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B3 3.2d – Student worksheet
Urea
When your body contains more amino acids than it needs, the excess amino acids are broken
down in the liver because your body cannot store them.
The first step in the breakdown of an amino acid produces a substance called ammonia. Ammonia
dissolves easily in water and is very toxic to the body. The ammonia is then converted into urea in
another reaction. Urea is also soluble in water but it is much less toxic than ammonia.
1
2
a
What are amino acids used for in the body? (1 mark)
b
Why are excess amino acids broken down? (1 mark)
a
Explain why mammals convert ammonia to urea before the urea travels in the blood from
the liver to the kidneys. (2 marks)
b
Explain why urea must be soluble. (1 mark)
c
Explain why the urea must be excreted, rather than stored in the body. (1 mark)
Not all animals excrete urea. The table shows several other substances produced from amino acid
breakdown that are excreted.
Excretory product
Animal
Habitat
urea
humans and other
mammals
land, water
ammonia
freshwater fish
fresh water such as
rivers and lakes
uric acid
birds, land-living insects
land
3
Suggest why freshwater fish do not convert ammonia to urea before excreting it. (2 marks)
4
Uric acid is non-toxic and nearly solid. It can be stored in the body for a long while without
damage.
a
Developing chicks, still in the egg, get more amino acids than they need from the egg yolk.
Suggest why uric acid is the excretory product in birds. (2 marks)
b
Many land-living insects don’t drink water. Suggest why uric acid is the best excretory
product for them. (2 marks)
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