23.7:
Comparative Excretory Systems
Biological macromolecules, carbohydrates, lipids, proteins, and nucleic acids, are the most important large molecules in the body.
Proteins and nucleic acids contain nitrogen that is often a byproduct when these molecules are broken down. Excess nitrogen in the body tends to form ammonia, which is highly toxic and must be removed either directly or after conversion to urea or uric acid.
Most aquatic animals directly release ammonia into their environment. Much of the ammonia is lost to diffusion, so this process is not energy-intensive. However, ammonia can only be tolerated at low concentrations, so these animals require lots of water to dilute it in.
For many organisms, this water cost is too hefty. Mammals, many adult amphibians, and some marine organisms convert ammonia into urea before expelling it from the body.
Urea is much less toxic than ammonia, so it requires less water for its removal. However, converting ammonia into urea requires energy.
Birds, reptiles, and insects convert ammonia primarily into uric acid. Uric acid can be excreted in a more solid form, requiring very little water. However, converting ammonia into uric acid is even more energy-intensive than conversion to urea.
The advantages and costs of these ammonia-removal methods, direct release or conversion to urea or uric acid, reflect the adaptations by organisms to different habitats.
Fossil evidence indicates that life began in water. As organisms moved to land, dry conditions likely spurred the evolution of the uric acid pathway, allowing animals to conserve more water.
Different reproductive characteristics also favored distinct methods of nitrogenous waste removal.
For example, the water-solubility of urea allows mammalian embryos to remove waste in their mother's blood.
On the other hand, the solid nature of uric acid allows waste from bird embryos to form harmless lumps inside of eggs, which have hard shells that urea would be unable to pass through.
23.7:
Comparative Excretory Systems
Animals have evolved different strategies for excretion, the removal of waste from the body. Most waste must be dissolved in water to be excreted, so an animal’s excretory strategy directly affects its water balance.
Nitrogenous wastes are some of the most significant forms of animal waste. Nitrogen is released when proteins and nucleic acids are broken down for energy or conversion into carbohydrates and fats. Proteins are broken down into amino acids and nucleic acids into nitrogenous bases. The nitrogen-containing amino groups of amino acids and nitrogenous bases are then converted into nitrogenous wastes.
Typical nitrogenous wastes released by animals include ammonia, urea, and uric acid. These excretory strategies involve tradeoffs between conserving energy and water.
The various nitrogenous wastes reflect distinct habitats and evolutionary histories. For example, most aquatic animals are ammonotelic, meaning they directly excrete ammonia. This approach is less energy-intensive than converting ammonia into urea or uric acid before excretion, but also requires more water. For terrestrial organisms, which face perhaps no more significant regulatory threat than dehydration, water conservation is worth the extra energy cost.
Ureotelic animals, like mammals and sharks, convert ammonia into urea before excretion. Urea is less toxic than ammonia and requires less water for removal from the body. Many amphibians that move from aquatic to terrestrial habitats excrete ammonia primarily as tadpoles but excrete mostly urea as adults on land.
Uricotelic organisms, including reptiles, birds, and many insects, convert ammonia into uric acid before excretion. Uric acid is not water-soluble and is excreted as a paste or powder, using very little water. Uric acid is even less toxic than urea. However, converting ammonia into uric acid requires even more energy than conversion into urea.
These different excretory strategies allow animals to meet the unique water and energy demands of their environments.
Suggested Reading
Wright, P A. “Nitrogen Excretion: Three End Products, Many Physiological Roles.” The Journal of Experimental Biology 198, no. 2 (February 1, 1995): 273.[Source]>
Spring, Jeffrey H., S. Renee Robichaux, and John A. Hamlin. “The Role of Aquaporins in Excretion in Insects.” Journal of Experimental Biology 212, no. 3 (February 1, 2009): 358. [Source]