Metabolism produces toxic by-products. Perhaps the most troublesome is the nitrogen-containing waste from the metabolism of proteins and nucleic acids. Nitrogen is removed from these nutrients when they are broken down for energy or when they are converted to carbohydrates or fats. The nitrogenous waste product is ammonia, a small and very toxic molecule. Some animals excrete their ammonia directly; others first convert it to less toxic wastes such as urea or uric acid (Figure 40.15). We shall see that the form of nitrogenous waste an animal excretes depends on both the animal's evolutionary history and its habitat. Ammonia Most aquatic animals excrete NITROGENOUS WASTES . as ammonia. Ammonia molecules are small and very
NITROGENOUS WASTES .. Ammonia is a toxic by-product of the metabolic removal of nitrogen from proteins and nucleic acids. Most aquatic animals get rid of ammonia by excreting it in very dilute solutions. Most terrestrial animals convert the ammonia to urea or uric acid, which conserves water because these less toxic wastes can be transported in the body in more concentrated form. soluble in water, so they easily permeate membranes. In soft-bodied invertebrates, ammonia diffuses across the whole body surface into the surrounding water. In freshwater fishes, most of the ammonia is lost as ammonium ions (NH4+) across the epithelium of the gills, with kidneys playing only a minor role in excretion of nitrogenous waste. The epithelium of the gills takes up Na + from the water in exchange for NH4 +, which helps freshwater fishes maintain Na + concentrations much higher than that in the surrounding water.
Ammonia excretion, though it works in water, is unsuitable for disposing of nitrogenous waste on land. A terrestrial animal would have to urinate copiously to get rid of ammonia, because a compound so toxic could only be transported in the animal and excreted in a very dilute solution. Instead, mammals and most 1 amphibians excrete urea. (Many marine fishes land turtles, which have the problem of conserving water in their hyperosmotic environment, also excrete ) This substance can be handled in much more concentrated form because it is about 100,000 times less toxic than ammonia. Urea excretion enables the animal to sacrifice less water to discard its nitrogenous waste, an important adaptation for living on land.
Urea is produced in the liver by a metabolic cycle that combines ammonia with carbon dioxide. The cir-culatory system carries the urea to the kidneys. As mentioned earlier, not all urea is excreted immediately by mammalian kidneys; some of it is retained in the kidneys, where it contributes to osmoregulation by helping to maintain the osmolarity gradient that func-tions in water reabsorption. Sharks, remember, also produce urea, which is retained at a relatively high concentration in the blood, which helps balance the osmolarity of body fluids with the surrounding seawater.
Amphibians that undergo metamorphosis generally switch from excreting ammonia to excreting urea during the transformation from an aquatic larva, the tadpole, to the terrestrial adult. This biochemical modification, however, is not inexorably coupled to metamorphosis. Frogs that remain aquatic, such as the South African clawed toad (Xenopus), continue excreting ammonia after metamorphosis. But if these animals are forced to stay out of water for several weeks, they begin to produce urea. Similarly, African lungfish switch from ammonia to urea excretion if their habitat dries up and they are forced to burrow in the mud and become inactive.
Land snails, insects, birds, and some reptiles excrete uric acid as the major nitrogenous waste. Because it is thousands of times less soluble in water than either ammonia or urea, uric acid can be excreted as a precipitate after nearly all the water has been reabsorbed from the urine. In birds and reptiles, the pastelike urine is excreted into the cloaca and eliminated along with feces from the intestine.
Uric acid and urea represent two different adaptations that enable terrestrial animals to excrete NITROGENOUS WASTES . with a minimal loss of water. One factor that seems to have been important in determining which of these alternatives evolved in a particular group of animals is the mode of reproduction. Soluble wastes can diffuse out of a shell-less amphibian egg or be carried away by the mother's blood in the case of a mammalian embryo. The vertebrates that excrete uric acid, however, produce shelled eggs, which are permeable to gases but not to liquids. If an embryo released ammonia or urea within a she]Jed egg, the soluble waste would accumulate to toxic concentrations. Uric acid precipitates out of solution and can be stored within the egg as a solid that is left behind when the animal hatches.
In grouping the various vertebrates according to the NITROGENOUS WASTES . they excrete, the boundaries are not drawn strictly along phylogenetic lines but depend also on habitat. Among reptiles, for instance, lizards, snakes, and terrestrial turtles excrete mainly uric acid; crocodiles excrete ammonia in addition to uric acid; and aquatic turtles excrete both urea and ammonia. In fact, individual turtles modify their NITROGENOUS WASTES . when their environment changes. A tortoise that usually produces urea can shift to uric acid production when the temperature increases and water becomes less available.
This is another example of how response to the environment occurs on two levels: Evolution determines the limits of physiological responses for a species, but individual organisms make adjustments within that range as the r environment changes. This principle also applies to the regulation of body temperature.