Questions: Osmoregulation and Nitrogenous Wastes Across Animals
4 questions to test your understanding
Score: 0 / 4
Question 1 Multiple Choice
Birds excrete most of their nitrogen as uric acid. Which pair of advantages best explains this?
AUric acid is very soluble and costs no energy to make
BUric acid has low toxicity and can be excreted as a paste with little water, and it can be stored safely inside a shelled egg
CUric acid is more toxic than ammonia and kills parasites
DUric acid is a gas and is breathed out
Uric acid is largely insoluble, so it does not reach toxic levels in solution and can be excreted with very little water. That saves water for a flying animal and lets the embryo inside a shelled egg store its waste as harmless crystals. The cost is that making uric acid uses more energy than making urea or ammonia.
Question 2 True / False
A freshwater bony fish drinks large amounts of water to stay hydrated.
TTrue
FFalse
Answer: False
A freshwater fish's body fluids are saltier than the water around it, so water constantly enters by osmosis, mainly through the gills. The fish drinks very little, makes large amounts of dilute urine and takes up salt actively through its gills.
Question 3 Short Answer
Describe the water and salt problem a marine bony fish faces and how it solves it.
Think about your answer, then reveal below.
Model answer: Its body fluids are less salty than seawater, so it loses water by osmosis and gains salt by diffusion. It drinks seawater, pumps the excess salt out through specialized gill cells, and produces only small amounts of urine.
The problem is the reverse of a freshwater fish's. Sharks and rays use a different solution: they keep high levels of urea in their blood, which brings their body fluids to about the same osmotic strength as seawater or slightly above.
Question 4 Multiple Choice
Which nitrogenous waste is most toxic but needs the least energy to produce, and is common in aquatic animals?
AUric acid
BUrea
CAmmonia
DCreatinine
Ammonia is very toxic but also very soluble, and aquatic animals can lose it by diffusion across gills or skin into large volumes of water. Converting it to urea or uric acid costs energy, which is worthwhile only when water is scarce.