Question

Why do we excrete excess nitrogen instead of recycling it? How can we explain that there has been no selection for more nitrogen recycling in animals, as this would probably constitue a selective advantage?

Answer 1

Animals excrete excess nitrogen to prevent the accumulation of toxic ammonia. Terrestrial organisms have evolved the urea cycle to convert ammonia into urea, which is less toxic and can be excreted in urine. Excreting urea is advantageous because it reduces the risk of disrupting homeostasis and allows animals to adapt to terrestrial environments.

Step-by-step explanation:

The excretion of excess nitrogen in animals is an essential process to maintain homeostasis and prevent the accumulation of toxic ammonia. Nitrogenous wastes are produced during the breakdown of proteins and nucleic acids. Aquatic animals, such as fish, can directly release ammonia into the water. However, terrestrial animals, including humans, have evolved the urea cycle, which converts ammonia into urea, a less toxic form that can be excreted in urine. This mechanism allows animals to excrete nitrogenous waste without losing excessive amounts of water.

In terms of evolutionary advantages, the urea cycle is beneficial for terrestrial organisms because urea is less toxic than ammonia. Excreting urea reduces the risk of raising the pH of body fluids and disrupting homeostasis. Additionally, the conversion of ammonia into urea requires ATP and water, indicating that excreting urea is energetically costly. However, this cost is outweighed by the advantages of excreting a less toxic waste product that allows animals to live successfully in terrestrial environments.