{"id":1666,"date":"2018-03-09T18:56:55","date_gmt":"2018-03-09T18:56:55","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/?post_type=chapter&#038;p=1666"},"modified":"2024-04-26T18:22:37","modified_gmt":"2024-04-26T18:22:37","slug":"plant-photorespiration","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/chapter\/plant-photorespiration\/","title":{"raw":"Plant Photorespiration","rendered":"Plant Photorespiration"},"content":{"raw":"<div class=\"textbox learning-objectives\">\r\n<h3>Learning Outcomes<\/h3>\r\n<ul>\r\n \t<li>Differentiate between C<sub>3<\/sub>, C<sub>4<\/sub>, and CAM plant approaches to photorespiration<\/li>\r\n<\/ul>\r\n<\/div>\r\nHigh crop yields are pretty important\u2014for keeping people fed, and also for keeping economies running. If you heard there was a single factor that reduced the yield of wheat by 20 percent and the yield of soybeans by 36 percent in the United States, for instance, you might be curious to know what it was[footnote]Walker, Berkeley J., VanLoocke, Andy, Bernacchi, Carl J., and Ort, Donald R. (2016). The cost of photorespiration to food production now and in the future. <em>Annual Review of Plant Biology<\/em> 67, 107. :\/\/dx.doi.org\/10.1146\/annurev-arplant-043015-111709.[\/footnote].\r\n\r\nAs it turns out, the factor behind those (real-life) numbers is photorespiration. This wasteful metabolic pathway begins when rubisco, the carbon-fixing enzyme of the Calvin cycle, grabs O<sub>2<\/sub>\u00a0rather than CO<sub>2<\/sub>. It uses up fixed carbon, wastes energy, and tends to happens when plants close their stomata (leaf pores) to reduce water loss. High temperatures make it even worse.\r\n\r\nSome plants, unlike wheat and soybean, can escape the worst effects of photorespiration. The C<sub>4<\/sub>\u00a0and CAM pathways are two adaptations\u2014beneficial features arising by natural selection\u2014that allow certain species to minimize photorespiration. These pathways work by ensuring that Rubisco always encounters high concentrations of CO<sub>2<\/sub>\u00a0making it unlikely to bind to O<sub>2<\/sub>.\r\n\r\nNow, let's take a closer look at the C<sub>3<\/sub>, C<sub>4<\/sub>\u00a0and CAM pathways and see how they do (or don't!) reduce photorespiration.\r\n<h2>C<sub>3<\/sub> plants<\/h2>\r\nA \"normal\" plant\u2014one that doesn't have photosynthetic adaptations to reduce photorespiration\u2014is called a C<sub>3<\/sub>\u00a0plant. The first step of the Calvin cycle is the fixation of carbon dioxide by rubisco, and plants that use only this \"standard\" mechanism of carbon fixation are called C<sub>3<\/sub>\u00a0plants, for the three-carbon compound (3-PGA) the reaction produces[footnote]Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., and Jackson, R. B. (2011). Alternative mechanisms of carbon fixation have evolved in hot, arid climates. In <em>Campbell biology<\/em> (10th ed.) San Francisco, CA: Pearson, 201.[\/footnote]. About 85 percent of the plant species on the planet are C<sub>3<\/sub>\u00a0plants, including rice, wheat, soybeans and all trees.\r\n\r\n[caption id=\"attachment_1668\" align=\"aligncenter\" width=\"373\"]<img class=\"wp-image-1668\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/2840\/2018\/03\/09182953\/c3plants_crop.png\" alt=\"Image of the c3 pathway. Carbon dioxide enters a mesophyll cell and is fixed immediately by rubisco, leading to the formation of three PGA molecules, which contain three carbons.\" width=\"373\" height=\"481\" \/> Figure 1. The C<sub>3<\/sub> Pathway[\/caption]\r\n<h2>C<sub>4<\/sub> plants<\/h2>\r\nIn C<sub>4<\/sub> plants, the light-dependent reactions and the Calvin cycle are physically separated, with the light-dependent reactions occurring in the mesophyll cells (spongy tissue in the middle of the leaf) and the Calvin cycle occurring in special cells around the leaf veins. These cells are called <strong>bundle-sheath<\/strong> cells.\r\n\r\nTo see how this division helps, let's look at an example of C<sub>4<\/sub> photosynthesis in action. First, atmospheric CO<sub>2<\/sub> is fixed in the mesophyll cells to form a simple, 4-carbon organic acid (oxaloacetate). This step is carried out by a non-rubisco enzyme, PEP carboxylase, that has no tendency to bind O<sub>2<\/sub>. Oxaloacetate is then converted to a similar molecule, malate, that can be transported in to the bundle-sheath cells. Inside the bundle sheath, malate breaks down, releasing a molecule of CO<sub>2<\/sub>. The CO<sub>2<\/sub> is then fixed by rubisco and made into sugars via the Calvin cycle, exactly as in C<sub>3<\/sub> photosynthesis.\r\n\r\n[caption id=\"attachment_1669\" align=\"aligncenter\" width=\"550\"]<img class=\"wp-image-1669\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/2840\/2018\/03\/09184139\/c4plants.png\" alt=\"Image of the c4 pathway. Initial carbon fixation takes place in mesophyll cells and the Calvin cycle takes place in bundle-sheath cells. PEP carboxylase attaches an incoming carbon dioxide molecule to the three carbon-molecule PEP producing oxaloacetate, a four-carbon molecule. The oxaloacetate is converted to malate, which travels out of the mesophyll cell and into a neighboring bundle-sheath cell. Inside the bundle-sheath cell, the malate is broken down to release carbon dioxide, which then enters the Calvin cycle. Pyruvate is also produced in this step and moves back into the mesophyll cell, where it is converted into PEP, a reaction that converts ATP and Pi into AMP and PPi.\" width=\"550\" height=\"642\" \/> Figure 2. The C<sub>4<\/sub> Pathway[\/caption]\r\n\r\nThis process isn't without its energetic price: ATP must be expended to return the three-carbon \u201cferry\u201d molecule from the bundle sheath cell and get it ready to pick up another molecule of atmospheric CO<sub>2<\/sub>. However, because the mesophyll cells constantly pump CO<sub>2<\/sub> into neighboring bundle-sheath cells in the form of malate, there\u2019s always a high concentration of CO<sub>2<\/sub>\u00a0relative to O<sub>2<\/sub> right around rubisco. This strategy minimizes photorespiration.\r\n\r\nThe C<sub>4<\/sub> pathway is used in about 3 percent of all vascular plants; some examples are crabgrass, sugarcane and corn. C<sub>4<\/sub> plants are common in habitats that are hot, but are less abundant in areas that are cooler. In hot conditions, the benefits of reduced photorespiration likely exceed the ATP cost of moving CO<sub>2<\/sub> from the mesophyll cell to the bundle-sheath cell.\r\n<h2>CAM plants<\/h2>\r\n[caption id=\"attachment_1671\" align=\"alignright\" width=\"325\"]<img class=\"wp-image-1671\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/2840\/2018\/03\/09185544\/crassulaceae.png\" alt=\"Image of a succulent plant\" width=\"325\" height=\"216\" \/> Figure 3. Crassulaceae[\/caption]\r\n\r\nSome plants that are adapted to dry environments, such as cacti and pineapples, use the crassulacean acid metabolism (CAM) pathway to minimize photorespiration. This name comes from the family of plants, the Crassulaceae, in which scientists first discovered the pathway.\r\n\r\nInstead of separating the light-dependent reactions and the use of CO<sub>2<\/sub>\u00a0in the Calvin cycle in space, CAM plants separate these processes in time. At night, CAM plants open their stomata, allowing CO<sub>2<\/sub>\u00a0to diffuse into the leaves. This CO<sub>2<\/sub>\u00a0is fixed into oxaloacetate by PEP carboxylase (the same step used by C<sub>4<\/sub> plants), then converted to malate or another type of organic acid[footnote]Crassulacean acid metabolism. (2016, May 29). Retrieved July 22, 2016 from Wikipedia: <a href=\"https:\/\/en.wikipedia.org\/wiki\/Crassulacean_acid_metabolism#Biochemistry\" target=\"_blank\" rel=\"noopener\">https:\/\/en.wikipedia.org\/wiki\/Crassulacean_acid_metabolism#Biochemistry<\/a>.[\/footnote].\r\n\r\nThe organic acid is stored inside vacuoles until the next day. In the daylight, the CAM plants do not open their stomata, but they can still photosynthesis. That's because the organic acids are transported out of the vacuole and broken down to release CO<sub>2<\/sub>, which enters the Calvin cycle. This controlled release maintains a high concentration of CO<sub>2<\/sub>\u00a0around rubisco[footnote]Raven, Peter H., Johnson, George B., Losos, Mason, Kenneth A., Losos, Jonathan B., and Singer, Susan R. (2014). Photorespiration. In <em>Biology<\/em> (10th ed., AP ed.). New York, NY: McGraw-Hill, 165.[\/footnote].\r\n\r\n[caption id=\"attachment_1670\" align=\"aligncenter\" width=\"527\"]<img class=\"wp-image-1670\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/2840\/2018\/03\/09185438\/camplants.png\" alt=\"Image of the C A M pathway. C A M plants temporally separate carbon fixation and the Calvin cycle. Carbon dioxide diffuses into leaves in the night when the stomata are open and is fixed into oxaloacetate by PEP carboxylase which attaches the carbon dioxide to the three-carbon molecule PEP. The oxaloacetate is converted into another organic acid, such as malate. The organic acid is stored until the next day and is then broken down releasing carbon dioxide that can be fixed by rubisco and enter the Calvin cycle to make sugars.\" width=\"527\" height=\"704\" \/> Figure 4. The CAM Pathway[\/caption]\r\n\r\nThe CAM pathway requires ATP at multiple steps (not shown above), so like C<span id=\"katex-45\"><sub>4<\/sub>\u00a0<\/span>photosynthesis, it is not an energetic \"freebie.\"[footnote]Crassulacean acid metabolism. (2016, May 29).[\/footnote]\u00a0However, plant species that use CAM photosynthesis not only avoid photorespiration, but are also very water-efficient. Their stomata only open at night, when humidity tends to be higher and temperatures are cooler, both factors that reduce water loss from leaves. CAM plants are typically dominant in very hot, dry areas, like deserts.\r\n<h2>Comparisons of C<sub>3<\/sub>, C<sub>4<\/sub>,\u00a0and CAM plants<\/h2>\r\nC<sub>3<\/sub>, C<sub>4<\/sub>,\u00a0and CAM plants all use the Calvin cycle to make sugars from CO<sub>2<\/sub>. These pathways for fixing CO<sub>2<\/sub>\u00a0have different advantages and disadvantages and make plants suited for different habitats. The C<sub>3<\/sub>\u00a0mechanism works well in cool environments, while C<sub>4<\/sub>\u00a0and CAM plants are adapted to hot, dry areas.\r\n\r\nBoth the C<sub>4<\/sub>\u00a0and CAM pathways have evolved independently over two dozen times, which suggests they may give plant species in hot climates a significant evolutionary advantage[footnote]Guralnick, Lonnie J., Amanda Cline, Monica Smith, and Rowan F. Sage. (2008). Evolutionary physiology: the extent of C<sub>4<\/sub> and CAM photosynthesis in the genera Anacampseros and Grahamia of the Portulacaceae. <em>Journal of Experimental Botany<\/em>, 59(7), 1735\u20131742. <a href=\"http:\/\/dx.doi.org\/10.1093\/jxb\/ern081\" target=\"_blank\" rel=\"noopener\">http:\/\/dx.doi.org\/10.1093\/jxb\/ern081<\/a>.[\/footnote].\r\n<table>\r\n<thead>\r\n<tr>\r\n<th width=\"10%\">Type<\/th>\r\n<th width=\"40%\">Separation of initial CO<sub>2<\/sub> fixation and Calvin cycle<\/th>\r\n<th width=\"20%\">Stomata open<\/th>\r\n<th width=\"40%\">Best adapted to<\/th>\r\n<\/tr>\r\n<\/thead>\r\n<tbody>\r\n<tr>\r\n<td>C<sub>3<\/sub><\/td>\r\n<td>No separation<\/td>\r\n<td>Day<\/td>\r\n<td>Cool, wet environments<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>C<sub>4<\/sub><\/td>\r\n<td>Between mesophyll and bundle-sheath cells (in space)<\/td>\r\n<td>Day<\/td>\r\n<td>Hot, sunny environments<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>CAM<\/td>\r\n<td>Between night and day (in time)<\/td>\r\n<td>Night<\/td>\r\n<td>Very hot, dry environments<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<iframe src=\"https:\/\/lumenlearning.h5p.com\/content\/1291235753785642518\/embed\" width=\"1088\" height=\"637\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><script src=\"https:\/\/lumenlearning.h5p.com\/js\/h5p-resizer.js\" charset=\"UTF-8\"><\/script>\r\n<div class=\"textbox tryit\">\r\n<h3>Try It<\/h3>\r\nhttps:\/\/assess.lumenlearning.com\/practice\/15be5587-58ca-4001-9bb1-4dd753896ada\r\n<\/div>","rendered":"<div class=\"textbox learning-objectives\">\n<h3>Learning Outcomes<\/h3>\n<ul>\n<li>Differentiate between C<sub>3<\/sub>, C<sub>4<\/sub>, and CAM plant approaches to photorespiration<\/li>\n<\/ul>\n<\/div>\n<p>High crop yields are pretty important\u2014for keeping people fed, and also for keeping economies running. If you heard there was a single factor that reduced the yield of wheat by 20 percent and the yield of soybeans by 36 percent in the United States, for instance, you might be curious to know what it was<a class=\"footnote\" title=\"Walker, Berkeley J., VanLoocke, Andy, Bernacchi, Carl J., and Ort, Donald R. (2016). The cost of photorespiration to food production now and in the future. Annual Review of Plant Biology 67, 107. :\/\/dx.doi.org\/10.1146\/annurev-arplant-043015-111709.\" id=\"return-footnote-1666-1\" href=\"#footnote-1666-1\" aria-label=\"Footnote 1\"><sup class=\"footnote\">[1]<\/sup><\/a>.<\/p>\n<p>As it turns out, the factor behind those (real-life) numbers is photorespiration. This wasteful metabolic pathway begins when rubisco, the carbon-fixing enzyme of the Calvin cycle, grabs O<sub>2<\/sub>\u00a0rather than CO<sub>2<\/sub>. It uses up fixed carbon, wastes energy, and tends to happens when plants close their stomata (leaf pores) to reduce water loss. High temperatures make it even worse.<\/p>\n<p>Some plants, unlike wheat and soybean, can escape the worst effects of photorespiration. The C<sub>4<\/sub>\u00a0and CAM pathways are two adaptations\u2014beneficial features arising by natural selection\u2014that allow certain species to minimize photorespiration. These pathways work by ensuring that Rubisco always encounters high concentrations of CO<sub>2<\/sub>\u00a0making it unlikely to bind to O<sub>2<\/sub>.<\/p>\n<p>Now, let&#8217;s take a closer look at the C<sub>3<\/sub>, C<sub>4<\/sub>\u00a0and CAM pathways and see how they do (or don&#8217;t!) reduce photorespiration.<\/p>\n<h2>C<sub>3<\/sub> plants<\/h2>\n<p>A &#8220;normal&#8221; plant\u2014one that doesn&#8217;t have photosynthetic adaptations to reduce photorespiration\u2014is called a C<sub>3<\/sub>\u00a0plant. The first step of the Calvin cycle is the fixation of carbon dioxide by rubisco, and plants that use only this &#8220;standard&#8221; mechanism of carbon fixation are called C<sub>3<\/sub>\u00a0plants, for the three-carbon compound (3-PGA) the reaction produces<a class=\"footnote\" title=\"Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., and Jackson, R. B. (2011). Alternative mechanisms of carbon fixation have evolved in hot, arid climates. In Campbell biology (10th ed.) San Francisco, CA: Pearson, 201.\" id=\"return-footnote-1666-2\" href=\"#footnote-1666-2\" aria-label=\"Footnote 2\"><sup class=\"footnote\">[2]<\/sup><\/a>. About 85 percent of the plant species on the planet are C<sub>3<\/sub>\u00a0plants, including rice, wheat, soybeans and all trees.<\/p>\n<div id=\"attachment_1668\" style=\"width: 383px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1668\" class=\"wp-image-1668\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/2840\/2018\/03\/09182953\/c3plants_crop.png\" alt=\"Image of the c3 pathway. Carbon dioxide enters a mesophyll cell and is fixed immediately by rubisco, leading to the formation of three PGA molecules, which contain three carbons.\" width=\"373\" height=\"481\" \/><\/p>\n<p id=\"caption-attachment-1668\" class=\"wp-caption-text\">Figure 1. The C<sub>3<\/sub> Pathway<\/p>\n<\/div>\n<h2>C<sub>4<\/sub> plants<\/h2>\n<p>In C<sub>4<\/sub> plants, the light-dependent reactions and the Calvin cycle are physically separated, with the light-dependent reactions occurring in the mesophyll cells (spongy tissue in the middle of the leaf) and the Calvin cycle occurring in special cells around the leaf veins. These cells are called <strong>bundle-sheath<\/strong> cells.<\/p>\n<p>To see how this division helps, let&#8217;s look at an example of C<sub>4<\/sub> photosynthesis in action. First, atmospheric CO<sub>2<\/sub> is fixed in the mesophyll cells to form a simple, 4-carbon organic acid (oxaloacetate). This step is carried out by a non-rubisco enzyme, PEP carboxylase, that has no tendency to bind O<sub>2<\/sub>. Oxaloacetate is then converted to a similar molecule, malate, that can be transported in to the bundle-sheath cells. Inside the bundle sheath, malate breaks down, releasing a molecule of CO<sub>2<\/sub>. The CO<sub>2<\/sub> is then fixed by rubisco and made into sugars via the Calvin cycle, exactly as in C<sub>3<\/sub> photosynthesis.<\/p>\n<div id=\"attachment_1669\" style=\"width: 560px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1669\" class=\"wp-image-1669\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/2840\/2018\/03\/09184139\/c4plants.png\" alt=\"Image of the c4 pathway. Initial carbon fixation takes place in mesophyll cells and the Calvin cycle takes place in bundle-sheath cells. PEP carboxylase attaches an incoming carbon dioxide molecule to the three carbon-molecule PEP producing oxaloacetate, a four-carbon molecule. The oxaloacetate is converted to malate, which travels out of the mesophyll cell and into a neighboring bundle-sheath cell. Inside the bundle-sheath cell, the malate is broken down to release carbon dioxide, which then enters the Calvin cycle. Pyruvate is also produced in this step and moves back into the mesophyll cell, where it is converted into PEP, a reaction that converts ATP and Pi into AMP and PPi.\" width=\"550\" height=\"642\" \/><\/p>\n<p id=\"caption-attachment-1669\" class=\"wp-caption-text\">Figure 2. The C<sub>4<\/sub> Pathway<\/p>\n<\/div>\n<p>This process isn&#8217;t without its energetic price: ATP must be expended to return the three-carbon \u201cferry\u201d molecule from the bundle sheath cell and get it ready to pick up another molecule of atmospheric CO<sub>2<\/sub>. However, because the mesophyll cells constantly pump CO<sub>2<\/sub> into neighboring bundle-sheath cells in the form of malate, there\u2019s always a high concentration of CO<sub>2<\/sub>\u00a0relative to O<sub>2<\/sub> right around rubisco. This strategy minimizes photorespiration.<\/p>\n<p>The C<sub>4<\/sub> pathway is used in about 3 percent of all vascular plants; some examples are crabgrass, sugarcane and corn. C<sub>4<\/sub> plants are common in habitats that are hot, but are less abundant in areas that are cooler. In hot conditions, the benefits of reduced photorespiration likely exceed the ATP cost of moving CO<sub>2<\/sub> from the mesophyll cell to the bundle-sheath cell.<\/p>\n<h2>CAM plants<\/h2>\n<div id=\"attachment_1671\" style=\"width: 335px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1671\" class=\"wp-image-1671\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/2840\/2018\/03\/09185544\/crassulaceae.png\" alt=\"Image of a succulent plant\" width=\"325\" height=\"216\" \/><\/p>\n<p id=\"caption-attachment-1671\" class=\"wp-caption-text\">Figure 3. Crassulaceae<\/p>\n<\/div>\n<p>Some plants that are adapted to dry environments, such as cacti and pineapples, use the crassulacean acid metabolism (CAM) pathway to minimize photorespiration. This name comes from the family of plants, the Crassulaceae, in which scientists first discovered the pathway.<\/p>\n<p>Instead of separating the light-dependent reactions and the use of CO<sub>2<\/sub>\u00a0in the Calvin cycle in space, CAM plants separate these processes in time. At night, CAM plants open their stomata, allowing CO<sub>2<\/sub>\u00a0to diffuse into the leaves. This CO<sub>2<\/sub>\u00a0is fixed into oxaloacetate by PEP carboxylase (the same step used by C<sub>4<\/sub> plants), then converted to malate or another type of organic acid<a class=\"footnote\" title=\"Crassulacean acid metabolism. (2016, May 29). Retrieved July 22, 2016 from Wikipedia: https:\/\/en.wikipedia.org\/wiki\/Crassulacean_acid_metabolism#Biochemistry.\" id=\"return-footnote-1666-3\" href=\"#footnote-1666-3\" aria-label=\"Footnote 3\"><sup class=\"footnote\">[3]<\/sup><\/a>.<\/p>\n<p>The organic acid is stored inside vacuoles until the next day. In the daylight, the CAM plants do not open their stomata, but they can still photosynthesis. That&#8217;s because the organic acids are transported out of the vacuole and broken down to release CO<sub>2<\/sub>, which enters the Calvin cycle. This controlled release maintains a high concentration of CO<sub>2<\/sub>\u00a0around rubisco<a class=\"footnote\" title=\"Raven, Peter H., Johnson, George B., Losos, Mason, Kenneth A., Losos, Jonathan B., and Singer, Susan R. (2014). Photorespiration. In Biology (10th ed., AP ed.). New York, NY: McGraw-Hill, 165.\" id=\"return-footnote-1666-4\" href=\"#footnote-1666-4\" aria-label=\"Footnote 4\"><sup class=\"footnote\">[4]<\/sup><\/a>.<\/p>\n<div id=\"attachment_1670\" style=\"width: 537px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1670\" class=\"wp-image-1670\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/2840\/2018\/03\/09185438\/camplants.png\" alt=\"Image of the C A M pathway. C A M plants temporally separate carbon fixation and the Calvin cycle. Carbon dioxide diffuses into leaves in the night when the stomata are open and is fixed into oxaloacetate by PEP carboxylase which attaches the carbon dioxide to the three-carbon molecule PEP. The oxaloacetate is converted into another organic acid, such as malate. The organic acid is stored until the next day and is then broken down releasing carbon dioxide that can be fixed by rubisco and enter the Calvin cycle to make sugars.\" width=\"527\" height=\"704\" \/><\/p>\n<p id=\"caption-attachment-1670\" class=\"wp-caption-text\">Figure 4. The CAM Pathway<\/p>\n<\/div>\n<p>The CAM pathway requires ATP at multiple steps (not shown above), so like C<span id=\"katex-45\"><sub>4<\/sub>\u00a0<\/span>photosynthesis, it is not an energetic &#8220;freebie.&#8221;<a class=\"footnote\" title=\"Crassulacean acid metabolism. (2016, May 29).\" id=\"return-footnote-1666-5\" href=\"#footnote-1666-5\" aria-label=\"Footnote 5\"><sup class=\"footnote\">[5]<\/sup><\/a>\u00a0However, plant species that use CAM photosynthesis not only avoid photorespiration, but are also very water-efficient. Their stomata only open at night, when humidity tends to be higher and temperatures are cooler, both factors that reduce water loss from leaves. CAM plants are typically dominant in very hot, dry areas, like deserts.<\/p>\n<h2>Comparisons of C<sub>3<\/sub>, C<sub>4<\/sub>,\u00a0and CAM plants<\/h2>\n<p>C<sub>3<\/sub>, C<sub>4<\/sub>,\u00a0and CAM plants all use the Calvin cycle to make sugars from CO<sub>2<\/sub>. These pathways for fixing CO<sub>2<\/sub>\u00a0have different advantages and disadvantages and make plants suited for different habitats. The C<sub>3<\/sub>\u00a0mechanism works well in cool environments, while C<sub>4<\/sub>\u00a0and CAM plants are adapted to hot, dry areas.<\/p>\n<p>Both the C<sub>4<\/sub>\u00a0and CAM pathways have evolved independently over two dozen times, which suggests they may give plant species in hot climates a significant evolutionary advantage<a class=\"footnote\" title=\"Guralnick, Lonnie J., Amanda Cline, Monica Smith, and Rowan F. Sage. (2008). Evolutionary physiology: the extent of C4 and CAM photosynthesis in the genera Anacampseros and Grahamia of the Portulacaceae. Journal of Experimental Botany, 59(7), 1735\u20131742. http:\/\/dx.doi.org\/10.1093\/jxb\/ern081.\" id=\"return-footnote-1666-6\" href=\"#footnote-1666-6\" aria-label=\"Footnote 6\"><sup class=\"footnote\">[6]<\/sup><\/a>.<\/p>\n<table>\n<thead>\n<tr>\n<th style=\"width: 10%;\">Type<\/th>\n<th style=\"width: 40%;\">Separation of initial CO<sub>2<\/sub> fixation and Calvin cycle<\/th>\n<th style=\"width: 20%;\">Stomata open<\/th>\n<th style=\"width: 40%;\">Best adapted to<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>C<sub>3<\/sub><\/td>\n<td>No separation<\/td>\n<td>Day<\/td>\n<td>Cool, wet environments<\/td>\n<\/tr>\n<tr>\n<td>C<sub>4<\/sub><\/td>\n<td>Between mesophyll and bundle-sheath cells (in space)<\/td>\n<td>Day<\/td>\n<td>Hot, sunny environments<\/td>\n<\/tr>\n<tr>\n<td>CAM<\/td>\n<td>Between night and day (in time)<\/td>\n<td>Night<\/td>\n<td>Very hot, dry environments<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p><iframe loading=\"lazy\" src=\"https:\/\/lumenlearning.h5p.com\/content\/1291235753785642518\/embed\" width=\"1088\" height=\"637\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><script src=\"https:\/\/lumenlearning.h5p.com\/js\/h5p-resizer.js\" charset=\"UTF-8\"><\/script><\/p>\n<div class=\"textbox tryit\">\n<h3>Try It<\/h3>\n<p>\t<iframe id=\"assessment_practice_15be5587-58ca-4001-9bb1-4dd753896ada\" class=\"resizable\" src=\"https:\/\/assess.lumenlearning.com\/practice\/15be5587-58ca-4001-9bb1-4dd753896ada?iframe_resize_id=assessment_practice_id_15be5587-58ca-4001-9bb1-4dd753896ada\" frameborder=\"0\" style=\"border:none;width:100%;height:100%;min-height:300px;\"><br \/>\n\t<\/iframe>\n<\/div>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-1666\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>C3, C4, and CAM plants. <strong>Provided by<\/strong>: Khan Academy. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"https:\/\/www.khanacademy.org\/science\/biology\/photosynthesis-in-plants\/photorespiration--c3-c4-cam-plants\/a\/c3-c4-and-cam-plants-agriculture\">https:\/\/www.khanacademy.org\/science\/biology\/photosynthesis-in-plants\/photorespiration--c3-c4-cam-plants\/a\/c3-c4-and-cam-plants-agriculture<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by-nc-sa\/4.0\/\">CC BY-NC-SA: Attribution-NonCommercial-ShareAlike<\/a><\/em><\/li><li>Crassulaceae. <strong>Authored by<\/strong>: Guyon Moree. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"https:\/\/flic.kr\/p\/4ZD6pP\">https:\/\/flic.kr\/p\/4ZD6pP<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em><\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section><hr class=\"before-footnotes clear\" \/><div class=\"footnotes\"><ol><li id=\"footnote-1666-1\">Walker, Berkeley J., VanLoocke, Andy, Bernacchi, Carl J., and Ort, Donald R. (2016). The cost of photorespiration to food production now and in the future. <em>Annual Review of Plant Biology<\/em> 67, 107. :\/\/dx.doi.org\/10.1146\/annurev-arplant-043015-111709. <a href=\"#return-footnote-1666-1\" class=\"return-footnote\" aria-label=\"Return to footnote 1\">&crarr;<\/a><\/li><li id=\"footnote-1666-2\">Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., and Jackson, R. B. (2011). Alternative mechanisms of carbon fixation have evolved in hot, arid climates. In <em>Campbell biology<\/em> (10th ed.) San Francisco, CA: Pearson, 201. <a href=\"#return-footnote-1666-2\" class=\"return-footnote\" aria-label=\"Return to footnote 2\">&crarr;<\/a><\/li><li id=\"footnote-1666-3\">Crassulacean acid metabolism. (2016, May 29). Retrieved July 22, 2016 from Wikipedia: <a href=\"https:\/\/en.wikipedia.org\/wiki\/Crassulacean_acid_metabolism#Biochemistry\" target=\"_blank\" rel=\"noopener\">https:\/\/en.wikipedia.org\/wiki\/Crassulacean_acid_metabolism#Biochemistry<\/a>. <a href=\"#return-footnote-1666-3\" class=\"return-footnote\" aria-label=\"Return to footnote 3\">&crarr;<\/a><\/li><li id=\"footnote-1666-4\">Raven, Peter H., Johnson, George B., Losos, Mason, Kenneth A., Losos, Jonathan B., and Singer, Susan R. (2014). Photorespiration. In <em>Biology<\/em> (10th ed., AP ed.). New York, NY: McGraw-Hill, 165. <a href=\"#return-footnote-1666-4\" class=\"return-footnote\" aria-label=\"Return to footnote 4\">&crarr;<\/a><\/li><li id=\"footnote-1666-5\">Crassulacean acid metabolism. (2016, May 29). <a href=\"#return-footnote-1666-5\" class=\"return-footnote\" aria-label=\"Return to footnote 5\">&crarr;<\/a><\/li><li id=\"footnote-1666-6\">Guralnick, Lonnie J., Amanda Cline, Monica Smith, and Rowan F. Sage. (2008). Evolutionary physiology: the extent of C<sub>4<\/sub> and CAM photosynthesis in the genera Anacampseros and Grahamia of the Portulacaceae. <em>Journal of Experimental Botany<\/em>, 59(7), 1735\u20131742. <a href=\"http:\/\/dx.doi.org\/10.1093\/jxb\/ern081\" target=\"_blank\" rel=\"noopener\">http:\/\/dx.doi.org\/10.1093\/jxb\/ern081<\/a>. <a href=\"#return-footnote-1666-6\" class=\"return-footnote\" aria-label=\"Return to footnote 6\">&crarr;<\/a><\/li><\/ol><\/div>","protected":false},"author":17,"menu_order":13,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"C3, C4, and CAM plants\",\"author\":\"\",\"organization\":\"Khan Academy\",\"url\":\"https:\/\/www.khanacademy.org\/science\/biology\/photosynthesis-in-plants\/photorespiration--c3-c4-cam-plants\/a\/c3-c4-and-cam-plants-agriculture\",\"project\":\"\",\"license\":\"cc-by-nc-sa\",\"license_terms\":\"\"},{\"type\":\"cc\",\"description\":\"Crassulaceae\",\"author\":\"Guyon Moree\",\"organization\":\"\",\"url\":\"https:\/\/flic.kr\/p\/4ZD6pP\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"\"}]","CANDELA_OUTCOMES_GUID":"ce4d9eff-b683-4075-9ccc-d439353c60f8, 9c627ac3-bfdc-4527-8d0d-fa077f9878c8","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-1666","chapter","type-chapter","status-publish","hentry"],"part":483,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/pressbooks\/v2\/chapters\/1666","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/wp\/v2\/users\/17"}],"version-history":[{"count":12,"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/pressbooks\/v2\/chapters\/1666\/revisions"}],"predecessor-version":[{"id":2996,"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/pressbooks\/v2\/chapters\/1666\/revisions\/2996"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/pressbooks\/v2\/parts\/483"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/pressbooks\/v2\/chapters\/1666\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/wp\/v2\/media?parent=1666"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/pressbooks\/v2\/chapter-type?post=1666"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/wp\/v2\/contributor?post=1666"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/wp\/v2\/license?post=1666"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}