The Ammonia Problem

How do animals manage toxic nitrogenous waste?

The breakdown of proteins by cells releases a nitrogen-containing, or nitrogenous, waste: ammonia. This creates a problem, because ammonia is poisonous! Even moderate concentrations of ammonia can kill most cells. Animal systems address this difficulty in one of two ways. Animals either eliminate ammonia from the body quickly or convert it into other nitrogenous compounds that are less toxic. The elimination of metabolic wastes, such as ammonia, is called excretion. Some small animals that live in wet environments rid their bodies of ammonia by allowing it to diffuse out of their body fluids across their skin. Most larger animals, and even some smaller ones that live in dry environments, have excretory systems that process ammonia and eliminate it from the body.

Storing Nitrogenous Wastes Animals that cannot dispose of ammonia continuously, as it is produced, have evolved ways to hold, or “store,” nitrogenous wastes until they can be eliminated. In most cases, ammonia itself cannot be stored in body fluids, because it is too toxic. Insects, reptiles, and birds typically solve this problem by converting ammonia into a sticky white compound called uric acid, which you can see in Figure 27–14. Uric acid is much less toxic than ammonia and is also less soluble in water. Mammals and some amphibians, on the other hand, convert ammonia to a different nitrogenous compound—urea. Like uric acid, urea is less toxic than ammonia, but unlike uric acid, urea is highly soluble in water.

FIGURE 27–13 Ammonia Some aquatic animals, such as this zebra flatworm, release ammonia as soon as they produce it.