Nitrogenous Wastes

Nitrogenous Waste in Terrestrial Animals: The Urea Cycle

Urea, a nitrogenous waste material, is the end product excreted in urine when ammonia is metabolized by animals, such as mammals.

Learning Objectives

Discuss the urea cycle

Key Takeaways

Key Points

  • Ureotelic animals, which includes mammals, produce urea as the main nitrogenous waste material.
  • 2 NH3 + CO2 + 3 ATP + H2O → H2N-CO-NH2 + 2 ADP + 4 Pi + AMP is the chemical reaction by which toxic ammonia is converted to urea.
  • The urea cycle involves the multi-step conversion (carried out by five different enzymes ) of the amino acid L- ornithine into different intermediates before being regenerated.

Key Terms

  • ureotelic: animals that secrete urea as the primary nitrogenous waste material
  • ornithine: an amino acid, which acts as an intermediate in the biosynthesis of urea
  • urea: a water-soluble organic compound, CO(NH2)2, formed by the metabolism of proteins and excreted in the urine

Nitrogenous Waste in Terrestrial Animals: The Urea Cycle

Mammals, including humans, are the primary producers of urea. Because they secrete urea as the primary nitrogenous waste product, they are called ureotelic animals. Urea serves an important role in the metabolism of nitrogen-containing compounds by animals. It is the main nitrogen-containing substance in the urine of mammals. Urea is a colorless, odorless solid, highly soluble in water, and practically non-toxic. Dissolved in water, it is neither acidic nor alkaline. 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 overall chemical reaction by which ammonia is converted to urea is 2 NH3 (ammonia) + CO2 + 3 ATP + H2O → H2N-CO-NH2 (urea) + 2 ADP + 4 Pi + AMP.

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.

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Urea Cycle: The urea cycle converts ammonia to urea in five steps that include the catalyzation of five different enzymes.

Nitrogenous Waste in Birds and Reptiles: Uric Acid

Birds and reptiles have evolved the ability to convert toxic ammonia into uric acid or guanine rather than urea.

Learning Objectives

Compare the major byproduct of ammonia metabolism in mammals to that of birds and reptiles

Key Takeaways

Key Points

  • Nitrogenous wastes in the body tend to form toxic ammonia, which must be excreted.
  • Mammals such as humans excrete urea, while birds, reptiles, and some terrestrial invertebrates produce uric acid as waste.
  • Uricothelic organisms tend to excrete uric acid waste in the form of a white paste or powder.
  • Conversion of ammonia into uric acid is more energy intensive than the conversion of ammonia into urea.
  • Producing uric acid instead of urea is advantageous because it is less toxic and reduces water loss and the subsequent need for water.

Key Terms

  • urea: a water-soluble organic compound, CO(NH2)2, formed by the metabolism of proteins and excreted in the urine
  • guano: the excrement of seabirds, cave-dwelling bats, pinnipeds, or birds more generally
  • purine: any of a class of organic heterocyclic base containing fused pyrimidine and imidazole rings; they are components of nucleic acids
  • xanthine: a precursor of uric acid found in many organs of the body
  • hypoxanthine: an intermediate in the biosynthesis of uric acid
  • uric acid: a bicyclic heterocyclic phenolic compound, formed in the body by the metabolism of protein and excreted in the urine

Nitrogenous Waste in Birds and Reptiles: Uric Acid

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. The formation of ammonia itself requires energy in the form of ATP and large quantities of water to dilute it out of a biological system.

While aquatic animals can easily excrete ammonia into their watery surroundings, terrestrial animals have evolved special mechanisms to eliminate the toxic ammonia from their systems. The animals must detoxify ammonia by converting it into a relatively-nontoxic form such as urea or uric acid.

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Nitrogen excretion: Nitrogenous waste is excreted in different forms by different species. These include (a) ammonia, (b) urea, and (c) uric acid.

Birds, reptiles, and most terrestrial arthropods, such as insects, are called uricothelic organisms because they convert toxic ammonia to uric acid or the closely-related compound guanine (guano), rather than urea. In contrast, mammals (including humans) produce urea from ammonia; however, they also form some uric acid during the breakdown of nucleic acids. In this case, uric acid is excreted in urine instead of in feces, as is done in birds and reptiles.

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. The production of uric acid involves a complex metabolic pathway that is energetically costly in comparison to processing of other nitrogenous wastes such as urea (from the urea cycle) or ammonia; however, it has the advantages of reducing water loss and, hence, reducing the need for water.

Uric acid is also less toxic than ammonia or urea. It contains four nitrogen atoms; only a small amount of water is needed for its excretion. Out of solute, it precipitates and forms crystals. The enzyme xanthine oxidase makes uric acid from xanthine and hypoxanthine, which in turn are produced from other purines. Xanthine oxidase is a large enzyme whose active site consists of the metal, molybdenum, bound to sulfur and oxygen. Uric acid is released in hypoxic conditions.