{"id":648,"date":"2018-05-03T17:58:07","date_gmt":"2018-05-03T17:58:07","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-osbiology2e\/chapter\/energy-in-living-systems\/"},"modified":"2018-06-12T13:37:16","modified_gmt":"2018-06-12T13:37:16","slug":"energy-in-living-systems","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-oneonta-osbiology2e-1\/chapter\/energy-in-living-systems\/","title":{"raw":"Energy in Living Systems","rendered":"Energy in Living Systems"},"content":{"raw":"<div class=\"textbox learning-objectives\">\r\n<h3>Learning Objectives<\/h3>\r\nBy the end of this section, you will be able to do the following:\r\n<ul>\r\n \t<li>Discuss the importance of electrons in the transfer of energy in living systems<\/li>\r\n \t<li>Explain how ATP is used by cells as an energy source<\/li>\r\n<\/ul>\r\n<\/div>\r\n<p id=\"fs-id1894837\">Energy production within a cell involves many coordinated chemical pathways. Most of these pathways are combinations of oxidation and reduction reactions, which occur at the same time. An oxidation reaction strips an electron from an atom in a compound, and the addition of this electron to another compound is a reduction reaction. Because oxidation and reduction usually occur together, these pairs of reactions are called oxidation reduction reactions, or redox reactions.<\/p>\r\n\r\n<div id=\"fs-id1798739\" class=\"bc-section section\">\r\n<h3>Electrons and Energy<\/h3>\r\n<p id=\"fs-id3112918\">The removal of an electron from a molecule (oxidizing it), results in a decrease in potential energy in the oxidized compound. The electron (sometimes as part of a hydrogen atom) does not remain unbonded, however, in the cytoplasm of a cell. Rather, the electron is shifted to a second compound, reducing the second compound. <em>The shift of an electron from one compound to another removes some potential energy from the first compound (the oxidized compound) and increases the potential energy of the second compound (the reduced compound).<\/em> The transfer of electrons between molecules is important because most of the energy stored in atoms and used to fuel cell functions is in the form of high-energy electrons. The transfer of energy in the form of high-energy electrons allows the cell to transfer and use energy in an incremental fashion\u2014in small packages rather than in a single, destructive burst. This chapter focuses on the extraction of energy from food; you will see that as you track the path of the transfers, you are tracking the path of electrons moving through metabolic pathways.<\/p>\r\n\r\n<div id=\"fs-id2763612\" class=\"bc-section section\">\r\n<h4>Electron Carriers<\/h4>\r\nIn living systems, a small class of compounds functions as electron shuttles: they bind and carry high-energy electrons between compounds in biochemical pathways. The principal electron carriers we will consider are derived from the B vitamin group and are derivatives of nucleotides. These compounds can be easily reduced (that is, they accept electrons) or oxidized (they lose electrons). Nicotinamide adenine dinucleotide (NAD) (<a class=\"autogenerated-content\" href=\"#fig-ch07_01_01\">(Figure)<\/a>) is derived from vitamin B<sub>3<\/sub>, niacin. NAD<sup>+<\/sup> is the oxidized form of the molecule; NADH is the reduced form of the molecule after it has accepted two electrons and a proton (which together are the equivalent of a hydrogen atom with an extra electron). Note that if a compound has an \u201cH\u201d on it, it is generally reduced (e.g., NADH is the reduced form of NAD).\r\n<p id=\"fs-id1464730\">NAD<sup>+<\/sup> can accept electrons from an organic molecule according to the general equation:<\/p>\r\n\r\n<div id=\"eip-739\">[latex]\\begin{array}{c}\\text{RH}\\\\ \\begin{array}{c}\\text{Reducing\u00a0}\\\\ \\text{agent}\\end{array}\\end{array}\\text{\u00a0+\u00a0}\\begin{array}{c}{\\text{NAD}}^{+}\\\\ \\begin{array}{c}\\text{Oxidizing}\\\\ \\text{agent}\\end{array}\\end{array}\\to \\text{\u00a0}\\begin{array}{c}\\text{NADH}\\\\ \\text{Reduced}\\end{array}\\text{\u00a0+}\\begin{array}{c}\\text{R}\\\\ \\text{Oxidized}\\end{array}[\/latex]<\/div>\r\n<p id=\"fs-id2073095\">When electrons are added to a compound, <em>it is reduced<\/em>. A compound that reduces another is called a reducing agent. In the above equation, RH is a reducing agent, and NAD<sup>+<\/sup> is reduced to NADH. When electrons are removed from a compound, <em>it is oxidized<\/em>. A compound that oxidizes another is called an oxidizing agent. In the above equation, NAD<sup>+<\/sup> is an oxidizing agent, and RH is oxidized to R.<\/p>\r\n<p id=\"fs-id2583910\">Similarly, flavin adenine dinucleotide (FAD<sup>+<\/sup>) is derived from vitamin B<sub>2<\/sub>, also called riboflavin. Its reduced form is FADH<sub>2<\/sub>. A second variation of NAD, NADP, contains an extra phosphate group. Both NAD<sup>+<\/sup> and FAD<sup>+<\/sup> are extensively used in energy extraction from sugars, and NADP plays an important role in anabolic reactions and photosynthesis in plants.<\/p>\r\n\r\n<div id=\"fig-ch07_01_01\" class=\"wp-caption aligncenter\"><span id=\"fs-id1937340\"><img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/3206\/2018\/05\/03175759\/Figure_07_01_01ab.jpg\" alt=\"This illustration shows the molecular structure of NAD^{+} and NADH. Both compounds are composed of an adenine nucleotide and a nicotinamide nucleotide, which bond together to form a dinucleotide. The nicotinamide nucleotide is at the 5' end, and the adenine nucleotide is at the 3\u2019 end. Nicotinamide is a nitrogenous base, meaning it has nitrogen in a six-membered carbon ring. In NADH, one extra hydrogen is associated with this ring, which is not found in NAD^{+}.\" width=\"500\" \/><\/span><\/div>\r\n<div class=\"wp-caption-text\">The oxidized form of the electron carrier (NAD<sup>+<\/sup>) is shown on the left, and the reduced form (NADH) is shown on the right. The nitrogenous base in NADH has one more hydrogen ion and two more electrons than in NAD<sup>+<\/sup>.<\/div>\r\n<\/div>\r\n<\/div>\r\n<div id=\"fs-id1511906\" class=\"bc-section section\">\r\n<h3>ATP in Living Systems<\/h3>\r\n<p id=\"fs-id1640338\">A living cell cannot store significant amounts of free energy. Excess free energy would result in an increase of heat in the cell, which would result in excessive thermal motion that could damage and then destroy the cell. Rather, a cell must be able to handle that energy in a way that enables the cell to store energy safely and release it for use only as needed. Living cells accomplish this by using the compound adenosine triphosphate (ATP). ATP is often called the \u201cenergy currency\u201d of the cell, and, like currency, this versatile compound can be used to fill any energy need of the cell. How? It functions similarly to a rechargeable battery.<\/p>\r\n<p id=\"fs-id1359897\">When ATP is broken down, usually by the removal of its terminal phosphate group, energy is released. The energy is used to do work by the cell, usually when the released phosphate binds to another molecule, thereby activating it. For example, in the mechanical work of muscle contraction, ATP supplies the energy to move the contractile muscle proteins. Recall the active transport work of the sodium-potassium pump in cell membranes. ATP alters the structure of the integral protein that functions as the pump, changing its affinity for sodium and potassium. In this way, the cell performs work, pumping ions against their electrochemical gradients.<\/p>\r\n\r\n<div id=\"fs-id2183864\" class=\"bc-section section\">\r\n<h4>ATP Structure and Function<\/h4>\r\n<p id=\"fs-id1595819\">At the heart of ATP is a molecule of adenosine monophosphate (AMP), which is composed of an adenine molecule bonded to a ribose molecule and to a single phosphate group (<a class=\"autogenerated-content\" href=\"#fig-ch07_01_02\">(Figure)<\/a>). Ribose is a five-carbon sugar found in RNA, and AMP is one of the nucleotides in RNA. The addition of a second phosphate group to this core molecule results in the formation of adenosine <u>di<\/u>phosphate (ADP); the addition of a third phosphate group forms adenosine <u>tri<\/u>phosphate (ATP).<\/p>\r\n\r\n<div id=\"fig-ch07_01_02\" class=\"wp-caption aligncenter\"><span id=\"fs-id2756859\"><img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/3206\/2018\/05\/03175801\/Figure_07_01_02.jpg\" alt=\"This illustration shows the molecular structure of ATP. This molecule is an adenine nucleotide with a string of three phosphate groups attached to it. The phosphate groups are named alpha, beta, and gamma in order of increasing distance from the ribose sugar to which they are attached.\" width=\"400\" \/><\/span><\/div>\r\n<div class=\"wp-caption-text\">ATP (adenosine triphosphate) has three phosphate groups that can be removed by hydrolysis (addition of H<sub>2<\/sub>O) to form ADP (adenosine diphosphate) or AMP (adenosine monophosphate). The negative charges on the phosphate group naturally repel each other, requiring energy to bond them together and releasing energy when these bonds are broken.<\/div>\r\n<p id=\"fs-id2308504\">The addition of a phosphate group to a molecule requires energy. Phosphate groups are negatively charged and thus repel one another when they are arranged in series, as they are in ADP and ATP. This repulsion makes the ADP and ATP molecules inherently unstable. The release of one or two phosphate groups from ATP, a process called dephosphorylation, releases energy.<\/p>\r\n\r\n<\/div>\r\n<div id=\"fs-id3570970\" class=\"bc-section section\">\r\n<h4>Energy from ATP<\/h4>\r\n<p id=\"fs-id1981342\">Hydrolysis is the process of breaking complex macromolecules apart. During hydrolysis, water is split, or lysed, and the resulting hydrogen atom (H<sup>+<\/sup>) and a hydroxyl group (OH<sup>-<\/sup>), or <em>hydroxide,<\/em> are added to the larger molecule. The hydrolysis of ATP produces ADP, together with an inorganic phosphate ion (P<sub>i<\/sub>), and the release of free energy. To carry out life processes, ATP is continuously broken down into ADP, and like a rechargeable battery, ADP is continuously regenerated into ATP by the reattachment of a third phosphate group. Water, which was broken down into its hydrogen atom and hydroxyl group (hydroxide) during ATP hydrolysis, is regenerated when a third phosphate is added to the ADP molecule, reforming ATP.<\/p>\r\n<p id=\"fs-id1798236\">Obviously, energy must be infused into the system to regenerate ATP. Where does this energy come from? In nearly every living thing on Earth, the energy comes from the metabolism of glucose, fructose, or galactose, all isomers with the chemical formula C<sub>6<\/sub>H<sub>12<\/sub>O<sub>6<\/sub> but different molecular configurations. In this way, ATP is a direct link between the limited set of exergonic pathways of glucose catabolism and the multitude of endergonic pathways that power living cells.<\/p>\r\n\r\n<\/div>\r\n<div id=\"fs-id1560590\" class=\"bc-section section\">\r\n<h4>Phosphorylation<\/h4>\r\n<p id=\"fs-id2348311\">Recall that, in some chemical reactions, enzymes may bind to several substrates that react with each other on the enzyme, forming an intermediate complex. An intermediate complex is a temporary structure, and it allows one of the substrates (such as ATP) and reactants to more readily react with each other; in reactions involving ATP, ATP is one of the substrates and ADP is a product. During an endergonic chemical reaction, ATP forms an intermediate complex with the substrate and enzyme in the reaction. This intermediate complex allows the ATP to transfer its third phosphate group, with its energy, to the substrate, a process called phosphorylation. Phosphorylation refers to the addition of the phosphate (~P). This is illustrated by the following generic reaction, in which A and B represent two different substrates:<\/p>\r\n\r\n<div id=\"eip-458\">[latex]\\text{A\u00a0+\u00a0enzyme\u00a0+\u00a0ATP}\\to \\text{\u00a0}\\left[\\text{A\u00a0}-\\text{\u00a0enzyme\u00a0}-\\text{\u00a0}\\sim \\text{P}\\right]\\text{\u00a0}\\to \\text{\u00a0B\u00a0+\u00a0enzyme\u00a0+\u00a0ADP\u00a0+\u00a0phosphate\u00a0ion}[\/latex]<\/div>\r\nWhen the intermediate complex breaks apart, the energy is used to modify the substrate and convert it into a product of the reaction. The ADP molecule and a free phosphate ion are released into the medium and are available for recycling through cell metabolism.\r\n\r\n<\/div>\r\n<div id=\"fs-id2195071\" class=\"bc-section section\">\r\n<h4>Substrate Phosphorylation<\/h4>\r\n<p id=\"fs-id1451024\">ATP is generated through two mechanisms during the breakdown of glucose. A few ATP molecules are generated (that is, regenerated from ADP) as a direct result of the chemical reactions that occur in the catabolic pathways. A phosphate group is removed from an intermediate reactant in the pathway, and the free energy of the reaction is used to add the third phosphate to an available ADP molecule, producing ATP (<a class=\"autogenerated-content\" href=\"#fig-ch07_01_03\">(Figure)<\/a>). This very direct method of phosphorylation is called substrate-level phosphorylation.<\/p>\r\n\r\n<div id=\"fig-ch07_01_03\" class=\"wp-caption aligncenter\"><span id=\"fs-id1414909\"><img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/3206\/2018\/05\/03175804\/Figure_07_01_03.jpg\" alt=\"This illustration shows a substrate-level phosphorylation reaction in which the gamma phosphate of ATP is attached to a protein.\" width=\"350\" \/><\/span><\/div>\r\n<div class=\"wp-caption-text\">In phosphorylation reactions, the gamma (third) phosphate of ATP is attached to a protein.<\/div>\r\n<\/div>\r\n<div id=\"fs-id2019618\" class=\"bc-section section\">\r\n<h4>Oxidative Phosphorylation<\/h4>\r\n<p id=\"fs-id2065719\">Most of the ATP generated during glucose catabolism, however, is derived from a much more complex process, chemiosmosis, which takes place in mitochondria (<a class=\"autogenerated-content\" href=\"#fig-ch07_01_04\">(Figure)<\/a>) within a eukaryotic cell or the plasma membrane of a prokaryotic cell. Chemiosmosis, a process of ATP production in cellular metabolism, is used to generate 90 percent of the ATP made during glucose catabolism and is also the method used in the light reactions of photosynthesis to harness the energy of sunlight. The production of ATP using the process of chemiosmosis is called oxidative phosphorylation because of the involvement of oxygen in the process.<\/p>\r\n\r\n<div id=\"fig-ch07_01_04\" class=\"wp-caption aligncenter\"><span id=\"fs-id1318782\"><img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/3206\/2018\/05\/03175806\/Figure_07_01_04.jpg\" alt=\"This illustration shows the structure of a mitochondrion, which has an outer membrane and an inner membrane. The inner membrane has many folds, called cristae. The space between the outer membrane and the inner membrane is called the intermembrane space, and the central space of the mitochondrion is called the matrix. ATP synthase enzymes and the electron transport chain are located in the inner membrane\" width=\"300\" \/><\/span><\/div>\r\n<div class=\"wp-caption-text\">In eukaryotes, oxidative phosphorylation takes place in mitochondria. In prokaryotes, this process takes place in the plasma membrane. (Credit: modification of work by Mariana Ruiz Villareal)<\/div>\r\n<div id=\"fs-id1912383\" class=\"career textbox examples\">\r\n<h3>Career Connections<\/h3>\r\n<strong>Mitochondrial Disease Physician<\/strong>\r\n\r\nWhat happens when the critical reactions of cellular respiration do not proceed correctly? This may happen in mitochondrial diseases, which are genetic disorders of metabolism. Mitochondrial disorders can arise from mutations in nuclear or mitochondrial DNA, and they result in the production of less energy than is normal in body cells. In type 2 diabetes, for instance, the oxidation efficiency of NADH is reduced, impacting oxidative phosphorylation but not the other steps of respiration. Symptoms of mitochondrial diseases can include muscle weakness, lack of coordination, stroke-like episodes, and loss of vision and hearing. Most affected people are diagnosed in childhood, although there are some adult-onset diseases. Identifying and treating mitochondrial disorders is a specialized medical field. The educational preparation for this profession requires a college education, followed by medical school with a specialization in medical genetics. Medical geneticists can be board certified by the American Board of Medical Genetics and go on to become associated with professional organizations devoted to the study of mitochondrial diseases, such as the Mitochondrial Medicine Society and the Society for Inherited Metabolic Disorders.\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<div class=\"summary textbox key-takeaways\">\r\n<h3>Section Summary<\/h3>\r\nATP functions as the energy currency for cells. It allows the cell to store energy briefly and transport it within the cell to support endergonic chemical reactions. The structure of ATP is that of an RNA nucleotide with three phosphates attached. As ATP is used for energy, a phosphate group or two are detached, and either ADP or AMP is produced. Energy derived from glucose catabolism is used to convert ADP into ATP. When ATP is used in a reaction, the third phosphate is temporarily attached to a substrate in a process called phosphorylation. The two processes of ATP regeneration that are used in conjunction with glucose catabolism are substrate-level phosphorylation and oxidative phosphorylation through the process of chemiosmosis.\r\n\r\n<\/div>\r\n<div id=\"fs-id1473147\" class=\"multiple-choice textbox exercises\">\r\n<h3>Review Questions<\/h3>\r\n<div id=\"fs-id1957993\">\r\n<div id=\"fs-id1276810\">\r\n<p id=\"fs-id2197455\">The energy currency used by cells is ________.<\/p>\r\n\r\n<ol id=\"fs-id1785724\" type=\"a\">\r\n \t<li>ATP<\/li>\r\n \t<li>ADP<\/li>\r\n \t<li>AMP<\/li>\r\n \t<li>adenosine<\/li>\r\n<\/ol>\r\n<\/div>\r\n<div id=\"fs-id1470656\">\r\n<p id=\"fs-id1760308\">\r\n[reveal-answer q=\"17792\"]Show Solution[\/reveal-answer]\r\n[hidden-answer a=\"17792\"]A[\/hidden-answer]<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n<div id=\"fs-id1419484\">\r\n<div id=\"fs-id2100818\">\r\n<p id=\"fs-id1450216\">A reducing chemical reaction ________.<\/p>\r\n\r\n<ol id=\"fs-id2962812\" type=\"a\">\r\n \t<li>reduces the compound to a simpler form<\/li>\r\n \t<li>adds an electron to the substrate<\/li>\r\n \t<li>removes a hydrogen atom from the substrate<\/li>\r\n \t<li>is a catabolic reaction<\/li>\r\n<\/ol>\r\n<\/div>\r\n<div>\r\n<p id=\"fs-id1961659\">\r\n[reveal-answer q=\"488992\"]Show Solution[\/reveal-answer]\r\n[hidden-answer a=\"488992\"]B[\/hidden-answer]<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<div id=\"fs-id1787762\" class=\"free-response textbox exercises\">\r\n<h3>Free Response<\/h3>\r\n<div id=\"fs-id2781180\">\r\n<div id=\"fs-id2642317\">\r\n<p id=\"fs-id1262550\">Why is it beneficial for cells to use ATP rather than energy directly from the bonds of carbohydrates? What are the greatest drawbacks to harnessing energy directly from the bonds of several different compounds?<\/p>\r\n\r\n<\/div>\r\n<div id=\"fs-id1380782\">\r\n\r\n[reveal-answer q=\"57732\"]Show Solution[\/reveal-answer]\r\n[hidden-answer a=\"57732\"]ATP provides the cell with a way to handle energy in an efficient manner. The molecule can be charged, stored, and used as needed. Moreover, the energy from hydrolyzing ATP is delivered as a consistent amount. Harvesting energy from the bonds of several different compounds would result in energy deliveries of different quantities.[\/hidden-answer]\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<div class=\"textbox shaded\">\r\n<h3>Glossary<\/h3>\r\n<dl id=\"fs-id1957966\">\r\n \t<dt>chemiosmosis<\/dt>\r\n \t<dd id=\"fs-id2148481\">process in which there is a production of adenosine triphosphate (ATP) in cellular metabolism by the involvement of a proton gradient across a membrane<\/dd>\r\n<\/dl>\r\n<dl id=\"fs-id2567941\">\r\n \t<dt>dephosphorylation<\/dt>\r\n \t<dd id=\"fs-id2020665\">removal of a phosphate group from a molecule<\/dd>\r\n<\/dl>\r\n<dl id=\"fs-id1428215\">\r\n \t<dt>oxidative phosphorylation<\/dt>\r\n \t<dd id=\"fs-id2315383\">production of ATP using the process of chemiosmosis in the presence of oxygen<\/dd>\r\n<\/dl>\r\n<dl id=\"fs-id2862440\">\r\n \t<dt>phosphorylation<\/dt>\r\n \t<dd id=\"fs-id2904661\">addition of a high-energy phosphate to a compound, usually a metabolic intermediate, a protein, or ADP<\/dd>\r\n<\/dl>\r\n<dl id=\"fs-id2205482\">\r\n \t<dt>redox reaction<\/dt>\r\n \t<dd id=\"fs-id2962857\">chemical reaction that consists of the coupling of an oxidation reaction and a reduction reaction<\/dd>\r\n<\/dl>\r\n<dl id=\"fs-id1468961\">\r\n \t<dt>substrate-level phosphorylation<\/dt>\r\n \t<dd id=\"fs-id2476144\">production of ATP from ADP using the excess energy from a chemical reaction and a phosphate group from a reactant<\/dd>\r\n<\/dl>\r\n<\/div>","rendered":"<div class=\"textbox learning-objectives\">\n<h3>Learning Objectives<\/h3>\n<p>By the end of this section, you will be able to do the following:<\/p>\n<ul>\n<li>Discuss the importance of electrons in the transfer of energy in living systems<\/li>\n<li>Explain how ATP is used by cells as an energy source<\/li>\n<\/ul>\n<\/div>\n<p id=\"fs-id1894837\">Energy production within a cell involves many coordinated chemical pathways. Most of these pathways are combinations of oxidation and reduction reactions, which occur at the same time. An oxidation reaction strips an electron from an atom in a compound, and the addition of this electron to another compound is a reduction reaction. Because oxidation and reduction usually occur together, these pairs of reactions are called oxidation reduction reactions, or redox reactions.<\/p>\n<div id=\"fs-id1798739\" class=\"bc-section section\">\n<h3>Electrons and Energy<\/h3>\n<p id=\"fs-id3112918\">The removal of an electron from a molecule (oxidizing it), results in a decrease in potential energy in the oxidized compound. The electron (sometimes as part of a hydrogen atom) does not remain unbonded, however, in the cytoplasm of a cell. Rather, the electron is shifted to a second compound, reducing the second compound. <em>The shift of an electron from one compound to another removes some potential energy from the first compound (the oxidized compound) and increases the potential energy of the second compound (the reduced compound).<\/em> The transfer of electrons between molecules is important because most of the energy stored in atoms and used to fuel cell functions is in the form of high-energy electrons. The transfer of energy in the form of high-energy electrons allows the cell to transfer and use energy in an incremental fashion\u2014in small packages rather than in a single, destructive burst. This chapter focuses on the extraction of energy from food; you will see that as you track the path of the transfers, you are tracking the path of electrons moving through metabolic pathways.<\/p>\n<div id=\"fs-id2763612\" class=\"bc-section section\">\n<h4>Electron Carriers<\/h4>\n<p>In living systems, a small class of compounds functions as electron shuttles: they bind and carry high-energy electrons between compounds in biochemical pathways. The principal electron carriers we will consider are derived from the B vitamin group and are derivatives of nucleotides. These compounds can be easily reduced (that is, they accept electrons) or oxidized (they lose electrons). Nicotinamide adenine dinucleotide (NAD) (<a class=\"autogenerated-content\" href=\"#fig-ch07_01_01\">(Figure)<\/a>) is derived from vitamin B<sub>3<\/sub>, niacin. NAD<sup>+<\/sup> is the oxidized form of the molecule; NADH is the reduced form of the molecule after it has accepted two electrons and a proton (which together are the equivalent of a hydrogen atom with an extra electron). Note that if a compound has an \u201cH\u201d on it, it is generally reduced (e.g., NADH is the reduced form of NAD).<\/p>\n<p id=\"fs-id1464730\">NAD<sup>+<\/sup> can accept electrons from an organic molecule according to the general equation:<\/p>\n<div id=\"eip-739\">[latex]\\begin{array}{c}\\text{RH}\\\\ \\begin{array}{c}\\text{Reducing\u00a0}\\\\ \\text{agent}\\end{array}\\end{array}\\text{\u00a0+\u00a0}\\begin{array}{c}{\\text{NAD}}^{+}\\\\ \\begin{array}{c}\\text{Oxidizing}\\\\ \\text{agent}\\end{array}\\end{array}\\to \\text{\u00a0}\\begin{array}{c}\\text{NADH}\\\\ \\text{Reduced}\\end{array}\\text{\u00a0+}\\begin{array}{c}\\text{R}\\\\ \\text{Oxidized}\\end{array}[\/latex]<\/div>\n<p id=\"fs-id2073095\">When electrons are added to a compound, <em>it is reduced<\/em>. A compound that reduces another is called a reducing agent. In the above equation, RH is a reducing agent, and NAD<sup>+<\/sup> is reduced to NADH. When electrons are removed from a compound, <em>it is oxidized<\/em>. A compound that oxidizes another is called an oxidizing agent. In the above equation, NAD<sup>+<\/sup> is an oxidizing agent, and RH is oxidized to R.<\/p>\n<p id=\"fs-id2583910\">Similarly, flavin adenine dinucleotide (FAD<sup>+<\/sup>) is derived from vitamin B<sub>2<\/sub>, also called riboflavin. Its reduced form is FADH<sub>2<\/sub>. A second variation of NAD, NADP, contains an extra phosphate group. Both NAD<sup>+<\/sup> and FAD<sup>+<\/sup> are extensively used in energy extraction from sugars, and NADP plays an important role in anabolic reactions and photosynthesis in plants.<\/p>\n<div id=\"fig-ch07_01_01\" class=\"wp-caption aligncenter\"><span id=\"fs-id1937340\"><img decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/3206\/2018\/05\/03175759\/Figure_07_01_01ab.jpg\" alt=\"This illustration shows the molecular structure of NAD^{+} and NADH. Both compounds are composed of an adenine nucleotide and a nicotinamide nucleotide, which bond together to form a dinucleotide. The nicotinamide nucleotide is at the 5' end, and the adenine nucleotide is at the 3\u2019 end. Nicotinamide is a nitrogenous base, meaning it has nitrogen in a six-membered carbon ring. In NADH, one extra hydrogen is associated with this ring, which is not found in NAD^{+}.\" width=\"500\" \/><\/span><\/div>\n<div class=\"wp-caption-text\">The oxidized form of the electron carrier (NAD<sup>+<\/sup>) is shown on the left, and the reduced form (NADH) is shown on the right. The nitrogenous base in NADH has one more hydrogen ion and two more electrons than in NAD<sup>+<\/sup>.<\/div>\n<\/div>\n<\/div>\n<div id=\"fs-id1511906\" class=\"bc-section section\">\n<h3>ATP in Living Systems<\/h3>\n<p id=\"fs-id1640338\">A living cell cannot store significant amounts of free energy. Excess free energy would result in an increase of heat in the cell, which would result in excessive thermal motion that could damage and then destroy the cell. Rather, a cell must be able to handle that energy in a way that enables the cell to store energy safely and release it for use only as needed. Living cells accomplish this by using the compound adenosine triphosphate (ATP). ATP is often called the \u201cenergy currency\u201d of the cell, and, like currency, this versatile compound can be used to fill any energy need of the cell. How? It functions similarly to a rechargeable battery.<\/p>\n<p id=\"fs-id1359897\">When ATP is broken down, usually by the removal of its terminal phosphate group, energy is released. The energy is used to do work by the cell, usually when the released phosphate binds to another molecule, thereby activating it. For example, in the mechanical work of muscle contraction, ATP supplies the energy to move the contractile muscle proteins. Recall the active transport work of the sodium-potassium pump in cell membranes. ATP alters the structure of the integral protein that functions as the pump, changing its affinity for sodium and potassium. In this way, the cell performs work, pumping ions against their electrochemical gradients.<\/p>\n<div id=\"fs-id2183864\" class=\"bc-section section\">\n<h4>ATP Structure and Function<\/h4>\n<p id=\"fs-id1595819\">At the heart of ATP is a molecule of adenosine monophosphate (AMP), which is composed of an adenine molecule bonded to a ribose molecule and to a single phosphate group (<a class=\"autogenerated-content\" href=\"#fig-ch07_01_02\">(Figure)<\/a>). Ribose is a five-carbon sugar found in RNA, and AMP is one of the nucleotides in RNA. The addition of a second phosphate group to this core molecule results in the formation of adenosine <u>di<\/u>phosphate (ADP); the addition of a third phosphate group forms adenosine <u>tri<\/u>phosphate (ATP).<\/p>\n<div id=\"fig-ch07_01_02\" class=\"wp-caption aligncenter\"><span id=\"fs-id2756859\"><img decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/3206\/2018\/05\/03175801\/Figure_07_01_02.jpg\" alt=\"This illustration shows the molecular structure of ATP. This molecule is an adenine nucleotide with a string of three phosphate groups attached to it. The phosphate groups are named alpha, beta, and gamma in order of increasing distance from the ribose sugar to which they are attached.\" width=\"400\" \/><\/span><\/div>\n<div class=\"wp-caption-text\">ATP (adenosine triphosphate) has three phosphate groups that can be removed by hydrolysis (addition of H<sub>2<\/sub>O) to form ADP (adenosine diphosphate) or AMP (adenosine monophosphate). The negative charges on the phosphate group naturally repel each other, requiring energy to bond them together and releasing energy when these bonds are broken.<\/div>\n<p id=\"fs-id2308504\">The addition of a phosphate group to a molecule requires energy. Phosphate groups are negatively charged and thus repel one another when they are arranged in series, as they are in ADP and ATP. This repulsion makes the ADP and ATP molecules inherently unstable. The release of one or two phosphate groups from ATP, a process called dephosphorylation, releases energy.<\/p>\n<\/div>\n<div id=\"fs-id3570970\" class=\"bc-section section\">\n<h4>Energy from ATP<\/h4>\n<p id=\"fs-id1981342\">Hydrolysis is the process of breaking complex macromolecules apart. During hydrolysis, water is split, or lysed, and the resulting hydrogen atom (H<sup>+<\/sup>) and a hydroxyl group (OH<sup>&#8211;<\/sup>), or <em>hydroxide,<\/em> are added to the larger molecule. The hydrolysis of ATP produces ADP, together with an inorganic phosphate ion (P<sub>i<\/sub>), and the release of free energy. To carry out life processes, ATP is continuously broken down into ADP, and like a rechargeable battery, ADP is continuously regenerated into ATP by the reattachment of a third phosphate group. Water, which was broken down into its hydrogen atom and hydroxyl group (hydroxide) during ATP hydrolysis, is regenerated when a third phosphate is added to the ADP molecule, reforming ATP.<\/p>\n<p id=\"fs-id1798236\">Obviously, energy must be infused into the system to regenerate ATP. Where does this energy come from? In nearly every living thing on Earth, the energy comes from the metabolism of glucose, fructose, or galactose, all isomers with the chemical formula C<sub>6<\/sub>H<sub>12<\/sub>O<sub>6<\/sub> but different molecular configurations. In this way, ATP is a direct link between the limited set of exergonic pathways of glucose catabolism and the multitude of endergonic pathways that power living cells.<\/p>\n<\/div>\n<div id=\"fs-id1560590\" class=\"bc-section section\">\n<h4>Phosphorylation<\/h4>\n<p id=\"fs-id2348311\">Recall that, in some chemical reactions, enzymes may bind to several substrates that react with each other on the enzyme, forming an intermediate complex. An intermediate complex is a temporary structure, and it allows one of the substrates (such as ATP) and reactants to more readily react with each other; in reactions involving ATP, ATP is one of the substrates and ADP is a product. During an endergonic chemical reaction, ATP forms an intermediate complex with the substrate and enzyme in the reaction. This intermediate complex allows the ATP to transfer its third phosphate group, with its energy, to the substrate, a process called phosphorylation. Phosphorylation refers to the addition of the phosphate (~P). This is illustrated by the following generic reaction, in which A and B represent two different substrates:<\/p>\n<div id=\"eip-458\">[latex]\\text{A\u00a0+\u00a0enzyme\u00a0+\u00a0ATP}\\to \\text{\u00a0}\\left[\\text{A\u00a0}-\\text{\u00a0enzyme\u00a0}-\\text{\u00a0}\\sim \\text{P}\\right]\\text{\u00a0}\\to \\text{\u00a0B\u00a0+\u00a0enzyme\u00a0+\u00a0ADP\u00a0+\u00a0phosphate\u00a0ion}[\/latex]<\/div>\n<p>When the intermediate complex breaks apart, the energy is used to modify the substrate and convert it into a product of the reaction. The ADP molecule and a free phosphate ion are released into the medium and are available for recycling through cell metabolism.<\/p>\n<\/div>\n<div id=\"fs-id2195071\" class=\"bc-section section\">\n<h4>Substrate Phosphorylation<\/h4>\n<p id=\"fs-id1451024\">ATP is generated through two mechanisms during the breakdown of glucose. A few ATP molecules are generated (that is, regenerated from ADP) as a direct result of the chemical reactions that occur in the catabolic pathways. A phosphate group is removed from an intermediate reactant in the pathway, and the free energy of the reaction is used to add the third phosphate to an available ADP molecule, producing ATP (<a class=\"autogenerated-content\" href=\"#fig-ch07_01_03\">(Figure)<\/a>). This very direct method of phosphorylation is called substrate-level phosphorylation.<\/p>\n<div id=\"fig-ch07_01_03\" class=\"wp-caption aligncenter\"><span id=\"fs-id1414909\"><img decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/3206\/2018\/05\/03175804\/Figure_07_01_03.jpg\" alt=\"This illustration shows a substrate-level phosphorylation reaction in which the gamma phosphate of ATP is attached to a protein.\" width=\"350\" \/><\/span><\/div>\n<div class=\"wp-caption-text\">In phosphorylation reactions, the gamma (third) phosphate of ATP is attached to a protein.<\/div>\n<\/div>\n<div id=\"fs-id2019618\" class=\"bc-section section\">\n<h4>Oxidative Phosphorylation<\/h4>\n<p id=\"fs-id2065719\">Most of the ATP generated during glucose catabolism, however, is derived from a much more complex process, chemiosmosis, which takes place in mitochondria (<a class=\"autogenerated-content\" href=\"#fig-ch07_01_04\">(Figure)<\/a>) within a eukaryotic cell or the plasma membrane of a prokaryotic cell. Chemiosmosis, a process of ATP production in cellular metabolism, is used to generate 90 percent of the ATP made during glucose catabolism and is also the method used in the light reactions of photosynthesis to harness the energy of sunlight. The production of ATP using the process of chemiosmosis is called oxidative phosphorylation because of the involvement of oxygen in the process.<\/p>\n<div id=\"fig-ch07_01_04\" class=\"wp-caption aligncenter\"><span id=\"fs-id1318782\"><img decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/3206\/2018\/05\/03175806\/Figure_07_01_04.jpg\" alt=\"This illustration shows the structure of a mitochondrion, which has an outer membrane and an inner membrane. The inner membrane has many folds, called cristae. The space between the outer membrane and the inner membrane is called the intermembrane space, and the central space of the mitochondrion is called the matrix. ATP synthase enzymes and the electron transport chain are located in the inner membrane\" width=\"300\" \/><\/span><\/div>\n<div class=\"wp-caption-text\">In eukaryotes, oxidative phosphorylation takes place in mitochondria. In prokaryotes, this process takes place in the plasma membrane. (Credit: modification of work by Mariana Ruiz Villareal)<\/div>\n<div id=\"fs-id1912383\" class=\"career textbox examples\">\n<h3>Career Connections<\/h3>\n<p><strong>Mitochondrial Disease Physician<\/strong><\/p>\n<p>What happens when the critical reactions of cellular respiration do not proceed correctly? This may happen in mitochondrial diseases, which are genetic disorders of metabolism. Mitochondrial disorders can arise from mutations in nuclear or mitochondrial DNA, and they result in the production of less energy than is normal in body cells. In type 2 diabetes, for instance, the oxidation efficiency of NADH is reduced, impacting oxidative phosphorylation but not the other steps of respiration. Symptoms of mitochondrial diseases can include muscle weakness, lack of coordination, stroke-like episodes, and loss of vision and hearing. Most affected people are diagnosed in childhood, although there are some adult-onset diseases. Identifying and treating mitochondrial disorders is a specialized medical field. The educational preparation for this profession requires a college education, followed by medical school with a specialization in medical genetics. Medical geneticists can be board certified by the American Board of Medical Genetics and go on to become associated with professional organizations devoted to the study of mitochondrial diseases, such as the Mitochondrial Medicine Society and the Society for Inherited Metabolic Disorders.<\/p>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"summary textbox key-takeaways\">\n<h3>Section Summary<\/h3>\n<p>ATP functions as the energy currency for cells. It allows the cell to store energy briefly and transport it within the cell to support endergonic chemical reactions. The structure of ATP is that of an RNA nucleotide with three phosphates attached. As ATP is used for energy, a phosphate group or two are detached, and either ADP or AMP is produced. Energy derived from glucose catabolism is used to convert ADP into ATP. When ATP is used in a reaction, the third phosphate is temporarily attached to a substrate in a process called phosphorylation. The two processes of ATP regeneration that are used in conjunction with glucose catabolism are substrate-level phosphorylation and oxidative phosphorylation through the process of chemiosmosis.<\/p>\n<\/div>\n<div id=\"fs-id1473147\" class=\"multiple-choice textbox exercises\">\n<h3>Review Questions<\/h3>\n<div id=\"fs-id1957993\">\n<div id=\"fs-id1276810\">\n<p id=\"fs-id2197455\">The energy currency used by cells is ________.<\/p>\n<ol id=\"fs-id1785724\" type=\"a\">\n<li>ATP<\/li>\n<li>ADP<\/li>\n<li>AMP<\/li>\n<li>adenosine<\/li>\n<\/ol>\n<\/div>\n<div id=\"fs-id1470656\">\n<p id=\"fs-id1760308\">\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q17792\">Show Solution<\/span><\/p>\n<div id=\"q17792\" class=\"hidden-answer\" style=\"display: none\">A<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div id=\"fs-id1419484\">\n<div id=\"fs-id2100818\">\n<p id=\"fs-id1450216\">A reducing chemical reaction ________.<\/p>\n<ol id=\"fs-id2962812\" type=\"a\">\n<li>reduces the compound to a simpler form<\/li>\n<li>adds an electron to the substrate<\/li>\n<li>removes a hydrogen atom from the substrate<\/li>\n<li>is a catabolic reaction<\/li>\n<\/ol>\n<\/div>\n<div>\n<p id=\"fs-id1961659\">\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q488992\">Show Solution<\/span><\/p>\n<div id=\"q488992\" class=\"hidden-answer\" style=\"display: none\">B<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div id=\"fs-id1787762\" class=\"free-response textbox exercises\">\n<h3>Free Response<\/h3>\n<div id=\"fs-id2781180\">\n<div id=\"fs-id2642317\">\n<p id=\"fs-id1262550\">Why is it beneficial for cells to use ATP rather than energy directly from the bonds of carbohydrates? What are the greatest drawbacks to harnessing energy directly from the bonds of several different compounds?<\/p>\n<\/div>\n<div id=\"fs-id1380782\">\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q57732\">Show Solution<\/span><\/p>\n<div id=\"q57732\" class=\"hidden-answer\" style=\"display: none\">ATP provides the cell with a way to handle energy in an efficient manner. The molecule can be charged, stored, and used as needed. Moreover, the energy from hydrolyzing ATP is delivered as a consistent amount. Harvesting energy from the bonds of several different compounds would result in energy deliveries of different quantities.<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"textbox shaded\">\n<h3>Glossary<\/h3>\n<dl id=\"fs-id1957966\">\n<dt>chemiosmosis<\/dt>\n<dd id=\"fs-id2148481\">process in which there is a production of adenosine triphosphate (ATP) in cellular metabolism by the involvement of a proton gradient across a membrane<\/dd>\n<\/dl>\n<dl id=\"fs-id2567941\">\n<dt>dephosphorylation<\/dt>\n<dd id=\"fs-id2020665\">removal of a phosphate group from a molecule<\/dd>\n<\/dl>\n<dl id=\"fs-id1428215\">\n<dt>oxidative phosphorylation<\/dt>\n<dd id=\"fs-id2315383\">production of ATP using the process of chemiosmosis in the presence of oxygen<\/dd>\n<\/dl>\n<dl id=\"fs-id2862440\">\n<dt>phosphorylation<\/dt>\n<dd id=\"fs-id2904661\">addition of a high-energy phosphate to a compound, usually a metabolic intermediate, a protein, or ADP<\/dd>\n<\/dl>\n<dl id=\"fs-id2205482\">\n<dt>redox reaction<\/dt>\n<dd id=\"fs-id2962857\">chemical reaction that consists of the coupling of an oxidation reaction and a reduction reaction<\/dd>\n<\/dl>\n<dl id=\"fs-id1468961\">\n<dt>substrate-level phosphorylation<\/dt>\n<dd id=\"fs-id2476144\">production of ATP from ADP using the excess energy from a chemical reaction and a phosphate group from a reactant<\/dd>\n<\/dl>\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-648\">\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>Biology 2e. <strong>Provided by<\/strong>: OpenStax. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"https:\/\/openstax.org\/details\/books\/biology-2e\">https:\/\/openstax.org\/details\/books\/biology-2e<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em>. <strong>License Terms<\/strong>: Download for free at http:\/\/cnx.org\/contents\/8d50a0af-948b-4204-a71d-4826cba765b8@8.19<\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section>","protected":false},"author":311,"menu_order":2,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"Biology 2e\",\"author\":\"\",\"organization\":\"OpenStax\",\"url\":\"https:\/\/openstax.org\/details\/books\/biology-2e\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"Download for free at http:\/\/cnx.org\/contents\/8d50a0af-948b-4204-a71d-4826cba765b8@8.19\"}]","CANDELA_OUTCOMES_GUID":"","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-648","chapter","type-chapter","status-publish","hentry"],"part":641,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/suny-oneonta-osbiology2e-1\/wp-json\/pressbooks\/v2\/chapters\/648","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/suny-oneonta-osbiology2e-1\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/suny-oneonta-osbiology2e-1\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-oneonta-osbiology2e-1\/wp-json\/wp\/v2\/users\/311"}],"version-history":[{"count":2,"href":"https:\/\/courses.lumenlearning.com\/suny-oneonta-osbiology2e-1\/wp-json\/pressbooks\/v2\/chapters\/648\/revisions"}],"predecessor-version":[{"id":2048,"href":"https:\/\/courses.lumenlearning.com\/suny-oneonta-osbiology2e-1\/wp-json\/pressbooks\/v2\/chapters\/648\/revisions\/2048"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/suny-oneonta-osbiology2e-1\/wp-json\/pressbooks\/v2\/parts\/641"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/suny-oneonta-osbiology2e-1\/wp-json\/pressbooks\/v2\/chapters\/648\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/suny-oneonta-osbiology2e-1\/wp-json\/wp\/v2\/media?parent=648"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-oneonta-osbiology2e-1\/wp-json\/pressbooks\/v2\/chapter-type?post=648"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-oneonta-osbiology2e-1\/wp-json\/wp\/v2\/contributor?post=648"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-oneonta-osbiology2e-1\/wp-json\/wp\/v2\/license?post=648"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}