Energy Metabolism I | Biology

<!DOCTYPE html> <html lang="en" dir="ltr"> <head>  <meta http-equiv="X-UA-Compatible" content="ie=edge"> <meta charset="utf-8"> <base href="https://www.visionlearning.com"> <title>Energy Metabolism I | Biology | Visionlearning</title> <link rel="canonical" href="https://www.visionlearning.com/en/library/biology/2/energy-metabolism-i/215"> <meta name="description" content="Food fuels our bodies, but how does our body convert food molecules into usable energy? This module looks at glycolysis and the Krebs cycle, two important stages of cellular respiration, the process by which cells harvest energy from food. It highlights the work of Sir Hans Adolf Krebs and his focus on cyclic pathways as he discovered the main biochemical pathway for breaking down fuel to produce energy."> <meta name="keywords" content="Krebs, Krebs cycle, citric acid cycle, energy metabolism, cellular metabolism, energy, glucose"> <meta name="viewport" content="width=device-width, initial-scale=1.0, shrink-to-fit=no"> <meta name="msvalidate.01" content="D8E20F39AD48052260032E56DE409970"> <script type="application/ld+json"> { "@context": "http://schema.org/", "@type": "Article", "mainEntityOfPage": { "@type": "WebPage", "@id": "https://visionlearning.com/en/library/biology/2/energy-metabolism-i/215" }, "name": "Energy Metabolism I", "headline": "Energy Metabolism I: Glycolosis and the Krebs cycle", "author": [ { "@type": "Person", "name": "David Warmflash, MD" } , { "@type": "Person", "name": "Nathan H Lents, Ph.D." }], "datePublished": "2015-07-15 07:49:39", "dateModified": "2017-02-12T08:30:00+05:00", "image": { "@type": "ImageObject", "url": "/img/library/moduleImages/featured_image_215-23061209062934.jpeg", "width": 696, "height": 464 }, "publisher": { "@type": "Organization", "name": "Visionlearning, Inc.", "logo": { "@type": "ImageObject", "url": "http://visionlearning.com/images/logo.png", "width": 278, "height": 60 } }, "description": "Food fuels our bodies, but how does our body convert food molecules into usable energy? This module looks at glycolysis and the Krebs cycle, two important stages of cellular respiration, the process by which cells harvest energy from food. It highlights the work of Sir Hans Adolf Krebs and his focus on cyclic pathways as he discovered the main biochemical pathway for breaking down fuel to produce energy.", "keywords": "Krebs, Krebs cycle, citric acid cycle, energy metabolism, cellular metabolism, energy, glucose", "inLanguage": { "@type": "Language", "name": "English", "alternateName": "en" }, "copyrightHolder": { "@type": "Organization", "name": "Visionlearning, Inc." }, "copyrightYear": "2015"} </script> <meta property="og:url" content="https://visionlearning.com/en/library/biology/2/energy-metabolism-i/215"> <meta property="og:title" content="Energy Metabolism I | Biology | Visionlearning" /> <meta property="og:type" content="website"> <meta property="og:site_name" content="Visionlearning"> <meta property="og:description" content="Food fuels our bodies, but how does our body convert food molecules into usable energy? This module looks at glycolysis and the Krebs cycle, two important stages of cellular respiration, the process by which cells harvest energy from food. It highlights the work of Sir Hans Adolf Krebs and his focus on cyclic pathways as he discovered the main biochemical pathway for breaking down fuel to produce energy."> <meta property="og:image" content="https://visionlearning.com/images/logo.png"> <meta property="fb:admins" content="100000299664514"> <link rel="stylesheet" type="text/css" href="/css/visionlearning.css">  <link rel="stylesheet" type="text/css" href="/css/visionlearning-icons.css">  <link rel="preconnect" as="font" href="https://fonts.gstatic.com" crossorigin> <link href="https://fonts.googleapis.com/css2?family=Open+Sans:ital,wght@0,400;0,700;1,400;1,700&family=Schoolbell&display=swap" rel="stylesheet"> <style> textarea.myEditor { width: 90%; height: 350px; } </style> <script type="text/x-mathjax-config" src="/js/mathjax-config.js"></script> <script id="MathJax-script" async src="/js/mathjax/tex-svg.js"></script> <script async 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<li><a href="/en/library/earth-science/6/factors-that-control-earths-temperature/234">Factors that Control Earth's Temperature</a></li> <li><a href="/en/library/earth-science/6/circulation-in-the-atmosphere/255">Circulation in the Atmosphere</a></li> </ul> </div> <button class="accordion__button" id="acc-button-hazards" data-accordion="button" aria-controls="acc-panel-hazards" aria-expanded="false"> <span class="accordion__button__label"> Hazards </span> </button> <div class="accordion__panel" id="acc-panel-hazards" data-accordion="panel" aria-labelledby="acc-button-hazards" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/earth-science/6/natural-hazards-and-risk/288">Natural Hazards and Risk</a></li> </ul> </div> <button class="accordion__button" id="acc-button-earth-history" data-accordion="button" aria-controls="acc-panel-earth-history" aria-expanded="false"> <span class="accordion__button__label"> Earth History </span> </button> <div class="accordion__panel" 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aria-controls="acc-panel-ecology" aria-expanded="false"> <span class="accordion__button__label"> Ecology </span> </button> <div class="accordion__panel" id="acc-panel-ecology" data-accordion="panel" aria-labelledby="acc-button-ecology" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/environmental-science/61/biodiversity-i/276">Biodiversity I</a></li> <li><a href="/en/library/environmental-science/61/biodiversity-ii/281">Biodiversity II</a></li> <li><a href="/en/library/environmental-science/61/ecosystem-services/279">Ecosystem Services</a></li> <li><a href="/en/library/environmental-science/61/population-biology/287">Population Biology</a></li> </ul> </div> <button class="accordion__button" id="acc-button-earth-cycles" data-accordion="button" aria-controls="acc-panel-earth-cycles" aria-expanded="false"> <span class="accordion__button__label"> Earth Cycles </span> </button> <div class="accordion__panel" id="acc-panel-earth-cycles" data-accordion="panel" aria-labelledby="acc-button-earth-cycles" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/environmental-science/61/the-nitrogen-cycle/98">The Nitrogen Cycle</a></li> <li><a href="/en/library/environmental-science/61/the-carbon-cycle/95">The Carbon Cycle</a></li> <li><a href="/en/library/environmental-science/61/the-phosphorus-cycle/197">The Phosphorus Cycle</a></li> </ul> </div> <button class="accordion__button" id="acc-button-scientific-research" data-accordion="button" aria-controls="acc-panel-scientific-research" aria-expanded="false"> <span class="accordion__button__label"> Scientific Research </span> </button> <div class="accordion__panel" id="acc-panel-scientific-research" data-accordion="panel" aria-labelledby="acc-button-scientific-research" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/environmental-science/61/collaborative-research-in-the-arctic-towards-understanding-climate-change/183">Collaborative Research in the Arctic Towards Understanding Climate Change</a></li> <li><a href="/en/library/environmental-science/61/atmospheric-chemistry-research-that-changed-global-policy/211">Atmospheric Chemistry Research that Changed Global Policy</a></li> </ul> </div> </div> </div> <button class="accordion__button" id="acc-button-general-science" data-accordion="button" aria-controls="acc-panel-general-science" aria-expanded="false"> <span class="accordion__button__label"> General Science </span> </button> <div class="accordion__panel" id="acc-panel-general-science" data-accordion="panel" aria-labelledby="acc-button-general-science" role="region"> <div class="accordion accordion--secondary"> <button class="accordion__button" id="acc-button-methods" data-accordion="button" aria-controls="acc-panel-methods" aria-expanded="false"> <span class="accordion__button__label"> Methods </span> </button> <div class="accordion__panel" id="acc-panel-methods" data-accordion="panel" aria-labelledby="acc-button-methods" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/general-science/3/the-scientific-method/45">The Scientific Method</a></li> </ul> </div> <button class="accordion__button" id="acc-button-measurement" data-accordion="button" aria-controls="acc-panel-measurement" aria-expanded="false"> <span class="accordion__button__label"> Measurement </span> </button> <div class="accordion__panel" id="acc-panel-measurement" data-accordion="panel" aria-labelledby="acc-button-measurement" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/general-science/3/the-metric-system/47">The Metric System</a></li> </ul> </div> <button class="accordion__button" id="acc-button-physical-properties" data-accordion="button" aria-controls="acc-panel-physical-properties" aria-expanded="false"> <span class="accordion__button__label"> Physical Properties </span> </button> <div class="accordion__panel" id="acc-panel-physical-properties" data-accordion="panel" aria-labelledby="acc-button-physical-properties" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/general-science/3/temperature/48">Temperature</a></li> <li><a href="/en/library/general-science/3/density-and-buoyancy/37">Density and Buoyancy</a></li> </ul> </div> </div> </div> <button class="accordion__button" id="acc-button-math-in-science" data-accordion="button" aria-controls="acc-panel-math-in-science" aria-expanded="false"> <span class="accordion__button__label"> Math in Science </span> </button> <div class="accordion__panel" id="acc-panel-math-in-science" data-accordion="panel" aria-labelledby="acc-button-math-in-science" role="region"> <div class="accordion accordion--secondary"> <button class="accordion__button" id="acc-button-equations" data-accordion="button" aria-controls="acc-panel-equations" aria-expanded="false"> <span class="accordion__button__label"> Equations </span> </button> <div class="accordion__panel" id="acc-panel-equations" data-accordion="panel" aria-labelledby="acc-button-equations" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/math-in-science/62/unit-conversion/144">Unit Conversion</a></li> <li><a href="/en/library/math-in-science/62/linear-equations/194">Linear Equations</a></li> <li><a href="/en/library/math-in-science/62/exponential-equations-i/206">Exponential Equations I</a></li> <li><a href="/en/library/math-in-science/62/exponential-equations-ii/210">Exponential Equations II</a></li> <li><a href="/en/library/math-in-science/62/scientific-notation/250">Scientific Notation</a></li> <li><a href="/en/library/math-in-science/62/measurement/257">Measurement</a></li> </ul> </div> <button class="accordion__button" id="acc-button-statistics" data-accordion="button" aria-controls="acc-panel-statistics" aria-expanded="false"> <span class="accordion__button__label"> Statistics </span> </button> <div class="accordion__panel" id="acc-panel-statistics" data-accordion="panel" aria-labelledby="acc-button-statistics" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/math-in-science/62/introduction-to-descriptive-statistics/218">Introduction to Descriptive Statistics</a></li> <li><a href="/en/library/math-in-science/62/introduction-to-inferential-statistics/224">Introduction to Inferential Statistics</a></li> <li><a href="/en/library/math-in-science/62/statistical-techniques/239">Statistical Techniques</a></li> </ul> </div> <button class="accordion__button" id="acc-button-trigonometric-functions" data-accordion="button" aria-controls="acc-panel-trigonometric-functions" aria-expanded="false"> <span class="accordion__button__label"> Trigonometric Functions </span> </button> <div class="accordion__panel" id="acc-panel-trigonometric-functions" data-accordion="panel" aria-labelledby="acc-button-trigonometric-functions" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/math-in-science/62/wave-mathematics/131">Wave Mathematics</a></li> </ul> </div> </div> </div> <button class="accordion__button" id="acc-button-physics" data-accordion="button" aria-controls="acc-panel-physics" aria-expanded="false"> <span class="accordion__button__label"> Physics </span> </button> <div class="accordion__panel" id="acc-panel-physics" data-accordion="panel" aria-labelledby="acc-button-physics" role="region"> <div class="accordion accordion--secondary"> <button class="accordion__button" id="acc-button-light-and-optics" data-accordion="button" aria-controls="acc-panel-light-and-optics" aria-expanded="false"> <span class="accordion__button__label"> Light and Optics </span> </button> <div class="accordion__panel" id="acc-panel-light-and-optics" data-accordion="panel" aria-labelledby="acc-button-light-and-optics" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/physics/24/the-nature-of-light/132">The Nature of Light</a></li> <li><a href="/en/library/physics/24/electromagnetism-and-light/138">Electromagnetism and Light</a></li> </ul> </div> <button class="accordion__button" id="acc-button-mechanics" data-accordion="button" aria-controls="acc-panel-mechanics" aria-expanded="false"> <span class="accordion__button__label"> Mechanics </span> </button> <div class="accordion__panel" id="acc-panel-mechanics" data-accordion="panel" aria-labelledby="acc-button-mechanics" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/physics/24/defining-energy/199">Defining Energy</a></li> <li><a href="/en/library/physics/24/waves-and-wave-motion/102">Waves and Wave Motion</a></li> <li><a href="/en/library/physics/24/gravity/118">Gravity</a></li> <li><a href="/en/library/physics/24/thermodynamics-i/200">Thermodynamics I</a></li> </ul> </div> </div> </div> <button class="accordion__button" id="acc-button-process-of-science" data-accordion="button" aria-controls="acc-panel-process-of-science" aria-expanded="false"> <span class="accordion__button__label"> Process of Science </span> </button> <div class="accordion__panel" id="acc-panel-process-of-science" data-accordion="panel" aria-labelledby="acc-button-process-of-science" role="region"> <div class="accordion accordion--secondary"> <button class="accordion__button" id="acc-button-introduction" data-accordion="button" aria-controls="acc-panel-introduction" aria-expanded="false"> <span class="accordion__button__label"> Introduction </span> </button> <div class="accordion__panel" id="acc-panel-introduction" data-accordion="panel" aria-labelledby="acc-button-introduction" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/process-of-science/49/the-process-of-science/176">The Process of Science</a></li> </ul> </div> <button class="accordion__button" id="acc-button-the-culture-of-science" data-accordion="button" aria-controls="acc-panel-the-culture-of-science" aria-expanded="false"> <span class="accordion__button__label"> The Culture of Science </span> </button> <div class="accordion__panel" id="acc-panel-the-culture-of-science" data-accordion="panel" aria-labelledby="acc-button-the-culture-of-science" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/process-of-science/49/the-nature-of-scientific-knowledge/185">The Nature of Scientific Knowledge</a></li> <li><a href="/en/library/process-of-science/49/scientists-and-the-scientific-community/172">Scientists and the Scientific Community</a></li> <li><a href="/en/library/process-of-science/49/scientific-ethics/161">Scientific Ethics</a></li> <li><a href="/en/library/process-of-science/49/scientific-institutions-and-societies/162">Scientific Institutions and Societies</a></li> </ul> </div> <button class="accordion__button" id="acc-button-ideas-in-science" data-accordion="button" aria-controls="acc-panel-ideas-in-science" aria-expanded="false"> <span class="accordion__button__label"> Ideas in Science </span> </button> <div class="accordion__panel" id="acc-panel-ideas-in-science" data-accordion="panel" aria-labelledby="acc-button-ideas-in-science" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/process-of-science/49/theories-hypotheses-and-laws/177">Theories, Hypotheses, and Laws</a></li> <li><a href="/en/library/process-of-science/49/scientific-controversy/181">Scientific Controversy</a></li> <li><a href="/en/library/process-of-science/49/creativity-in-science/182">Creativity in Science</a></li> </ul> </div> <button class="accordion__button" id="acc-button-research-methods" data-accordion="button" aria-controls="acc-panel-research-methods" aria-expanded="false"> <span class="accordion__button__label"> Research Methods </span> </button> <div class="accordion__panel" id="acc-panel-research-methods" data-accordion="panel" aria-labelledby="acc-button-research-methods" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/process-of-science/49/the-practice-of-science/148">The Practice of Science</a></li> <li><a href="/en/library/process-of-science/49/experimentation-in-scientific-research/150">Experimentation in Scientific Research</a></li> <li><a href="/en/library/process-of-science/49/description-in-scientific-research/151">Description in Scientific Research</a></li> <li><a href="/en/library/process-of-science/49/comparison-in-scientific-research/152">Comparison in Scientific Research</a></li> <li><a href="/en/library/process-of-science/49/modeling-in-scientific-research/153">Modeling in Scientific Research</a></li> </ul> </div> <button class="accordion__button" id="acc-button-data" data-accordion="button" aria-controls="acc-panel-data" aria-expanded="false"> <span class="accordion__button__label"> Data </span> </button> <div class="accordion__panel" id="acc-panel-data" data-accordion="panel" aria-labelledby="acc-button-data" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/process-of-science/49/data-analysis-and-interpretation/154">Data Analysis and Interpretation</a></li> <li><a href="/en/library/process-of-science/49/uncertainty-error-and-confidence/157">Uncertainty, Error, and Confidence</a></li> <li><a href="/en/library/process-of-science/49/statistics-in-science/155">Statistics in Science</a></li> <li><a href="/en/library/process-of-science/49/using-graphs-and-visual-data-in-science/156">Using Graphs and Visual Data in Science</a></li> </ul> </div> <button class="accordion__button" id="acc-button-scientific-communication" data-accordion="button" aria-controls="acc-panel-scientific-communication" aria-expanded="false"> <span class="accordion__button__label"> Scientific Communication </span> </button> <div class="accordion__panel" id="acc-panel-scientific-communication" data-accordion="panel" aria-labelledby="acc-button-scientific-communication" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/process-of-science/49/understanding-scientific-journals-and-articles/158">Understanding Scientific Journals and Articles</a></li> <li><a href="/en/library/process-of-science/49/utilizing-the-scientific-literature/173">Utilizing the Scientific Literature</a></li> <li><a href="/en/library/process-of-science/49/peer-review-in-scientific-publishing/159">Peer Review in Scientific Publishing</a></li> <li><a href="/en/library/process-of-science/49/the-how-and-why-of-scientific-meetings/186">The How and Why of Scientific Meetings</a></li> </ul> </div> </div> </div> <button class="accordion__button" id="acc-button-scientists-and-research" data-accordion="button" aria-controls="acc-panel-scientists-and-research" aria-expanded="false"> <span class="accordion__button__label"> Scientists and Research </span> </button> <div class="accordion__panel" id="acc-panel-scientists-and-research" data-accordion="panel" aria-labelledby="acc-button-scientists-and-research" role="region"> <div class="accordion accordion--secondary"> <button class="accordion__button" id="acc-button-scientific-research" data-accordion="button" aria-controls="acc-panel-scientific-research" aria-expanded="false"> <span class="accordion__button__label"> Scientific Research </span> </button> <div class="accordion__panel" id="acc-panel-scientific-research" data-accordion="panel" aria-labelledby="acc-button-scientific-research" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/scientists-and-research/58/collaborative-research-in-the-arctic-towards-understanding-climate-change/183">Collaborative Research in the Arctic Towards Understanding Climate Change</a></li> <li><a href="/en/library/scientists-and-research/58/from-stable-chromosomes-to-jumping-genes/184">From Stable Chromosomes to Jumping Genes</a></li> <li><a href="/en/library/scientists-and-research/58/an-elegant-experiment-to-test-the-process-of-dna-replication/187">An Elegant Experiment to Test the Process of DNA Replication</a></li> <li><a href="/en/library/scientists-and-research/58/the-founding-of-neuroscience/233">The Founding of Neuroscience</a></li> <li><a href="/en/library/scientists-and-research/58/tracking-endangered-jaguars-across-the-border/189">Tracking Endangered Jaguars across the Border</a></li> <li><a href="/en/library/scientists-and-research/58/atmospheric-chemistry-research-that-changed-global-policy/211">Atmospheric Chemistry Research that Changed Global Policy</a></li> <li><a href="/en/library/scientists-and-research/58/revolutionizing-medicine-with-monoclonal-antibodies/220">Revolutionizing Medicine with Monoclonal Antibodies</a></li> <li><a href="/en/library/scientists-and-research/58/uncovering-the-mysteries-of-chronic-mountain-sickness/238">Uncovering the Mysteries of Chronic Mountain Sickness</a></li> </ul> </div> <button class="accordion__button" id="acc-button-profiles-in-science" data-accordion="button" aria-controls="acc-panel-profiles-in-science" aria-expanded="false"> <span class="accordion__button__label"> Profiles in Science </span> </button> <div class="accordion__panel" id="acc-panel-profiles-in-science" data-accordion="panel" aria-labelledby="acc-button-profiles-in-science" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/scientists-and-research/58/luis-e.-miramontes/232">Luis E. Miramontes</a></li> <li><a href="/en/library/scientists-and-research/58/bernardo-houssay/237">Bernardo Houssay</a></li> <li><a href="/en/library/scientists-and-research/58/craig-lee/256">Craig Lee</a></li> <li><a href="/en/library/scientists-and-research/58/david-ho/241">David Ho</a></li> <li><a href="/en/library/scientists-and-research/58/louis-tompkins-wright/244">Louis Tompkins Wright</a></li> <li><a href="/en/library/scientists-and-research/58/carlos-j.-finlay/217">Carlos J. Finlay</a></li> <li><a href="/en/library/scientists-and-research/58/cecilia-payne/290">Cecilia Payne</a></li> <li><a href="/en/library/scientists-and-research/58/jazmin-scarlett/291">Jazmin Scarlett</a></li> <li><a href="/en/library/scientists-and-research/58/ramari-stewart/292">Ramari Stewart</a></li> <li><a href="/en/library/scientists-and-research/58/johnson-cerda/300">Johnson Cerda</a></li> <li><a href="/en/library/scientists-and-research/58/ellen-ochoa/201">Ellen Ochoa</a></li> <li><a href="/en/library/scientists-and-research/58/ruth-benerito/205">Ruth Benerito</a></li> <li><a href="/en/library/scientists-and-research/58/franklin-chang-díaz/219">Franklin Chang Díaz</a></li> <li><a href="/en/library/scientists-and-research/58/percy-lavon-julian/221">Percy Lavon Julian</a></li> <li><a href="/en/library/scientists-and-research/58/luis-walter-alvarez/229">Luis Walter Alvarez</a></li> <li><a href="/en/library/scientists-and-research/58/france-anne-dominic-córdova/230">France Anne-Dominic Córdova</a></li> </ul> </div> </div> </div> </div> </div> </li> <li>  <button class="button" data-toggle="dropdown">Biology </button> <div class="nav__dropdown box-shadow-1 padding-1"> <div class="accordion accordion--secondary font-size-sm"> <div class="accordion accordion--secondary"> <button class="accordion__button" id="acc-sub-button-biological-molecules" data-accordion="button" aria-controls="acc-sub-panel-biological-molecules" aria-expanded="false"> <span class="accordion__button__label"> Biological Molecules </span> </button> <div class="accordion__panel" id="acc-sub-panel-biological-molecules" data-accordion="panel" aria-labelledby="acc-sub-button-biological-molecules" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/biology/2/carbohydrates/61">Carbohydrates</a></li> <li><a href="/en/library/biology/2/fats-and-proteins/62">Fats and Proteins</a></li> <li><a href="/en/library/biology/2/biological-proteins/243">Biological Proteins</a></li> <li><a href="/en/library/biology/2/blood-biology-i/242">Blood Biology I</a></li> <li><a href="/en/library/biology/2/lipids/207">Lipids</a></li> </ul> </div> <button class="accordion__button" id="acc-sub-button-cell-biology" data-accordion="button" aria-controls="acc-sub-panel-cell-biology" aria-expanded="false"> <span class="accordion__button__label"> Cell Biology </span> </button> <div class="accordion__panel" id="acc-sub-panel-cell-biology" data-accordion="panel" aria-labelledby="acc-sub-button-cell-biology" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/biology/2/discovery-and-structure-of-cells/64">Discovery and Structure of Cells</a></li> <li><a href="/en/library/biology/2/respiration/285">Respiration</a></li> <li><a href="/en/library/biology/2/membranes-i/198">Membranes I</a></li> <li><a href="/en/library/biology/2/membranes-ii/204">Membranes II</a></li> <li><a href="/en/library/biology/2/cellular-organelles-i/195">Cellular Organelles I</a></li> <li><a href="/en/library/biology/2/cell-division-i/196">Cell Division I</a></li> <li><a href="/en/library/biology/2/cell-division-ii/212">Cell Division II</a></li> <li><a href="/en/library/biology/2/membranes-and-chemical-transport/106">Membranes and Chemical Transport</a></li> </ul> </div> <button class="accordion__button" id="acc-sub-button-energy-in-living-systems" data-accordion="button" aria-controls="acc-sub-panel-energy-in-living-systems" aria-expanded="false"> <span class="accordion__button__label"> Energy in Living Systems </span> </button> <div class="accordion__panel" id="acc-sub-panel-energy-in-living-systems" data-accordion="panel" aria-labelledby="acc-sub-button-energy-in-living-systems" role="region"> <ul class="nav text-color-link"> <li class="current">Energy Metabolism I</li> <li><a href="/en/library/biology/2/energy-metabolism-ii/225">Energy Metabolism II</a></li> <li><a href="/en/library/biology/2/photosynthesis-i/192">Photosynthesis I</a></li> </ul> </div> <button 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data-accordion="panel" aria-labelledby="acc-sub-button-ecology" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/biology/2/biodiversity-i/276">Biodiversity I</a></li> <li><a href="/en/library/biology/2/ecosystem-services/279">Ecosystem Services</a></li> <li><a href="/en/library/biology/2/animal-ecology/283">Animal Ecology</a></li> <li><a href="/en/library/biology/2/biodiversity-ii/281">Biodiversity II</a></li> <li><a href="/en/library/biology/2/animal-behavior/286">Animal Behavior</a></li> <li><a href="/en/library/biology/2/population-biology/287">Population Biology</a></li> <li><a href="/en/library/biology/2/trophic-ecology/293">Trophic Ecology</a></li> </ul> </div> </div> </div> </div> </li> </ul> </nav>  <div id="theTop"></div> <main id="skip-header-content"> <div class="margin-bottom-5"> <div class="container narrow wide--lg margin-y-4"> <article class="module"> <header class="module__header"> <span class="subcategory"> <strong><em>Energy in Living Systems</em></strong> </span> <h1>Energy Metabolism I: <sub><em>Glycolosis and the Krebs cycle</em></sub></h1> <p class="byline">by David Warmflash, MD, Nathan H Lents, Ph.D.</p> </header> <nav class="module__tabs"> <ul class="tabs-nav tabs-nav--pill tabs-nav--horizontal--md library"> <li> <a href="/en/library/biology/2/energy-metabolism-i/215/reading" class="is-active" aria-current="page" >Reading</a> </li> <li> <a href="/en/library/biology/2/energy-metabolism-i/215/quiz" >Quiz</a> </li> <li> <a href="/en/library/biology/2/energy-metabolism-i/215/resources" >Teach with this</a> </li> </ul> </nav> <script type="application/ld+json"> { "@context": "http://schema.org", "@type": "AudioObject", "contentUrl": "https://www.visionlearning.com/img/library/moduleAudio/module_215.mp3", "description": "Recording of Energy Metabolism I : Food fuels our bodies, but how does our body convert food molecules into usable energy? This module looks at glycolysis and the Krebs cycle, two important stages of cellular respiration, the process by which cells harvest energy from food. It highlights the work of Sir Hans Adolf Krebs and his focus on cyclic pathways as he discovered the main biochemical pathway for breaking down fuel to produce energy.", "encodingFormat": "mp3", "name": "module_215.mp3" } </script> <div class="module__audio"> <div class="audio-player border border-radius"> <audio id="audio"> <source src="https://www.visionlearning.com/img/library/moduleAudio/module_215.mp3" type="audio/mpeg"> Your browser does not support the audio element. </audio> <div class="audio-player__title"> <p>Listen to this reading</p> <span class="audio-player__timestamp" id="timestamp"> 00:00 </span> </div> <div class="audio-player__controls" id="controls"> <button class="button button--icon-only" id="play-pause-button"> <span class="icon icon-play" aria-hidden="true"></span> </button> <div class="audio-player__progress" id="progress-bar" tabindex="0" aria-valuemin="0" aria-valuemax="100" aria-valuenow="0" aria-label="Use arrow keys to forward or rewind the audio" role="slider"> <div class="audio-player__progress__fill"> <span class="audio-player__thumb"></span> </div> </div> <div class="audio-player__volume-container"> <button id="mute-button"> <span class="icon icon-volume"></span> </button> <div class="audio-player__volume" tabindex="0" aria-valuemin="0" aria-valuemax="100" aria-valuenow="100" aria-label="Use arrow keys to adjust volume" role="slider"> <div class="audio-player__volume__fill"> <span class="audio-player__thumb"></span> </div> </div> </div> </div> </div> </div> <hr class="module__divider" />  <div class="module__tools"> <aside class="module__tools__container border-radius box-shadow-1"> <div class="tabs tabs--toggle-mobile--lg" role="tablist"> <ul class="tab__buttons"> <li> <button class="button button--icon-over-text" aria-label="In this module" aria-controls="tab-panel-module__tools" aria-selected="true" role="tab"> <span class="button__icon"> <span class="icon icon-list" aria-hidden="true"></span> </span> <span class="button__text">Contents</span> </button> </li> <li> <button class="button button--icon-over-text" aria-controls="tab-panel-toggle-terms" aria-selected="false" role="tab"> <span class="button__icon"> <span class="icon icon-glossary-highlight"></span> </span> <span class="button__text">Glossary Terms</span> </button> </li> </ul> <div class="tabs__panel shown" id="tab-panel-module__tools" aria-labelledby="tab-button-module__tools" role="tabpanel"> <div class="table-of-contents"> <p class="table-of-contents__title"> Table of Contents </p> <ul class="table-of-contents__nav"> <li><a href="/en/library/biology/2/energy-metabolism-i/215#toc_1">Breaking down fuel for cellular energy</a> </li> <li><a href="/en/library/biology/2/energy-metabolism-i/215#toc_2">Cyclic assembly lines</a> </li> <li> <ul> <li><a href="/en/library/biology/2/energy-metabolism-i/215#toc2_1">Krebs discovers the urea cycle</a> </li> </ul> </li> <li> <ul> <li><a href="/en/library/biology/2/energy-metabolism-i/215#toc2_2">The glycolysis pathway: Embden and Meyerhof</a> </li> </ul> </li> <li> <ul> <li><a href="/en/library/biology/2/energy-metabolism-i/215#toc2_3">The Krebs cycle</a> </li> </ul> </li> <li><a href="/en/library/biology/2/energy-metabolism-i/215#toc_3">Moving molecules from workstation to workstation</a> </li> <li> <ul> <li><a href="/en/library/biology/2/energy-metabolism-i/215#toc2_4">ATP, the cellular energy currency</a> </li> </ul> </li> <li><a href="/en/library/biology/2/energy-metabolism-i/215#toc_4">Two new energy carriers</a> </li> </ul> </div> </div> <div class="tabs__panel" id="tab-panel-toggle-terms" aria-labelledby="tab-button-toggle-terms" role="tabpanel"> <div class="reading-toggle"> <div class="reading-toggle__switch"> <div class="form-entry__option__switch"> <label> <input type="checkbox" name="termsToggleSwitch" id="terms-toggle-switch" /> <span class="switch__slider"></span> <span class="option__label text-decoration-none font-size-md"> Highlight Glossary Terms </span> </label> </div> </div> <div class="reading-toggle__help"> <p> <em> Activate glossary term highlighting to easily identify key terms within the module. Once highlighted, you can click on these terms to view their definitions. </em> </p> </div> </div> <div class="glossary-container"></div> </div> <div class="tabs__panel" id="tab-panel-toggle-ngss" aria-labelledby="tab-button-toggle-ngss" role="tabpanel"> <div class="reading-toggle"> <div class="reading-toggle__switch"> <div class="form-entry__option__switch"> <label> <input type="checkbox" name="ngssToggleSwitch" id="ngss-toggle-switch" /> <span class="switch__slider"></span> <span class="option__label text-decoration-none font-size-md"> Show NGSS Annotations </span> </label> </div> </div> <div class="reading-toggle__help"> <p> <em> Activate NGSS annotations to easily identify NGSS standards within the module. Once highlighted, you can click on them to view these standards. </em> </p> </div> </div> <div class="ngss-container"></div> </div> </div> </aside> <div class="margin-3"> <script async src="https://pagead2.googlesyndication.com/pagead/js/adsbygoogle.js?client=ca-pub-9561344156007092" crossorigin="anonymous"></script>  <ins class="adsbygoogle" style="display:block" data-ad-client="ca-pub-9561344156007092" data-ad-slot="7634263342" data-ad-format="auto" data-full-width-responsive="true"></ins> <script> (adsbygoogle = window.adsbygoogle || []).push({}); </script> </div> </div>     <div class="module__main"> <div class="module__main__container"> <div class="accordion">  <button class="accordion__button" id="acc-button-key-concepts" data-accordion="button" aria-controls="acc-panel-key-concepts" aria-expanded="true" tabindex="0"> Did you know? </button> <div class="accordion__panel shown show" id="acc-panel-key-concepts" data-accordion="panel" aria-labelledby="acc-button-key-concepts" role="region"> <div class="accordion__panel__content"> <p>Did you know similar to the way cars are manufactured, chemical compounds in living cells are built up, broken down, and moved around in assembly-line fashion? In living organisms, a series of reactions is needed to get energy from food molecules. One such important chemical pathway is the circular assembly line known as the Krebs cycle.</p> </div> </div>  <button class="accordion__button" id="acc-button-table-of-contents" data-accordion="button" aria-controls="acc-panel-table-of-contents" aria-expanded="false" tabindex="0"> Key concepts </button> <div class="accordion__panel" id="acc-panel-table-of-contents" data-accordion="panel" aria-labelledby="acc-button-table-of-contents" role="region" aria-hidden="true"> <div class="accordion__panel__content"> <ul class="bulleted"> <li><p>In a cell, chemical compounds are put together, taken apart, and moved around through pathways that resemble moving assembly lines.</p></li> <li><p>The main types of biological macromolecules that cells use for fuel are sugars, fats, and proteins.</p></li> <li><p>The main biochemical pathway where the breakdown of biological fuels comes together is called the Krebs cycle. Named for its discoverer, Sir Hans Adolf Krebs, this pathway is like a circular assembly line.</p></li> </ul> </div> </div>  <button class="accordion__button" id="acc-button-terms-you-should-know" data-accordion="button" aria-controls="acc-panel-terms-you-should-know" aria-expanded="false" tabindex="0"> Terms you should know </button> <div class="accordion__panel" id="acc-panel-terms-you-should-know" data-accordion="panel" aria-labelledby="acc-button-terms-you-should-know" role="region" aria-hidden="true"> <div class="accordion__panel__content"> <dl> <dt>enzymes </dt> <dd> protein-based molecules in our cells that cause or speed up biochemical reactions </dd> <dt><a href="/en/glossary/view/glucose">glucose </a></dt> <dd> the primary form of sugar stored in the human body for energy. </dd> <dt><a href="/en/glossary/view/reaction">reaction </a></dt> <dd> a chemical change that happens when two or more atoms or molecules combine to form a new substance</dd> </dl> </div> </div> </div> <section> <p>Inspiration can come from many places. Sometimes, inventors are inspired by new discoveries in science, and sometimes it’s the other way around – scientists are inspired by new developments in industry. This is what happened in the early 20th century after the moving assembly line came of age. </p> <p>First introduced by Henry Ford in 1907, the assembly line was not just a single stream of automobile parts flowing from one worker to the next. Instead, it was a multi-path system of many assembly groups. Of course, there was a main assembly line that began with the wheels and the bottom of each car and ended with the completed vehicle, but there were also additional tributary lines feeding into the main line at different points. These tributary lines developed components that needed to be pre-assembled individually before they could go into each car. There was a special line for the engine, for the car body, the seats and doors, and movement of parts through each was timed so as to provide the components to the main assembly line in a coordinated fashion (Figure 1).</p>  <div class="figure"> <figure> <button class="lightbox-button" data-lightbox-src="/img/library/large_images/image_8586.jpg" data-lightbox="image"> <img src="/img/library/modules/mid215/Image/VLObject-8586-150712090717.jpg" alt="Figure 1: Workers on the first moving assembly line put together magnetos and flywheels for 1913 Ford automobiles." /> </button> <figcaption> <p><strong>Figure 1</strong>: Workers on the first moving assembly line put together magnetos and flywheels for 1913 Ford automobiles.</p> <span class="credit">image ©NARA</span> </figcaption> </figure> </div> <p>If anything slowed down one group – a shortage of parts, for instance – the entire system would slow. In such cases, the completed components from the other groups would accumulate, since they could not be put into new cars on the main line. But when all sections operated on schedule, the new Model T cars took shape very rapidly. In fact, operating like this, Ford could produce thousands of cars per day, which was a striking advance over earlier, custom made cars that were hugely expensive and available only to the wealthy. Other industries quickly adopted the assembly line approach. </p><p>By the start of the 20<sup>th</sup> century, scientists in various fields were already realizing that nature works in cycles. Geologists knew that water must cycle through the ground, oceans, and clouds; astronomers were figuring out that giant clouds of <mark class="term" data-term="gas" data-term-def="The state of matter characterized by its non-condensed nature and ability to flow. Unlike liquids, molecules within a gas remain far&hellip;" data-term-url="/en/glossary/view/gas/8725">gas</mark> were giving birth to stars one by one; and chemists and biologists were starting to think in this way too. But with scientists now seeing how efficiently the assembly lines could produce cars and other big machines of the era, that cycling aspect of nature now moved to center stage. Might the assembly of stars from gas, clouds from water, or rocks from lava work like a kind of natural assembly line? Moreover, at the <mark class="term" data-term="microscopic" data-term-def="That which cannot be seen with the unaided eye and requires a tool (such as a microscope) to view. Many scientific&hellip;" data-term-url="/en/glossary/view/microscopic/10590">microscopic</mark> level within <mark class="term" data-term="cell" data-term-def="The basic structural unit of all living things." data-term-url="/en/glossary/view/cell/8286">cells</mark>, might the processing of <mark class="term" data-term="molecule" data-term-def="A particle formed by the chemical bonding of two or more atoms. The molecule is the smallest particle of a&hellip;" data-term-url="/en/glossary/view/molecule/1518">molecules</mark> also proceed in an organized fashion, as if moving through a tiny factory? </p> <div class="container margin-y-4 text-align-center"> <script async src="https://pagead2.googlesyndication.com/pagead/js/adsbygoogle.js?client=ca-pub-9561344156007092" crossorigin="anonymous"></script>  <ins class="adsbygoogle" style="display:inline-block;width:300px;height:250px" data-ad-client="ca-pub-9561344156007092" data-ad-slot="9090201191"></ins> <script> (adsbygoogle = window.adsbygoogle || []).push({}); </script> </div> <p>In the 1920s and 1930s, biochemists began discovering <em>enzymes</em> – <mark class="term" data-term="protein" data-term-def="Macromolecules that are polymers of individual amino acids arranged in a chain and joined together by peptide bonds (and so also&hellip;" data-term-url="/en/glossary/view/protein/1594">proteins</mark> in our <mark class="term" data-term="cell" data-term-def="The basic structural unit of all living things." data-term-url="/en/glossary/view/cell/8286">cells</mark> that catalyze <mark class="term" data-term="chemical reaction" data-term-def="A process in which atoms and molecules recombine by forming or breaking chemical bonds. Chemical reactions form new products that&hellip;" data-term-url="/en/glossary/view/chemical+reaction/1547">chemical reactions</mark>. Simple <mark class="term" data-term="reaction" data-term-def="A chemical change when substances come into contact with each other." data-term-url="/en/glossary/view/reaction/8263">reactions</mark> were worked out rather quickly, but more complicated chemical reactions were difficult to study. For example, if a person consumed <mark class="term" data-term="compound" data-term-def="A material formed by the chemical combination of elements in defined proportions. Compounds can be chemically decomposed into simpler substances." data-term-url="/en/glossary/view/compound/1517">compound</mark> A in the diet and then excreted compound E in the urine, how exactly did that happen? Was compound A transformed into E directly? Or, did the <mark class="term" data-term="process" data-term-def="Method, procedure; series of actions or steps." data-term-url="/en/glossary/view/process/8256">process</mark> occur in steps, like on an assembly line, with compounds B, C, and D, created along the way as intermediaries? Were there tributary lines generating various components that were needed at different points? </p> <p><section id="toc_1" class=""> <h2>Breaking down fuel for cellular energy</h2></p> <p>In many cases, the assembly line idea seemed to be the only one that made sense conceptually. Consider <em>glucose</em>, for example, commonly known as blood <mark class="term" data-term="sugar" data-term-def="A water-soluble crystalline carbohydrate. There are many types of sugar of varying degrees of sweetness, including fructose, which occurs naturally in&hellip;" data-term-url="/en/glossary/view/sugar/5309">sugar</mark>. (See the structure of a <mark class="term" data-term="glucose" data-term-def="The primary form of sugar stored in the human body for energy: C<sub>6</sub>H<sub>12</sub>O<sub>6</sub>." data-term-url="/en/glossary/view/glucose/8735">glucose</mark> <mark class="term" data-term="molecule" data-term-def="A particle formed by the chemical bonding of two or more atoms. The molecule is the smallest particle of a&hellip;" data-term-url="/en/glossary/view/molecule/1518">molecule</mark> in Figure 2.) By the turn of the 20<sup>th</sup> century, scientists knew that glucose was one of the main fuels, or sources of <mark class="term" data-term="energy" data-term-def="An abstract property defined as the capacity to do work. The basic forms of energy include chemical, electrical, mechanical, nuclear, and&hellip;" data-term-url="/en/glossary/view/energy/1497">energy</mark>, for animals, <mark class="term" data-term="bacteria" data-term-def="(plural of bacterium) A large group of one-celled organisms that are found almost everywhere." data-term-url="/en/glossary/view/bacteria/8679">bacteria</mark>, and <mark class="term" data-term="yeast" data-term-def="Eukaryotic, single-celled organisms belonging to the fungi kingdom." data-term-url="/en/glossary/view/yeast/10190">yeast</mark>. Setting glucose on fire in the laboratory produced carbon dioxide (CO<sub>2</sub>) and water (H<sub>2</sub>O), the same <mark class="term" data-term="compound" data-term-def="A material formed by the chemical combination of elements in defined proportions. Compounds can be chemically decomposed into simpler substances." data-term-url="/en/glossary/view/compound/1517">compounds</mark> that animals produced when they exercised. However, no one believed that <mark class="term" data-term="cell" data-term-def="The basic structural unit of all living things." data-term-url="/en/glossary/view/cell/8286">cells</mark> could have tiny fires inside. <mark class="term" data-term="observation" data-term-def="1. The act of noticing something. 2. A record of that which has been noticed." data-term-url="/en/glossary/view/observation/8255">Observations</mark> under the microscope certainly did not show any flames. Nevertheless, people do feel a burning sensation in their muscles during heavy exercise. </p>  <div class="figure"> <figure> <button class="lightbox-button" data-lightbox="image"> <img src="/img/library/modules/mid215/Image/VLObject-8587-150715070755.jpg" alt="Figure 2: D-glucose with the formula C6H12O6." /> </button> <figcaption> <p><strong>Figure 2</strong>: D-glucose with the formula C<sub>6</sub>H<sub>12</sub>O<sub>6</sub>.</p> </figcaption> </figure> </div> <p>Realizing that that the breakdown of body fuels probably took place in a controlled series of steps, researchers imagined <mark class="term" data-term="enzyme" data-term-def="Molecules produced by living organisms that help catalyze biochemical reactions. Enzymes are predominantly protein or protein-based molecules and are highly&hellip;" data-term-url="/en/glossary/view/enzyme/1595">enzymes</mark> working like factory workers, modifying different parts of a particular chemical <mark class="term" data-term="compound" data-term-def="A material formed by the chemical combination of elements in defined proportions. Compounds can be chemically decomposed into simpler substances." data-term-url="/en/glossary/view/compound/1517">compound</mark>. Like the workers on an assembly line, each enzyme would make one special change to each <mark class="term" data-term="molecule" data-term-def="A particle formed by the chemical bonding of two or more atoms. The molecule is the smallest particle of a&hellip;" data-term-url="/en/glossary/view/molecule/1518">molecule</mark>. The altered molecule would then be further modified, step by step by different enzymes, and this could happen not only during the breakdown of fuels; it also could happen during the production, or <em>synthesis</em>, of needed biological molecules using simpler chemicals as building material.</p><p>Glucose and other <mark class="term" data-term="sugar" data-term-def="A water-soluble crystalline carbohydrate. There are many types of sugar of varying degrees of sweetness, including fructose, which occurs naturally in&hellip;" data-term-url="/en/glossary/view/sugar/5309">sugars</mark> belong to the class of macromolecules called carbohydrates (see our <a href="/library/module_viewer.php?mid=61">Carbohydrates</a> module). Along with <mark class="term" data-term="lipids" data-term-def="A diverse group of organic molecules that contain long hydrocarbon chains or rings and are hydrophobic. Examples are fats, oils, waxes,&hellip;" data-term-url="/en/glossary/view/lipids/1599">lipids</mark> and <mark class="term" data-term="protein" data-term-def="Macromolecules that are polymers of individual amino acids arranged in a chain and joined together by peptide bonds (and so also&hellip;" data-term-url="/en/glossary/view/protein/1594">proteins</mark>, carbohydrates play a variety of roles in <mark class="term" data-term="organism" data-term-def="Any connected living system, such as an animal, plant, fungus, or bacterium. Organisms may be composed of a single cell or&hellip;" data-term-url="/en/glossary/view/organism/2171">organisms</mark>, and one role is providing <mark class="term" data-term="cell" data-term-def="The basic structural unit of all living things." data-term-url="/en/glossary/view/cell/8286">cells</mark> with <mark class="term" data-term="energy" data-term-def="An abstract property defined as the capacity to do work. The basic forms of energy include chemical, electrical, mechanical, nuclear, and&hellip;" data-term-url="/en/glossary/view/energy/1497">energy</mark>. While <mark class="term" data-term="glucose" data-term-def="The primary form of sugar stored in the human body for energy: C<sub>6</sub>H<sub>12</sub>O<sub>6</sub>." data-term-url="/en/glossary/view/glucose/8735">glucose</mark> and fats (a class of lipid) are the preferred energy <mark class="term" data-term="compound" data-term-def="A material formed by the chemical combination of elements in defined proportions. Compounds can be chemically decomposed into simpler substances." data-term-url="/en/glossary/view/compound/1517">compounds</mark>, proteins also can be used as fuel (see our modules <a href="/library/module_viewer.php?mid=207">Lipids</a> and <a href="/library/module_viewer.php?mid=62">Fats and Proteins</a> to learn more). Like the logs of a cabin, proteins are made from building blocks called <em>amino acids</em>, which can be used in multiple ways. They can be put together giving the cabin structure, but if needed they can also be burned as firewood to keep the cabin warm.</p> <div class="comprehension-checkpoint margin-y-4"> <h6 class="comprehension-checkpoint__header"> <span> <span class="icon icon-question"></span> </span> Comprehension Checkpoint </h6> <form name="cc8600"> <div class="form-entry"> <div class="form-entry__field"> <span class="form-entry__field__label">Which of the following works to break down or build up chemicals in the body?</span> <div class="form-entry__option"> <div class="form-entry__option__radio" data-answer="correct"> <label> <input id="q1-8600-0-option-a" name="quiz-option-8600" type="radio" value="enzymes" > <span class="option__label"> <span class="screen-reader-only">a.</span> enzymes </span> </label> <span class="quiz__response" id="response-8600-0"> <strong>Correct!</strong> </span> </div> <div class="form-entry__option__radio" data-answer="incorrect"> <label> <input id="q1-8600-1-option-b" name="quiz-option-8600" type="radio" value="glucose" > <span class="option__label"> <span class="screen-reader-only">b.</span> glucose </span> </label> <span class="quiz__response" id="response-8600-1"> <strong>Incorrect.</strong> </span> </div> </div> </div> </div> </form> </div> <p>Within structures called <em>mitochondria</em>, <mark class="term" data-term="microscopic" data-term-def="That which cannot be seen with the unaided eye and requires a tool (such as a microscope) to view. Many scientific&hellip;" data-term-url="/en/glossary/view/microscopic/10590">microscopic</mark> power plants in the <mark class="term" data-term="cell" data-term-def="The basic structural unit of all living things." data-term-url="/en/glossary/view/cell/8286">cells</mark> of <em>eukaryotes</em> (Figure 3), broken down bits of carbohydrates, fats, and <mark class="term" data-term="protein" data-term-def="Macromolecules that are polymers of individual amino acids arranged in a chain and joined together by peptide bonds (and so also&hellip;" data-term-url="/en/glossary/view/protein/1594">proteins</mark> all come together, feeding into a kind of reverse assembly line that goes around and around in a cycle. As the cycle goes around, the various energy-rich bits are incorporated at different stations. At the same time, the cycle sends other products away to other areas of the power plant. The pathway has many names, including the <em>citric <mark class="term" data-term="acid" data-term-def="Generally, a substance that reacts with bases to form a salt, several different definitions of acids have been proposed by different&hellip;" data-term-url="/en/glossary/view/acid/1573">acid</mark> cycle</em> and the <em>tricarboxylic acid cycle (TCA)</em>, because of the <mark class="term" data-term="compound" data-term-def="A material formed by the chemical combination of elements in defined proportions. Compounds can be chemically decomposed into simpler substances." data-term-url="/en/glossary/view/compound/1517">compounds</mark> that cycle within it. However, it’s also known as the <em>Krebs cycle</em>, for its discoverer Sir <mark class="term" data-term="Hans Adolf Krebs" data-term-def="1900–1981). One of thousands of scientists who fled Germany because of their Jewish heritage when the Nazis came to power, Krebs&hellip;" data-term-url="/en/glossary/view/Krebs%2C+Hans+Adolf/10184">Hans Adolf Krebs</mark>. </p>  <div class="figure"> <figure> <button class="lightbox-button" data-lightbox-src="/img/library/large_images/image_7395.png" data-lightbox="image"> <img src="/img/library/modules/mid215/Image/VLObject-7395-150714080738.png" alt="Figure 3: A diagram of a typical animal cell. Number 9 indicates the mitochondria structures in the cell." /> </button> <figcaption> <p><strong>Figure 3</strong>: A diagram of a typical animal cell. Number 9 indicates the mitochondria structures in the cell.</p> <span class="credit">image ©Kelvinsong</span> </figcaption> </figure> </div> </section> <section id="toc_2"> <h2>Cyclic assembly lines</h2><p>Born August 25, 1900, in Germany, Krebs earned his MD and began his <mark class="term" data-term="research" data-term-def="A study or an investigation." data-term-url="/en/glossary/view/research/8257">research</mark> career working with Otto Heinrich Warburg. A pioneer in biochemistry, Warburg was the inventor of the manometer, an instrument that could measure oxygen and other gasses in blood and other <mark class="term" data-term="fluid" data-term-def="Able to flow because the intermolecular forces allow the molecules to move around in relation to one another. Both liquids and&hellip;" data-term-url="/en/glossary/view/fluid/8724">fluids</mark>. Warburg was one of the lead biochemists worldwide, and in the early 20th century his country was the best place for emerging researchers like Krebs to get an education. Germany in this era was the global center of scientific research, especially in all areas of chemistry. So frequent were German publications in research journals that students aspiring to science worldwide would learn German just to be prepared to read the new articles. This was the world in which Krebs came of age.</p></section> <section id="toc2_1"><h3>Krebs discovers the urea cycle</h3><p>Using the Warburg manometer, Krebs made his first big discovery, the urea cycle (also called the <em>ornithine cycle</em>). By the late 1920s, it was well know that the breakdown of amino <mark class="term" data-term="acid" data-term-def="Generally, a substance that reacts with bases to form a salt, several different definitions of acids have been proposed by different&hellip;" data-term-url="/en/glossary/view/acid/1573">acids</mark> in animals must release ammonia (NH<sub>3</sub>). Krebs new that ammonia is toxic, yet somehow the body is able to convert it to <em>urea</em>, a chemical that is easily excreted in urine. Thinking assembly line style, Krebs and his student, Kurt Henseleit, came up with a hypothetical set of <mark class="term" data-term="reaction" data-term-def="A chemical change when substances come into contact with each other." data-term-url="/en/glossary/view/reaction/8263">reactions</mark>, beginning with the conversion of <em>ornithine</em> into another chemical by receiving a piece of the <mark class="term" data-term="amino acid" data-term-def="Biochemical molecules that contain at least one amine group (-NH<sub>2</sub>) and at least one carboxylic acid group (-COOH) and conform&hellip;" data-term-url="/en/glossary/view/amino+acid/1596">amino acids</mark> containing the ammonia. The manometer allowed Krebs to analyze samples of animal liver exposed to the intermediary chemicals that they suspected were made from ornithine. Krebs and Henseleit, were able to test and tweak their <mark class="term" data-term="hypothesis" data-term-def="From the Greek word <em>hypothesis</em> meaning assumption or the basis of an argument, a hypothesis is a proposal intended to explain&hellip;" data-term-url="/en/glossary/view/hypothesis/3727">hypothesis</mark>, reaction by reaction. The pathway of reactions was a cycle, because, after a bunch of steps, ornithine was re-created. As this happened, more and more ammonia was converted to urea. Thus, as long as <mark class="term" data-term="amino acid" data-term-url="/en/glossary/view/amino+acid" data-term-def="Biochemical molecules that contain at least one amine group (-NH<sub>2</sub>) and at least one carboxylic acid group (-COOH) and conform&hellip;">amino acids</mark> were continuously broken down in the liver, the urea cycle would spin around and around, removing ammonia so that it did not accumulate and kill the <mark class="term" data-term="organism" data-term-def="Any connected living system, such as an animal, plant, fungus, or bacterium. Organisms may be composed of a single cell or&hellip;" data-term-url="/en/glossary/view/organism/2171">organism</mark>. It was a milestone discovery that made Krebs world famous when he published his findings in 1932 (Figure 4).</p>  <div class="figure"> <figure> <button class="lightbox-button" data-lightbox-src="/img/library/large_images/image_8588.jpg" data-lightbox="image"> <img src="/img/library/modules/mid215/Image/VLObject-8588-150714080744.jpg" alt="Figure 4: Hans Adolf Krebs" /> </button> <figcaption> <p><strong>Figure 4</strong>: Hans Adolf Krebs</p> </figcaption> </figure> </div> <p>Soon after that he was fired. Like many other academics in Germany, Krebs was dismissed from his position when the Nazis came to power in 1933, either because they were Jewish, as Krebs was, or because they opposed the Nazis. Prior to 1933, Germany was a powerhouse in all areas of science with a plethora of <mark class="term" data-term="Nobel Prize" data-term-def="Awards made annually, beginning in 1901, from funds originally established by Alfred B. Nobel for outstanding achievement in physics, chemistry, medicine&hellip;" data-term-url="/en/glossary/view/Nobel+Prize/3843">Nobel prizes</mark> going to Germans. That abruptly ended with the rise of Adolf Hitler.</p><p>Krebs relocated to England, along with many other academics escaping from Nazi controlled lands. Although he was unable to bring most of his personal possessions, he did take most of his lab equipment, including the Warburg manometer that had proven so useful in unlocking the secrets of the urea cycle.</p> <div class="comprehension-checkpoint margin-y-4"> <h6 class="comprehension-checkpoint__header"> <span> <span class="icon icon-question"></span> </span> Comprehension Checkpoint </h6> <form name="cc8602"> <div class="form-entry"> <div class="form-entry__field"> <span class="form-entry__field__label">Krebs lost his job because</span> <div class="form-entry__option"> <div class="form-entry__option__radio" data-answer="correct"> <label> <input id="q1-8602-0-option-a" name="quiz-option-8602" type="radio" value="he was Jewish." > <span class="option__label"> <span class="screen-reader-only">a.</span> he was Jewish. </span> </label> <span class="quiz__response" id="response-8602-0"> <strong>Correct!</strong> </span> </div> <div class="form-entry__option__radio" data-answer="incorrect"> <label> <input id="q1-8602-1-option-b" name="quiz-option-8602" type="radio" value="his ideas were too far-fetched." > <span class="option__label"> <span class="screen-reader-only">b.</span> his ideas were too far-fetched. </span> </label> <span class="quiz__response" id="response-8602-1"> <strong>Incorrect.</strong> </span> </div> </div> </div> </div> </form> </div> </section> <section id="toc2_2"><h3>The glycolysis pathway: Embden and Meyerhof</h3><p>With the urea cycle behind him, Krebs wanted to focus on tracking what happened to carbohydrates in the <mark class="term" data-term="cell" data-term-def="The basic structural unit of all living things." data-term-url="/en/glossary/view/cell/8286">cell</mark>. While at the University of Sheffield, Krebs set up the manometer and started working out the chemistry. One of his major goals was to map out the ultimate fate of <mark class="term" data-term="glucose" data-term-def="The primary form of sugar stored in the human body for energy: C<sub>6</sub>H<sub>12</sub>O<sub>6</sub>." data-term-url="/en/glossary/view/glucose/8735">glucose</mark> in the presence of oxygen. By this time, the initial breakdown of glucose was already understood, step-by-step. Known as glycolysis, this initial <mark class="term" data-term="process" data-term-def="Method, procedure; series of actions or steps." data-term-url="/en/glossary/view/process/8256">process</mark> splits each glucose <mark class="term" data-term="molecule" data-term-def="A particle formed by the chemical bonding of two or more atoms. The molecule is the smallest particle of a&hellip;" data-term-url="/en/glossary/view/molecule/1518">molecule</mark> into two smaller molecules called <em>pyruvate</em>.</p><p>The steps of glycolysis were worked out by two biochemists, Gustav Embden and Otto Fritz Meyerhof. (A few years after Krebs, Meyerhof also fled Nazi Germany for being Jewish.) Unlike burning <mark class="term" data-term="glucose" data-term-def="The primary form of sugar stored in the human body for energy: C<sub>6</sub>H<sub>12</sub>O<sub>6</sub>." data-term-url="/en/glossary/view/glucose/8735">glucose</mark> to a crisp in the laboratory, the conversion of glucose to pyruvate in <mark class="term" data-term="cell" data-term-def="The basic structural unit of all living things." data-term-url="/en/glossary/view/cell/8286">cells</mark> is carefully controlled by <mark class="term" data-term="enzyme" data-term-def="Molecules produced by living organisms that help catalyze biochemical reactions. Enzymes are predominantly protein or protein-based molecules and are highly&hellip;" data-term-url="/en/glossary/view/enzyme/1595">enzymes</mark>. Each step in the Embden-Meyerhof glycolysis pathway has its own enzyme that performs a specialized procedure on one <mark class="term" data-term="molecule" data-term-def="A particle formed by the chemical bonding of two or more atoms. The molecule is the smallest particle of a&hellip;" data-term-url="/en/glossary/view/molecule/1518">molecule</mark> after another, like the factory worker at a particular workstation.</p> <div class="container margin-y-4 text-align-center"> <script async src="https://pagead2.googlesyndication.com/pagead/js/adsbygoogle.js?client=ca-pub-9561344156007092" crossorigin="anonymous"></script>  <ins class="adsbygoogle" style="display:inline-block;width:300px;height:250px" data-ad-client="ca-pub-9561344156007092" data-ad-slot="3321739899"></ins> <script> (adsbygoogle = window.adsbygoogle || []).push({}); </script> </div> <p>In the course of breaking down <mark class="term" data-term="glucose" data-term-def="The primary form of sugar stored in the human body for energy: C<sub>6</sub>H<sub>12</sub>O<sub>6</sub>." data-term-url="/en/glossary/view/glucose/8735">glucose</mark> into pyruvate, glycolysis provides the <mark class="term" data-term="cell" data-term-def="The basic structural unit of all living things." data-term-url="/en/glossary/view/cell/8286">cell</mark> with <em>some</em> <mark class="term" data-term="energy" data-term-def="An abstract property defined as the capacity to do work. The basic forms of energy include chemical, electrical, mechanical, nuclear, and&hellip;" data-term-url="/en/glossary/view/energy/1497">energy</mark>, and does not require oxygen (Figure 5). This is good, since many <mark class="term" data-term="organism" data-term-def="Any connected living system, such as an animal, plant, fungus, or bacterium. Organisms may be composed of a single cell or&hellip;" data-term-url="/en/glossary/view/organism/2171">organisms</mark> live in <mark class="term" data-term="environment" data-term-def="The conditions that surround and affect an organism." data-term-url="/en/glossary/view/environment/8270">environments</mark> where oxygen is not even available. In fact, today we know that the <mark class="term" data-term="enzyme" data-term-def="Molecules produced by living organisms that help catalyze biochemical reactions. Enzymes are predominantly protein or protein-based molecules and are highly&hellip;" data-term-url="/en/glossary/view/enzyme/1595">enzymes</mark> controlling glycolysis emerged extremely early in the history of life, before there was any oxygen <mark class="term" data-term="gas" data-term-def="The state of matter characterized by its non-condensed nature and ability to flow. Unlike liquids, molecules within a gas remain far&hellip;" data-term-url="/en/glossary/view/gas/8725">gas</mark> in Earth’s oceans or <mark class="term" data-term="atmosphere" data-term-def="The collective mass of gases that surrounds the Earth or another planet." data-term-url="/en/glossary/view/atmosphere/8529">atmosphere</mark>.</p>  <div class="figure"> <figure> <button class="lightbox-button" data-lightbox-src="/img/library/large_images/image_8589.jpg" data-lightbox="image"> <img src="/img/library/modules/mid215/Image/VLObject-8589-150714090706.jpg" alt="Figure 5: A diagram of the glycolysis process that occurs in the cytoplasm of a cell." /> </button> <figcaption> <p><strong>Figure 5</strong>: A diagram of the glycolysis process that occurs in the cytoplasm of a cell.</p> <span class="credit">image ©RegisFrey</span> </figcaption> </figure> </div> <p>The understanding of glycolysis left a big question: What happens to the pyruvate after it is produced from the breakdown of glucose? By Krebs’ time, it was known that the answer depended on whether or not oxygen was available. It was also known that certain microorganisms, as well as animal muscles, produce a chemical <mark class="term" data-term="compound" data-term-def="A material formed by the chemical combination of elements in defined proportions. Compounds can be chemically decomposed into simpler substances." data-term-url="/en/glossary/view/compound/1517">compound</mark> called <em>lactic acid</em>. The reason, it turns out, is that lactic <mark class="term" data-term="acid" data-term-def="Generally, a substance that reacts with bases to form a salt, several different definitions of acids have been proposed by different&hellip;" data-term-url="/en/glossary/view/acid/1573">acid</mark> is very similar to pyruvate. When no oxygen is available – or in <mark class="term" data-term="organism" data-term-def="Any connected living system, such as an animal, plant, fungus, or bacterium. Organisms may be composed of a single cell or&hellip;" data-term-url="/en/glossary/view/organism/2171">organisms</mark> that don't have the ability to use oxygen even if it is available – pyruvate is converted to lactic acid as a waste <mark class="term" data-term="product" data-term-def="The material that is formed as a result of a chemical reaction. Written on the right side of a chemical equation.&hellip;" data-term-url="/en/glossary/view/product/1569">product</mark>. This is what happens in muscle <mark class="term" data-term="cell" data-term-def="The basic structural unit of all living things." data-term-url="/en/glossary/view/cell/8286">cells</mark> during intensive exercise, especially in an individual who has not warmed up sufficiently. </p><p>However, as Krebs knew, something bigger must have been happening in <mark class="term" data-term="cell" data-term-def="The basic structural unit of all living things." data-term-url="/en/glossary/view/cell/8286">cells</mark> when oxygen <em>was</em> available. One reason warming up helps muscles is that it brings more oxygen into the muscle cells, allowing for conversion of pyruvate to something other than lactic <mark class="term" data-term="acid" data-term-def="Generally, a substance that reacts with bases to form a salt, several different definitions of acids have been proposed by different&hellip;" data-term-url="/en/glossary/view/acid/1573">acid</mark>. Oxygen, it turns out, allows cells to activate a highly efficient <mark class="term" data-term="system" data-term-def="A group of interacting, interrelated or interdependent components that form a complex whole. The size of the system is defined for&hellip;" data-term-url="/en/glossary/view/system/3904">system</mark> to break down fuel to the ultimate end product: carbon dioxide (CO2).</p> <div class="comprehension-checkpoint margin-y-4"> <h6 class="comprehension-checkpoint__header"> <span> <span class="icon icon-question"></span> </span> Comprehension Checkpoint </h6> <form name="cc8605"> <div class="form-entry"> <div class="form-entry__field"> <span class="form-entry__field__label">Pyruvate is similar to</span> <div class="form-entry__option"> <div class="form-entry__option__radio" data-answer="correct"> <label> <input id="q1-8605-0-option-a" name="quiz-option-8605" type="radio" value="lactic acid." > <span class="option__label"> <span class="screen-reader-only">a.</span> lactic acid. </span> </label> <span class="quiz__response" id="response-8605-0"> <strong>Correct!</strong> </span> </div> <div class="form-entry__option__radio" data-answer="incorrect"> <label> <input id="q1-8605-1-option-b" name="quiz-option-8605" type="radio" value="oxygen." > <span class="option__label"> <span class="screen-reader-only">b.</span> oxygen. </span> </label> <span class="quiz__response" id="response-8605-1"> <strong>Incorrect.</strong> </span> </div> </div> </div> </div> </form> </div> </section> <section id="toc2_3"><h3>The Krebs cycle</h3><p>When 19<sup>th</sup> century researchers burned <mark class="term" data-term="sugar" data-term-def="A water-soluble crystalline carbohydrate. There are many types of sugar of varying degrees of sweetness, including fructose, which occurs naturally in&hellip;" data-term-url="/en/glossary/view/sugar/5309">sugar</mark> in the lab, they knew that oxygen was required to fuel the fire. This suggested that the <mark class="term" data-term="metabolism" data-term-def="A sequence of biochemical reactions in living organisms that converts food into energy used to drive other biological processes. Also, the&hellip;" data-term-url="/en/glossary/view/metabolism/1606">metabolism</mark> of <mark class="term" data-term="glucose" data-term-def="The primary form of sugar stored in the human body for energy: C<sub>6</sub>H<sub>12</sub>O<sub>6</sub>." data-term-url="/en/glossary/view/glucose/8735">glucose</mark> also required oxygen, at least when glucose was broken down all the way to CO<sub>2</sub> and H<sub>2</sub>O. Krebs knew that the key to understanding how most of the <mark class="term" data-term="energy" data-term-def="An abstract property defined as the capacity to do work. The basic forms of energy include chemical, electrical, mechanical, nuclear, and&hellip;" data-term-url="/en/glossary/view/energy/1497">energy</mark> was extracted from glucose was to understand what happened to pyruvate when oxygen was present. Clearly, it was something different than what happened in the absence of oxygen. Think of a fork in the road at the point that pyruvate is created from the breakdown of glucose. Without oxygen, pyruvate is converted to lactic <mark class="term" data-term="acid" data-term-def="Generally, a substance that reacts with bases to form a salt, several different definitions of acids have been proposed by different&hellip;" data-term-url="/en/glossary/view/acid/1573">acid</mark>, but the presence of oxygen opens the gate to an alternate route that ends, not with lactic acid, but with CO<sub>2</sub>. All that Krebs needed to do was figure out the various steps that occurred along the way. Luckily, he still had his handy manometer, and luckily, he didn't need to start from scratch. A few <mark class="term" data-term="reaction" data-term-def="A chemical change when substances come into contact with each other." data-term-url="/en/glossary/view/reaction/8263">reactions</mark> that Krebs was about to discover as steps in his new cycle were known already as independent reactions from <mark class="term" data-term="research" data-term-def="A study or an investigation." data-term-url="/en/glossary/view/research/8257">research</mark> of an older biochemist, Albert Szent-Györgyi. It was Krebs who postulated that the reactions might be connected in a cycle, just like the reactions of the urea cycle that he'd discovered back in Germany.</p><p>Krebs’ <mark class="term" data-term="research" data-term-def="A study or an investigation." data-term-url="/en/glossary/view/research/8257">research</mark> <mark class="term" data-term="method" data-term-def="A procedure or process; a systematic way of performing a task or conducting research." data-term-url="/en/glossary/view/method/8238">method</mark> was to let slices of beef liver soak in <mark class="term" data-term="solution" data-term-def="A mixture of more than one substance with properties that do not vary within the sample. Commonly used to describe a&hellip;" data-term-url="/en/glossary/view/solution/1571">solutions</mark> of various chemicals. Using the Warburg manometer, Krebs could then see how the unidentified liver <mark class="term" data-term="enzyme" data-term-def="Molecules produced by living organisms that help catalyze biochemical reactions. Enzymes are predominantly protein or protein-based molecules and are highly&hellip;" data-term-url="/en/glossary/view/enzyme/1595">enzymes</mark> would change the different chemicals in the solutions. Testing the <mark class="term" data-term="reaction" data-term-def="A chemical change when substances come into contact with each other." data-term-url="/en/glossary/view/reaction/8263">reactions</mark> one by one, he discovered that the breakdown of carbohydrates, <mark class="term" data-term="lipids" data-term-def="A diverse group of organic molecules that contain long hydrocarbon chains or rings and are hydrophobic. Examples are fats, oils, waxes,&hellip;" data-term-url="/en/glossary/view/lipids/1599">lipids</mark>, and <mark class="term" data-term="protein" data-term-def="Macromolecules that are polymers of individual amino acids arranged in a chain and joined together by peptide bonds (and so also&hellip;" data-term-url="/en/glossary/view/protein/1594">proteins</mark> did indeed proceed in a cyclic fashion. Bigger and more complex than the urea cycle, this cycle turned out to be the central route of all metabolic activity in the <mark class="term" data-term="cell" data-term-def="The basic structural unit of all living things." data-term-url="/en/glossary/view/cell/8286">cell</mark>. Krebs identified the cycle’s reactions by 1937, although he tweaked it over the course of the following decade. Part of that tweaking led him to discover yet one more cycle, a little one called the <em>glyoxylate cycle</em> that acted as a bypass route for a section of the Krebs cycle.</p> <div class="comprehension-checkpoint margin-y-4"> <h6 class="comprehension-checkpoint__header"> <span> <span class="icon icon-question"></span> </span> Comprehension Checkpoint </h6> <form name="cc8606"> <div class="form-entry"> <div class="form-entry__field"> <span class="form-entry__field__label">Krebs is famous for discovering</span> <div class="form-entry__option"> <div class="form-entry__option__radio" data-answer="incorrect"> <label> <input id="q1-8606-0-option-a" name="quiz-option-8606" type="radio" value="each reaction in the cycle that would be named after him." > <span class="option__label"> <span class="screen-reader-only">a.</span> each reaction in the cycle that would be named after him. </span> </label> <span class="quiz__response" id="response-8606-0"> <strong>Incorrect.</strong> </span> </div> <div class="form-entry__option__radio" data-answer="correct"> <label> <input id="q1-8606-1-option-b" name="quiz-option-8606" type="radio" value="that the reactions proceeded in a cyclic fashion." > <span class="option__label"> <span class="screen-reader-only">b.</span> that the reactions proceeded in a cyclic fashion. </span> </label> <span class="quiz__response" id="response-8606-1"> <strong>Correct!</strong> </span> </div> </div> </div> </div> </form> </div> </section> <section id="toc_3"> <h2>Moving molecules from workstation to workstation</h2><p>Thinking about the Krebs cycle in terms of workstations is a way to remember broadly what types of chemical <mark class="term" data-term="compound" data-term-def="A material formed by the chemical combination of elements in defined proportions. Compounds can be chemically decomposed into simpler substances." data-term-url="/en/glossary/view/compound/1517">compounds</mark> enter the cycle at certain points, what they are changed into as a result of entering the cycle, and what compounds then leave the cycle at different points. Since it is a circular pathway, there is no beginning or end. For the sake of learning the Krebs cycle, however, the “first” <mark class="term" data-term="reaction" data-term-def="A chemical change when substances come into contact with each other." data-term-url="/en/glossary/view/reaction/8263">reaction</mark> – is the conversion of oxaloacetate into citrate. While there are several differences between oxaloacetate and citrate, the most important difference is that citrate is the bigger <mark class="term" data-term="molecule" data-term-def="A particle formed by the chemical bonding of two or more atoms. The molecule is the smallest particle of a&hellip;" data-term-url="/en/glossary/view/molecule/1518">molecule</mark>. Its “backbone” is built of six carbon <mark class="term" data-term="atom" data-term-def="The smallest unit of an element that retains the chemical properties of the element. Atoms can exist alone or in&hellip;" data-term-url="/en/glossary/view/atom/1509">atoms</mark>, while oxaloacetate has just four. What is the source of the two extra carbon atoms? The chemical equations that Krebs wrote out told him that the source of the two carbon atoms could be acetate, which had to come from outside the cycle. Mixing oxaloacetate with his liver specimens, Krebs could test his <mark class="term" data-term="hypothesis" data-term-def="From the Greek word <em>hypothesis</em> meaning assumption or the basis of an argument, a hypothesis is a proposal intended to explain&hellip;" data-term-url="/en/glossary/view/hypothesis/3727">hypothesis</mark>. Using the Warburg manometer to measure changes in oxygen and CO<sub>2</sub> in his mixture, Krebs could tell when the cycle was turning around. The liver specimens supplied the <mark class="term" data-term="enzyme" data-term-def="Molecules produced by living organisms that help catalyze biochemical reactions. Enzymes are predominantly protein or protein-based molecules and are highly&hellip;" data-term-url="/en/glossary/view/enzyme/1595">enzymes</mark> that controlled the reactions, including the enzyme that adds two carbons to oxaloacetate, forming citrate. This meant that Krebs could add different carbon sources, one by one, to the mixture, and see which, if any allowed the cycle to go around. Doing this, he confirmed that acetate was the needed substrate. In a test tube, as in the <mark class="term" data-term="cell" data-term-def="The basic structural unit of all living things." data-term-url="/en/glossary/view/cell/8286">cells</mark> of his liver specimens, acetate had to be supplied from outside the cycle. Otherwise, the cycle would come to a halt.</p><p>The discovery that acetate joined a cycle of <mark class="term" data-term="reaction" data-term-def="A chemical change when substances come into contact with each other." data-term-url="/en/glossary/view/reaction/8263">reactions</mark> that led to the extraction of <mark class="term" data-term="energy" data-term-def="An abstract property defined as the capacity to do work. The basic forms of energy include chemical, electrical, mechanical, nuclear, and&hellip;" data-term-url="/en/glossary/view/energy/1497">energy</mark> from food was a major insight, because both <mark class="term" data-term="sugar" data-term-def="A water-soluble crystalline carbohydrate. There are many types of sugar of varying degrees of sweetness, including fructose, which occurs naturally in&hellip;" data-term-url="/en/glossary/view/sugar/5309">sugars</mark> and fats – the major sources of dietary energy for all <mark class="term" data-term="organism" data-term-def="Any connected living system, such as an animal, plant, fungus, or bacterium. Organisms may be composed of a single cell or&hellip;" data-term-url="/en/glossary/view/organism/2171">organisms</mark> – can be broken down to acetate, as can some amino <mark class="term" data-term="acid" data-term-def="Generally, a substance that reacts with bases to form a salt, several different definitions of acids have been proposed by different&hellip;" data-term-url="/en/glossary/view/acid/1573">acids</mark>. Krebs realized that pyruvate, the <mark class="term" data-term="product" data-term-def="The material that is formed as a result of a chemical reaction. Written on the right side of a chemical equation.&hellip;" data-term-url="/en/glossary/view/product/1569">product</mark> of the initial breakdown of <mark class="term" data-term="glucose" data-term-def="The primary form of sugar stored in the human body for energy: C<sub>6</sub>H<sub>12</sub>O<sub>6</sub>." data-term-url="/en/glossary/view/glucose/8735">glucose</mark>, is very similar to acetate, except that pyruvate has one additional carbon. If the extra carbon from pyruvate were removed, the remaining <mark class="term" data-term="molecule" data-term-def="A particle formed by the chemical bonding of two or more atoms. The molecule is the smallest particle of a&hellip;" data-term-url="/en/glossary/view/molecule/1518">molecule</mark> was easily converted to acetate.</p> <div class="comprehension-checkpoint margin-y-4"> <h6 class="comprehension-checkpoint__header"> <span> <span class="icon icon-question"></span> </span> Comprehension Checkpoint </h6> <form name="cc8607"> <div class="form-entry"> <div class="form-entry__field"> <span class="form-entry__field__label">Most of our energy comes from the _____ in our diet.</span> <div class="form-entry__option"> <div class="form-entry__option__radio" data-answer="incorrect"> <label> <input id="q1-8607-0-option-a" name="quiz-option-8607" type="radio" value="protein" > <span class="option__label"> <span class="screen-reader-only">a.</span> protein </span> </label> <span class="quiz__response" id="response-8607-0"> <strong>Incorrect.</strong> </span> </div> <div class="form-entry__option__radio" data-answer="correct"> <label> <input id="q1-8607-1-option-b" name="quiz-option-8607" type="radio" value="sugars and fats" > <span class="option__label"> <span class="screen-reader-only">b.</span> sugars and fats </span> </label> <span class="quiz__response" id="response-8607-1"> <strong>Correct!</strong> </span> </div> </div> </div> </div> </form> </div> <p>The acetate <mark class="term" data-term="molecule" data-term-def="A particle formed by the chemical bonding of two or more atoms. The molecule is the smallest particle of a&hellip;" data-term-url="/en/glossary/view/molecule/1518">molecule</mark> itself is small, and highly diffusible, so it must be chaperoned around the <mark class="term" data-term="cell" data-term-def="The basic structural unit of all living things." data-term-url="/en/glossary/view/cell/8286">cell</mark> by a much larger carrier molecule, called co-enzyme A (Co-A). Once the two-carbon acetate is linked up with the four-carbon oxaloacetate, however, the Co-A is free to pick up another acetate and repeat the <mark class="term" data-term="process" data-term-def="Method, procedure; series of actions or steps." data-term-url="/en/glossary/view/process/8256">process</mark>. Meanwhile, the cell has a new molecule of citrate with its six carbon <mark class="term" data-term="atom" data-term-def="The smallest unit of an element that retains the chemical properties of the element. Atoms can exist alone or in&hellip;" data-term-url="/en/glossary/view/atom/1509">atoms</mark>.</p><p>Realizing that he was dealing with a cyclic pathway, Krebs discovered that citrate does not remain for very long. After its shape is changed around, it is cut down to a five-carbon <mark class="term" data-term="molecule" data-term-def="A particle formed by the chemical bonding of two or more atoms. The molecule is the smallest particle of a&hellip;" data-term-url="/en/glossary/view/molecule/1518">molecule</mark> and then again to a four-carbon molecule, which then is modified several times until oxaloacetate is produced, all ready to be combined with a new acetate to produce more citrate, and the cycle goes around another time. It’s a true cycle, because the <mark class="term" data-term="product" data-term-def="The material that is formed as a result of a chemical reaction. Written on the right side of a chemical equation.&hellip;" data-term-url="/en/glossary/view/product/1569">product</mark> of the cycle – oxaloacetate – is also the first ingredient for the next cycle. (See Figure 6.)</p>  <div class="figure"> <figure> <button class="lightbox-button" data-lightbox-src="/img/library/large_images/image_8590.jpg" data-lightbox="image"> <img src="/img/library/modules/mid215/Image/VLObject-8590-150714090756.jpg" alt="Figure 6: The detailed Krebs cycle." /> </button> <figcaption> <p><strong>Figure 6</strong>: The detailed Krebs cycle.</p> <span class="credit">image ©Agrotman</span> </figcaption> </figure> </div> <p>Where do the carbon <mark class="term" data-term="atom" data-term-def="The smallest unit of an element that retains the chemical properties of the element. Atoms can exist alone or in&hellip;" data-term-url="/en/glossary/view/atom/1509">atoms</mark> go when they get cut off as the cycle goes from six to five and back to four-carbon units? The cycle occurs only in <mark class="term" data-term="aerobic" data-term-def="An organism or cell that requires oxygen to carry out its metabolic processes; a process that requires oxygen." data-term-url="/en/glossary/view/aerobic/6538">aerobic</mark> <mark class="term" data-term="organism" data-term-def="Any connected living system, such as an animal, plant, fungus, or bacterium. Organisms may be composed of a single cell or&hellip;" data-term-url="/en/glossary/view/organism/2171">organisms</mark>, life forms that use oxygen, and using the Warburg manometer Krebs discovered that a portion of the carbon removed from the main <mark class="term" data-term="compound" data-term-def="A material formed by the chemical combination of elements in defined proportions. Compounds can be chemically decomposed into simpler substances." data-term-url="/en/glossary/view/compound/1517">compound</mark> was combining with oxygen atoms to generate CO<sub>2</sub>.</p> <div class="comprehension-checkpoint margin-y-4"> <h6 class="comprehension-checkpoint__header"> <span> <span class="icon icon-question"></span> </span> Comprehension Checkpoint </h6> <form name="cc8609"> <div class="form-entry"> <div class="form-entry__field"> <span class="form-entry__field__label">The Krebs cycle occurs in</span> <div class="form-entry__option"> <div class="form-entry__option__radio" data-answer="incorrect"> <label> <input id="q1-8609-0-option-a" name="quiz-option-8609" type="radio" value="all living organisms." > <span class="option__label"> <span class="screen-reader-only">a.</span> all living organisms. </span> </label> <span class="quiz__response" id="response-8609-0"> <strong>Incorrect.</strong> </span> </div> <div class="form-entry__option__radio" data-answer="correct"> <label> <input id="q1-8609-1-option-b" name="quiz-option-8609" type="radio" value="only those lifeforms that use oxygen." > <span class="option__label"> <span class="screen-reader-only">b.</span> only those lifeforms that use oxygen. </span> </label> <span class="quiz__response" id="response-8609-1"> <strong>Correct!</strong> </span> </div> </div> </div> </div> </form> </div> </section> <section id="toc2_4"><h3>ATP, the cellular <mark class="term" data-term="energy" data-term-def="An abstract property defined as the capacity to do work. The basic forms of energy include chemical, electrical, mechanical, nuclear, and&hellip;" data-term-url="/en/glossary/view/energy/1497">energy</mark> currency</h3><p>The <mark class="term" data-term="energy" data-term-def="An abstract property defined as the capacity to do work. The basic forms of energy include chemical, electrical, mechanical, nuclear, and&hellip;" data-term-url="/en/glossary/view/energy/1497">energy</mark> contained in fatty <mark class="term" data-term="acid" data-term-def="Generally, a substance that reacts with bases to form a salt, several different definitions of acids have been proposed by different&hellip;" data-term-url="/en/glossary/view/acid/1573">acids</mark>, <mark class="term" data-term="glucose" data-term-def="The primary form of sugar stored in the human body for energy: C<sub>6</sub>H<sub>12</sub>O<sub>6</sub>." data-term-url="/en/glossary/view/glucose/8735">glucose</mark>, and <mark class="term" data-term="amino acid" data-term-def="Biochemical molecules that contain at least one amine group (-NH<sub>2</sub>) and at least one carboxylic acid group (-COOH) and conform&hellip;" data-term-url="/en/glossary/view/amino+acid/1596">amino acids</mark> is held in the various chemical <mark class="term" data-term="bond" data-term-def="The force that holds together units such as atoms or molecules. <br> <b>[verb]</b> To hold or fasten units such as atoms or molecules together." data-term-url="/en/glossary/view/bond/8297">bonds</mark> that keep the individual <mark class="term" data-term="atom" data-term-def="The smallest unit of an element that retains the chemical properties of the element. Atoms can exist alone or in&hellip;" data-term-url="/en/glossary/view/atom/1509">atoms</mark> together. The most common way that we store the energy harvested from those <mark class="term" data-term="chemical bond" data-term-def="A link between atoms. See <a href="http://www.visionlearning.com/en/glossary/index/I#term-1550">ionic bond</a> and <a href="http://www.visionlearning.com/en/glossary/index/C#term-1545">covalent bond</a>." data-term-url="/en/glossary/view/chemical+bond/1548">chemical bonds</mark> is within a <mark class="term" data-term="molecule" data-term-def="A particle formed by the chemical bonding of two or more atoms. The molecule is the smallest particle of a&hellip;" data-term-url="/en/glossary/view/molecule/1518">molecule</mark> called <mark class="term" data-term="adenosine triphosphate" data-term-url="/en/glossary/view/adenosine+triphosphate" data-term-def="(ATP) Molecules that provide energy for important chemical reactions within the cell; the main energy currency of the cell.">ATP</mark>. <mark class="term" data-term="ATP" data-term-def="Adenosine triphosphate. Molecules that provide energy for important chemical reactions within the cell." data-term-url="/en/glossary/view/ATP/6545">ATP</mark> is often called the cellular currency of energy. Just like economic currency, such as a dollar bill, the energy currency of ATP can be used to “purchase” whatever <mark class="term" data-term="reaction" data-term-def="A chemical change when substances come into contact with each other." data-term-url="/en/glossary/view/reaction/8263">reactions</mark> or activities the <mark class="term" data-term="cell" data-term-def="The basic structural unit of all living things." data-term-url="/en/glossary/view/cell/8286">cell</mark> needs to perform. Energy <mark class="term" data-term="metabolism" data-term-def="A sequence of biochemical reactions in living organisms that converts food into energy used to drive other biological processes. Also, the&hellip;" data-term-url="/en/glossary/view/metabolism/1606">metabolism</mark> also depends on a chemical <mark class="term" data-term="compound" data-term-def="A material formed by the chemical combination of elements in defined proportions. Compounds can be chemically decomposed into simpler substances." data-term-url="/en/glossary/view/compound/1517">compound</mark> called GTP, which is nearly the same as ATP. Continuing with the dollar bill analogy, one can imagine GTP as a silver dollar coin. It’s not encountered as often as the normal paper dollar, but it has the same <mark class="term" data-term="value" data-term-def="A number that is assigned based on measurement or a calculation. In mathematics, an unknown value that is commonly represented by&hellip;" data-term-url="/en/glossary/view/value/8254">value</mark> and the two can be exchanged easily. As biological fuel such as glucose is broken down, ATP and GTP molecules are produced at different points in the assembly line.</p><p>Krebs found that the breakdown of <mark class="term" data-term="sugar" data-term-def="A water-soluble crystalline carbohydrate. There are many types of sugar of varying degrees of sweetness, including fructose, which occurs naturally in&hellip;" data-term-url="/en/glossary/view/sugar/5309">sugars</mark> and fats into CO<sub>2</sub>, through a cycle of many <mark class="term" data-term="chemical reaction" data-term-def="A process in which atoms and molecules recombine by forming or breaking chemical bonds. Chemical reactions form new products that&hellip;" data-term-url="/en/glossary/view/chemical+reaction/1547">chemical reactions</mark>, produced <mark class="term" data-term="adenosine triphosphate" data-term-url="/en/glossary/view/adenosine+triphosphate" data-term-def="(ATP) Molecules that provide energy for important chemical reactions within the cell; the main energy currency of the cell.">ATP</mark> and GTP that the <mark class="term" data-term="cell" data-term-def="The basic structural unit of all living things." data-term-url="/en/glossary/view/cell/8286">cell</mark> could use to drive all sorts of <mark class="term" data-term="reaction" data-term-def="A chemical change when substances come into contact with each other." data-term-url="/en/glossary/view/reaction/8263">reactions</mark> and activities. However, something was clearly missing. For one thing, the amount of <mark class="term" data-term="ATP" data-term-def="Adenosine triphosphate. Molecules that provide energy for important chemical reactions within the cell." data-term-url="/en/glossary/view/ATP/6545">ATP</mark> and GTP was much smaller than he predicted. Scientists knew how many calories sugars and fats provided and most of that <mark class="term" data-term="energy" data-term-def="An abstract property defined as the capacity to do work. The basic forms of energy include chemical, electrical, mechanical, nuclear, and&hellip;" data-term-url="/en/glossary/view/energy/1497">energy</mark> was still unaccounted for in the reactions that Krebs discovered. Secondly, oxygen was not directly required for any of the reactions that Krebs discovered. Why was oxygen so important for harvesting the energy of sugars and fats if it wasn't required in their breakdown?</p></section> <section id="toc_4"> <h2>Two new energy carriers</h2><p>Looking more closely at chemical <mark class="term" data-term="bond" data-term-def="The force that holds together units such as atoms or molecules. <br> <b>[verb]</b> To hold or fasten units such as atoms or molecules together." data-term-url="/en/glossary/view/bond/8297">bonds</mark> of food <mark class="term" data-term="molecule" data-term-def="A particle formed by the chemical bonding of two or more atoms. The molecule is the smallest particle of a&hellip;" data-term-url="/en/glossary/view/molecule/1518">molecules</mark>, the <mark class="term" data-term="energy" data-term-def="An abstract property defined as the capacity to do work. The basic forms of energy include chemical, electrical, mechanical, nuclear, and&hellip;" data-term-url="/en/glossary/view/energy/1497">energy</mark> that is first present in those bonds is actually carried by the <mark class="term" data-term="electron" data-term-def="A subatomic particle with a negative charge of 1.60 × 10<sup>-19</sup> coulombs and a mass of 9.11 × 10<sup>-31</sup> kg. Electrons&hellip;" data-term-url="/en/glossary/view/electron/852">electrons</mark> that form the <mark class="term" data-term="covalent bond" data-term-def="A very strong chemical bond formed by the sharing of a pair of electrons. Multiple covalent bonds can be formed&hellip;" data-term-url="/en/glossary/view/covalent+bond/1550">covalent bonds</mark>. Each bond is made by a pair of electrons, and depending on how the various <mark class="term" data-term="atom" data-term-def="The smallest unit of an element that retains the chemical properties of the element. Atoms can exist alone or in&hellip;" data-term-url="/en/glossary/view/atom/1509">atoms</mark> are arranged, the electron pairs can hold different amounts of energy. In the course of <mark class="term" data-term="chemical reaction" data-term-def="A process in which atoms and molecules recombine by forming or breaking chemical bonds. Chemical reactions form new products that&hellip;" data-term-url="/en/glossary/view/chemical+reaction/1547">chemical reactions</mark> that harvest the energy from the food molecules to form <mark class="term" data-term="adenosine triphosphate" data-term-url="/en/glossary/view/adenosine+triphosphate" data-term-def="(ATP) Molecules that provide energy for important chemical reactions within the cell; the main energy currency of the cell.">ATP</mark> and GTP, the energy-holding electron pairs are physically transferred from one chemical <mark class="term" data-term="compound" data-term-def="A material formed by the chemical combination of elements in defined proportions. Compounds can be chemically decomposed into simpler substances." data-term-url="/en/glossary/view/compound/1517">compound</mark> to another, and various compounds that carry the electrons around are called <em>electron carriers</em>.</p><p>During the course of the Krebs cycle, two <mark class="term" data-term="compound" data-term-def="A material formed by the chemical combination of elements in defined proportions. Compounds can be chemically decomposed into simpler substances." data-term-url="/en/glossary/view/compound/1517">compounds</mark> are produced that do not feed back into the cycle. They are not <mark class="term" data-term="adenosine triphosphate" data-term-url="/en/glossary/view/adenosine+triphosphate" data-term-def="(ATP) Molecules that provide energy for important chemical reactions within the cell; the main energy currency of the cell.">ATP</mark> or GTP, and their function was not obvious to Krebs. These compounds were NADH and FADH<sub>2</sub>, made from their precursors NAD<sup>+</sup> and FADH<sup>+</sup>, as shown in Figure 7 (learn more about these compounds in our <a href="https://www.visionlearning.com/en/library/Biology/2/Energy-Metabolism-II/225">Energy <mark class="term" data-term="metabolism" data-term-def="A sequence of biochemical reactions in living organisms that converts food into energy used to drive other biological processes. Also, the&hellip;" data-term-url="/en/glossary/view/metabolism/1606">Metabolism</mark> II</a> module). Krebs knew that NADH was also made in glycolysis, so he suspected that finding out what they do would probably answer the question of where the rest of the <mark class="term" data-term="ATP" data-term-def="Adenosine triphosphate. Molecules that provide energy for important chemical reactions within the cell." data-term-url="/en/glossary/view/ATP/6545">ATP</mark> in cellular <mark class="term" data-term="respiration" data-term-def="The physical and chemical processes by which an organism supplies its cells and tissues with the oxygen needed for metabolism and&hellip;" data-term-url="/en/glossary/view/respiration/2195">respiration</mark> comes from.</p>  <div class="figure"> <figure> <button class="lightbox-button" data-lightbox="image"> <img src="/img/library/modules/mid215/Image/VLObject-8591-150715060733.jpg" alt="Figure 7: A diagram of the action inside a mitochondria, showing the Krebs cycle (also called the citric acid cycle) and the electron transport chain." /> </button> <figcaption> <p><strong>Figure 7</strong>: A diagram of the action inside a mitochondria, showing the Krebs cycle (also called the citric acid cycle) and the electron transport chain.</p> <span class="credit">image ©RegisFrey</span> </figcaption> </figure> </div> <p>The discovery of the Krebs cycle would earn <mark class="term" data-term="Hans Adolf Krebs" data-term-def="1900–1981). One of thousands of scientists who fled Germany because of their Jewish heritage when the Nazis came to power, Krebs&hellip;" data-term-url="/en/glossary/view/Krebs%2C+Hans+Adolf/10184">Hans Adolf Krebs</mark> the <mark class="term" data-term="Nobel Prize" data-term-def="Awards made annually, beginning in 1901, from funds originally established by Alfred B. Nobel for outstanding achievement in physics, chemistry, medicine&hellip;" data-term-url="/en/glossary/view/Nobel+Prize/3843">Nobel Prize</mark> in Physiology and Medicine in 1953, and five years later a knighthood. Even though Krebs did not discover the next phase of cellular <mark class="term" data-term="respiration" data-term-def="The physical and chemical processes by which an organism supplies its cells and tissues with the oxygen needed for metabolism and&hellip;" data-term-url="/en/glossary/view/respiration/2195">respiration</mark> – oxidative phosphorylation - his work with the urea cycle and the Krebs cycle probably helped to inspire those discoveries, because, as it turned out, oxidative phosphorylation was also a kind of assembly line. </p></section> <footer class="module__main__footer"> <hr class="border-color-dark"> <p class="citation"> <em> David Warmflash, MD, Nathan H Lents, Ph.D. “Energy Metabolism I” Visionlearning Vol. BIO-4 (3), 2015. </em> </p>  <div class="title-list" name="further"> <p class="h6 title-list__title"> Further Reading </p> <ul class="grid grid--column-2--md grid--column-3--md gap-1"> <li> <a class="no-hover-focus height-100" href="/en/library/Biology/2/Energy-Metabolism-II/225"> <article class="flex-row align-items-center flex-column--md align-items-start--md height-100 theme-light padding-2 gap-2"> <div class="width-30 width-auto--md"> <img class="border-radius box-shadow-1" src="/img/library/moduleImages/featured_image_225-23061209062951.jpeg" alt="Energy Metabolism II"> </div> <div class="flex-grow-shrink"> <h2 class="h6 font-weight-normal"> Energy Metabolism II: <em>The Generation of ATP</em> </h2> </div> </article> </a> </li> <li> <a class="no-hover-focus height-100" href="/en/library/Biology/2/Carbohydrates/61"> <article class="flex-row align-items-center flex-column--md align-items-start--md height-100 theme-light padding-2 gap-2"> <div class="width-30 width-auto--md"> <img class="border-radius box-shadow-1" src="/img/library/moduleImages/featured_image_61-23061209062220.jpeg" alt="Carbohydrates"> </div> <div class="flex-grow-shrink"> <h2 class="h6 font-weight-normal"> Carbohydrates: <em>Simple sugars and complex carbohydrates</em> </h2> </div> </article> </a> </li> </ul> </div> </footer> </div>    </article> </div> </div> </main>   <footer class="position-relative box-shadow-1 font-size-md" id="global-footer"> <h2 class="screen-reader-only">Page Footer</h2> <div class="back-to-top"> <div class="container wide"> <button class="button button--has-icon font-size-sm"> <span class="icon icon-arrow-up"></span> <span class="button__text">Back to top</span> </button> </div> </div> <div class="container wide padding-y-2"> <div class="grid grid--column-2--md grid--column-4--lg gap-4 grid--divider--fill-x"> <nav> <ul class="nav font-weight-bold"> <li> <a href="/en/library" title="Readings & quizzes"> Library </a> </li> <li> <a href="/en/glossary" title="Science terms"> Glossary </a> </li> <li> <a href="/en/classroom" title="Courses & bookmarks"> Classroom </a> </li> </ul> </nav> <nav> <ul class="nav"> <li><a href="/en/about">About</a></li> <li><a href="/en/help">Contact</a></li> <li><a href="/en/about/jobs">Jobs</a></li> <li><a href="/en/help/faq">FAQ</a></li> </ul> </nav> <div> <ul class="nav nav--horizontal margin-bottom-2"> <li> <a class="display-flex" href="https://www.nsf.gov" target="_blank" rel="noopener"> <img src="/images/sponsor-nsf.png" width="60" height="60" alt="US Education Department Logo" /> </a> </li> <li> <a class="display-flex" href="https://www.ed.gov/" target="_blank" rel="noopener"> <img src="/images/sponsor-doe.png" width="60" height="60" alt="US Education Department Logo" /> </a> </li> </ul> <p>Visionlearning is supported by the The National Science Foundation and the U.S. Department of Education. 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Energy Metabolism I | Biology | Visionlearning