Ammonia is a common metabolic waste product of nitrogen-
learning outcomes
You should be able to:
Compare the properties of ammonia, urea, and uric acid as nitrogenous waste products.
Describe the relative advantages and disadvantages of excreting nitrogenous waste as urea or as uric acid.
When amino acids are broken down, a first step is removal of the amine group, forming ammonia. What happens to that ammonia in a fish and in a mammal? Explain why this happens.
Ammonia is toxic and must be either excreted rapidly or detoxified. Ammonia is highly soluble in water and diffuses rapidly, so in fish, ammonia can diffuse into the blood and then be lost to the environment by continuous diffusion across the gill membranes. Because mammals are tidal breathers, the ammonia in their blood would have to reach higher levels to be lost by diffusion across alveolar membranes, so mammals detoxify ammonia by converting it into urea or uric acid.
Why do sharks and rays produce urea rather than uric acid to raise the osmotic concentration of their extracellular fluids?
Uric acid is considerably less soluble than urea. It precipitates out of solution in humans, in whom the total osmolarity of extracellular fluid is 300 mosm/L and therefore the contribution of uric acid to that concentration is considerably less than 300 mosm/L. Sharks and rays could not produce an extracellular osmolarity greater than that of seawater by retaining uric acid.
For terrestrial animals, what is the advantage of excreting uric acid as the nitrogenous waste product?
Most terrestrial animals have to conserve water. The advantage of excreting uric acid is that it can be precipitated out of solution in the excretory system, enabling the reabsorption, and therefore conservation, of water.
Animals exhibit a variety of adaptations for dealing with the challenges of salt and water balance in different environments. All of these adaptations, however, are based on two basic mechanisms—