Final answer:
Urea and uric acid are both involved in the excretion of nitrogenous wastes but are formed and excreted differently. Urea is less energy-intensive to produce and is excreted by mammals, while uric acid is excreted by birds, reptiles, and some insects as an adaptation to conserve water. These compounds are both produced in the liver but have different solubility properties and energetic costs for their production.
Step-by-step explanation:
The formation of urea and uric acid are similar in that both are processes for excreting nitrogenous waste, but they differ in where and how they occur. In mammals, including humans, the urea cycle converts ammonia into urea in the liver, which is then excreted through the kidneys. This cycle requires less energy and is more complex than the formation of uric acid.
In contrast, birds, reptiles, and some insects convert ammonia to uric acid via the uric acid cycle. Uric acid is less water-soluble than urea, making it a more effective form of nitrogenous waste in environments where conserving water is crucial. Therefore, the presence of the urea cycle or the uric acid cycle in different organisms can be seen as an evolutionary adaptation to their respective environments.
Ammonia is transferred to the liver in various forms, such as alanine and glutamine, where it is then converted into either urea or uric acid depending on the species. Because of this variance, the correct answer to how urea and uric acid are formed and excreted would be option 'a': Urea and uric acid both form in the liver but are excreted differently.