The principal metabolic waste products in most animals are water, carbon dioxide, and nitrogenous wastes. Carbon dioxide and some water re eliminated from the bodies mainly by respiratory structures, whereas most of the water and nitrogenous wastes are excreted by special excretory organs.
A nitrogenous waste includes ammonia, uric acid and urea, recall that amino acids and nucleic acids contain nitrogen. During the breakdown of amino acids, the nitrogen-containing amino group is removed by a process called deamination and converted to ammonia. However, ammonia is highly toxic. Some aquatic simple animals excrete it is dissolved form into the surrounding water through the process of diffusion before it can build up to toxic levels in their tissues. But in complex terrestrial organisms, including humans, ammonia being very toxic, is converted to some less toxic nitrogenous wastes such as urea or uric acid.
Uric acid is produced both from ammonia and by the breakdown of nucleotides from nucleic acids. Uric acid is the least toxic and can be excreted in a crystalline form with a little use of water. This is an important adaptation for conserving water in many terrestrial animals such as insects, reptiles and birds.
In birds, the frequent excretion of uric acid in the faeces and the absence of urinary bladder keeps the body weight light which is very important for flight.
Urea is the principal nitrogenous waste product of amphibians and mammals. It is produced mainly in the liver. Urea is far less toxic than ammonia and so can accumulate to some extant in the body without causing tissue damage. Because urea is highly soluble, it is excreted dissolved in water. For its excretion, urea needs less water than ammonia but need more water than uric acid.
13.3.1 Excretion in amoeba
In amoeba and other unicellular freshwater animals contractile vacuoles are present. These contractile vacuoles are osmoregulatory in function as they are involved in the removal of excess water from the body. This water however also carries with it some of the ammonia and CO2 which are produced in metabolism. However, most of the ammonia and CO2 diffuse out of the body through cell membrane in the dissolved form (fig 13.2)
