Nitrogenous wastes refer to the nitrogen-derived compounds that are excreted from living organisms. These are broadly classified as urea, uric acid, and ammonia. There is a distinction between uric acid and ammonia, as ammonia comes out in a gaseous state while the other two are dissolved in water. While the gaseous waste is excreted through the lungs, the liquid waste is excreted via the kidneys.
Nitrogenous wastes come in different dimensions. The most common ones are urea, uric acid, ammonia, and creatinine. Urea is a salt of carbonic acid derived from amino acids in the body, while uric acid is a salt of carbonic acid from purine. The amount of nitrogenous waste excreted at each stage of digestion is not the same all the time. It depends on many factors like age, sex, health condition, etc.
The liver mainly produces nitrogenous wastes. The amino acids in the food we eat are converted to urea and uric acid, forming ammonia. The ammonia is then released into the urine, while the uric acid is excreted through sweat. Water and other compounds consumed in the diet are then used to dilute the waste.
Ammonia is toxic, and it’s from proteins and nucleic acids. During catabolism, it is released when the cells break down. It also acts as a weak base, lowering body pH and facilitating the breakdown of more cells and tissues.
In plants, nitrogenous wastes are produced from their amino acid composition, converted to proteins, and stored. In animals, these wastes are produced from their food intake amount. The waste is formed by the liver and released into the bloodstream.
The nitrogenous wastes are produced in the body in different digestion stages and are excreted out in different ways. Creatine is a nitrogenous waste produced inside the muscles during the digestion process and gets transported in the blood. From there, it is excreted out via the kidneys.
Urea is another nitrogenous waste that is produced inside both muscles and the liver. It is excreted out via kidneys. Ammonia is released during the digestion process in large amounts by being derived from purine. It is not harmful to the body because it is quickly converted into urea for excretion.
Aquatic animals can eliminate toxic ammonia from their systems without converting it into urea or uric acid. They do this because ammonia dissolves easily in water and will not form crystals. Still, when it has been converted to urea or uric acid, crystallization has already begun, making it more difficult for them to eliminate.
Ammonia is released from aquatic animals via their gills, and the surrounding water carries it away. Some marine animals like fish also secrete ammonia through their skin and scales, which are then washed away by the water.
Amphibians, like frogs, can eliminate ammonia through their skin, gills, and feces. Aquatic animals can’t eliminate ammonia from their systems through breathing as they do not have lungs that can process oxygen.
In reptiles, uric acid is the main nitrogenous waste that is excreted by birds as well. It acts as a concentrated form of nitrogen and is eliminated from the body in solid urea. In reptiles, it is released through their skin, which has pores that permit them.
To regulate their body temperature, birds can excrete uric acid through their sweat glands and beaks. When uric acid is released from a bird’s beak, it creates foam and blocks the nostrils. To prevent this, some birds have developed a nasal membrane that can absorb the uric acid before it reaches their nostrils.
Birds produce uric acid as a byproduct of ammonia metabolism. Nucleic acids contain nitrogen which makes ammonia. Birds convert toxic ammonia to uric acid for excretion via the kidneys.
Excretion in different birds
Some birds like the ostrich and emu have developed a way to eliminate uric acid while eating food that contains a large amount of purine. In these birds, the uric acid is converted into allantoin, allowing them to excrete it through their urine.
The conversion of ammonia to uric acid is done by a family of enzymes called carbonyl reductase. This enzyme is also used to convert uric acid to allantoin and is also used to convert allantoin back to uric acid.
In birds, the enzyme has the additional function of converting toxic ammonia to uric acid. This conversion helps birds in excreting excessive amounts of ammonia from their bodies. The elimination prevents acidosis, which is a build-up of toxic ammonia in the body.
Alkaline birds like the flamingo can produce uric acid without the presence of ammonia. This is due to the conversion of allantoin to uric acid by a different enzyme called uricase.
The urea cycle is the main process by which terrestrial animals convert ammonia to urea. The chemical reaction for the conversion of ammonia is
2 NH3 (ammonia) +CO2 + 3ATP+ H2O→ H2N-CO-NH2 (urea) + 2 ADP+ 4 Pi + AMP.
Nitrogenous waste in terrestrial animals is composed of ammonia ions, carbon dioxide, and water. The nitrogen waste is converted to urea that is then excreted from the body in urine. The urea cycle is the process that takes place in the liver and also involves other organs like the kidneys and the intestines.
The liver produces two compounds that are essential in converting ammonia to urea: carbamoyl phosphate (CP) and ornithine.
Adenosine triphosphate (ATP) is the energy molecule of cells. It is produced by breaking down nucleic acids into smaller units called adenosine diphosphate (ADP). ATP is also used to produce energy. It does this by joining with the phosphate of adenosine diphosphate during cellular respiration.
The Urea Cycle Process
In the urea cycle, ATP is used to produce carbamoyl phosphate (CP) from ornithine and ammonia. This process is facilitated by the enzyme carbamoyl phosphate synthetase, which is located in the liver’s mitochondrial membrane.
Enzyme ornithine transcarbamylase catalyzes the formation of carbamoyl phosphate from two molecules of ornithine and one molecule of ammonia. ATP facilitates the process.
It is essential to generate a lot of ATP so that there will be enough of it to aid in producing carbamoyl phosphate. The latter helps to convert toxic ammonia to urea.
The urea cycle converts ammonia to urea then terrestrial animals can eliminate urea in the form of a concentrated stream of urine. The conversion of ammonia to urea is facilitated by CP produced in the liver and kidneys (the source of the enzymes that convert toxic ammonia to urea).
The urine is first passed through the kidneys, where further water and excess ions are excreted. The remaining stream of urine is then passed into the intestines. The water content in this stream of urine is removed through absorption in the intestines. What remains is excreted as nitrogen waste in the form of urea.
Aside from the conversion of ammonia to urea, other aspects of nitrogenous waste are also handled through the urea cycle. The cycles in which nitrogenous waste is converted to ammonia and then back to uric acid are called the ‘urea cycle’ because it involves urea.
Certain microbes also use the urea cycle to convert ammonia to amino acids. However, the most common use of the urea cycle is to convert ammonia to urea.
Every organism needs to excrete nitrogenous wastes. If these wastes are not eliminated, it might harm the health of an organism. For example, urea and ammonia are used as nitrogen sources by plants that might lead to overgrowth of these by algae, which is unsuitable for the aquatic environment.
If the nitrogenous waste levels increase in the body, it might cause hypertension and increase blood serum levels. The wastes can also damage the cells and tissues in the body.
Excessive uric acid causes gouty arthritis, and in large quantities, the uric acid becomes a solid that forms crystals. The crystals then settle in joints, where they cause damage, inflammation, and pain.
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Nitrogenous wastes are compounds that contain the element nitrogen. These wastes can be converted to urea and then excreted from the body in the form of a concentrated stream of urine. Urea is the end product because it is easier and safer to expel than waste containing ammonia or uric acid.
These wastes are produced every time an organism uses energy. ATP and ADP contain nitrogen in their structures, so breaking down either of these compounds necessitates the release of nitrogenous wastes.
Both aquatic animals and terrestrial animals produce nitrogenous wastes in the form of urea. However, they differ in how these waste products are excreted from their bodies.