Excretion in Plants:
Plants, unlike animals don't contain distinguished problems about excretion. This is mainly due to the basic differences which exist in physiology and mode of life between plants and animals. Plants are producers and they synthesize all organic requirements according to metabolic demands. Plants produce only amount of protein essential to satisfy immediate demand. There is never excess of protein and thus extremely small excretion of nitrogenous waste substances. If proteins are broken down in amino acids, latter can be recycled in new proteins. Three of waste substances made by certain metabolic activities in plants (oxygen, CO2 and water) are raw materials for other reactions in plants. Excess CO2 and water are utilized up while excess oxygen is given off to atmosphere as excretory products by diffusion.
Though, many organic waste products of plants are stored in dead permanent tissues in plants. Bulk of most perennial plants is made of dead tissues in which excretory materials are passed. In this condition they have no unfavorable effects on activities of living tissues. Apart from this, several mineral salts, taken up in form of ions may collect organic acids, that might prove harmful to plants, frequently combine with excess cations and precipitate out as insoluble crystals that can be carefully stored in plant cells. For instance, calcium ion and sulphate ions are taken up together but sulphate ions is utilized up immediately in amino acid synthesis leaving the excess of calcium ions. These combine with oxalic and pectic acids to create harmless insoluble products like calcium oxalate and calcium pectate. Substances aren't only removed through leaf loss but also through fruits, petals and seeds, though excretory function is not main function of the dispersal. Though, in aquatic plants, most of the metabolic wastes are lost in their surrounding water through diffusion.
Excretory Products in Animals:
Excretory substances are made during metabolism of nitrogenous substances like amino acids and nucleic acids. Metabolism of carbohydrates and fats produces CO2 and H2O that are simple to remove. Protein metabolism creates nitrogenous waste material like ammonia, uric acid and urea.
Nitrogenous Waste Products:
In living organisms, proteins act as significant dietary constituents required for building, growth and repair of body cells. Proteins are nitrogen containing compounds that are metabolized to form end products like urea, ammonia and uric acid. These end products are derived from degradation of amino acids, proteins, pyrimidines and purines in body.
Nitrogenous excretory products are made by breakdown of proteins, nucleic acids and excess amino acids. First product of breakdown of excess amino acid is ammonia. Ammonia is made by removal of amino group from amino acid by the process known as deamination. Ammonia may be excreted instantly or converted or converted to urea or uric acid. Deamination of amino acid happens in liver. Ammonia is toxic substance and is continuously being produced in tissue by deamination of amino acids. It is eliminated rapidly from body. Mammals can't withstand ammonia in the blood, whereas reptiles, amphibians, and fishes can withstand the higher concentration of ammonia. Also, several invertebrates have shown high tolerance of ammonia in blood.
This is derived metabolism of amino acids and purines in liver. It is very soluble in water and less toxic than ammonia. Human blood generally has high tolerance of urea concentration. Urea is made in body by metabolism of 3 amino acids, ornithine, citrulline, and arginine in most vertebrates and mammals.
Uric acid is the most significant nitrogenous waste product in uterine of reptiles, birds, insects and snails. It is formed from ammonia. It is less toxic, insoluble in water and possibly stored or excreted in crystalline form. Its formation is the adaptation for conservation of water as its elimination needs extremely little water.
Other nitrogenous constituents:
Spiders excrete approximately exclusively chemical known as guanine. It is even less soluble as compared to uric acid and therefore needs no water for elimination. It is metabolic waste of nucleotide metabolism. It is also found in penguins.
Xanthine and hypoxanthine:
These are excreted by the number of insects.
Marine teleost fishes excrete the large proportion of the nitrogen as trimethylamine oxide (TMAO). The large amount of the compound is also stored in the body for osmoregulation, that is to minimize loss of water and entry of salts.
Hippuric acid and ornithuric acid:
Hippuric acid is formed in mammals. Diets of mammals have traces of benzoic acid which is a toxic substance. This benzoic acid combines with amino acid glycine to create the less toxic substance hippuric acid. In birds, dietary benzoic acid combines with ornithine and is excreted in form ornithuric acid.
Creatine and creatinine:
These are excreted from brain, muscle, blood, and urine in animals.
Pterydine are considered as excretory products that are significant pigments in insects. In some insects, traces of pterydine are excreted in faeces, wings, fat body and urine.
Patterns of Excretion:
On the basis of type of nitrogenous waste products excreted from body, animals can be categorized in different patterns of excretion
Animals excreting their nitrogenous waste in form of ammonia are called as ammonotelic. This phenomenon is called as ammonotelism.
Ammonia, the primary metabolic waste product of protein metabolism is most toxic form and needs large amount of water for elimination. It is very soluble in water with that it forms ammonium hydroxide (NH4OH) that injures cells directly by alkaline caustic action. It is most appropriate for aquatic organisms that have constant access to water. No energy is needed to produce ammonia. Kidneys don't play any important role in its removal. Several aquatic amphibians, bony fishes, and aquatic insects are ammonotelic in nature. In anurans (amphibians) larval tadpoles excrete ammonia, while adults generate urea.
Animals that excrete nitrogenous waste mostly in form of uric acid and urates are called as uricotelic. Phenomenon is called as uricotelism. Every terrestrial animal such as reptiles, insects, and birds excrete uric acid. Uric acid (that needs more energy) is made by degradation of purines (like guanine) in liver and kidneys to some extent. In uricotelic animals, excess nitrogen is first utilized in synthesis of purines. The purine is changed to xanthine (from hypoxanthine or guanine) that is then oxidized to uric acid. Part of uric acid is oxidized further to create allantoin and allantoic acid. Teleost fish excrete allantoate or hydration product of allantoin. In many fishes and amphibians, allantoate is hydrolyzed to urea and glyoxylate. Few marine invertebrates have gone a step ahead by hydrolysing urea to ammonia and carbon dioxide.
Animals which excrete nitrogenous waste mostly in form of urea are called ureotelic and phenomenon is called ureotelism. Terrestrial adaptation required production of lesser toxic nitrogenous wastes like urea and uric acid for conservation of water. Mammals, several terrestrial amphibians and marine fishes mostly excrete urea. Ammonia made by metabolism is converted in urea in liver of these animals and released in blood that is filtered excreted out by kidneys. Urea can be stored in body for significant periods of time, and is least toxic. It is eradicated in form of urine. Ureotelism is shown by semi-terrestrial animals, like few earthworms, adult amphibians, elasmobranch (cartilagineous fishes) and mammals.
In few arthropods like spiders, guanine is predominant excretory products elaborated by Malpighibian tubules and cloacal sacs. This pattern of excretion is called as Guanotelism and these animals are said to be guanotelic.
Marine teleost excrete trimethylamine oxide as main nitrogenous product that is soluble in water and nontoxic in nature. It is not present in marine elasmobranch. Marine teloests are faced with problem of keeping osmotic balance by retaining water in body that is aided by trimethylamine oxide. This compound is present in little quantities in muscle, and blood of marine fish that diffuses out through membrane. This compound has foul smell and is possibly derived from breakdown products of lipoproteins. Significant amounts of trimethylamine oxides are formed in squids, octopus, crabs and barnacles. It is found in urine of certain animals such as oysters, echinoderm, tunicates and gastropods.
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