Excretion is how organisms get rid of nitrogen-containing waste products of metabolism.
Of all waste products, nitrogenous waste (nitrogen-containing waste) is one of the most important because of its effect on osmoregulation. The most prominent of these nitrogenous wastes are proteins. When proteins get broken down in the body, enzymes remove the nitrogen in the form of ammonia (NH2) , a very toxic molecule.
Ammonia, a very dangerous substance
However, because it requires a lot of water, releasing ammonia directly is most common in aquatic species. Many animals first combine it with one of two substances:
- Urea is produced in the liver and is a combination of ammonia and carbon dioxide (CO2), and also requires energy in the form of ATP to be produced. It's mostly excreted by mammals, most adult amphibians (animals capable of breathing both on land and in water), sharks, and some marine bony fishes and turtles. Since urea is much less toxic than ammonia alone, it [urea] can be transported and stored safely at high concentrations.
- Uric acid is a relativlely non-toxic substance that is insoluble in water, so it is great for animals with little access to water. However, uric acid is very energy consuming and even requires more ATP than urea in order to produce. Animals that excrete uric acid are insects, landsnails, many reptiles, and birds.