{"id":716,"date":"2016-05-02T14:04:44","date_gmt":"2016-05-02T18:04:44","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/conceptsofbiologygunness\/?post_type=chapter&#038;p=716"},"modified":"2016-05-02T14:04:44","modified_gmt":"2016-05-02T18:04:44","slug":"22-4-nitrogenous-wastes","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/conceptsofbiologygunness\/chapter\/22-4-nitrogenous-wastes\/","title":{"raw":"22.4.\u00a0Nitrogenous Wastes","rendered":"22.4.\u00a0Nitrogenous Wastes"},"content":{"raw":"<div xml:lang=\"en\" class=\"section module\" title=\"41.4.&#xA0;Nitrogenous Wastes\">\n<div class=\"titlepage\">\n<div class=\"abstract\">\n<div class=\"bcc-box bcc-highlight\">\n<h3>Learning Objectives<\/h3>\nBy the end of this section, you will be able to:\n<div class=\"itemizedlist\">\n<ul class=\"itemizedlist\"><li class=\"listitem\">Compare and contrast the way in which aquatic animals and terrestrial animals can eliminate toxic ammonia from their systems<\/li>\n\t<li class=\"listitem\">Compare the major byproduct of ammonia metabolism in vertebrate animals to that of birds, insects, and reptiles<\/li>\n<\/ul><\/div>\n<\/div>\n<\/div>\n<\/div>\n<span id=\"m44811-fs-idp247785536\"> <\/span>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 <span id=\"m44811-autoid-cnx2dbk-id1499754\"> <\/span><strong>ammonia<\/strong><a id=\"id841641\" class=\"indexterm\">, 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. Animals that live in aquatic environments tend to release ammonia into the water. Animals that excrete ammonia are said to be <span id=\"m44811-autoid-cnx2dbk-id1499760\"> <\/span><strong>ammonotelic<\/strong><\/a><a id=\"id841657\" class=\"indexterm\">. Terrestrial organisms have evolved other mechanisms to excrete nitrogenous wastes. The animals must detoxify ammonia by converting it into a relatively nontoxic form such as urea or uric acid. Mammals, including humans, produce urea, whereas reptiles and many terrestrial invertebrates produce uric acid. Animals that secrete urea as the primary nitrogenous waste material are called <span id=\"m44811-autoid-cnx2dbk-id1499767\"> <\/span><strong>ureotelic<\/strong><\/a><a id=\"id841674\" class=\"indexterm\"> animals.\n<div class=\"section\" title=\"Nitrogenous Waste in Terrestrial Animals: The Urea Cycle\">\n<div class=\"titlepage\">\n<div>\n<div>\n<h2 id=\"m44811-fs-idp85160304\"><span class=\"cnx-gentext-section cnx-gentext-autogenerated\"><span class=\"cnx-gentext-section cnx-gentext-t\">Nitrogenous Waste in Terrestrial Animals: The Urea Cycle<\/span><\/span><\/h2>\n<\/div>\n<\/div>\n<\/div>\n<span id=\"m44811-fs-idp71399856\"> <\/span>The <span id=\"m44811-autoid-cnx2dbk-id1500767\"> <\/span><strong>urea cycle<\/strong><\/div><\/a><a id=\"id841703\" class=\"indexterm\"> 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 NH<sub>3<\/sub> (ammonia) + CO<sub>2<\/sub> + 3 ATP + H<sub>2<\/sub>O \u2192 H<sub>2<\/sub>N-CO-NH<sub>2<\/sub> (urea) + 2 ADP + 4 P<sub>i<\/sub> + AMP.\n\n<span id=\"m44811-fs-idp160881968\"> <\/span>The urea cycle utilizes five intermediate steps, catalyzed by five different enzymes, to convert ammonia to urea, as shown in <\/a><a class=\"xref target-figure\" href=\"ch41.html#m44811-fig-ch41_04_01\" title=\"Figure&#xA0;41.12.&#xA0;\">Figure 22.12<\/a>. 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 <span id=\"m44811-autoid-cnx2dbk-id1500811\"> <\/span><strong>blood urea nitrogen<\/strong><a id=\"id841759\" class=\"indexterm\"> or BUN, is used as an indicator of kidney function.\n\n[caption id=\"attachment_1351\" align=\"aligncenter\" width=\"600\"]<\/a><a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_41_04_01.jpg\"><img src=\"https:\/\/pressbooks.bccampus.ca\/conceptsofbiologygunness\/wp-content\/uploads\/sites\/34\/2016\/05\/Figure_41_04_01-920x1024.jpg\" alt=\"Figure_41_04_01\" class=\"wp-image-1351\" height=\"668\" width=\"600\"\/><\/a> Figure 22.12.\u00a0 The urea cycle converts ammonia to urea.[\/caption]\n\n\u00a0\n<div class=\"title\">\n<div class=\"body\">\n<div class=\"mediaobject\">\n<h2 class=\"title\">Excretion of Nitrogenous Waste<\/h2>\n<p title=\"Excretion of Nitrogenous Waste\">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.<\/p>\n\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"section\" title=\"Nitrogenous Waste in Birds and Reptiles: Uric Acid\">\n<div class=\"titlepage\">\n<div>\n<div>\n<h2 id=\"m44811-fs-idp206279600\"><span class=\"cnx-gentext-section cnx-gentext-autogenerated\"><span class=\"cnx-gentext-section cnx-gentext-t\">Nitrogenous Waste in Birds and Reptiles: Uric Acid<\/span><\/span><\/h2>\n<\/div>\n<\/div>\n<\/div>\n<span id=\"m44811-fs-idp105475568\"> <\/span>Birds, reptiles, and most terrestrial arthropods convert toxic ammonia to <span id=\"m44811-autoid-cnx2dbk-id1266296\"> <\/span><strong>uric acid<\/strong><a id=\"id841851\" class=\"indexterm\"> 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 <\/a><a class=\"xref target-figure\" href=\"ch41.html#m44811-fig-ch41_04_02\" title=\"Figure&#xA0;41.13.&#xA0;\">Figure 22.13<\/a>.\n\n[caption id=\"attachment_1352\" align=\"aligncenter\" width=\"600\"]<a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_41_04_02abc.jpg\"><img src=\"https:\/\/pressbooks.bccampus.ca\/conceptsofbiologygunness\/wp-content\/uploads\/sites\/34\/2016\/05\/Figure_41_04_02abc-1024x256.jpg\" alt=\"Figure_41_04_02abc\" class=\"wp-image-1352\" height=\"150\" width=\"600\"\/><\/a> Figure 22.13.\u00a0 Nitrogenous waste is excreted in different forms by different species. These include (a) ammonia, (b) urea, and (c) uric acid. (credit a: modification of work by Eric Engbretson, USFWS; credit b: modification of work by B. \"Moose\" Peterson, USFWS; credit c: modification of work by Dave Menke, USFWS)[\/caption]\n\n<div id=\"m44811-fig-ch41_04_02\" class=\"figure\" title=\"Figure&#xA0;41.13.&#xA0;\">\n<h2 class=\"title\">\n<\/h2><\/div>\n<div id=\"m44811-fs-idp161007280\" class=\"note everyday\">\n<div class=\"title\">\n<div class=\"body\">\n<h2 class=\"title\"><b>Gout<\/b><\/h2>\n<p title=\"Gout\">Mammals use uric acid crystals as an <span id=\"m44811-autoid-cnx2dbk-id1376543\"> <\/span><strong>antioxidant<\/strong><a id=\"id841939\" class=\"indexterm\"> 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 <\/a><a class=\"xref target-figure\" href=\"ch41.html#m44811-fig-ch41_04_03\" title=\"Figure&#xA0;41.14.&#xA0;\">Figure 22.14<\/a>. Food choices that reduce the amount of nitrogenous bases in the diet help reduce the risk of gout. For example, tea, coffee, and chocolate have purine-like compounds, called xanthines, and should be avoided by people with gout and kidney stones.<\/p>\n\n\n[caption id=\"attachment_1353\" align=\"aligncenter\" width=\"300\"]<a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_41_04_03.jpg\"><img src=\"https:\/\/pressbooks.bccampus.ca\/conceptsofbiologygunness\/wp-content\/uploads\/sites\/34\/2016\/05\/Figure_41_04_03.jpg\" alt=\"Figure_41_04_03\" class=\"wp-image-1353\" height=\"190\" width=\"300\"\/><\/a> Figure 22.14.\u00a0 Gout causes the inflammation visible in this person\u2019s left big toe joint. (credit: \"Gonzosft\"\/Wikimedia Commons)[\/caption]\n<h2 title=\"Gout\">\u00a0Summary<\/h2>\n<p title=\"Gout\">Ammonia is the waste produced by metabolism of nitrogen-containing compounds like proteins and nucleic acids. 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. Urea is the major byproduct of ammonia metabolism in vertebrate animals. Uric acid is the major byproduct of ammonia metabolism in birds, terrestrial arthropods, and reptiles.<\/p>\n\n<\/div>\n<\/div>\n<div class=\"title\">\n<div class=\"bcc-box bcc-info\">\n<h3>Exercises<\/h3>\n<div id=\"m44811-fs-idp207067392\" class=\"exercise\">\n<div class=\"title\">\n<div class=\"section empty\">\n<div class=\"section empty\">\n<div class=\"cnx-eoc multiple-choice\">\n<div class=\"section empty\">\n<div class=\"section\">\n<div class=\"body\">\n<div id=\"m44811-fs-idp143558720\" class=\"exercise\">\n<div class=\"title\">1. BUN is ________.<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"orderedlist\">\n\nA) blood urea nitrogen\n\nB) blood uric acid nitrogen\n\nC) an indicator of blood volume\n\nD) an indicator of blood pressure\n\n<\/div>\n<\/div>\n<div id=\"m44811-fs-idp223796272\" class=\"solution labeled\">\n<div class=\"title\">Answer:\u00a0<span style=\"line-height: 1.5\">A<\/span><\/div>\n<\/div>\n<\/div>\n<div id=\"m44811-fs-idp57677728\" class=\"exercise\">\n<div class=\"title\">\n<div class=\"title\"><span style=\"line-height: 1.5\">2. Human beings accumulate ________ before excreting nitrogenous waste.<\/span><\/div>\n<div class=\"body\">\n<div class=\"problem\">\n<div class=\"orderedlist\">\n\nA) nitrogen\n\nB) ammonia\n\nC) urea\n\nD) uric acid\n\n<\/div>\n<\/div>\n<div id=\"m44811-fs-idp43720320\" class=\"solution labeled\">\n<div class=\"body\">\n\n<span id=\"m44811-fs-idp145426304\"><span>Answer:\u00a0<\/span><\/span>C\n\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n3. In terms of evolution, why might the urea cycle have evolved in organisms?\n\n<\/div>\n<div class=\"body\">\n<div id=\"m44811-fs-idp41307024\" class=\"solution labeled\">\n<div class=\"title\"><span><span class=\"epub-only pre-text\"> <\/span><\/span><\/div>\n<div class=\"title\">It is believed that 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.<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div id=\"m44811-fs-idm18879616\" class=\"exercise\">\n<div class=\"body\">\n<div class=\"problem\">\n<div class=\"problem\">4. Compare and contrast the formation of urea and uric acid.<\/div>\n<div id=\"m44811-fs-idp26505088\" class=\"solution labeled\">\n<div class=\"body\">\n\n<span id=\"m44811-fs-idp74294320\"> <\/span>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. Birds, reptiles, and insects, on the other hand, convert toxic ammonia to uric acid instead of urea. Conversion of ammonia to uric acid requires more energy and is much more complex than conversion of ammonia to urea.\n\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n\u00a0\n\n<\/div>\n<\/div>\n<div xml:lang=\"en\" class=\"section module\" title=\"41.5.&#xA0;Hormonal Control of Osmoregulatory Functions\">\n<div class=\"titlepage\">\n<div class=\"bcc-box bcc-success\">\n<h3>Glossary<\/h3>\n<dl><dt><strong>ammonia<\/strong><\/dt><dd>compound made of one nitrogen atom and three hydrogen atoms<\/dd><dt><strong>ammonotelic<\/strong><\/dt><dd>describes an animal that excretes ammonia as the primary waste material<\/dd><dt><strong>antioxidant<\/strong><\/dt><dd>agent that prevents cell destruction by reactive oxygen species<\/dd><dt><strong>blood urea nitrogen (BUN)<\/strong><\/dt><dd>estimate of urea in the blood and an indicator of kidney function<\/dd><dt><strong>urea cycle<\/strong><\/dt><dd>pathway by which ammonia is converted to urea<\/dd><dt><strong>ureotelic<\/strong><\/dt><dd>describes animals that secrete urea as the primary nitrogenous waste material<\/dd><dt><strong>uric acid<\/strong><\/dt><dd>byproduct of ammonia metabolism in birds, insects, and reptiles<\/dd><\/dl><\/div>\n\u00a0\n\n<\/div>\n<\/div><\/div><\/div><\/div>","rendered":"<div xml:lang=\"en\" class=\"section module\" title=\"41.4.&#xa0;Nitrogenous Wastes\">\n<div class=\"titlepage\">\n<div class=\"abstract\">\n<div class=\"bcc-box bcc-highlight\">\n<h3>Learning Objectives<\/h3>\n<p>By the end of this section, you will be able to:<\/p>\n<div class=\"itemizedlist\">\n<ul class=\"itemizedlist\">\n<li class=\"listitem\">Compare and contrast the way in which aquatic animals and terrestrial animals can eliminate toxic ammonia from their systems<\/li>\n<li class=\"listitem\">Compare the major byproduct of ammonia metabolism in vertebrate animals to that of birds, insects, and reptiles<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<p><span id=\"m44811-fs-idp247785536\"> <\/span>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 <span id=\"m44811-autoid-cnx2dbk-id1499754\"> <\/span><strong>ammonia<\/strong><a id=\"id841641\" class=\"indexterm\">, 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. Animals that live in aquatic environments tend to release ammonia into the water. Animals that excrete ammonia are said to be <span id=\"m44811-autoid-cnx2dbk-id1499760\"> <\/span><strong>ammonotelic<\/strong><\/a><a id=\"id841657\" class=\"indexterm\">. Terrestrial organisms have evolved other mechanisms to excrete nitrogenous wastes. The animals must detoxify ammonia by converting it into a relatively nontoxic form such as urea or uric acid. Mammals, including humans, produce urea, whereas reptiles and many terrestrial invertebrates produce uric acid. Animals that secrete urea as the primary nitrogenous waste material are called <span id=\"m44811-autoid-cnx2dbk-id1499767\"> <\/span><strong>ureotelic<\/strong><\/a><a id=\"id841674\" class=\"indexterm\"> animals.<\/p>\n<div class=\"section\" title=\"Nitrogenous Waste in Terrestrial Animals: The Urea Cycle\">\n<div class=\"titlepage\">\n<div>\n<div>\n<h2 id=\"m44811-fs-idp85160304\"><span class=\"cnx-gentext-section cnx-gentext-autogenerated\"><span class=\"cnx-gentext-section cnx-gentext-t\">Nitrogenous Waste in Terrestrial Animals: The Urea Cycle<\/span><\/span><\/h2>\n<\/div>\n<\/div>\n<\/div>\n<p><span id=\"m44811-fs-idp71399856\"> <\/span>The <span id=\"m44811-autoid-cnx2dbk-id1500767\"> <\/span><strong>urea cycle<\/strong><\/div>\n<p><\/a><a id=\"id841703\" class=\"indexterm\"> 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 NH<sub>3<\/sub> (ammonia) + CO<sub>2<\/sub> + 3 ATP + H<sub>2<\/sub>O \u2192 H<sub>2<\/sub>N-CO-NH<sub>2<\/sub> (urea) + 2 ADP + 4 P<sub>i<\/sub> + AMP.<\/p>\n<p><span id=\"m44811-fs-idp160881968\"> <\/span>The urea cycle utilizes five intermediate steps, catalyzed by five different enzymes, to convert ammonia to urea, as shown in <\/a><a class=\"xref target-figure\" href=\"ch41.html#m44811-fig-ch41_04_01\" title=\"Figure&#xa0;41.12.&#xa0;\">Figure 22.12<\/a>. 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 <span id=\"m44811-autoid-cnx2dbk-id1500811\"> <\/span><strong>blood urea nitrogen<\/strong><a id=\"id841759\" class=\"indexterm\"> or BUN, is used as an indicator of kidney function.<\/p>\n<figure id=\"attachment_1351\" aria-describedby=\"caption-attachment-1351\" style=\"width: 600px\" class=\"wp-caption aligncenter\"><a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_41_04_01.jpg\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/conceptsofbiologygunness\/wp-content\/uploads\/sites\/34\/2016\/05\/Figure_41_04_01-920x1024.jpg\" alt=\"Figure_41_04_01\" class=\"wp-image-1351\" height=\"668\" width=\"600\" \/><\/a><figcaption id=\"caption-attachment-1351\" class=\"wp-caption-text\">Figure 22.12.\u00a0 The urea cycle converts ammonia to urea.<\/figcaption><\/figure>\n<p>\u00a0<\/p>\n<div class=\"title\">\n<div class=\"body\">\n<div class=\"mediaobject\">\n<h2 class=\"title\">Excretion of Nitrogenous Waste<\/h2>\n<p title=\"Excretion of Nitrogenous Waste\">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.<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"section\" title=\"Nitrogenous Waste in Birds and Reptiles: Uric Acid\">\n<div class=\"titlepage\">\n<div>\n<div>\n<h2 id=\"m44811-fs-idp206279600\"><span class=\"cnx-gentext-section cnx-gentext-autogenerated\"><span class=\"cnx-gentext-section cnx-gentext-t\">Nitrogenous Waste in Birds and Reptiles: Uric Acid<\/span><\/span><\/h2>\n<\/div>\n<\/div>\n<\/div>\n<p><span id=\"m44811-fs-idp105475568\"> <\/span>Birds, reptiles, and most terrestrial arthropods convert toxic ammonia to <span id=\"m44811-autoid-cnx2dbk-id1266296\"> <\/span><strong>uric acid<\/strong><a id=\"id841851\" class=\"indexterm\"> 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 <\/a><a class=\"xref target-figure\" href=\"ch41.html#m44811-fig-ch41_04_02\" title=\"Figure&#xa0;41.13.&#xa0;\">Figure 22.13<\/a>.<\/p>\n<figure id=\"attachment_1352\" aria-describedby=\"caption-attachment-1352\" style=\"width: 600px\" class=\"wp-caption aligncenter\"><a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_41_04_02abc.jpg\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/conceptsofbiologygunness\/wp-content\/uploads\/sites\/34\/2016\/05\/Figure_41_04_02abc-1024x256.jpg\" alt=\"Figure_41_04_02abc\" class=\"wp-image-1352\" height=\"150\" width=\"600\" \/><\/a><figcaption id=\"caption-attachment-1352\" class=\"wp-caption-text\">Figure 22.13.\u00a0 Nitrogenous waste is excreted in different forms by different species. These include (a) ammonia, (b) urea, and (c) uric acid. (credit a: modification of work by Eric Engbretson, USFWS; credit b: modification of work by B. &#8220;Moose&#8221; Peterson, USFWS; credit c: modification of work by Dave Menke, USFWS)<\/figcaption><\/figure>\n<div id=\"m44811-fig-ch41_04_02\" class=\"figure\" title=\"Figure&#xa0;41.13.&#xa0;\">\n<h2 class=\"title\">\n<\/h2>\n<\/div>\n<div id=\"m44811-fs-idp161007280\" class=\"note everyday\">\n<div class=\"title\">\n<div class=\"body\">\n<h2 class=\"title\"><b>Gout<\/b><\/h2>\n<p title=\"Gout\">Mammals use uric acid crystals as an <span id=\"m44811-autoid-cnx2dbk-id1376543\"> <\/span><strong>antioxidant<\/strong><a id=\"id841939\" class=\"indexterm\"> 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 <\/a><a class=\"xref target-figure\" href=\"ch41.html#m44811-fig-ch41_04_03\" title=\"Figure&#xa0;41.14.&#xa0;\">Figure 22.14<\/a>. Food choices that reduce the amount of nitrogenous bases in the diet help reduce the risk of gout. For example, tea, coffee, and chocolate have purine-like compounds, called xanthines, and should be avoided by people with gout and kidney stones.<\/p>\n<figure id=\"attachment_1353\" aria-describedby=\"caption-attachment-1353\" style=\"width: 300px\" class=\"wp-caption aligncenter\"><a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_41_04_03.jpg\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/conceptsofbiologygunness\/wp-content\/uploads\/sites\/34\/2016\/05\/Figure_41_04_03.jpg\" alt=\"Figure_41_04_03\" class=\"wp-image-1353\" height=\"190\" width=\"300\" \/><\/a><figcaption id=\"caption-attachment-1353\" class=\"wp-caption-text\">Figure 22.14.\u00a0 Gout causes the inflammation visible in this person\u2019s left big toe joint. (credit: &#8220;Gonzosft&#8221;\/Wikimedia Commons)<\/figcaption><\/figure>\n<h2 title=\"Gout\">\u00a0Summary<\/h2>\n<p title=\"Gout\">Ammonia is the waste produced by metabolism of nitrogen-containing compounds like proteins and nucleic acids. 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. Urea is the major byproduct of ammonia metabolism in vertebrate animals. Uric acid is the major byproduct of ammonia metabolism in birds, terrestrial arthropods, and reptiles.<\/p>\n<\/div>\n<\/div>\n<div class=\"title\">\n<div class=\"bcc-box bcc-info\">\n<h3>Exercises<\/h3>\n<div id=\"m44811-fs-idp207067392\" class=\"exercise\">\n<div class=\"title\">\n<div class=\"section empty\">\n<div class=\"section empty\">\n<div class=\"cnx-eoc multiple-choice\">\n<div class=\"section empty\">\n<div class=\"section\">\n<div class=\"body\">\n<div id=\"m44811-fs-idp143558720\" class=\"exercise\">\n<div class=\"title\">1. BUN is ________.<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"orderedlist\">\n<p>A) blood urea nitrogen<\/p>\n<p>B) blood uric acid nitrogen<\/p>\n<p>C) an indicator of blood volume<\/p>\n<p>D) an indicator of blood pressure<\/p>\n<\/div>\n<\/div>\n<div id=\"m44811-fs-idp223796272\" class=\"solution labeled\">\n<div class=\"title\">Answer:\u00a0<span style=\"line-height: 1.5\">A<\/span><\/div>\n<\/div>\n<\/div>\n<div id=\"m44811-fs-idp57677728\" class=\"exercise\">\n<div class=\"title\">\n<div class=\"title\"><span style=\"line-height: 1.5\">2. Human beings accumulate ________ before excreting nitrogenous waste.<\/span><\/div>\n<div class=\"body\">\n<div class=\"problem\">\n<div class=\"orderedlist\">\n<p>A) nitrogen<\/p>\n<p>B) ammonia<\/p>\n<p>C) urea<\/p>\n<p>D) uric acid<\/p>\n<\/div>\n<\/div>\n<div id=\"m44811-fs-idp43720320\" class=\"solution labeled\">\n<div class=\"body\">\n<p><span id=\"m44811-fs-idp145426304\"><span>Answer:\u00a0<\/span><\/span>C<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<p>3. In terms of evolution, why might the urea cycle have evolved in organisms?<\/p>\n<\/div>\n<div class=\"body\">\n<div id=\"m44811-fs-idp41307024\" class=\"solution labeled\">\n<div class=\"title\"><span><span class=\"epub-only pre-text\"> <\/span><\/span><\/div>\n<div class=\"title\">It is believed that 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.<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div id=\"m44811-fs-idm18879616\" class=\"exercise\">\n<div class=\"body\">\n<div class=\"problem\">\n<div class=\"problem\">4. Compare and contrast the formation of urea and uric acid.<\/div>\n<div id=\"m44811-fs-idp26505088\" class=\"solution labeled\">\n<div class=\"body\">\n<p><span id=\"m44811-fs-idp74294320\"> <\/span>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. Birds, reptiles, and insects, on the other hand, convert toxic ammonia to uric acid instead of urea. Conversion of ammonia to uric acid requires more energy and is much more complex than conversion of ammonia to urea.<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<p>\u00a0<\/p>\n<\/div>\n<\/div>\n<div xml:lang=\"en\" class=\"section module\" title=\"41.5.&#xa0;Hormonal Control of Osmoregulatory Functions\">\n<div class=\"titlepage\">\n<div class=\"bcc-box bcc-success\">\n<h3>Glossary<\/h3>\n<dl>\n<dt><strong>ammonia<\/strong><\/dt>\n<dd>compound made of one nitrogen atom and three hydrogen atoms<\/dd>\n<dt><strong>ammonotelic<\/strong><\/dt>\n<dd>describes an animal that excretes ammonia as the primary waste material<\/dd>\n<dt><strong>antioxidant<\/strong><\/dt>\n<dd>agent that prevents cell destruction by reactive oxygen species<\/dd>\n<dt><strong>blood urea nitrogen (BUN)<\/strong><\/dt>\n<dd>estimate of urea in the blood and an indicator of kidney function<\/dd>\n<dt><strong>urea cycle<\/strong><\/dt>\n<dd>pathway by which ammonia is converted to urea<\/dd>\n<dt><strong>ureotelic<\/strong><\/dt>\n<dd>describes animals that secrete urea as the primary nitrogenous waste material<\/dd>\n<dt><strong>uric acid<\/strong><\/dt>\n<dd>byproduct of ammonia metabolism in birds, insects, and reptiles<\/dd>\n<\/dl>\n<\/div>\n<p>\u00a0<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n","protected":false},"author":16,"menu_order":74,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-716","chapter","type-chapter","status-publish","hentry"],"part":307,"_links":{"self":[{"href":"https:\/\/pressbooks.bccampus.ca\/conceptsofbiologygunness\/wp-json\/pressbooks\/v2\/chapters\/716","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/pressbooks.bccampus.ca\/conceptsofbiologygunness\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/pressbooks.bccampus.ca\/conceptsofbiologygunness\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/conceptsofbiologygunness\/wp-json\/wp\/v2\/users\/16"}],"version-history":[{"count":1,"href":"https:\/\/pressbooks.bccampus.ca\/conceptsofbiologygunness\/wp-json\/pressbooks\/v2\/chapters\/716\/revisions"}],"predecessor-version":[{"id":939,"href":"https:\/\/pressbooks.bccampus.ca\/conceptsofbiologygunness\/wp-json\/pressbooks\/v2\/chapters\/716\/revisions\/939"}],"part":[{"href":"https:\/\/pressbooks.bccampus.ca\/conceptsofbiologygunness\/wp-json\/pressbooks\/v2\/parts\/307"}],"metadata":[{"href":"https:\/\/pressbooks.bccampus.ca\/conceptsofbiologygunness\/wp-json\/pressbooks\/v2\/chapters\/716\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/pressbooks.bccampus.ca\/conceptsofbiologygunness\/wp-json\/wp\/v2\/media?parent=716"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/conceptsofbiologygunness\/wp-json\/pressbooks\/v2\/chapter-type?post=716"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/conceptsofbiologygunness\/wp-json\/wp\/v2\/contributor?post=716"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/conceptsofbiologygunness\/wp-json\/wp\/v2\/license?post=716"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}