Question

2. Excretion of nitrogen-containing wastes is a problem for animals because nitrogen-containing waste products can be toxic to cells. Three different mechanisms have evolved in animals for removal of nitrogenous waste:

• ammonotelism: excretion of ammonia
• ureotelism: excretion of urea
• uricotelism: excretion of uric acid

The least energy-consuming mechanism is ammonotelism, or ammonia excretion. However, ammonia is highly toxic to animal cells and must be eliminated quickly to preserve cell integrity. Urea is less toxic to cells than ammonia, but producing it requires more energy expenditure than producing ammonia. Production of uric acid is the most energy consumptive of the three, but uric acid is the least toxic of the three waste forms. Cells can be exposed to uric acid for long periods of time without harm.  

In addition, ammonia and urea are highly soluble in water. Dilution of these compounds reduces their toxicity and can help to move them quickly away from cells. Uric acid is insoluble in water and tends to precipitate out as a semi-solid when it builds up in some quantity.

a. The data in the table below show the results of chemical analysis of animal excrement for compounds containing nitrogen. The quantities listed in the table reflect the percentage of the total nitrogen in the form of ammonia, urea, and uric acid. Analyze the data and describe the relationship between the type of excretion and the type of organism and its environment.

	Species name	% of Total Nitrogen
		Ammonia	Urea	Uric Acid
Aquatic turtles	Kinosternon subrubrum	24	23	<1
	Pelusios derbianus	19	24	5
	Chelydra serpentina	11	80	10
	Chelodina longicollus	15	78	4
Terrestrial tortoises	Testudo sulcata	3	20	55
	Testudo elegans	6	9	56
	Gopherus berlandieri	9	4	86
	Gopherus agassizii	3	15	20
Snakes	Zamensis diadema	4	2	69
	Eryx thebacious	6	0	63
	Coluber constrictor	0	0	58

b. The type of excretion mechanism used by an animal reflects a balance among the amount of energy needed to produce the waste, the toxicity of the waste, and the availability of water in the animal’s environment that could be used to flush the waste away. Propose an explanation for the observed excretion mechanisms in turtles, tortoises, and snakes that takes into account the toxicity, water solubility, and energy requirements related to each mechanism.

Answer 1

	Species name	% of Total Nitrogen
Ammonia	Urea	Uric Acid
Aquatic turtles	Kinosternon subrubrum	24	23	<1
Pelusios derbianus	19	24	5
Chelydra serpentina	11	80	10
Chelodina longicollus	15	78	4
Terrestrial tortoises	Testudo sulcata	3	20	55
Testudo elegans	6	9	56
Gopherus berlandieri	9	4	86
Gopherus agassizii	3	15	20
Snakes	Zamensis diadema	4	2	69
Eryx thebacious	6	0	63
Coluber constrictor	0	0	58

a) The auqatic animals produce higher percents of urea, while uric acid levels are very low. On the other hand, the terrestrial organisms, both turtoises and snakes, produce greater percents of uric acid. Snakes, specially, produce extremely low levels of urea.

Aquatic turtles may be have opt to produce urea, to avoid exposing themselves to the toxic ammonia, and don't go into the high energy consumption of uric acid. In this way the turtles managed to avoid toxicity and stay in an intermediate state of energy consumption for the purpose.

On the opposite side, uric acid is widely produced by terresttrial reptiles. The reason of this lies in the fact that, as uric acid is less soluble in water, the organisms need to spend more water in urine in order to excrete it. The terrestrial species here opt to excrete uric acid, even when it is more energy consuming, because it allows them to preserve more water in their bodies.

b) The mechanism is, aquatic turtles follow the path of avoiding ammonia toxicity and, since they don't have the issue of lack of water, they can opt for the urea, as it is less energy consuming than uric acid. The terrestrial reptiles don't have the same problem about ammonia because their excretions are not going to dilute in their surroundings, but they have an issue about preserving their water resource, that's why the opt for the uric acid, since excreting it won't make them waste as much water, even when it is more energy consuming.