Metabolic wastes or excrements are

substance Substance may refer to: * Matter, anything that has mass and takes up space Chemistry * Chemical substance, a material with a definite chemical composition * Drug substance ** Substance abuse, drug-related healthcare and social policy diagnosis ...

s left over from

metabolic Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...

processes (such as cellular respiration) which cannot be used by the organism (they are surplus or toxic), and must therefore be

excreted Excretion is a process in which metabolic waste is eliminated from an organism. In vertebrates this is primarily carried out by the lungs, kidneys, and skin. This is in contrast with secretion, where the substance may have specific tasks after lea ...

. This includes nitrogen compounds, water, CO₂, phosphates, sulphates, etc. Animals treat these compounds as excretes. Plants have chemical "machinery" which transforms some of them (primarily the nitrogen compounds) into useful substances. All the metabolic wastes are excreted in a form of water solutes through the excretory organs ( nephridia, Malpighian tubules, kidneys), with the exception of CO₂, which is excreted together with the water vapor throughout the

lung The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of t ...

s. The elimination of these compounds enables the chemical homeostasis of the organism.

Nitrogen wastes

The nitrogen compounds through which excess nitrogen is eliminated from organisms are called nitrogenous wastes () 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, it is the

feces Feces ( or faeces), known colloquially and in slang as poo and poop, are the solid or semi-solid remains of food that was not digested in the small intestine, and has been broken down by bacteria in the large intestine. Feces contain a relati ...

Ammonotelism

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. Thus, the marine organisms excrete ammonia directly into the water and are called ammonotelic. Ammonotelic animals include crustaceans, platyhelminths,

cnidarian Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter. Their distinguishing feature is cnidocytes, specialized cells that th ...

porifera Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through th ...

ns, echinoderms, and other aquatic invertebrates.

Ureotelism

The excretion of urea is called ureotelism. Land animals, mainly

amphibian Amphibians are tetrapod, four-limbed and ectothermic vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terres ...

s and

mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...

s, 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.

Uricotelism

Uricotelism is the excretion of excess nitrogen in the form of uric acid. Uricotelic animals include insects, birds and most

reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...

s. 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. Notably however, great apes and humans, while ureotelic, are also uricotelic to a small extent, with uric acid potentially causing problems such as kidney stones and gout, but also functioning as a blood antioxidant.

Water and gases

These compounds form during the

catabolism Catabolism () is the set of metabolic pathways that breaks down molecules into smaller units that are either oxidized to release energy or used in other anabolic reactions. Catabolism breaks down large molecules (such as polysaccharides, lipids, ...

of carbohydrates and lipids in condensation reactions, and in some other metabolic

reactions Reaction may refer to a process or to a response to an action, event, or exposure: Physics and chemistry *Chemical reaction *Nuclear reaction *Reaction (physics), as defined by Newton's third law *Chain reaction (disambiguation). Biology and me ...

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."excretion." Encyclopædia Britannica. Encyclopædia Britannica Ultimate Reference Suite. Chicago: Encyclopædia Britannica, 2010.

Solids

Nitrate Nitrate is a polyatomic ion A polyatomic ion, also known as a molecular ion, is a covalent bonded set of two or more atoms, or of a metal complex, that can be considered to behave as a single unit and that has a net charge that is not zer ...

s and

nitrite The nitrite polyatomic ion, ion has the chemical formula . Nitrite (mostly sodium nitrite) is widely used throughout chemical and pharmaceutical industries. The nitrite anion is a pervasive intermediate in the nitrogen cycle in nature. The name ...

s are wastes produced by nitrifying bacteria, just as sulfur and sulfates are produced by the sulfur-reducing bacteria and sulfate-reducing bacteria. Insoluble iron waste can be made by

iron bacteria Iron-oxidizing bacteria are chemotrophic bacteria that derive energy by oxidizing dissolved ferrous iron. They are known to grow and proliferate in waters containing iron concentrations as low as 0.1 mg/L. However, at least 0.3 ppm of dissolved ox ...

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. Animals dispose of solid waste as

References

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