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Removal of nitrogenous compounds
You cannot store excess amino acids.
Amino acids contain almost as much energy as carbohydrates.
It would be wasteful to simply excrete excess amino acids.
Instead, excess amino acids are transported to the liver.
Amino acids are made of an amino group and a carbon skeleton called a keto acid.
In the liver, the amino group is removed from the carbon skeleton through a process called
deamination.
The keto acid is either used in respiration or recycled to form carbohydrates or fat for storage.
The amino group initially forms the very soluble and highly toxic compound ammonia.
Ammonia must be dealt with quickly because of its toxicity.
Ammonia can simply be excreted in water if the organism has a sufficient source of water.
Ammonia will diffuse passively out of respiratory structures such as gills.
It takes a lot of water to dissolve and flush ammonia, however, and each ammonia molecule
carries only one nitrogen atom.
Marine organisms and all terrestrial organisms will often invest some energy to convert the
ammonia into urea.
Urea is less soluble and less toxic than ammonia.
Because urea is less toxic, it can be allowed to accumulate in the blood to some extent.
Urea is transported to the kidneys for excretion.
Urea has two nitrogen atoms, and therefore takes less water to excrete.