Why do we excrete urea

The urea cycle utilizes five intermediate steps, catalyzed by five different enzymes, to convert ammonia to urea, as shown in Figure The amino acid L-ornithine gets converted into different intermediates before being regenerated at the end of the urea cycle. Hence, the urea cycle is also referred to as the ornithine cycle. The enzyme ornithine transcarbamylase catalyzes a key step in the urea cycle and its deficiency can lead to accumulation of toxic levels of ammonia in the body.

The first two reactions occur in the mitochondria and the last three reactions occur in the cytosol. Urea concentration in the blood, called blood urea nitrogen or BUN, is used as an indicator of kidney function. The theory of evolution proposes that life started in an aquatic environment.

It is not surprising to see that biochemical pathways like the urea cycle evolved to adapt to a changing environment when terrestrial life forms evolved. Arid conditions probably led to the evolution of the uric acid pathway as a means of conserving water. Birds, reptiles, and most terrestrial arthropods convert toxic ammonia to uric acid or the closely related compound guanine guano instead of urea. Mammals also form some uric acid during breakdown of nucleic acids.

Uric acid is a compound similar to purines found in nucleic acids. It is water insoluble and tends to form a white paste or powder; it is excreted by birds, insects, and reptiles. Conversion of ammonia to uric acid requires more energy and is much more complex than conversion of ammonia to urea Figure Mammals use uric acid crystals as an antioxidant in their cells.

However, too much uric acid tends to form kidney stones and may also cause a painful condition called gout, where uric acid crystals accumulate in the joints, as illustrated in Figure Food choices that reduce the amount of nitrogenous bases in the diet help reduce the risk of gout. The body uses it in many processes, the most notable one being nitrogen excretion. Urea is widely used in fertilizers as a convenient source of nitrogen. It is also an important raw material for the chemical industry.

Apart from mammals, urea is also found in the urine of amphibians, as well as some fish. Interestingly, tadpoles excrete ammonia, but shift to urea production during metamorphosis. In humans, apart from being a carrier of waste nitrogen, urea also plays a role in the countercurrent exchange system of the nephrons, which allows for re-absorption of water and critical ions from the excreted urine. This mechanism, controlled by an anti-diuretic hormone, allows the body to create hyperosmotic urine, which has a higher concentration of dissolved substances than the blood plasma.

This mechanism is important to prevent the loss of water, to maintain blood pressure, and to maintain a suitable concentration of sodium ions in the blood plasmas. The urea cycle is the primary mechanism by which mammals convert ammonia to urea. Urea is made in the liver and excreted in urine. The urea cycle utilizes five intermediate steps, catalyzed by five different enzymes, to convert ammonia to urea. The amino acid L-ornithine is converted into different intermediates before being regenerated at the end of the urea cycle.

Hence, the urea cycle is also referred to as the ornithine cycle. The enzyme ornithine transcarbamylase catalyzes a key step in the urea cycle. Its deficiency can lead to accumulation of toxic levels of ammonia in the body.

The first two reactions occur in the mitochondria, while the last three reactions occur in the cytosol. Urea Cycle : The urea cycle converts ammonia to urea in five steps that include the catalyzation of five different enzymes. Birds and reptiles have evolved the ability to convert toxic ammonia into uric acid or guanine rather than urea. Of the four major macromolecules in biological systems, both proteins and nucleic acids contain nitrogen. During the catabolism, or breakdown, of nitrogen-containing macromolecules, carbon, hydrogen, and oxygen are extracted and stored in the form of carbohydrates and fats.

Excess nitrogen is excreted from the body. Nitrogenous wastes tend to form toxic ammonia, which raises the pH of body fluids. Urea is produced commercially by several steps, which begin with the direct reaction of ammonia with carbon dioxide in a high pressure, high temperature reactor. It is a very important starting material in a number of chemical syntheses, and is used on an industrial scale for the manufacture of fertilisers, pharmaceuticals and resins.

For example, urea is one of the precursors to the various barbiturates , which are widely used as sedatives and sleeping pills. Urea is also used in the production of urethanes , which are then polymerised to form polyurethane foams. Another important application is in the manufacture of resins and polymers. Urea can react with formaldehyde to make the urea-formaldehyde resins, which are highly important in moulded plastics.

Another product is, melamine , which is formed by the dehydration of urea, and is used primarily in the production of melamine-formaldehyde resins which have much greater hardness and stain resistance than urea-formaldehyde resins. Both of these types of polymer resin have very varied uses including adhesives, laminates, moulding compounds, coatings and textile finishes. As well as its uses as a chemical precursor, urea has properties which make it useful in its own right.

For example, because of its high nitrogen content, urea is often used as a component of agricultural fertilizers. Also, urea has the ability to weakly bond to many large organic compounds, surrounding them in an inert 'shell' and so inhibiting intermolecular hydrogen bonding.