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2. 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

3. Nitrogen is excreted as ammonia, urea or uric acid

4. 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

5. Freshwater fish live in a hypotonic environment and excrete much of their nitrogenous waste in the form of ammonia, through their gills

6. 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

7. Urea is the principal nitrogenous excretory product for terrestrial mammals and amphibians

8. 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

9. 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

10. The formation and excretion of uric acid is an adaptation for water conservation in terrestrial habitats

11. 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

12. 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

13. 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

14. Nitrogenous excretory products correlate with the mode of reproduction exhibited by various groups of animals

15. 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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