Download 15.3 Nitrogenous Excretion

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Nitrogenous Excretion
Carbohydrates, proteins and fats form the major
components of the diet for most animals
Carbohydrates and fats are metabolised to carbon
dioxide and water; proteins and nucleic acids yield
carbon dioxide and water but they also give rise to
nitrogen-containing excretory products
The metabolism of amino acids involves the removal
of the amino group (deamination) to form ammonia
In some animals, the ammonia is excreted unchanged
whereas in others it is synthesised into
either urea or uric acid
Nitrogen is excreted as ammonia,
urea or uric acid
Nitrogenous Excretion
The form in which nitrogen is excreted is closely
related to the habitat of the animal and the
availability of water
Most aquatic organisms excrete their
nitrogenous waste as ammonia; ammonia is a
highly toxic compound and requires large
amounts of water for its excretion
The high solubility and small molecular size of
ammonia means that it can diffuse rapidly through
any surface in contact with water; ammonia does not
need to be excreted through the kidneys
Freshwater fish live in a hypotonic environment and excrete
much of their nitrogenous waste in the form
of ammonia, through their gills
Nitrogenous Excretion
Urea is formed in the liver of vertebrates by the
ornithine cycle; urea is less soluble than ammonia
but its toxicity is relatively low
The low toxicity of urea means that less water is
required for its excretion, and it is the main excretory
product for adult amphibians and mammals
The ammonia, produced from the deamination of
amino acids, enters the ornithine cycle
where it is converted into urea
Urea is the principal nitrogenous excretory product for
terrestrial mammals and amphibians
The aquatic larvae of the frog excrete
ammonia, and the semi-aquatic adult
excretes urea; this is a reflection of
changes in the habitat and water
availability; at the onset of
metamorphosis, there is a marked
increase in the activity of liver enzymes
that regulate the ornithine cycle
Nitrogenous Excretion
Uric acid is the principal, nitrogenous excretory
product in insects, land snails, most reptiles and birds
The formation of uric acid by these groups of
animals is an adaptation for water conservation
in a terrestrial habitat
Uric acid is a relatively non-toxic compound of
low solubility in water and requires
little water for its excretion
Uric acid can be stored in body tissues, and at
high concentrations it precipitates out of
solution to form a white solid
The formation and excretion of uric acid is an
adaptation for water conservation in terrestrial habitats
The semi-solid portion of the bird
droppings in this nesting material consists
principally of uric acid; very little water
is used by birds for nitrogenous excretion
The success of insects in terrestrial habitats is largely due
to their ability to conserve water; some insects do not
excrete uric acid but store this compound in structures
called 'fat bodies', and therefore require no water for its
elimination
Nitrogenous Excretion and Reproduction
There is a correlation between the mode of
reproduction of the higher vertebrates and the nature
of their nitrogenous excretory products
The mammalian embryo and the amphibian egg both
develop in a watery environment that allows for the
efficient elimination of nitrogenous waste
The development of reptiles and birds takes place
within a shelled, closed egg (the ‘cleidoic egg’)
where water supply is limited
Unlike ammonia or urea, uric acid can be precipitated and
deposited as crystals within the egg membranes; in
precipitate form, the uric acid is rendered non-toxic, and
this is equivalent to elimination of the nitrogenous waste
Nitrogenous excretory products correlate with the mode of
reproduction exhibited by various groups of animals
Nitrogenous Excretion and Metabolism
In a metabolic sense, the least expensive nitrogenous
waste product is ammonia
Ammonia is the immediate product of the
deamination of amino acids and requires no further
modification when excreted by aquatic animals
In terrestrial animals, where water conservation is
essential to life, the conversion of toxic ammonia into
less toxic urea and uric acid requires ATP, and hence
places metabolic demands on the organisms concerned
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