
Start with water and salt. Many marine invertebrates, such as sea stars and jellyfish, are osmoconformers: their body fluids have the same osmotic strength as seawater, so they gain or lose little water. Osmoregulators keep their internal fluids different from the outside, which costs energy.
Fish show the two opposite problems. A marine bony fish has body fluids less salty than seawater. Water leaves by osmosis and salt diffuses in. The fish drinks seawater, gets rid of excess salt through specialized cells in its gills, and makes little urine. A freshwater bony fish has the reverse problem. Its fluids are saltier than the water, so water floods in and salt leaks out. It drinks very little, produces large volumes of dilute urine, and actively absorbs salt through its gills. Land animals lose water by evaporation and must conserve it, using waterproof coverings, behavior and excretory organs that save water.
Now consider nitrogen. Removing the amino group from an amino acid produces ammonia, NH3. Ammonia is very toxic, but it is very soluble. Most aquatic animals, including most bony fish, simply let it diffuse away across their gills into the surrounding water.
On land, there is not enough water to flush ammonia away safely, so animals convert it into less toxic forms. Mammals, most adult amphibians, and sharks make urea in the liver through the urea cycle. Urea is far less toxic than ammonia, so it can be stored in the blood at higher levels and excreted in concentrated urine, but making it costs ATP. Birds, insects, many reptiles and land snails excrete uric acid. It is almost insoluble, so it leaves the body as a paste with very little water. It is also the most energy-costly of the three to make.
Egg type helps explain the pattern. An embryo in a shelled egg cannot get rid of waste. Soluble ammonia or urea would build up to toxic levels, but uric acid precipitates as harmless crystals.
Excretory organs vary across phyla. Flatworms use protonephridia, earthworms use metanephridia, insects use Malpighian tubules, and vertebrates use kidneys. The mammalian kidney, with its long loops of Henle, is one of the most effective at saving water.