Excretory System

Nitrogenous waste products have their origin in the breakdown of proteins by cells. Cells catabolize amino acids to obtain energy. The first step of this process is deamination. During deamination, enzymes remove the amino group as ammonia (NH₃). Ammonia is toxic, even at low concentrations, and requires large amounts of water to flush it out of the body. Many animals, including humans, create a less poisonous substance, urea, by combining ammonia with carbon dioxide. An animal can retain urea for some time before excreting it, but it requires water to remove it from the body as urine. Birds, insects, land snails, and most reptiles convert ammonia into an insoluble substance, uric acid. This way, water is not required water to remove urea from the body. This method of ammonia excretion is particularly advantageous for animals, since they all lay eggs. If the embryo excreted ammonia inside the egg, it would rapidly poison its environment. Even urea would be dangerous. However, uric acid in solid form is a safe way to store nitrogenous wastes in the egg.