Types of Nitrogenous Waste

Metabolic processes produce waste, and a great deal of this is in the form of nitrogen.
This combines with water to produce toxic ammonia.

Animals must safely excrete these toxins. This is the function of the excretory system.

x

x

x

x

x

x

    Ammonia

    • contains one nitrogen per molecule
    • is highly toxic
    • requires large amounts of water for dilution and flushing from the body.
    • Animals with constant access to fresh water
      excrete much of their nitrogenous waste as ammonia.
    • Examples: freshwater fish, amphibian larvae

x

    Urea

    Terrestrial animals have less access to water, and have been selected to
    • repackage ammonia as less toxic compounds
    • flush those compounds with less water

    Urea...

    • requires metabolic energy to build (unlike ammonia)
    • is manufactured in the liver, excreted by kidneys
    • contains two nitrogens per molecule
    • is less toxic than ammonia
    • requires less water to dilute and flush from the body
    • is excreted by a wide variety of animals
    • Examples: adult amphibians, mammals, marine fish

x

    Uric acid

    Animals that evolved in very dry environments have been
    naturally selected to excrete nitrogen using even less water.

    Uric acid...

    • requires more metabolic energy to build than either ammonia or urea
    • contains four nitrogens per molecule
    • has very low toxicity, so does not need to be diluted
    • is not readily water soluble
    • requires little water for flushing from the body
    • is the primary nitrogenous waste product of reptiles (including birds)

    Poop or Pee?

    The white, crystalline substance you see in a typical bird or lizard poop is actually urine.
    • white is uric acid crystals
    • brown/green is fecal matter

    Animals have evolved to produce the nitrogenous waste
    products most adaptive in their particular environment.

    Consider what type of environmental pressures would
    drive natural selection for each of these excretory products.