<P> The nitrogen compounds through which excess nitrogen is eliminated from organisms are called nitrogenous wastes (/ naɪˈtrɒdʒɪnəs /) or nitrogen wastes . They are ammonia, urea, uric acid, and creatinine . All of these substances are produced from protein metabolism . In many animals, the urine is the main route of excretion for such wastes; in some, the feces is . </P> <P> Ammonotelism is the excretion of ammonia and ammonium ions . Ammonia (NH) forms with the oxidation of amino groups. (- NH), which are removed from the proteins when they convert into carbohydrates . It is a very toxic substance to tissues and extremely soluble in water . Only one nitrogen atom is removed with it . A lot of water is needed for the excretion of ammonia, about 0.5 L of water is needed per 1 g of nitrogen to maintain ammonia levels in the excretory fluid below the level in body fluids to prevent toxicity . Thus, the marine organisms excrete ammonia directly into the water and are called ammonotelic . Ammonotelic animals include protozoans, crustaceans, platyhelminths, cnidarians, poriferans, echinoderms, and other aquatic invertebrates . </P> <P> The excretion of urea is called ureotelism . Land animals, mainly amphibians and mammals, convert ammonia into urea, a process which occurs in the liver and kidney . These animals are called ureotelic . Urea is a less toxic compound than ammonia; two nitrogen atoms are eliminated through it and less water is needed for its excretion . It requires 0.05 L of water to excrete 1 g of nitrogen, approximately only 10% of that required in ammonotelic organisms . </P> <P> Uricotelism is the ridding of excess nitrogen using uric acid . This method is used by birds and diapsids, insects, lizards, and snakes, and these animals are called uricotelic . Uric acid is less toxic than ammonia or urea . It contains four nitrogen atoms and only a small amount of water (about 0.001 L per 1 g of nitrogen) is needed for its excretion . Uric acid is the least soluble in water and can be stored in cells and body tissues without toxic effects . A single molecule of uric acid can remove four atoms of nitrogen making uricotelism more efficient than ammonotelism or ureotelism . </P>

Explain why insects excrete uric acid as their principal nitrogenous waste