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Metabolic wastes orexcrements aresubstances left over frommetabolic processes (such ascellular respiration) which cannot be used by theorganism (they are surplus ortoxic), and must therefore beexcreted. This includesnitrogen compounds,water,CO2,phosphates,sulphates, etc.Animals treat these compounds as excretes.Plants havemetabolic pathways which transforms some of them (primarily the oxygen compounds) into useful substances.
All the metabolic wastes are excreted in a form ofwatersolutes through the excretoryorgans (nephridia,Malpighian tubules,kidneys), with the exception of CO2, which is excreted together with thewater vapor throughout thelungs. The elimination of these compounds enables the chemicalhomeostasis of the organism.
The nitrogen compounds through which excess nitrogen is eliminated from organisms are callednitrogenous wastes (/naɪˈtrɒdʒɪnəs/) ornitrogen wastes. They areammonia,urea,uric acid, andcreatinine. All of these substances are produced fromprotein metabolism. In many animals, theurine is the main route ofexcretion for such wastes; in some, it is thefeces.
Ammonotelism is the excretion of ammonia and ammonium ions. Ammonia (NH3) forms with the oxidation of amino groups.(-NH2), 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.[citation needed] Thus, the marine organisms excrete ammonia directly into the water and are calledammonotelic.[2] Ammonotelic animals include mostfish as well ascrustaceans,platyhelminths,cnidarians,poriferans,echinoderms and otheraquaticinvertebrates.[3]
The excretion of urea is called ureotelism. Land animals, mainlyamphibians andmammals, convert ammonia into urea, a process which occurs in theliver and kidney. These animals are calledureotelic.[3] 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.[citation needed]
Uricotelism is the excretion of excess nitrogen in the form ofuric acid.[4]Uricotelic animals includeinsects,birds and mostreptiles. Though requiring more metabolic energy to make than urea, uric acid's low toxicity and low solubility in water allow it to be concentrated into a small volume of pasty white suspension in feces, compared to the liquid urine of mammals.[3] Notably however, great apes and humans, while ureotelic, are also uricotelic to a small extent, with uric acid potentially causing problems such askidney stones andgout, but also functioning as a blood antioxidant.
These compounds form during thecatabolism of carbohydrates andlipids in condensation reactions, and in some other metabolicreactions of the amino acids. Oxygen is produced by plants and some bacteria in photosynthesis, while CO2 is a waste product of all animals and plants. Nitrogen gases are produced by denitrifying bacteria and as a waste product, and bacteria for decaying yield ammonia, as do most invertebrates and vertebrates. Water is the only liquid waste from animals and photosynthesizing plants.[5]
Nitrates andnitrites are wastes produced bynitrifying bacteria, just as sulfur and sulfates are produced by thesulfur-reducing bacteria andsulfate-reducing bacteria. Insoluble iron waste can be made byiron bacteria by using soluble forms. In plants, resins, fats, waxes, and complex organic chemicals are exuded from plants, e.g., the latex from rubber trees and milkweeds. Solid waste products may be manufactured as organic pigments derived from breakdown of pigments like hemoglobin, and inorganic salts like carbonates, bicarbonates, and phosphate, whether in ionic or in molecular form, are excreted as solids.[5]
Animals dispose of solid waste asfeces.