A trout lives in a stream. A dog lives in a house…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A trout lives in a stream. A dog lives in a house. A sparrow lays an egg with a hard shell in its nest.
- Which animal needs the most water to remove its nitrogenous waste? Name the waste.
- Which animal needs the least water? Name the waste.
- Why can the embryo of the sparrow store its waste inside the shell?
- Why does the dog not simply excrete ammonia, which is cheaper to make?
Need a reminder? The key ideaShow
The three nitrogenous wastes are ammonia (one nitrogen atom in a molecule), urea (two) and uric acid (four). Ammonia is the most toxic and needs the most water. Uric acid is the least toxic and leaves as a paste. Ammonotelic animals are bony fishes, aquatic amphibians and aquatic insects. Ureotelic animals are mammals, many terrestrial amphibians and marine fishes. Uricotelic animals are reptiles, birds, land snails and insects. Use u, ammonia u, urea u and uric acid u.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. (a) The trout. It excretes ammonia, which is very toxic and needs a great deal of water. The stream supplies it.
Step 2. (b) The sparrow. It excretes uric acid, a paste with minimum loss of water.
Step 3. (c) Uric acid is almost insoluble, so it does not harm the embryo. It can be stored as a solid inside the egg.
Step 4. (d) Ammonia is very toxic. A land animal would need a large volume of water to keep it dilute enough, and water is scarce. So the liver of the dog makes urea, which is far less toxic.
Answer: (a) trout, ammonia; (b) sparrow, uric acid; (c) uric acid is insoluble and harmless; (d) ammonia is too toxic for the water that a land animal can spend.
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