Short notes on the metabolic waste products

Metabolic waste products:

They may be grouped under the following headings:

1. Respiratory waste products:

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Catabolic waste products of food stuffs are CO2 and water. In higher animals CO2 is excreted with the expired air through lungs. Excess of water is eliminated in the form of urine and sweat.

2. Nitrogenous waste products:

Nitrogenous waste products are de­rived from the deamination of the excess of amino acids taken in with the diet and also from the breakdown of animal proteins and nucleic acids. These are of following types:

a. Ammonia:

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It is formed by the deamination of amino acids during pro­tein metabolism. Ammonia is highly toxic and its rapid elimination is essential. It is highly soluble in water and is quickly eliminated into sur­rounding water in acquatic animals such as Protozoans, Hydra, etc.

b. Urea:

It is less toxic than ammonia. Most mammals, fishes and am­phibians excrete their nitrogenous waste products in the form of urea.

c. Uric acid:

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Insects, some snails, birds and reptiles with few exceptions, excrete uric acid. Uric acid is excreted in solid form. As uric acid is in­soluble in water, it crystallizes out from the fluid urine and water is reab­sorbed, thereby allowing conservation of water.

d. Amino Acid:

In certain animals like molluses and echinoderms, the excess of amino acids is removed as such without undergoing any change.

e. Other nitrogenous compounds:

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Allantoin and allantoic acid are other nitrogenous excretory products. These are insoluble in water and are formed during embryonic development by amniotes with shelled eggs.

There are certain other nitrogenous waste products such as guanine and adenine from nucleic acid break down and creatine from the creatinine of muscles that are excreted along with other nitrogenous waste products in the urine.

3. Mineral Ions:

Sodium, potassium, calcium, magensium, chloride and ammonia are the essential mineral ions of the animals. The excess of these ions taken in with the diet are excreted by one means or the other.