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width:17px; height:17px; overflow:hidden; text-decoration:none;"><div style="position:absolute; font-size:17px; overflow:hidden; line-height:17px; letter-spacing:17px;"><a rel="nofollow" class="external text" href="https://en.citizendium.org/wiki/index.php?title=Metabolism/Citable_Version&printable=yes"><span title="https://en.citizendium.org/wiki/index.php?title=Metabolism/Citable_Version&printable=yes" style="text-decoration:none;">   </span></a></div><a href="/wiki/File:Print_button.png" class="image" title="https://en.citizendium.org/wiki/index.php?title=Metabolism/Citable_Version&printable=yes"><img alt="https://en.citizendium.org/wiki/index.php?title=Metabolism/Citable_Version&printable=yes" src="/wiki/images/thumb/0/07/Print_button.png/17px-Print_button.png" decoding="async" width="17" height="17" srcset="/wiki/images/thumb/0/07/Print_button.png/26px-Print_button.png 1.5x, /wiki/images/thumb/0/07/Print_button.png/34px-Print_button.png 2x" data-file-width="45" data-file-height="45" /></a></div> </td></tr></tbody></table><font size="2"><b>This version <a href="/wiki/CZ:Approval_Process" title="CZ:Approval Process">approved</a></b> either by <a href="/wiki/User:Chris_Day" title="User:Chris Day">the Approvals Committee, or an Editor</a> from the listed workgroup. The <a href="/wiki/CZ:Biology_Workgroup" title="CZ:Biology Workgroup">Biology Workgroup</a> is responsible for this citable version. <i>While we have done conscientious work, we cannot guarantee that this version is wholly free of mistakes.</i> See <span class="plainlinks plainlinksneverexpand"><a rel="nofollow" class="external text" href="https://citizendium.org/wiki/index.php?title=Metabolism&action=history">here</a></span> (not <i>History</i>) for authorship.</font><br /> <div align="center"><b>Help improve this work further on the <a href="/wiki/Metabolism" title="Metabolism">editable Main Article</a>!</b></div> </td></tr></tbody></table></div> <p><b>Metabolism</b> (from <a href="/wiki/Greek_language" title="Greek language">Greek</a> <i>μεταβολισμός</i> "metabolismos") is the <a href="/wiki/Biochemistry" title="Biochemistry">biochemical</a> modification of <a href="/wiki/Chemical_compound" title="Chemical compound">chemical compounds</a> by <a href="/wiki/Life" title="Life">living</a> <a href="/wiki/Organism" title="Organism">organisms</a> and <a href="/wiki/Cell_(biology)" title="Cell (biology)">cells</a>. In common usage, the word is often used to refer to the <a href="/wiki/index.php?title=Basal_metabolic_rate&action=edit&redlink=1" class="new" title="Basal metabolic rate (page does not exist)">basal metabolic rate</a>, the "set point" that each person has in breaking down food energy and building up their own body. In <a href="/wiki/index.php?title=Multicellular&action=edit&redlink=1" class="new" title="Multicellular (page does not exist)">multicellular</a> creatures like humans, its meaning encompasses the <i>overall</i> ingestion of <a href="/wiki/Food" title="Food">food</a> and excretion of wastes, as well as the building up of <a href="/wiki/index.php?title=Muscles&action=edit&redlink=1" class="new" title="Muscles (page does not exist)">muscles</a> and the <a href="/wiki/Growth" title="Growth">growth</a> of the body. In terms of the whole organism, metabolism includes the chemical conversion of ingested items <i>other</i> than food, like drugs and <a href="/wiki/index.php?title=Poisons&action=edit&redlink=1" class="new" title="Poisons (page does not exist)">poisons</a> (see <a href="/wiki/index.php?title=Drug_metabolism&action=edit&redlink=1" class="new" title="Drug metabolism (page does not exist)">Drug metabolism</a>). This article describes the actual biology of metabolism at a cellular level, which explains just how those processes are carried out. </p><p>Metabolism includes: (1) <a href="/wiki/Anabolism" title="Anabolism">anabolism</a>, in which a cell uses chemical energy and <a href="/wiki/index.php?title=Reducing_power&action=edit&redlink=1" class="new" title="Reducing power (page does not exist)">reducing power</a> to <i>construct</i> complex molecules, and perform life functions such as <i>creating</i> cellular structure; and (2) <a href="/wiki/Catabolism" title="Catabolism">catabolism</a>, in which a cell <i>breaks down</i> complex molecules to yield the chemical energy and reducing power. Cell metabolism involves complex sequences of controlled chemical reactions called <a href="/wiki/index.php?title=Metabolic_pathway&action=edit&redlink=1" class="new" title="Metabolic pathway (page does not exist)">metabolic pathways</a>. Just as the word metabolism can be used to describe processes in a whole organism, the terms "anabolism" and "catabolism" can similarly be used in this way. For example, anabolic processes can also refer to building up muscle and adding body weight, while catabolic processes can refer to the loss of muscle mass and body <a href="/wiki/Fat" title="Fat">fat</a>. </p> <div class="thumb tright"><div class="thumbinner" style="width:207px;"><a href="/wiki/File:Weightlifter.jpg" class="image"><img alt="" src="/wiki/images/5/57/Weightlifter.jpg" decoding="async" width="205" height="282" class="thumbimage" data-file-width="205" data-file-height="282" /></a> <div class="thumbcaption">With proper training and nutrition, weight lifting promotes the anabolic process of bodybuilding. Natural hormones, produced in both men and women, aid muscle development in response to weight bearing exercise.</div></div></div> <div id="toc" class="toc" role="navigation" aria-labelledby="mw-toc-heading"><input type="checkbox" role="button" id="toctogglecheckbox" class="toctogglecheckbox" style="display:none" /><div class="toctitle" lang="en" dir="ltr"><h2 id="mw-toc-heading">Contents</h2><span class="toctogglespan"><label class="toctogglelabel" for="toctogglecheckbox"></label></span></div> <ul> <li class="toclevel-1 tocsection-1"><a href="#History"><span class="tocnumber">1</span> <span class="toctext">History</span></a></li> <li class="toclevel-1 tocsection-2"><a href="#Overview:_Harnessing_energy_and_making_chemical_bonds"><span class="tocnumber">2</span> <span class="toctext">Overview: Harnessing energy and making chemical bonds</span></a> <ul> <li class="toclevel-2 tocsection-3"><a href="#ATP:_the_energy_currency_of_cells"><span class="tocnumber">2.1</span> <span class="toctext">ATP: the energy currency of cells</span></a> <ul> <li class="toclevel-3 tocsection-4"><a href="#Phototrophic"><span class="tocnumber">2.1.1</span> <span class="toctext">Phototrophic</span></a></li> <li class="toclevel-3 tocsection-5"><a href="#Chemotrophic"><span class="tocnumber">2.1.2</span> <span class="toctext">Chemotrophic</span></a></li> </ul> </li> <li class="toclevel-2 tocsection-6"><a href="#Reducing_Power:_obtaining_electrons_for_chemical_bonds"><span class="tocnumber">2.2</span> <span class="toctext">Reducing Power: obtaining electrons for chemical bonds</span></a> <ul> <li class="toclevel-3 tocsection-7"><a href="#Organotrophic"><span class="tocnumber">2.2.1</span> <span class="toctext">Organotrophic</span></a></li> <li class="toclevel-3 tocsection-8"><a href="#Lithotrophic"><span class="tocnumber">2.2.2</span> <span class="toctext">Lithotrophic</span></a></li> </ul> </li> </ul> </li> <li class="toclevel-1 tocsection-9"><a href="#Regulation_of_metabolism_in_animals"><span class="tocnumber">3</span> <span class="toctext">Regulation of metabolism in animals</span></a></li> </ul> </div> <h2><span class="mw-headline" id="History">History</span></h2> <div class="thumb tright"><div class="thumbinner" style="width:152px;"><a href="/wiki/File:SantoriosMeal.jpg" class="image"><img alt="" src="/wiki/images/thumb/3/32/SantoriosMeal.jpg/150px-SantoriosMeal.jpg" decoding="async" width="150" height="228" class="thumbimage" srcset="/wiki/images/thumb/3/32/SantoriosMeal.jpg/225px-SantoriosMeal.jpg 1.5x, /wiki/images/thumb/3/32/SantoriosMeal.jpg/300px-SantoriosMeal.jpg 2x" data-file-width="310" data-file-height="472" /></a> <div class="thumbcaption"><div class="magnify"><a href="/wiki/File:SantoriosMeal.jpg" class="internal" title="Enlarge"></a></div><a href="/wiki/index.php?title=Santorio_Santorio&action=edit&redlink=1" class="new" title="Santorio Santorio (page does not exist)">Santorio Santorio</a> (1561-1636) in his steelyard balance, from <i>Ars de statica medecina</i>, first published in 1614.</div></div></div> <p>The first controlled experiments on human metabolism were published by <a href="/wiki/index.php?title=Santorio_Santorio&action=edit&redlink=1" class="new" title="Santorio Santorio (page does not exist)">Santorio Santorio</a> in 1614 in his book <i>Ars de statica medecina</i>, in which he described experiments in which he weighed himself in a chair suspended from a steelyard balance (see image), before and after eating, sleeping, working, sex, fasting, depriving from drinking, and excreting. He found that by far the greatest part of the food he took in was lost from the body through <i>perspiratio insensibilis</i> (insensible perspiration). In <a href="/wiki/Medicine" title="Medicine">medicine</a> and the <a href="/wiki/Health_sciences" class="mw-redirect" title="Health sciences">health sciences</a>, the term "insensible losses" is still used to refer to fluids that escape the body without leaving easily-measurable traces behind. </p><p>At about the same time, <a href="/wiki/index.php?title=Jan_Baptist_van_Helmont&action=edit&redlink=1" class="new" title="Jan Baptist van Helmont (page does not exist)">Jan Baptist van Helmont</a> made the first observations regarding <a href="/wiki/Photosynthesis" title="Photosynthesis">photosynthesis</a>, when he discovered that growing plants drew almost no matter from the surrounding soil. The physical source of the plant's growth was not obvious until later experiments, which delved into the process now known as <a href="/wiki/Photosynthesis" title="Photosynthesis">photosynthesis</a>. </p><p>In the 18th century, <a href="/wiki/index.php?title=Joseph_Priestley&action=edit&redlink=1" class="new" title="Joseph Priestley (page does not exist)">Joseph Priestley</a> discovered that green plants released a substance (later found to be <a href="/wiki/Oxygen" title="Oxygen">oxygen</a>) that could sustain the life of a mouse in an enclosed chamber. <a href="/wiki/index.php?title=Jan_Ingenhousz&action=edit&redlink=1" class="new" title="Jan Ingenhousz (page does not exist)">Jan Ingenhousz</a> extended Priestley's experiments to show that oxygen was produced when light was cast on the plant, while <a href="/wiki/index.php?title=Jean_Senebier&action=edit&redlink=1" class="new" title="Jean Senebier (page does not exist)">Jean Senebier</a> showed that <a href="/wiki/Carbon_dioxide" title="Carbon dioxide">carbon dioxide</a> was absorbed by plants during <a href="/wiki/Photosynthesis" title="Photosynthesis">photosynthesis</a>. In 1804, <a href="/wiki/index.php?title=Nicolas-Th%C3%A9odore_de_Saussure&action=edit&redlink=1" class="new" title="Nicolas-Théodore de Saussure (page does not exist)">Nicolas de Saussure</a> discovered that plant growth was the result of both the fixation of atmospheric carbon dioxide (<span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle CO_{2}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mi>C</mi> <msub> <mi>O</mi> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> </msub> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle CO_{2}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/bb6f61d9e9b0384c5055e3907c9268633562ba33" class="mwe-math-fallback-image-inline" aria-hidden="true" style="vertical-align: -0.671ex; width:4.594ex; height:2.509ex;" alt="{\displaystyle CO_{2}}"/></span>) into the plant, and the incorporation of water. </p><p>Between 1854 and 1864, <a href="/wiki/Louis_Pasteur" title="Louis Pasteur">Louis Pasteur</a> discovered that <a href="/wiki/Glucose" title="Glucose">glucose</a> <a href="/wiki/index.php?title=Fermentation&action=edit&redlink=1" class="new" title="Fermentation (page does not exist)">fermentation</a> is due to <a href="/wiki/index.php?title=Microorganisms&action=edit&redlink=1" class="new" title="Microorganisms (page does not exist)">microorganisms</a>, and, in 1897, <a href="/wiki/index.php?title=Eduard_Buchner&action=edit&redlink=1" class="new" title="Eduard Buchner (page does not exist)">Eduard Buchner</a> proved that <i>cell-free</i> yeast extracts could also perform these reactions, and so the ability to ferment was not limited to entire living creatures (cells)- but included certain portions of their physical contents. Subsequent investigations showed that living organisms, with few exceptions, metabolize glucose using the same mechanism, namely, by a <a href="/wiki/Glycolysis" title="Glycolysis">biochemical pathway</a> that breaks down sugar. </p> <h2><span class="mw-headline" id="Overview:_Harnessing_energy_and_making_chemical_bonds">Overview: Harnessing energy and making chemical bonds</span></h2> <div class="thumb tleft"><div class="thumbinner" style="width:352px;"><a href="/wiki/File:Metabolism_scheme.GIF" class="image"><img alt="" src="/wiki/images/thumb/1/10/Metabolism_scheme.GIF/350px-Metabolism_scheme.GIF" decoding="async" width="350" height="290" class="thumbimage" srcset="/wiki/images/thumb/1/10/Metabolism_scheme.GIF/525px-Metabolism_scheme.GIF 1.5x, /wiki/images/1/10/Metabolism_scheme.GIF 2x" data-file-width="580" data-file-height="480" /></a> <div class="thumbcaption"><div class="magnify"><a href="/wiki/File:Metabolism_scheme.GIF" class="internal" title="Enlarge"></a></div>A few of the catabolic pathways in a cell. <a href="/wiki/Protein" title="Protein">Proteins</a> are broken down into <a href="/wiki/Amino_acids" class="mw-redirect" title="Amino acids">amino acids</a>, and fats into glycerol and <a href="/wiki/index.php?title=Fatty_acids&action=edit&redlink=1" class="new" title="Fatty acids (page does not exist)">fatty acids</a>. <a href="/wiki/index.php?title=Carbohydrates&action=edit&redlink=1" class="new" title="Carbohydrates (page does not exist)">Carbohydrates</a> (mostly sugars and starch) are hydrolyzed into monosacharides like glucose. The <a href="/wiki/Mitochondrion" title="Mitochondrion">mitochondrion</a> (in green) contains the enzymes that catalyze the <a href="/wiki/Citric_acid_cycle" title="Citric acid cycle">citric acid cycle</a> and <a href="/wiki/index.php?title=%CE%92-oxidation&action=edit&redlink=1" class="new" title="Β-oxidation (page does not exist)">beta-oxidation</a>, as well as the <a href="/wiki/Electron_transport_chain" title="Electron transport chain">electron transport chain</a> (where respiration occurs). <a href="/wiki/index.php?title=ATP&action=edit&redlink=1" class="new" title="ATP (page does not exist)">ATP</a> is a high-energy molecule. See text for details</div></div></div> <p>Living things, like all things, obey the laws of <a href="/wiki/Thermodynamics" title="Thermodynamics">thermodynamics</a>. That means that energy and matter cannot be created from <i>nothing</i>; cool things always get colder rather than warmer, and each fragment of a whole are smaller than the whole itself. But, unlike inanimate things, cells and tissues are able to harness energy and matter to change in ways that give the illusion of defying those laws. A baby does grow. A walrus' body is <i>warmer</i> than its icy surroundings. An <a href="/wiki/index.php?title=Amoeba&action=edit&redlink=1" class="new" title="Amoeba (page does not exist)">amoeba</a> can divide and shortly be <i>two</i> amoebas, each one the same size of the original cell that split. The metabolism of the baby, the walrus, and the amoeba is responsible for all these processes. Of course, rather than defy the laws of thermodynamics, the chemical reactions that make up metabolic processes always obey them. </p><p><a href="/wiki/Enzyme" title="Enzyme">Enzymes</a> present in <a href="/wiki/Cell_(biology)" title="Cell (biology)">cells</a> can catalyze a large variety of chemical reactions with exquisite specificity. Generally, enzymes are protein molecules that make reactions go faster by bringing the reactant molecules close together in just the right orientation for a chemical change to occur. Sometimes these enzymes are floating free in the cytoplasm of the cell, other times they are corralled together within a compartment of the cell, a special organelle. For example, the <a href="/wiki/Mitochondrion" title="Mitochondrion">mitochondrion</a> of cells contains enzymes for <a href="/wiki/Oxidative_phosphorylation" title="Oxidative phosphorylation">oxidative phosphorylation</a> (a catabolic process). The <a href="/wiki/Golgi_apparatus" title="Golgi apparatus">Golgi apparatus</a> of cells contains many of the enzymes used for protein <a href="/wiki/index.php?title=Posttranslational_modification&action=edit&redlink=1" class="new" title="Posttranslational modification (page does not exist)">posttranslational modification</a> (an anabolic process). </p><p>Often, the chemical reactions needed to synthesize useful cell components require energy. Chemists describe these reactions as involving a positive change in <a href="/wiki/index.php?title=Gibbs_free_energy&action=edit&redlink=1" class="new" title="Gibbs free energy (page does not exist)">free energy</a>. Such chemical transformations are not spontaneous, but "uphill", requiring more than just the mixing of the substrates. In these cases, specific enzymes may couple each "uphill" (non-spontaneous or energy requiring) reaction to a second, steep "downhill" (very spontaneous or energy releasing) reaction. Thus, thermodynamically favorable reactions can be used to "drive" each thermodynamically unfavorable one - such that the the overall process goes on its own, as a spontaneous <i>series of reactions</i>. </p> <h3><span class="mw-headline" id="ATP:_the_energy_currency_of_cells">ATP: the energy currency of cells</span></h3> <p>There is one particular energetically favourable reaction that is repeatedly used to drive "uphill" reactions in metabolism: </p> <dl><dd>Adenosine triphosphate + water → Adenosine diphosphate + phosphate ion + hydrogen ion</dd></dl> <p>This reaction, the hydrolysis of <a href="/wiki/index.php?title=ATP&action=edit&redlink=1" class="new" title="ATP (page does not exist)">Adenosine triphosphate</a> (ATP) into <a href="/wiki/index.php?title=ADP&action=edit&redlink=1" class="new" title="ADP (page does not exist)">Adenosine diphoshate</a> and two ions, occurs often in metabolic pathways. ATP is sometimes called the "energy currency" of cells because it is so often used to "finance" uphill reactions. To restore ATP, energy must be added to the products of the reaction, shown on the right side of the above equation. This is done by coupling the uphill synthesis of ATP to additional energy-releasing reactions. ATP synthesis is so ubiquitous that organisms can be classified according to how they derive energy for the process. Organisms can be classified as either <a href="#Phototrophic">Phototrophic</a> or <a href="#Chemotrophic">Chemotrophic</a>. </p> <h4><span class="mw-headline" id="Phototrophic">Phototrophic</span></h4> <p><a href="/wiki/index.php?title=Phototroph&action=edit&redlink=1" class="new" title="Phototroph (page does not exist)">Phototrophic</a> organisms can obtain energy from light. In these reactions, excitation of a <a href="/wiki/index.php?title=Photosynthetic_reaction_centre&action=edit&redlink=1" class="new" title="Photosynthetic reaction centre (page does not exist)">photosynthetic reaction centre</a> is caused by the absorption of a light photon. During the process, the reaction center loses an electron that excites (reduces) an electron acceptor, such as <a href="/wiki/index.php?title=Pheophytin&action=edit&redlink=1" class="new" title="Pheophytin (page does not exist)">pheophytin</a>, initiating a flow of electrons down an <a href="/wiki/Electron_transport_chain" title="Electron transport chain">electron transport chain</a> present in the <a href="/wiki/Thylakoid" title="Thylakoid">thylakoid</a> membrane. The energy released in the electron transfer steps serves to create a proton gradient across the membrane; its dissipation is used by <a href="/wiki/index.php?title=ATP_synthase&action=edit&redlink=1" class="new" title="ATP synthase (page does not exist)">ATP synthase</a> as the energy to synthesise <a href="/wiki/Adenosine_triphosphate" title="Adenosine triphosphate">ATP</a> from <a href="/wiki/index.php?title=ADP&action=edit&redlink=1" class="new" title="ADP (page does not exist)">ADP</a> and a <a href="/wiki/Phosphate" title="Phosphate">phosphate</a> anion by <a href="/wiki/index.php?title=Photophosphorylation&action=edit&redlink=1" class="new" title="Photophosphorylation (page does not exist)">photophosphorylation</a> (see <a href="/wiki/index.php?title=Chemiosmotic_hypothesis&action=edit&redlink=1" class="new" title="Chemiosmotic hypothesis (page does not exist)">Chemiosmotic hypothesis</a>). Depending on the organism, the reaction center regains the lost electron by either recycling the excited electrons or taking one from an electron donor. In plants, a water molecule serves as the electron donor through a process called <a href="/wiki/Photolysis" title="Photolysis">photolysis</a>, that releases <a href="/wiki/Oxygen" title="Oxygen">oxygen</a> gas as a waste product. </p> <div class="thumb tleft"><div class="thumbinner" style="width:352px;"><a href="/wiki/File:Metabolism_scheme_anabolism.GIF" class="image"><img alt="" src="/wiki/images/thumb/f/f1/Metabolism_scheme_anabolism.GIF/350px-Metabolism_scheme_anabolism.GIF" decoding="async" width="350" height="290" class="thumbimage" srcset="/wiki/images/thumb/f/f1/Metabolism_scheme_anabolism.GIF/525px-Metabolism_scheme_anabolism.GIF 1.5x, /wiki/images/f/f1/Metabolism_scheme_anabolism.GIF 2x" data-file-width="580" data-file-height="480" /></a> <div class="thumbcaption"><div class="magnify"><a href="/wiki/File:Metabolism_scheme_anabolism.GIF" class="internal" title="Enlarge"></a></div>A few of the anabolic pathways in a cell. Glucose can be stored as a <a href="/wiki/Glycogen" title="Glycogen">glycogen</a> polymer, or synthesized from lower molecular weight precursors. Excess acetyl-CoA can be stored as fatty acids, or converted into <a href="/wiki/index.php?title=Ketone_bodies&action=edit&redlink=1" class="new" title="Ketone bodies (page does not exist)">ketone bodies</a>.</div></div></div> <h4><span class="mw-headline" id="Chemotrophic">Chemotrophic</span></h4> <p>Chemotrophic organisms obtain energy from chemical reactions. For example, <a href="/wiki/Glucose" title="Glucose">glucose</a> can be oxidized to <a href="/wiki/index.php?title=Pyruvate&action=edit&redlink=1" class="new" title="Pyruvate (page does not exist)">pyruvate</a> through <a href="/wiki/Glycolysis" title="Glycolysis">glycolysis</a>. This yields two molecules of ATP for each molecule of glucose, by <a href="/wiki/index.php?title=Substrate-level_phosphorylation&action=edit&redlink=1" class="new" title="Substrate-level phosphorylation (page does not exist)">substrate-level phosphorylation</a>, and four electrons, which reduce two NAD<sup>+</sup> molecules to <a href="/wiki/index.php?title=NADH&action=edit&redlink=1" class="new" title="NADH (page does not exist)">NADH</a>. </p><p>For glycolysis to continue, the NADH must be recycled to NAD<sup>+</sup> by donating the electrons to an electron acceptor. <a href="/wiki/index.php?title=Respiration&action=edit&redlink=1" class="new" title="Respiration (page does not exist)">Respiration</a> is said to occur if this electron acceptor is <i>external</i> to the metabolism, and may be either <a href="/wiki/index.php?title=Anaerobic_respiration&action=edit&redlink=1" class="new" title="Anaerobic respiration (page does not exist)">anaerobic</a> or <a href="/wiki/index.php?title=Aerobic_respiration&action=edit&redlink=1" class="new" title="Aerobic respiration (page does not exist)">aerobic</a>. <a href="/wiki/index.php?title=Fermentation&action=edit&redlink=1" class="new" title="Fermentation (page does not exist)">Fermentation</a>, on the other hand, does not use an external electron acceptor: in this case, the electron acceptor is a product of glycolysis, usually pyruvate or a pyruvate derivative. </p><p><a href="/wiki/index.php?title=Acetyl-CoA&action=edit&redlink=1" class="new" title="Acetyl-CoA (page does not exist)">Acetyl-CoA</a> is a pivotal molecule during aerobic respiration. Acetyl-CoA is derived from pyruvate, but can also be formed through <a href="/wiki/index.php?title=Beta-oxidation&action=edit&redlink=1" class="new" title="Beta-oxidation (page does not exist)">β-oxidation</a> of <a href="/wiki/index.php?title=Fatty_acids&action=edit&redlink=1" class="new" title="Fatty acids (page does not exist)">fatty acids</a> or through the catabolism of amino acids, and is oxidized to CO<sub>2</sub> through the <a href="/wiki/Krebs_cycle" class="mw-redirect" title="Krebs cycle">Krebs cycle</a>. The Krebs cycle releases eight electrons from each acetyl-CoA molecule, which are eventually used in aerobic organisms to reduce oxygen (terminal electron acceptor) via an electron transport chain. This is part of the process to synthesis more ATP, known as <a href="/wiki/Oxidative_phosphorylation" title="Oxidative phosphorylation">oxidative phosphorylation</a>, and is very similar to photophosphorylation in phototrophs. </p> <h3><span class="mw-headline" id="Reducing_Power:_obtaining_electrons_for_chemical_bonds">Reducing Power: obtaining electrons for chemical bonds</span></h3> <p>Reducing power is an important input into many anabolic pathways, including the <a href="/wiki/Calvin_cycle" title="Calvin cycle">Calvin cycle</a> of photosynthesis, the <a href="/wiki/Biosynthesis" title="Biosynthesis">biosynthesis</a> of <a href="/wiki/Amino_acid" title="Amino acid">amino acids</a>, and the biosynthesis of <a href="/wiki/Fatty_acid" title="Fatty acid">fatty acids</a>. Reducing power is usually supplied as hydrogen equivalents carried by <a href="/wiki/NADPH" title="NADPH">NADPH</a>. Organisms can be classified according to the primary source of this reducing power as: </p> <h4><span class="mw-headline" id="Organotrophic"><a href="/wiki/index.php?title=Organotroph&action=edit&redlink=1" class="new" title="Organotroph (page does not exist)">Organotrophic</a></span></h4> <p>These organisms use organic compounds (e.g. <a href="/wiki/Glucose" title="Glucose">glucose</a>) as the primary electron source. </p> <h4><span class="mw-headline" id="Lithotrophic"><a href="/wiki/index.php?title=Lithotrophy&action=edit&redlink=1" class="new" title="Lithotrophy (page does not exist)">Lithotrophic</a></span></h4> <p>These organisms use inorganic compounds (e.g. Fe<sup>2+</sup>, (iron ions)) as primary electron source. </p> <h2><span class="mw-headline" id="Regulation_of_metabolism_in_animals">Regulation of metabolism in animals</span></h2> <p>In animals, metabolism is controlled by the <a href="/wiki/index.php?title=Endocrine_system&action=edit&redlink=1" class="new" title="Endocrine system (page does not exist)">endocrine system</a> through the secretion of <a href="/wiki/Hormone" title="Hormone">hormones</a>. Some hormones have anabolic actions on the body, others have mainly catabolic actions. For example, <a href="/wiki/Testosterone" title="Testosterone">testosterone</a> is an anabolic hormone, and synthetic steroids that produce the anabolic actions are known as <a href="/wiki/Anabolic_steroids" title="Anabolic steroids">anabolic steroids</a>. <a href="/wiki/Cortisol" title="Cortisol">Cortisol</a> on the other hand, which is a steroid hormone produced by the <a href="/wiki/Adrenal_gland" title="Adrenal gland">adrenal gland</a>, is a catabolic hormone. </p><p>Two hormones synthesized by the <a href="/wiki/index.php?title=Pancreas&action=edit&redlink=1" class="new" title="Pancreas (page does not exist)">pancreas</a>, <a href="/wiki/Insulin" title="Insulin">insulin</a> and <a href="/wiki/index.php?title=Glucagon&action=edit&redlink=1" class="new" title="Glucagon (page does not exist)">glucagon</a>, are particularly important. Insulin is secreted when blood glucose levels are high, and it stimulates glucose uptake by muscle, glycogen synthesis, and triacylglyceride synthesis by <a href="/wiki/Adipose_tissue" title="Adipose tissue">adipose tissue</a> (fat). It also inhibits gluconeogenesis and glycogen degradation. Glucagon is secreted when blood glucose levels are low, and its effects are opposite to those of insulin. In the liver, glucagon stimulates glycogen degradation and the absorption of gluconeogenic aminoacids, and it inhibits glycogen synthesis and promotes the release of <a href="/wiki/index.php?title=Fatty_acids&action=edit&redlink=1" class="new" title="Fatty acids (page does not exist)">fatty acids</a> by adipose tissue. </p><p>In mammals and other warm blooded animals, many metabolic process are ultimately controlled by the <a href="/wiki/Central_nervous_system" title="Central nervous system">central nervous system</a>, which regulates the endocrine system. The central nervous system and endocrine system are influenced by the balance between the <i>energy demands</i> of the organism, and its <i>energy stores</i> (see also <a href="/wiki/Hunger" title="Hunger">Hunger</a>). For example, fat stores secrete a hormone called <a href="/wiki/Leptin" title="Leptin">leptin</a> that acts at the <a href="/wiki/Hypothalamus" title="Hypothalamus">hypothalamus</a> to regulate hormone secretion. The hypothalamus is also sensitive to circulating concentrations of glucose and insulin, and to body temperature. When the ambient temperature is low, the metabolic rate of an <a href="/wiki/index.php?title=Endotherm&action=edit&redlink=1" class="new" title="Endotherm (page does not exist)">endothermic</a> animal will <i>increase</i> in order to generate more body heat (<a href="/wiki/index.php?title=Thermogenesis&action=edit&redlink=1" class="new" title="Thermogenesis (page does not exist)">thermogenesis</a>). In animals that <a href="/wiki/index.php?title=Hibernate&action=edit&redlink=1" class="new" title="Hibernate (page does not exist)">hibernate</a>, the body temperature drops down enough that the basal metabolic rate is quite low, conserving energy over a winter period of inactivity. </p><p>Some <a href="/wiki/index.php?title=Ectotherm&action=edit&redlink=1" class="new" title="Ectotherm (page does not exist)">ectothermic</a> animals, like reptiles, regulate their body temperature by their behavior. These "cold blooded" creatures, including lizards, snakes, and turtles, keep at an optimum body temperature by heating up in the sun (basking) and cooling down in the shade or the cool earth of a burrow. The metabolism of these animals also changes with body temperature, and explains the sluggish movements of an ectotherm in colder seasons or times of day. </p>  </div> <div class="printfooter" data-nosnippet="">Retrieved from "<a dir="ltr" href="https://citizendium.org/wiki/index.php?title=Metabolism/Citable_Version&oldid=5155">https://citizendium.org/wiki/index.php?title=Metabolism/Citable_Version&oldid=5155</a>"</div></div> <div id="catlinks" class="catlinks" data-mw="interface"><div id="mw-normal-catlinks" class="mw-normal-catlinks"><a href="/wiki/Special:Categories" title="Special:Categories">Categories</a>: <ul><li><a href="/wiki/Category:Subpages" title="Category:Subpages">Subpages</a></li><li><a href="/wiki/Category:Biology_Extra_Subpages" title="Category:Biology Extra Subpages">Biology Extra Subpages</a></li><li><a href="/wiki/Category:Biology_Approved_Extra_Subpages" title="Category:Biology Approved Extra Subpages">Biology Approved Extra Subpages</a></li><li><a href="/wiki/Category:Citable_versions_of_articles" title="Category:Citable versions of articles">Citable versions of articles</a></li><li><a href="/wiki/Category:Biology_Citable_Version_Subpages" title="Category:Biology Citable Version Subpages">Biology Citable Version Subpages</a></li><li><a href="/wiki/Category:All_Content" title="Category:All Content">All Content</a></li><li><a href="/wiki/Category:Biology_Content" title="Category:Biology Content">Biology Content</a></li></ul></div><div id="mw-hidden-catlinks" class="mw-hidden-catlinks mw-hidden-cats-hidden">Hidden category: <ul><li><a href="/wiki/Category:Biology_tag" title="Category:Biology tag">Biology tag</a></li></ul></div></div> </div> </div> <div id="mw-navigation"> <h2>Navigation menu</h2> <div id="mw-head"> <nav id="p-personal" class="vector-menu mw-portlet mw-portlet-personal vector-user-menu-legacy" aria-labelledby="p-personal-label" role="navigation" > <h3 id="p-personal-label" class="vector-menu-heading " > <span class="vector-menu-heading-label">Personal tools</span> </h3> <div class="vector-menu-content"> <ul class="vector-menu-content-list"><li id="pt-login" class="mw-list-item"><a href="/wiki/index.php?title=Special:UserLogin&returnto=Metabolism%2FCitable+Version" title="To edit pages, [[Special:RequestAccount|get a contributor account]]. 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