Discovery and Structure of Cells | 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>Discovery and Structure of Cells | Biology | Visionlearning</title> <link rel="canonical" href="https://www.visionlearning.com/en/library/biology/2/discovery-and-structure-of-cells/64"> <meta name="description" content="Cells are the basic structural and functional unit of life. This module traces the discovery of the cell in the 1600s and the development of modern cell theory. The module looks at similarities and differences between different types of cells and the relationship between cell structure and function. The Theory of Universal Common Descent is presented along with evidence that all living things on Earth descended from a common ancestor."> <meta name="keywords" content="cell structure, discovery of cells, organelles, cells, membrane, eukaryote, prokaryote"> <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/discovery-and-structure-of-cells/64" }, "name": "Discovery and Structure of Cells", "headline": "Discovery and Structure of Cells: Cell theory, prokaryotes, and eukaryotes", "author": [ { "@type": "Person", "name": "Heather MacNeill Falconer, M.A./M.S." } , { "@type": "Person", "name": "Nathan H Lents, Ph.D." }], "datePublished": "2003-03-24 11:23:38", "dateModified": "2017-02-12T08:30:00+05:00", "image": { "@type": "ImageObject", "url": "/img/library/moduleImages/featured_image_64-23061209062524.jpg", "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": "Cells are the basic structural and functional unit of life. This module traces the discovery of the cell in the 1600s and the development of modern cell theory. The module looks at similarities and differences between different types of cells and the relationship between cell structure and function. The Theory of Universal Common Descent is presented along with evidence that all living things on Earth descended from a common ancestor.", "keywords": "cell structure, discovery of cells, organelles, cells, membrane, eukaryote, prokaryote", "inLanguage": { "@type": "Language", "name": "English", "alternateName": "en" }, "copyrightHolder": { "@type": "Organization", "name": "Visionlearning, Inc." }, "copyrightYear": "2003"} </script> <meta property="og:url" content="https://visionlearning.com/en/library/biology/2/discovery-and-structure-of-cells/64"> <meta property="og:title" content="Discovery and Structure of Cells | Biology | Visionlearning" /> <meta property="og:type" content="website"> <meta property="og:site_name" content="Visionlearning"> <meta property="og:description" content="Cells are the basic structural and functional unit of life. This module traces the discovery of the cell in the 1600s and the development of modern cell theory. The module looks at similarities and differences between different types of cells and the relationship between cell structure and function. The Theory of Universal Common Descent is presented along with evidence that all living things on Earth descended from a common ancestor."> <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 src="https://pagead2.googlesyndication.com/pagead/js/adsbygoogle.js?client=ca-pub-9561344156007092" 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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 class="current">Discovery and Structure of Cells</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><a href="/en/library/biology/2/energy-metabolism-i/215">Energy Metabolism I</a></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>Cell Biology</em></strong> </span> <h1>Discovery and Structure of Cells: <sub><em>Cell theory, prokaryotes, and eukaryotes</em></sub></h1> <p class="byline">by Heather MacNeill Falconer, M.A./M.S., 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/discovery-and-structure-of-cells/64/reading" class="is-active" aria-current="page" >Reading</a> </li> <li> <a href="/en/library/biology/2/discovery-and-structure-of-cells/64/quiz" >Quiz</a> </li> <li> <a href="/en/library/biology/2/discovery-and-structure-of-cells/64/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_64.mp3", "description": "Recording of Discovery and Structure of Cells : Cells are the basic structural and functional unit of life. This module traces the discovery of the cell in the 1600s and the development of modern cell theory. The module looks at similarities and differences between different types of cells and the relationship between cell structure and function. The Theory of Universal Common Descent is presented along with evidence that all living things on Earth descended from a common ancestor.", "encodingFormat": "mp3", "name": "module_64.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_64.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/discovery-and-structure-of-cells/64#toc_1">Antony van Leeuwenhoek improves microscopy</a> </li> <li><a href="/en/library/biology/2/discovery-and-structure-of-cells/64#toc_2">The beginnings of cell theory</a> </li> <li><a href="/en/library/biology/2/discovery-and-structure-of-cells/64#toc_3">Prokaryotes and eukaryotes: The case for a shared ancestry</a> </li> <li><a href="/en/library/biology/2/discovery-and-structure-of-cells/64#toc_4">Differences between cell types: Organelles and their functions</a> </li> <li><a href="/en/library/biology/2/discovery-and-structure-of-cells/64#toc_5">Cell theory expands</a> </li> <li><a href="/en/library/biology/2/discovery-and-structure-of-cells/64#toc_6">Cell variety within organisms</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 that the human body consists of trillions of individual cells and 200 distinct types of cells? Human cells range in size from 1/12,000 of an inch (a few micrometers) to over 39 inches (more than a meter) long. All living things are made of cells, but in spite of vast differences in size, shape, and function, these building blocks of life share remarkable similarities.</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>Cells are the basic structural and functional unit of all living things and contain inheritable genetic material.</p></li> <li><p>The activity of a cell is carried out by the sub-cellular structures it possesses.</p></li> <li><p>Cells possess an outer boundary layer, called a cell membrane, cytoplasm, which contains organelles, and genetic material.</p></li> <li><p> There is considerable variety among living cells, including the function of membranes and subcellular structures, and the different types of functions the cells carry out, such as chemical transport, support, and other functions.</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><a href="/en/glossary/view/function">function </a></dt> <dd> special purpose; designated action </dd> <dt><a href="/en/glossary/view/membrane">membrane </a></dt> <dd> a thin layer of tissue </dd> <dt>structure </dt> <dd> physical form; the arrangement of parts; the way something is built</dd> </dl> </div> </div> </div> <section> <p>There are many discoveries that have changed the course of science and the world. Nikola Tesla’s discovery of alternating <mark class="term" data-term="current" data-term-def="a flow, as of electricity or water. In oceanography and hydrology, a channel of water that flows together at the same velocity" data-term-url="/en/glossary/view/current/8278">currents</mark>, for example, helped pave the way for widespread access to electricity, and Louis Pasteur’s discovery that <mark class="term" data-term="heat" data-term-def="A measure of the total internal energy of a substance that can be increased or decreased when objects with different temperatures&hellip;" data-term-url="/en/glossary/view/heat/1506">heat</mark> and disinfectant could kill <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> improved food safety and saved millions of lives. In 1655, the English scientist <mark class="term" data-term="Robert Hooke" data-term-def="An English physicist, born on the Isle of Wight (1635-1703). Hooke's studies were extremely diverse, encompassing biology, geology, physics, chemistry, and&hellip;" data-term-url="/en/glossary/view/Hooke%2C+Robert/4460">Robert Hooke</mark> made an <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">observation</mark> that would change the study of biology forever. While examining a thin, dried section of cork tree with a crude <mark class="term" data-term="light" data-term-def="A form of electromagnetic radiation. Visible light is that associated with stimulating the organs of sight, which for normal human&hellip;" data-term-url="/en/glossary/view/light/1498">light</mark> microscope, Hooke observed that he could plainly see the cork to be made up of tiny spaces surrounded by walls, much like a honeycomb, but that the spaces were irregular and shallow (Figure 1). Further, Hooke noted that these "little Boxes" were so numerous that there were "in a square Inch above a Million... and in a Cubick Inch, above twelve hundred Millions [sic]" (Hooke, 1655).</p>  <div class="figure"> <figure> <button class="lightbox-button" data-lightbox-src="/img/library/large_images/image_14015.jpg" data-lightbox="image"> <img src="/img/library/modules/mid64/Image/VLObject14015-24122010124924.jpeg" alt="Figure 1: The cork described in Micrographia by Robert Hooke. " /> </button> <figcaption> <p><strong>Figure 1:</strong> The cork described in Micrographia by Robert Hooke. </p> <span class="credit">image ©Public Domain</span> </figcaption> </figure> </div> <p>In his landmark book <em>Micrographia</em>, Hooke called these spaces "cells" because they resembled the small rooms monks lived in (<em>cella</em> in Latin). What Hooke’s samples were not able to reveal at the time, though, was 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> are not in fact empty. Though he was diligent in looking at his samples through different magnifications and with various <mark class="term" data-term="light" data-term-def="A form of electromagnetic radiation. Visible light is that associated with stimulating the organs of sight, which for normal human&hellip;" data-term-url="/en/glossary/view/light/1498">light</mark> sources and angles, there were two major obstacles that stood in Hooke’s way of discovering subcellular structures. The first was that the microscope he was using at the time was still too low of a magnification to show that much was contained within the walls of the cells. The second: His samples were of cork – composed of long-dead cells, absent of any <mark class="term" data-term="cytosol" data-term-def="The fluid portion of the cytoplasm that is in every animal and plant cell. It surrounds all of the organelles of&hellip;" data-term-url="/en/glossary/view/cytosol/9843">cytosol</mark> or <mark class="term" data-term="organelle" data-term-def="Structure or compartment within a cell that performs a specialized function such as respiration or photosynthesis. An organelle is analogous to&hellip;" data-term-url="/en/glossary/view/organelle/5281">organelles</mark>.</p> <p><section id="toc_1" class=""> <h2>Antony van Leeuwenhoek improves microscopy</h2></p> <p>In the years immediately following, other scientists would build on the work of Hooke, including Antony van Leeuwenhoek (1632 – 1723), a cloth merchant in Delft, Nederland. Van Leeuwenhoek was not a scientist by formal training, but he was an industrious and curious individual who took great joy in observing the world around him (Anderson, 2009). While working in his haberdashery in the 1670s, van Leeuwenhoek began to <mark class="term" data-term="experiment" data-term-def="A test or trial carried out under controlled conditions so that specific actions can be performed and the results can be observed." data-term-url="/en/glossary/view/experiment/8292">experiment</mark> with glass-blowing and the construction of microscopes (Figure 2). Using the designs described by Hooke in <em>Micrographia</em>, van Leeuwenhoek built his own microscopes by hand, fabricating every element from the highly-refined lens to the screws used to hold the instrument together (Anderson, 2009).</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>  <div class="figure"> <figure> <button class="lightbox-button" data-lightbox-src="/img/library/large_images/image_14016.jpeg" data-lightbox="image"> <img src="/img/library/modules/mid64/Image/VLObject14016-24122010124948.jpg" alt="Figure 2: van Leeuwenhoek's simple microscope. On the brass plate is a small magnifying lens mounted and a sharp point that would hold the specimen. Turning the screws would adjust the position and focus. " /> </button> <figcaption> <p><strong>Figure 2:</strong> van Leeuwenhoek's simple microscope. On the brass plate is a small magnifying lens mounted and a sharp point that would hold the specimen. Turning the screws would adjust the position and focus. </p> <span class="credit">image ©<a href="http://wellcomeimages.org/"> CC BY 4.0 Wellcome Images</a> </span> </figcaption> </figure> </div> <p>During his lifetime, van Leeuwenhoek constructed hundreds of microscopes and lenses by hand, each one unique. It was with these microscopes and improved lenses that he began to study the world around him and share these <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> with institutions like the English <mark class="term" data-term="Royal Society of London" data-term-url="/en/glossary/view/Royal+Society+of+London" data-term-def="The longest continuously existing scientific society, established at Cambridge University in 1660. More information about the Royal Society is available on&hellip;">Royal Society</mark>. One of his first important observations came in August 1674, when he looked at water samples from Berkelse Meer, a lake two miles outside of Delft. In a letter to <mark class="term" data-term="Henry Oldenburg" data-term-def="German natural philosopher and diplomat, born in Bremen (1619-1677). Oldenburg was an original fellow of the Royal Society, served as its&hellip;" data-term-url="/en/glossary/view/Oldenburg%2C+Henry/4502">Henry Oldenburg</mark> that September, and published in <em>Philosophical Transactions of the Royal Society</em>, van Leeuwenhoek noted:</p><blockquote class="blockquote"> <p>I took up some of it [the water] in a Glass-vessel which having viewed the next day, I found moving in it several Earthy particles, and some green streaks, spirally ranged, ... among all of which there crawled abundance of little animals some of which were roundish; those that were somewhat bigger than others were of an Oval figure: On these latter I saw two legs near the head and two little fins on the other end of their body…. The motion of most of them in the water was so swift, and so various, upwards, downwards, and round about, that I confess I could not but wonder at it. I judge, that some of these little creatures were above a thousand times smaller than the smallest ones, which I have hitherto seen.</p> </p></blockquote><p>What van Leeuwenhoek was seeing, we can now presume, were some of the smallest forms of life: protozoa, rotifers, ciliates, and <mark class="term" data-term="phytoplankton" data-term-def="Microscopic plants, such as algae, that float freely in the water column of an aquatic system (e.g., lake, ocean)." data-term-url="/en/glossary/view/phytoplankton/7074">phytoplankton</mark>. Van Leeuwenhoek’s descriptions are among the first to identify the unique features of these different <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> <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 was the beginning of the discipline we now call <em>microbiology</em> – the study of microscopic organisms.</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="cc7424"> <div class="form-entry"> <div class="form-entry__field"> <span class="form-entry__field__label">Antony van Leeuwenhoek's work led to the field of</span> <div class="form-entry__option"> <div class="form-entry__option__radio" data-answer="correct"> <label> <input id="q1-7424-0-option-a" name="quiz-option-7424" type="radio" value="microbiology." > <span class="option__label"> <span class="screen-reader-only">a.</span> microbiology. </span> </label> <span class="quiz__response" id="response-7424-0"> <strong>Correct!</strong> </span> </div> <div class="form-entry__option__radio" data-answer="incorrect"> <label> <input id="q1-7424-1-option-b" name="quiz-option-7424" type="radio" value="botany." > <span class="option__label"> <span class="screen-reader-only">b.</span> botany. </span> </label> <span class="quiz__response" id="response-7424-1"> <strong>Incorrect.</strong> </span> </div> </div> </div> </div> </form> </div> <p>In the years immediately following van Leeuwenhoek’s discovery of microorganisms in the Berkelse Meer water, his studies unearthed some very important cellular distinctions. Among them was the discovery of single-celled <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> (Figure 3) and structures existing within the walls of Hooke’s originally-thought-empty plant <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> (large <mark class="term" data-term="organelle" data-term-def="Structure or compartment within a cell that performs a specialized function such as respiration or photosynthesis. An organelle is analogous to&hellip;" data-term-url="/en/glossary/view/organelle/5281">organelles</mark> called <em>vacuoles</em>).</p>  <div class="figure"> <figure> <button class="lightbox-button" data-lightbox-src="/img/library/large_images/image_14017.jpeg" data-lightbox="image"> <img src="/img/library/modules/mid64/Image/VLObject14017-24122010125038.jpg" alt="Figure 3: Leeuwenhoek's drawing of protozoa. " /> </button> <figcaption> <p><strong>Figure 3:</strong> Leeuwenhoek's drawing of protozoa. </p> <span class="credit">image ©Public Domain</span> </figcaption> </figure> </div> </section> <section id="toc_2"> <h2>The beginnings of cell theory</h2><p>Science is not a solitary endeavor, and many important “Aha!” moments have been the result of being in the right place at the right time. Such was the case with the <mark class="term" data-term="development" data-term-def="The gradual exposure to stimuli in the early-developmental stages that influences the size, shape, and function of animal once mature." data-term-url="/en/glossary/view/development/13147">development</mark> of <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> <mark class="term" data-term="theory" data-term-def="A scientific theory is an explanation inferred from multiple lines of evidence for some broad aspect of the natural world and&hellip;" data-term-url="/en/glossary/view/theory/4854">theory</mark>. During a dinner conversation in 1838, German botanist <mark class="term" data-term="Matthias Jacob Schleiden" data-term-def="German botanist born in Hamburg (1804-1881). Schleiden determined the importance of the nucleus in cellular propagation and, with Theodor Schwann, is&hellip;" data-term-url="/en/glossary/view/Schleiden%2C+Matthias+Jacob/4555">Matthias Jacob Schleiden</mark> mentioned to the German physiologist Theodore Schwann that he believed all plants, and plant parts, were composed of cells. Schwann had come to a similar conclusion about animals through his own <mark class="term" data-term="research" data-term-def="A study or an investigation." data-term-url="/en/glossary/view/research/8257">research</mark> and immediately saw the value in sharing such an idea publicly in the scientific sector (and sharing it quickly, before someone else got credit for it). In 1839, his work <em>Microscopic Investigations on the Accordance in the Structure and Growth of Plants and Animals</em> was published and, without giving any credit to Schleiden, formally made the first statement in cell theory: All living things are composed of cells and cell products.</p><p>This simple statement was another important turning point in biology. At the time of the book’s publication, Schwann was working in the laboratory of Johannes Peter Müller at the University of Berlin. In a true symbiotic relationship, Müller used his leverage as the University’s Chair of the Anatomy and Physiology Department to promote the text; Schwann continued to work in the laboratory and helped solidify Müller’s lab as the <mark class="term" data-term="research" data-term-def="A study or an investigation." data-term-url="/en/glossary/view/research/8257">research</mark> center of <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> <mark class="term" data-term="theory" data-term-def="A scientific theory is an explanation inferred from multiple lines of evidence for some broad aspect of the natural world and&hellip;" data-term-url="/en/glossary/view/theory/4854">theory</mark>.</p><p>The belief that all living things are composed of <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> was soon widely accepted in the scientific community, but how these cells came into being was still an area of unknowing. In the heavily religious <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> of the 18th and 19th centuries, any suggestion that life was the result of something <em>other than</em> a Divine Creation was met with great resistance. So most scientists of the time subscribed to the Theory of Spontaneous Generation while they continued to <mark class="term" data-term="research" data-term-def="A study or an investigation." data-term-url="/en/glossary/view/research/8257">research</mark> the structure and <mark class="term" data-term="function" data-term-def="Adaptations that influence how the animal interacts with other species. For example, animal function typically serves genetic and reproductive success." data-term-url="/en/glossary/view/function/13151">function</mark> of life (see our <a href="/en/library/Process-of-Science/49/Experimentation-in-Scientific-Research/150">Experimentation in Scientific Research</a> module).</p><p>As the quality and magnification ability of microscopes continued to improve, the discoveries about the diversity of <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> and cell structures also increased. It wasn't long before it was clear that there were two major types of cells – those that are an <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> unto themselves (unicellular), and those that are most often part of a larger body (multicellular). Further, within these two types it was noticed that some had a membrane-bound <mark class="term" data-term="nucleus" data-term-def="1. [Atomic] A tiny, dense positively charged mass at the heart of an atom. The nucleus is composed of protons and&hellip;" data-term-url="/en/glossary/view/nucleus/1526">nucleus</mark>, and some did not. These two major types were given the names <em>prokaryote</em> (meaning, "before the nut" or "kernel") and <em>eukaryote</em> (meaning, "with the nut" or "kernel"). The "nut" or "kernel" refers to the nucleus (Figure 4).</p>  <div class="figure"> <figure> <button class="lightbox-button" data-lightbox-src="/img/library/large_images/image_14010.jpg" data-lightbox="image"> <img src="/img/library/modules/mid64/Image/VLObject14010-24122010122807.jpg" alt="Figure 4: The cells of prokaryotes (left) and eukaryotes (right)." /> </button> <figcaption> <p><strong>Figure 4:</strong> The cells of prokaryotes (left) and eukaryotes (right).</p> <span class="credit">image ©Visionlearning</span> </figcaption> </figure> </div> <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="cc7431"> <div class="form-entry"> <div class="form-entry__field"> <span class="form-entry__field__label">The first formal statement in cell theory was that:</span> <div class="form-entry__option"> <div class="form-entry__option__radio" data-answer="incorrect"> <label> <input id="q1-7431-0-option-a" name="quiz-option-7431" type="radio" value="Every cell has a nucleus." > <span class="option__label"> <span class="screen-reader-only">a.</span> Every cell has a nucleus. </span> </label> <span class="quiz__response" id="response-7431-0"> <strong>Incorrect.</strong> </span> </div> <div class="form-entry__option__radio" data-answer="correct"> <label> <input id="q1-7431-1-option-b" name="quiz-option-7431" type="radio" value="All living things are made of cells." > <span class="option__label"> <span class="screen-reader-only">b.</span> All living things are made of cells. </span> </label> <span class="quiz__response" id="response-7431-1"> <strong>Correct!</strong> </span> </div> </div> </div> </div> </form> </div> </section> <section id="toc_3"> <h2>Prokaryotes and eukaryotes: The case for a shared ancestry</h2><p>While all life is made up of <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>, not all cells have the same structure. In the organization of living things, <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> fall into one of two groups: <mark class="term" data-term="prokaryote" data-term-def="An organism, generally one-celled, whose cells do not have a nucleus or other membrane-bound organelles, such as bacteria." data-term-url="/en/glossary/view/prokaryote/8677">prokaryotes</mark> and <mark class="term" data-term="eukaryote" data-term-def="A single- or multi-cellular organism whose cells contain a distinct nucleus that encloses the organism's genetic material." data-term-url="/en/glossary/view/eukaryote/5297">eukaryotes</mark>. Prokaryotes (archaea and bacteria) and eukaryotes (fungi, plants, animals, and protists) have many defining factors that are different from one another, but their similarities are <em>very</em> important and form the foundation on which a <mark class="term" data-term="theory" data-term-def="A scientific theory is an explanation inferred from multiple lines of evidence for some broad aspect of the natural world and&hellip;" data-term-url="/en/glossary/view/theory/4854">theory</mark> of shared ancestry is built.</p><p>All <mark class="term" data-term="prokaryote" data-term-def="An organism, generally one-celled, whose cells do not have a nucleus or other membrane-bound organelles, such as bacteria." data-term-url="/en/glossary/view/prokaryote/8677">prokaryotes</mark> and <mark class="term" data-term="eukaryote" data-term-def="A single- or multi-cellular organism whose cells contain a distinct nucleus that encloses the organism's genetic material." data-term-url="/en/glossary/view/eukaryote/5297">eukaryotes</mark> consist of <mark class="term" data-term="cytosol" data-term-def="The fluid portion of the cytoplasm that is in every animal and plant cell. It surrounds all of the organelles of&hellip;" data-term-url="/en/glossary/view/cytosol/9843">cytosol</mark> with ribosomes suspended in it, the genetic material of <mark class="term" data-term="DNA" data-term-def="Deoxyribonucleic acid. A double-stranded nucleic acid containing the sugar 2-deoxy-D-ribose. A constituent of cellular nuclear material responsible for encoding&hellip;" data-term-url="/en/glossary/view/DNA/1604">DNA</mark> and <mark class="term" data-term="RNA" data-term-def="Ribonucleic acid. A single-stranded nucleic acid containing the sugar ribose. In most organisms, a molecule responsible for transfer of the&hellip;" data-term-url="/en/glossary/view/RNA/1605">RNA</mark>, and are enclosed in a <mark class="term" data-term="membrane" data-term-def="A thin layer of tissue that forms a boundary of a cell or cell part." data-term-url="/en/glossary/view/membrane/8282">membrane</mark>. These common <mark class="term" data-term="element" data-term-def="One of fewer than 118 pure chemical substances. An element is a substance composed of atoms with identical atomic number." data-term-url="/en/glossary/view/element/1510">elements</mark> are chemically and structurally almost indistinguishable. The <mark class="term" data-term="plasma membrane" data-term-def="The semi-permeable layer of tissue enclosing the cytoplasm of a cell. The plasma membrane separates and protects the cell's interior from&hellip;" data-term-url="/en/glossary/view/plasma+membrane/5282">plasma membrane</mark> consists of a <mark class="term" data-term="phospholipid" data-term-def="A lipid or glyceride that contains a phosphate group. The phosphate group imparts a polar side to the molecule, while&hellip;" data-term-url="/en/glossary/view/phospholipid/2567">phospholipid</mark> bilayer, which is a fatty film that surrounds 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> (see our <a href="/en/library/Biology/2/Membranes-I/198">Membranes I: Introduction to Biological Membranes</a> module to learn more). This membrane contains several structures that allow the cell to perform necessary tasks, including pumps and channels that allow substances to move into and out of the cell, and receptors that allow the cell to sense what is in its surroundings and be recognized by other cells (see our <a href="/en/library/Biology/2/Membranes-II/204">Membranes II: Passive and Active Transporters</a> module). This plasma membrane forms a semi-permeable barrier that keeps the cell’s cytosol from leaking out and the surrounding <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">environment</mark> from leaking in.</p><p>Cytosol is a gel-like <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">fluid</mark> consisting of water packed with dissolved <mark class="term" data-term="nutrient" data-term-def="A chemical substance (e.g., minerals, vitamins, proteins) that is needed by an organism to survive and grow. See also: macronutrient and micronutrient." data-term-url="/en/glossary/view/nutrient/7058">nutrients</mark>, wastes, <mark class="term" data-term="ion" data-term-def="An atom or molecule that has acquired an electrical charge by either gaining or losing electrons. A cation is an ion&hellip;" data-term-url="/en/glossary/view/ion/853">ions</mark>, <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>, <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>, and many other <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>. 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> take place in the <mark class="term" data-term="cytosol" data-term-def="The fluid portion of the cytoplasm that is in every animal and plant cell. It surrounds all of the organelles of&hellip;" data-term-url="/en/glossary/view/cytosol/9843">cytosol</mark> and it contains <mark class="term" data-term="particle" data-term-def="A tiny piece of matter." data-term-url="/en/glossary/view/particle/8259">particles</mark> and filaments that provide shape to <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>. Suspended within the cytosol are ribosomes – large molecular machines responsible for translating the information contained in <mark class="term" data-term="RNA" data-term-def="Ribonucleic acid. A single-stranded nucleic acid containing the sugar ribose. In most organisms, a molecule responsible for transfer of the&hellip;" data-term-url="/en/glossary/view/RNA/1605">RNA</mark> into proteins. The number of ribosomes in a cell depends largely on the cell’s <mark class="term" data-term="function" data-term-def="Adaptations that influence how the animal interacts with other species. For example, animal function typically serves genetic and reproductive success." data-term-url="/en/glossary/view/function/13151">function</mark>. (See Figures 5 and 6 for illustrations of the cell structures.)</p>  <div class="figure"> <figure> <button class="lightbox-button" data-lightbox-src="/img/library/large_images/image_14011.jpg" data-lightbox="image"> <img src="/img/library/modules/mid64/Image/VLObject14011-24122010122917.jpg" alt="Figure 5: A diagram of a typical animal cell. " /> </button> <figcaption> <p><strong>Figure 5:</strong> A diagram of a typical animal cell. </p> <span class="credit">image ©Visionlearning</span> </figcaption> </figure> </div>  <div class="figure"> <figure> <button class="lightbox-button" data-lightbox-src="/img/library/large_images/image_14012.jpg" data-lightbox="image"> <img src="/img/library/modules/mid64/Image/VLObject14012-24122010122955.jpg" alt="Figure 6: A diagram of a typical plant cell." /> </button> <figcaption> <p><strong>Figure 6:</strong> A diagram of a typical plant cell.</p> <span class="credit">image ©Visionlearning</span> </figcaption> </figure> </div> <p>Both <mark class="term" data-term="eukaryote" data-term-def="A single- or multi-cellular organism whose cells contain a distinct nucleus that encloses the organism's genetic material." data-term-url="/en/glossary/view/eukaryote/5297">eukaryotes</mark> and <mark class="term" data-term="prokaryote" data-term-def="An organism, generally one-celled, whose cells do not have a nucleus or other membrane-bound organelles, such as bacteria." data-term-url="/en/glossary/view/prokaryote/8677">prokaryotes</mark> also have genetic material (DNA and RNA), which carries the instructions for the production of <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> (see our <mark class="term" data-term="DNA" data-term-def="Deoxyribonucleic acid. A double-stranded nucleic acid containing the sugar 2-deoxy-D-ribose. A constituent of cellular nuclear material responsible for encoding&hellip;" data-term-url="/en/glossary/view/DNA/1604">DNA</mark> module series). However, likely the most important distinction between the two taxa of <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 that the genetic material of eukaryotes is enclosed within a double <mark class="term" data-term="membrane" data-term-def="A thin layer of tissue that forms a boundary of a cell or cell part." data-term-url="/en/glossary/view/membrane/8282">membrane</mark>, creating a <mark class="term" data-term="nucleus" data-term-def="1. [Atomic] A tiny, dense positively charged mass at the heart of an atom. The nucleus is composed of protons and&hellip;" data-term-url="/en/glossary/view/nucleus/1526">nucleus</mark>. Prokaryotes have no such membrane-bound nucleus; their genetic material exists in a nucleoid – an irregularly shaped region within the <mark class="term" data-term="cytosol" data-term-def="The fluid portion of the cytoplasm that is in every animal and plant cell. It surrounds all of the organelles of&hellip;" data-term-url="/en/glossary/view/cytosol/9843">cytosol</mark>.</p><p>Although a bacterium seems much different than a mold, and a tree seems very different from a human, inside 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 all these <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> many things are very much the same. This argues that all living things on Earth are related and descended from a common ancestor. This is called the <strong>Theory of Universal Common Descent</strong>. Consider the following:</p>  <div class="figure"> <figure> <button class="lightbox-button" data-lightbox-src="/img/library/large_images/image_14013.jpg" data-lightbox="image"> <img src="/img/library/modules/mid64/Image/VLObject14013-24122010123112.svg" alt="Figure 7: A comparison of Ribonucleic acid (RNA) and Deoxyribonucleic acid (DNA). " /> </button> <figcaption> <p><strong>Figure 7:</strong> A comparison of Ribonucleic acid (RNA) and Deoxyribonucleic acid (DNA). </p> <span class="credit">image ©Visionlearning</span> </figcaption> </figure> </div> <ol> <li><strong>All living things use <mark class="term" data-term="DNA" data-term-def="Deoxyribonucleic acid. A double-stranded nucleic acid containing the sugar 2-deoxy-D-ribose. A constituent of cellular nuclear material responsible for encoding&hellip;" data-term-url="/en/glossary/view/DNA/1604">DNA</mark> for their genetic material</strong>. Hypothetically speaking, there are dozens 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> that could <mark class="term" data-term="function" data-term-def="Adaptations that influence how the animal interacts with other species. For example, animal function typically serves genetic and reproductive success." data-term-url="/en/glossary/view/function/13151">function</mark> as a repository of genetic information. In fact, <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> and <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> might have been "better" choices than DNA, since they would allow much more information to be stored in the same size molecule. However, every living <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> stores their genetic information in the form of <mark class="term" data-term="chromosome" data-term-def="The organized genetic structure of DNA with associated proteins that contains the hereditary information necessary for reproduction, protein manufacture, and other functions." data-term-url="/en/glossary/view/chromosome/3760">chromosomes</mark> made of DNA. In addition, all living things use the same four <mark class="term" data-term="nucleotide" data-term-def="The building blocks of DNA and RNA, consisting of a nitrogen base, a five-carbon sugar, and one or more phosphate groups." data-term-url="/en/glossary/view/nucleotide/1603">nucleotides</mark> as the building blocks of DNA. Nucleotides could be built in an almost <mark class="term" data-term="infinite" data-term-def="Having the quality of being endless or limitless. Something that is immeasurable or boundless." data-term-url="/en/glossary/view/infinite/11225">infinite</mark> number of ways, but only four are used by life on Earth (Figure 6).</li> <li><strong>The genetic code is universal</strong>. Not only do all living things store their genetic information using the same molecule, the code for reading the information is identical as well. For example, in a given DNA sequence, Cytosine-Thymine-Cytosine (CTC) codes for the 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">acid</mark> leucine. This is true in every living cell from <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> to humans. There is no reason why this would have to be true. The genetic code is like Morse Code: it is purely arbitrary. Any number and combination of DNA nucleotides could serve as a code for any given amino acid. And yet, all life uses the exact same code. (There are a couple of exceptions, but these are very rare.) This common feature of life is what allows us to insert <mark class="term" data-term="gene" data-term-def="Material (usually DNA) that is inherited from a parent and which encodes for a cellular component important for some cellular function." data-term-url="/en/glossary/view/gene/3294">genes</mark> from one <mark class="term" data-term="species" data-term-def="1. In biological classifications, it is the lowest and most basic unit of the Linnaean taxonomic hierarchy (although it is also&hellip;" data-term-url="/en/glossary/view/species/893">species</mark> into another and have those genes still work properly. For example, it would be extremely expensive to harvest insulin from human donors in order to treat patients with diabetes. Instead, scientists have engineered bacteria that contain the human insulin gene. The gene is read and interpreted the same way in both cells, so the bacteria build a perfectly functional human insulin molecule.</li> <li><strong>All cells convert chemical <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> in similar ways</strong>. The energy that reaches the planet from the sun could be harvested in an almost infinite number of ways. However, 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> and <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> for <mark class="term" data-term="photosynthesis" data-term-def="Formation of carbohydrates from carbon dioxide and a source of hydrogen (as water) in the chlorophyll-containing tissues of plants exposed to light." data-term-url="/en/glossary/view/photosynthesis/2194">photosynthesis</mark> are strikingly similar among all photosynthetic cells, from cyanobacteria and plankton to oak trees and lily pads. Similarly, all cells consume macromolecules and convert their energy in astonishingly similar ways. The enzymes of glycolysis, the process 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>, are shared among all living cells. In addition, all cells make and use <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> molecules as their general “currency” for transferring energy in its 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>. There are literally thousands of molecules that could be used for this purpose, including many that would function more efficiently than <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>. The chemical <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> of energy conversion are remarkably similar in all cells on Earth.</li> <li><strong>All ribosomes are structurally and functionally similar</strong>. Structures called ribosomes are responsible for interpreting the genetic code of DNA, received in the form of mRNA, and building proteins according to that code. The ribosomes of all <mark class="term" data-term="prokaryote" data-term-def="An organism, generally one-celled, whose cells do not have a nucleus or other membrane-bound organelles, such as bacteria." data-term-url="/en/glossary/view/prokaryote/8677">prokaryotes</mark> are almost exactly the same, and so are the ribosomes of all <mark class="term" data-term="eukaryote" data-term-def="A single- or multi-cellular organism whose cells contain a distinct nucleus that encloses the organism's genetic material." data-term-url="/en/glossary/view/eukaryote/5297">eukaryotes</mark>. Between eukaryotes and prokaryotes, although there are differences, the overall structure is remarkably similar. All ribosomes have two parts: a big subunit and a small subunit. They operate in nearly identical mechanisms.</li> <li><strong>All biological <mark class="term" data-term="membrane" data-term-def="A thin layer of tissue that forms a boundary of a cell or cell part." data-term-url="/en/glossary/view/membrane/8282">membranes</mark> are similar</strong>. From the <mark class="term" data-term="plasma membrane" data-term-def="The semi-permeable layer of tissue enclosing the cytoplasm of a cell. The plasma membrane separates and protects the cell's interior from&hellip;" data-term-url="/en/glossary/view/plasma+membrane/5282">plasma membrane</mark> of bacteria to the nuclear envelope of animals, the water-tight membranes that establish separate compartments within and around a living cell are extremely similar. On the one hand, this is no surprise because the properties of <mark class="term" data-term="phospholipid" data-term-def="A lipid or glyceride that contains a phosphate group. The phosphate group imparts a polar side to the molecule, while&hellip;" data-term-url="/en/glossary/view/phospholipid/2567">phospholipids</mark> are quite unique. On the other hand, starting with very basic chemical building blocks, like those found on the early Earth, many possible membrane-forming molecules could have emerged. In fact, scientists can now synthesize much better, simpler, and more stable membranes. That all life uses the same basic membrane structure is strong <mark class="term" data-term="evidence" data-term-def="Support for an idea, opinion, or hypothesis." data-term-url="/en/glossary/view/evidence/8243">evidence</mark> that once membranes first evolved, they were passed to descendants with little changing along the way.</li> </ol><p>These are but a few of the pieces of <mark class="term" data-term="evidence" data-term-def="Support for an idea, opinion, or hypothesis." data-term-url="/en/glossary/view/evidence/8243">evidence</mark> for the Theory of Universal Common <mark class="term" data-term="descent" data-term-def="Ancestry or heritage." data-term-url="/en/glossary/view/descent/8285">Descent</mark>.</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>Prokaryotes have been documented in the <mark class="term" data-term="fossil" data-term-def="The preserved impression or remains of an animal or plant whose living tissue has been replaced by minerals." data-term-url="/en/glossary/view/fossil/8558">fossil</mark> <mark class="term" data-term="record" data-term-def="A written account or description. <br> <b>[verb]</b> To write an account or description." data-term-url="/en/glossary/view/record/8239">record</mark> as far back as 4.2 billion years (see Figure 8). <mark class="term" data-term="eukaryote" data-term-def="A single- or multi-cellular organism whose cells contain a distinct nucleus that encloses the organism's genetic material." data-term-url="/en/glossary/view/eukaryote/5297">Eukaryotes</mark>, however, can only be documented as far back as 2.7 billion years, leaving 1.5 billion years of <mark class="term" data-term="evolution" data-term-def="Change in the gene pool of a population from generation to generation by such processes as mutation, natural selection, and genetic drift." data-term-url="/en/glossary/view/evolution/5284">evolution</mark> to take place between. For this reason, scientists believe that eukaryotes evolved from <mark class="term" data-term="prokaryote" data-term-def="An organism, generally one-celled, whose cells do not have a nucleus or other membrane-bound organelles, such as bacteria." data-term-url="/en/glossary/view/prokaryote/8677">prokaryotes</mark> long after the central features of living <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> had already emerged.</p>  <div class="figure"> <figure> <button class="lightbox-button" data-lightbox-src="/img/library/large_images/image_14018.jpeg" data-lightbox="image"> <img src="/img/library/modules/mid64/Image/VLObject14018-24122010125204.jpg" alt="Figure 8: A timescale of prokaryote evolution. From the article by Battistuzzi, F.U., Feijao, A., and Hedges, S.B. 2004. A genomic timescale of prokaryote evolution: insights into the origin of methanogenesis, phototrophy, and the colonization of land." /> </button> <figcaption> <p><strong>Figure 8:</strong> A timescale of prokaryote evolution. From the article by Battistuzzi, F.U., Feijao, A., and Hedges, S.B. 2004. A genomic timescale of prokaryote evolution: insights into the origin of methanogenesis, phototrophy, and the colonization of land.</p> <span class="credit">image ©<a href="https://www.researchgate.net/publication/51367242_A_genomic_time_scale_of_prokaryote_evolution_insights_into_the_origin_of_Methanogenesis_phototrophy_and_the_colonization_of_land_BMC"> Battistuzzi et al., 2004</a> </span> </figcaption> </figure> </div> <p>The defining feature of <mark class="term" data-term="eukaryote" data-term-def="A single- or multi-cellular organism whose cells contain a distinct nucleus that encloses the organism's genetic material." data-term-url="/en/glossary/view/eukaryote/5297">eukaryotes</mark>, the <mark class="term" data-term="nucleus" data-term-def="1. [Atomic] A tiny, dense positively charged mass at the heart of an atom. The nucleus is composed of protons and&hellip;" data-term-url="/en/glossary/view/nucleus/1526">nucleus</mark>, first evolved as an in-folding of the plasma <mark class="term" data-term="membrane" data-term-def="A thin layer of tissue that forms a boundary of a cell or cell part." data-term-url="/en/glossary/view/membrane/8282">membrane</mark>, which formed a compartment to house and protect the <mark class="term" data-term="DNA" data-term-def="Deoxyribonucleic acid. A double-stranded nucleic acid containing the sugar 2-deoxy-D-ribose. A constituent of cellular nuclear material responsible for encoding&hellip;" data-term-url="/en/glossary/view/DNA/1604">DNA</mark>. This in-folding evolved into the nuclear envelope – the double membrane of the nucleus (Figure 9). In addition, as discussed in our <a href="/en/library/Biology/2/Cellular-Organelles-I/195">Cellular Organelles I: Membrane-bound Organelles</a> module, the Theory of Endosymbiosis suggests that a small <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="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> was able to penetrate the <mark class="term" data-term="plasma membrane" data-term-def="The semi-permeable layer of tissue enclosing the cytoplasm of a cell. The plasma membrane separates and protects the cell's interior from&hellip;" data-term-url="/en/glossary/view/plasma+membrane/5282">plasma membrane</mark> of a larger, anaerobic <mark class="term" data-term="prokaryote" data-term-def="An organism, generally one-celled, whose cells do not have a nucleus or other membrane-bound organelles, such as bacteria." data-term-url="/en/glossary/view/prokaryote/8677">prokaryote</mark>, probably an archeon, and survive, living symbiotically with the host <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>. Over time, the small aerobic bacteria evolved into the <mark class="term" data-term="organelle" data-term-def="Structure or compartment within a cell that performs a specialized function such as respiration or photosynthesis. An organelle is analogous to&hellip;" data-term-url="/en/glossary/view/organelle/5281">organelle</mark> we know today as the mitochondrion.</p>  <div class="figure"> <figure> <button class="lightbox-button" data-lightbox-src="/img/library/large_images/image_14014.jpg" data-lightbox="image"> <img src="/img/library/modules/mid64/Image/VLObject14014-24122010123151.jpg" alt="Figure 9: The cell membrane in-folding and creating a nuclear envelope." /> </button> <figcaption> <p><strong>Figure 9:</strong> The cell membrane in-folding and creating a nuclear envelope.</p> <span class="credit">image ©Visionlearning</span> </figcaption> </figure> </div> <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="cc7443"> <div class="form-entry"> <div class="form-entry__field"> <span class="form-entry__field__label">The Theory of Universal Common Descent argues that</span> <div class="form-entry__option"> <div class="form-entry__option__radio" data-answer="correct"> <label> <input id="q1-7443-0-option-a" name="quiz-option-7443" type="radio" value="all living things descended from a single ancestor." > <span class="option__label"> <span class="screen-reader-only">a.</span> all living things descended from a single ancestor. </span> </label> <span class="quiz__response" id="response-7443-0"> <strong>Correct!</strong> </span> </div> <div class="form-entry__option__radio" data-answer="incorrect"> <label> <input id="q1-7443-1-option-b" name="quiz-option-7443" type="radio" value="cells without a nucleus evolved long after the existence of cells that contain a nucleus." > <span class="option__label"> <span class="screen-reader-only">b.</span> cells without a nucleus evolved long after the existence of cells that contain a nucleus. </span> </label> <span class="quiz__response" id="response-7443-1"> <strong>Incorrect.</strong> </span> </div> </div> </div> </div> </form> </div> </section> <section id="toc_4"> <h2>Differences between cell types: Organelles and their functions</h2><p>All basic chemical and physiological <mark class="term" data-term="function" data-term-def="Adaptations that influence how the animal interacts with other species. For example, animal function typically serves genetic and reproductive success." data-term-url="/en/glossary/view/function/13151">functions</mark> – repair, growth, movement, <mark class="term" data-term="immunity" data-term-def="Resistance against infection by a particular disease; the body’s ability to protect against a particular infectious disease." data-term-url="/en/glossary/view/immunity/8745">immunity</mark>, communication, digestion – are carried out inside of <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>, and the activities of cells depends on the activities of the structures <em>within</em> the cell (including the organelles). This means cells can convert <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 one form (which, depending on the cell type, can be in the form of <mark class="term" data-term="light" data-term-def="A form of electromagnetic radiation. Visible light is that associated with stimulating the organs of sight, which for normal human&hellip;" data-term-url="/en/glossary/view/light/1498">light</mark>, <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>, or other compounds) into another. For example, cells can digest the building blocks of other <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 it has eaten and used the released energy to build its own materials such as <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, and fats.</p><p>Most of the activities of a <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> are carried out via the production of <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>. Proteins are large <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> that are made by specific <mark class="term" data-term="organelle" data-term-def="Structure or compartment within a cell that performs a specialized function such as respiration or photosynthesis. An organelle is analogous to&hellip;" data-term-url="/en/glossary/view/organelle/5281">organelles</mark> within the cell using the instructions contained within its genetic material (see our series on DNA: <a href=/en/library/Biology/2/DNA-I/149">DNA I: The Genetic Material</a>, <a href="/en/library/Biology/2/DNA-II/160">DNA II: The Structure of DNA</a>, <a href="/en/library/Biology/2/DNA-III/180">DNA III: The Replication of DNA</a>). Depending on the type of <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>, specific organelles may or may not be present in a cell.</p><p>In addition to the plasma (cell) <mark class="term" data-term="membrane" data-term-def="A thin layer of tissue that forms a boundary of a cell or cell part." data-term-url="/en/glossary/view/membrane/8282">membrane</mark>, <mark class="term" data-term="cytosol" data-term-def="The fluid portion of the cytoplasm that is in every animal and plant cell. It surrounds all of the organelles of&hellip;" data-term-url="/en/glossary/view/cytosol/9843">cytosol</mark>, ribosomes, and <mark class="term" data-term="nucleus" data-term-def="1. [Atomic] A tiny, dense positively charged mass at the heart of an atom. The nucleus is composed of protons and&hellip;" data-term-url="/en/glossary/view/nucleus/1526">nucleus</mark>, the typical components of <mark class="term" data-term="eukaryotic" data-term-def="Of cells with a nucleus and other organelles that are surrounded by lipid membranes" data-term-url="/en/glossary/view/eukaryotic/6539">eukaryotic</mark> <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> include: <mark class="term" data-term="mitochondria" data-term-def="Organelles that convert energy from food molecules into ATP, the main energy currency inside cells." data-term-url="/en/glossary/view/mitochondria/6542">mitochondria</mark>, transport <mark class="term" data-term="vesicle" data-term-def="A small, self-enclosed sac inside of cells containing fluid and other material. The vesicles organize the cell's metabolism, help store enzymes&hellip;" data-term-url="/en/glossary/view/vesicle/2610">vesicles</mark>, endoplasmic reticulum, Golgi bodies, and lysosomes. In addition to these, photosynthetic (plant) cells will have a cell wall, <mark class="term" data-term="chloroplast" data-term-def="Organelle in plant and algae cells where photosynthesis occurs." data-term-url="/en/glossary/view/chloroplast/6543">chloroplasts</mark>, and a central vacuole.</p><p>Prokaryotic <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>, however, do not contain any membrane-bound <mark class="term" data-term="organelle" data-term-def="Structure or compartment within a cell that performs a specialized function such as respiration or photosynthesis. An organelle is analogous to&hellip;" data-term-url="/en/glossary/view/organelle/5281">organelles</mark>. Instead, they can include plasmids, a cell wall, and in the case of photosynthetic <mark class="term" data-term="prokaryote" data-term-def="An organism, generally one-celled, whose cells do not have a nucleus or other membrane-bound organelles, such as bacteria." data-term-url="/en/glossary/view/prokaryote/8677">prokaryotes</mark>, thylakoids. Table 1, below, lists the <mark class="term" data-term="function" data-term-def="Adaptations that influence how the animal interacts with other species. For example, animal function typically serves genetic and reproductive success." data-term-url="/en/glossary/view/function/13151">function</mark> of each type of organelle and which group of cells it is found in.</p> <a class="interactive-animation" href="https://www.visionlearning.com/library/animations/Cell_Biology/Cell_Animal.html" target="_blank"> <img class="interactive-animation__image" src="/images/anim-snaps/ia-animal-cell.png" width="200" alt="The Structure of Animal Cells" /> <p class="interactive-animation__title"> <em>Interactive Animation:</em> <strong class="link-new-window"> <span class="link__text">The Structure of Animal Cells</span> </strong> </p> </a> <a class="interactive-animation" href="https://www.visionlearning.com/library/animations/Cell_Biology/Cell_Plant.html" target="_blank"> <img class="interactive-animation__image" src="/images/anim-snaps/ia-plant-cell.png" width="200" alt="The Structure of Plant Cells" /> <p class="interactive-animation__title"> <em>Interactive Animation:</em> <strong class="link-new-window"> <span class="link__text">The Structure of Plant Cells</span> </strong> </p> </a> <div class='"figure"'><table class="table"> <colgroup> <col style="width:20%"> <col style="width:80%"> </colgroup> <tbody> <tr> <th colspan="2" style="text-align:center;"><strong>Membrane-bound Organelles</strong></th> </tr> <tr> <td colspan="2" style="text-align:center;"><em>Independently replicating</em></td> </tr> <tr> <td><img src="https://www.visionlearning.com/images/figure-images/cell-thumbnails-01.jpg"></td> <td><p><strong>Mitochondrion<em> (plural: mitochondria)</em></strong></p> <p>The "power supplier" for the cell, generating most of the ATP used in cell processes through the conversion of nutrients into energy. Also involved in cell signaling, controlling the cell cycle and cell growth, and cellular differentiation. Found in all eukaryotes.</p> </td> </tr> <tr> <td><img src="https://www.visionlearning.com/images/figure-images/cell-thumbnails-02.jpg"></td> <td><p><strong>Chloroplasts</strong></p> <p>A chlorophyll-containing plastid responsible for converting sunlight and carbon dioxide into oxygen and sugar. Found in plants and algae.</p> </td> </tr> <tr> <td colspan="2" style="text-align:center;"><em>Endomembrane System</em></td> </tr> <tr> <td><img src="https://www.visionlearning.com/images/figure-images/cell-thumbnails-03.jpg"></td> <td><p><strong>Smooth Endoplasmic Reticulum</strong></p> <p>A series of sac-like membranes responsible for the synthesis and storage of lipids, phospholipids, and steroids, as well as the metabolism of carbohydrates.</p> </td> </tr> <tr> <td><img src="https://www.visionlearning.com/images/figure-images/cell-thumbnails-04.jpg"></td> <td><p><strong>Rough Endoplasmic Reticulum</strong></p> <p>A series of sac-like membranes studded with ribosomes, responsible for the synthesis and export of proteins</p> </td> </tr> <tr> <td><img src="https://www.visionlearning.com/images/figure-images/cell-thumbnails-05.jpg"></td> <td><p><strong>Golgi Apparatus</strong></p> <p>Functioning like a distribution center, the Golgi Apparatus gathers simple molecules and creates more complex molecules. Once created, those complex molecules are transported to other organelles, stored in vesicles, or exported from the cell.</p> </td> </tr> <tr> <td><img src="https://www.visionlearning.com/images/figure-images/cell-thumbnails-06.jpg"></td> <td><p><strong>Plasma Membrane</strong></p> <p>A layer of phospholipids and proteins that forms a barrier between the inside of the cell and the outside environment.</p> </td> </tr> <tr> <td><img src="https://www.visionlearning.com/images/figure-images/cell-thumbnails-07.jpg"></td> <td><p><strong>Cell Wall</strong></p> <p>Found in plants, fungi, and some protists, a structure outside of the plasma membrane that provides strength, support, and protection.</p> </td> </tr> <tr> <td><img src="https://www.visionlearning.com/images/figure-images/cell-thumbnails-08.jpg"></td> <td><p><strong>Lysosomes</strong></p> <p>A specialized compartment containing hydrolytic enzymes. The role of lysosomes is to digest sugars, proteins, and other "foodsâ a cell absorbs.</p> </td> </tr> <tr> <td><img src="https://www.visionlearning.com/images/figure-images/cell-thumbnails-09.jpg"></td> <td><p><strong>Nuclear Envelope</strong></p> <p>A double-membrane surrounding the nucleus. This membrane provides a barrier between the nucleus and the cytosol.</p> </td> </tr> <tr> <td><img src="https://www.visionlearning.com/images/figure-images/cell-thumbnails-10.jpg"></td> <td><p><strong>Peroxisome</strong></p> <p>Similar to lysosomes, these contain enzymes used in a variety of reactions, including oxidation reactions. In plant seeds, peroxisomes convert stored fatty acids to carbohydrates, providing energy for germination. In plant leaves, they are involved in photorespiration.</p> </td> </tr> <tr> <td><img src="https://www.visionlearning.com/images/figure-images/cell-thumbnails-11.jpg"></td> <td><p><strong>Vacuoles</strong></p> <p>A compartment used for storage of nutrients, water, and waste. In plants, the central vacuole plays an important role in providing structure.</p> </td> </tr> <tr> <td><img src="https://www.visionlearning.com/images/figure-images/cell-thumbnails-12.jpg"></td> <td><p><strong>Vesicles</strong></p> <p>A small spherical compartment composed of a lipid bilayer and internal fluid used to exchange cargo between organelles of the endomembrane system. Specialized vesicles play a variety of roles.</p> </td> </tr> <tr> <td><img src="https://www.visionlearning.com/images/figure-images/cell-thumbnails-13.jpg"></td> <td><p><strong>Cytosol</strong></p> <p>Otherwise known as intracellular fluid (ICF), the liquid matrix found within a cell that holds other organelles and allows intracellular processes to take place.</p> </td> </tr> </tbody> </table></div><div class='"figure"'><table class="table"> <colgroup> <col style="width:20%"> <col style="width:80%"> </colgroup> <tbody> <tr> <th colspan="2" style="text-align:center;"><strong>Non-membrane-bound Organelles</strong></th> </tr> <tr> <td><img src="https://www.visionlearning.com/images/figure-images/cell-thumbnails-14.jpg"></td> <td><p><strong>Cytoskeleton</strong></p> <p>A complex network of protein fibers that give the cell its shape. These fibers also play important roles in assisting vesicles and organelles to move around the cell, as well as the movement of chromosomes during cell division.</p> </td> </tr> <tr> <td><img src="https://www.visionlearning.com/images/figure-images/cell-thumbnails-15.jpg"></td> <td><p><strong>Ribosome</strong></p> <p>The protein builders of the cell. Ribosomes are responsible for constructing amino acids to build amino acid chains.</p> </td> </tr> <tr> <td><img src="https://www.visionlearning.com/images/figure-images/cell-thumbnails-16.jpg"></td> <td><p><strong>Nucleolus</strong></p> <p>Where ribosomes are made inside the nucleus.</p> </td> </tr> </tbody> </table></div> <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="cc7451"> <div class="form-entry"> <div class="form-entry__field"> <span class="form-entry__field__label">The production of ______ allows the cell to carry out most of its activities.</span> <div class="form-entry__option"> <div class="form-entry__option__radio" data-answer="incorrect"> <label> <input id="q1-7451-0-option-a" name="quiz-option-7451" type="radio" value="sugars" > <span class="option__label"> <span class="screen-reader-only">a.</span> sugars </span> </label> <span class="quiz__response" id="response-7451-0"> <strong>Incorrect.</strong> </span> </div> <div class="form-entry__option__radio" data-answer="correct"> <label> <input id="q1-7451-1-option-b" name="quiz-option-7451" type="radio" value="proteins" > <span class="option__label"> <span class="screen-reader-only">b.</span> proteins </span> </label> <span class="quiz__response" id="response-7451-1"> <strong>Correct!</strong> </span> </div> </div> </div> </div> </form> </div> </section> <section id="toc_5"> <h2>Cell theory expands</h2><p>In 1855 German biologist Rudolf Virchow realized that the widely held idea that <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> spontaneously generate out of non-living <mark class="term" data-term="matter" data-term-def="The substance that makes up physical objects." data-term-url="/en/glossary/view/matter/8264">matter</mark> did not make sense, and an idea proposed by Polish-German embryologist <mark class="term" data-term="Robert Remak" data-term-def="Polish microscopist and doctor (1815-1865) who lived and worked primarily in Berlin, Germany. Under his microscope, Remak discovered that new cells&hellip;" data-term-url="/en/glossary/view/Remak%2C+Robert/5339">Robert Remak</mark> might be correct. Remak, a friend and colleague of Virchow, had put forth the idea 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> generate from preexisting cells, and not from things like dust and dead fish. Plagiarizing Remak’s idea, Virchow officially added to cell <mark class="term" data-term="theory" data-term-def="A scientific theory is an explanation inferred from multiple lines of evidence for some broad aspect of the natural world and&hellip;" data-term-url="/en/glossary/view/theory/4854">theory</mark> in 1858 with the statement: Every cell originates from another existing cell like it. This statement, along with Schwann's declaration that "All living things are made of cells," forms the basis of modern cell theory.</p><p>While it is true that all <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> are produced by the division of preexisting cells (in other words, through reproduction), we now know that 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> of how cell reproduction takes place differs between <mark class="term" data-term="prokaryote" data-term-def="An organism, generally one-celled, whose cells do not have a nucleus or other membrane-bound organelles, such as bacteria." data-term-url="/en/glossary/view/prokaryote/8677">prokaryotes</mark> and <mark class="term" data-term="eukaryote" data-term-def="A single- or multi-cellular organism whose cells contain a distinct nucleus that encloses the organism's genetic material." data-term-url="/en/glossary/view/eukaryote/5297">eukaryotes</mark> – and that the speed at which it takes place within those two groups <em>also</em> differs.</p><p>Prokaryotes replicate through <mark class="term" data-term="binary fission" data-term-def="A form of asexual reproduction where a cell divides itself into two identical daughter cells." data-term-url="/en/glossary/view/binary+fission/6547">binary fission</mark>, a <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> by which a single <mark class="term" data-term="prokaryote" data-term-def="An organism, generally one-celled, whose cells do not have a nucleus or other membrane-bound organelles, such as bacteria." data-term-url="/en/glossary/view/prokaryote/8677">prokaryote</mark> <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> simply divides itself in half. Because prokaryotes are single-celled <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>, every time a <mark class="term" data-term="prokaryotic" data-term-def="Cells that do not have a nuclei, nor any other membrane bound organelles." data-term-url="/en/glossary/view/prokaryotic/6540">prokaryotic</mark> cell divides it is reproducing. Prior to the fission, the genetic material (DNA) is copied within the cell; then the two <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> of <mark class="term" data-term="DNA" data-term-def="Deoxyribonucleic acid. A double-stranded nucleic acid containing the sugar 2-deoxy-D-ribose. A constituent of cellular nuclear material responsible for encoding&hellip;" data-term-url="/en/glossary/view/DNA/1604">DNA</mark> attach to opposite sides of the <mark class="term" data-term="membrane" data-term-def="A thin layer of tissue that forms a boundary of a cell or cell part." data-term-url="/en/glossary/view/membrane/8282">membrane</mark>. The membrane then grows between the two molecules, forming a separation. When the prokaryote has doubled in size, the cell begins to pinch inward and a cell wall forms, dividing the cell in half (see animation).</p><p>Eukaryotes, however, reproduce through a more complicated <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> involving several phases, including interphase, <mark class="term" data-term="mitosis" data-term-def="Process occurring as part of the eukaryotic cell cycle. In mitosis, chromosomes in the cell nucleus separate into two identical chromosome&hellip;" data-term-url="/en/glossary/view/mitosis/10188">mitosis</mark>, and <mark class="term" data-term="cytokinesis" data-term-def="The process of cell division in the eukaryotic cell cycle, characterized by the cytoplasm dividing to form two daughter cells." data-term-url="/en/glossary/view/cytokinesis/10178">cytokinesis</mark> (see our <a href="/en/library/Biology/2/Cell-Division-I/196">Cell Division I: The Cell Cycle</a> module). During the first half of interphase, called the G1 phase, <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> take in <mark class="term" data-term="nutrient" data-term-def="A chemical substance (e.g., minerals, vitamins, proteins) that is needed by an organism to survive and grow. See also: macronutrient and micronutrient." data-term-url="/en/glossary/view/nutrient/7058">nutrients</mark> and nearly double in size. Then, in S-phase, the <mark class="term" data-term="DNA" data-term-def="Deoxyribonucleic acid. A double-stranded nucleic acid containing the sugar 2-deoxy-D-ribose. A constituent of cellular nuclear material responsible for encoding&hellip;" data-term-url="/en/glossary/view/DNA/1604">DNA</mark> inside the cell’s <mark class="term" data-term="nucleus" data-term-def="1. [Atomic] A tiny, dense positively charged mass at the heart of an atom. The nucleus is composed of protons and&hellip;" data-term-url="/en/glossary/view/nucleus/1526">nucleus</mark> replicates, making a complete copy of itself. Following this is the G2 phase, where the cell checks and corrects any errors that may have occurred in that DNA <mark class="term" data-term="replication" data-term-def="In science, quality research studies are designed such that the exact procedures and methods followed by the scientists are clear and&hellip;" data-term-url="/en/glossary/view/replication/3730">replication</mark> and grows a little bit more. If all has gone well, the cell proceeds to mitosis, or M phase, when the two DNA copies are separated and the nucleus splits to create two identical <mark class="term" data-term="daughter" data-term-def="A material that is derived from the breakdown or division of another. For example, a product of the radioactive decay of&hellip;" data-term-url="/en/glossary/view/daughter/10177">daughter</mark> nuclei. Finally, the rest of the cell splits in two, each with its own new nucleus, in a process called cytokinesis.</p> <a class="interactive-animation" href="https://www.visionlearning.com/library/animations/Prok_Vs_Euk_r5/Prok_Vs_Euk_r5.html" target="_blank"> <img class="interactive-animation__image" src="/images/anim-snaps/ia-prokaryote.jpg" width="200" alt="Prokaryote vs. Eukaryote Cell Division" /> <p class="interactive-animation__title"> <em>Interactive Animation:</em> <strong class="link-new-window"> <span class="link__text">Prokaryote vs. Eukaryote Cell Division</span> </strong> </p> </a> <p>All <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> contain genetic material in the form of <mark class="term" data-term="DNA" data-term-def="Deoxyribonucleic acid. A double-stranded nucleic acid containing the sugar 2-deoxy-D-ribose. A constituent of cellular nuclear material responsible for encoding&hellip;" data-term-url="/en/glossary/view/DNA/1604">DNA</mark> that is passed from the <mark class="term" data-term="parent" data-term-def="The material or source from which something is derived." data-term-url="/en/glossary/view/parent/1618">parent</mark> cell to the two <mark class="term" data-term="daughter" data-term-def="A material that is derived from the breakdown or division of another. For example, a product of the radioactive decay of&hellip;" data-term-url="/en/glossary/view/daughter/10177">daughter</mark> cells. However, in <mark class="term" data-term="prokaryote" data-term-def="An organism, generally one-celled, whose cells do not have a nucleus or other membrane-bound organelles, such as bacteria." data-term-url="/en/glossary/view/prokaryote/8677">prokaryotes</mark> that genetic material is in a circular form, while in <mark class="term" data-term="eukaryote" data-term-def="A single- or multi-cellular organism whose cells contain a distinct nucleus that encloses the organism's genetic material." data-term-url="/en/glossary/view/eukaryote/5297">eukaryotes</mark> the genetic material is in linear strands. For prokaryotes, <mark class="term" data-term="binary fission" data-term-def="A form of asexual reproduction where a cell divides itself into two identical daughter cells." data-term-url="/en/glossary/view/binary+fission/6547">binary fission</mark> is a means of reproducing and increasing numbers of the <mark class="term" data-term="population" data-term-def="In biology, the population is all individuals of a certain kind of plant or animal that live in a particular habitat.&hellip;" data-term-url="/en/glossary/view/population/8283">population</mark>. For example, <em>Salmonella</em> <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> can grow so fast that the population doubles every thirty minutes. This means that a single <em>Salmonella</em> bacterium sitting on a tiny piece of raw chicken left in the kitchen sink after dinner can give rise to a million descendants by the next morning. </p><p>However, most <mark class="term" data-term="eukaryote" data-term-def="A single- or multi-cellular organism whose cells contain a distinct nucleus that encloses the organism's genetic material." data-term-url="/en/glossary/view/eukaryote/5297">eukaryotes</mark> are multicellular. Most of 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> division that takes place is not to produce more <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>, but rather to allow the organism to grow and develop and to repair and renew tissues. Nevertheless, each and every cell division follows the same complex pattern. Even the most rapidly dividing human cells take more than twenty hours to a complete a single division cycle (with only very rare exceptions). In addition, it is not just the <mark class="term" data-term="DNA" data-term-def="Deoxyribonucleic acid. A double-stranded nucleic acid containing the sugar 2-deoxy-D-ribose. A constituent of cellular nuclear material responsible for encoding&hellip;" data-term-url="/en/glossary/view/DNA/1604">DNA</mark> and the <mark class="term" data-term="cytosol" data-term-def="The fluid portion of the cytoplasm that is in every animal and plant cell. It surrounds all of the organelles of&hellip;" data-term-url="/en/glossary/view/cytosol/9843">cytosol</mark> that must be duplicated and split between the two <mark class="term" data-term="daughter" data-term-def="A material that is derived from the breakdown or division of another. For example, a product of the radioactive decay of&hellip;" data-term-url="/en/glossary/view/daughter/10177">daughter</mark> cells: <mark class="term" data-term="eukaryotic" data-term-def="Of cells with a nucleus and other organelles that are surrounded by lipid membranes" data-term-url="/en/glossary/view/eukaryotic/6539">Eukaryotic</mark> cells contain many specialized <mark class="term" data-term="organelle" data-term-def="Structure or compartment within a cell that performs a specialized function such as respiration or photosynthesis. An organelle is analogous to&hellip;" data-term-url="/en/glossary/view/organelle/5281">organelles</mark> suspended in their cytosol. During cell division, the many organelles must expand, split up, and be distributed more or less evenly between the two daughter cells. Unlike the other eukaryotic organelles, <mark class="term" data-term="mitochondria" data-term-def="Organelles that convert energy from food molecules into ATP, the main energy currency inside cells." data-term-url="/en/glossary/view/mitochondria/6542">mitochondria</mark> and <mark class="term" data-term="chloroplast" data-term-def="Organelle in plant and algae cells where photosynthesis occurs." data-term-url="/en/glossary/view/chloroplast/6543">chloroplasts</mark> contain their own unique genetic material. They replicate themselves independently when more are needed and are then passively distributed to the two daughter cells during <mark class="term" data-term="cytokinesis" data-term-def="The process of cell division in the eukaryotic cell cycle, characterized by the cytoplasm dividing to form two daughter cells." data-term-url="/en/glossary/view/cytokinesis/10178">cytokinesis</mark>.</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="cc7459"> <div class="form-entry"> <div class="form-entry__field"> <span class="form-entry__field__label">In eukaryotes, the main purpose of cell division is to</span> <div class="form-entry__option"> <div class="form-entry__option__radio" data-answer="correct"> <label> <input id="q1-7459-0-option-a" name="quiz-option-7459" type="radio" value="repair and renew tissues in the organism." > <span class="option__label"> <span class="screen-reader-only">a.</span> repair and renew tissues in the organism. </span> </label> <span class="quiz__response" id="response-7459-0"> <strong>Correct!</strong> </span> </div> <div class="form-entry__option__radio" data-answer="incorrect"> <label> <input id="q1-7459-1-option-b" name="quiz-option-7459" type="radio" value="make new organisms." > <span class="option__label"> <span class="screen-reader-only">b.</span> make new organisms. </span> </label> <span class="quiz__response" id="response-7459-1"> <strong>Incorrect.</strong> </span> </div> </div> </div> </div> </form> </div> </section> <section id="toc_6"> <h2>Cell variety within organisms</h2><p>Cell diversity extends beyond the differences between <mark class="term" data-term="prokaryote" data-term-def="An organism, generally one-celled, whose cells do not have a nucleus or other membrane-bound organelles, such as bacteria." data-term-url="/en/glossary/view/prokaryote/8677">prokaryotes</mark> and <mark class="term" data-term="eukaryote" data-term-def="A single- or multi-cellular organism whose cells contain a distinct nucleus that encloses the organism's genetic material." data-term-url="/en/glossary/view/eukaryote/5297">eukaryotes</mark>, and between the different kingdoms of <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> (plants, animals, etc.). There are also major differences 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> within an individual organism, reflecting the different <mark class="term" data-term="function" data-term-def="Adaptations that influence how the animal interacts with other species. For example, animal function typically serves genetic and reproductive success." data-term-url="/en/glossary/view/function/13151">functions</mark> cells perform. For example, the human body consists of trillions of cells, including some 200 different cell types that vary greatly in size, shape, and function. The smallest human cells, sperm cells, are a few micrometers wide (1/12,000 of an inch), whereas the longest cells, the neurons that run from the tip of the big toe to the spinal cord, are over a meter long in an average adult.</p><p>Human <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> also vary significantly in structure and <mark class="term" data-term="function" data-term-def="Adaptations that influence how the animal interacts with other species. For example, animal function typically serves genetic and reproductive success." data-term-url="/en/glossary/view/function/13151">function</mark>. For example, only muscle cells contain myofilaments – protein-containing structures that allow the cells to contract (shorten) and, as a result, cause movement. The eye contains specialized cells called <em>photoreceptors</em> that have the ability to detect <mark class="term" data-term="light" data-term-def="A form of electromagnetic radiation. Visible light is that associated with stimulating the organs of sight, which for normal human&hellip;" data-term-url="/en/glossary/view/light/1498">light</mark>. These cells contain special chemicals called <mark class="term" data-term="pigment" data-term-def="A light-absorbing molecule that gathers energy from the sun; a molecule that gives color and is involved in vital functions within&hellip;" data-term-url="/en/glossary/view/pigment/8522">pigments</mark> that can <mark class="term" data-term="absorb" data-term-def="Take in or soak up (energy, liquids, or other substances), usually gradually, through a chemical or physical action." data-term-url="/en/glossary/view/absorb/11219">absorb</mark> light and special structures that release chemicals onto other cells which can then send electrochemical <mark class="term" data-term="current" data-term-def="a flow, as of electricity or water. In oceanography and hydrology, a channel of water that flows together at the same velocity" data-term-url="/en/glossary/view/current/8278">currents</mark> to the brain, a <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> we perceive as vision.</p><p>Plants also contain a wide variety of <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> types. There are specialized cells called <em>collenchyma</em> that provide structure without restricting growth and flexibility. These cells lack secondary cell walls, and their primary cell walls lack a hardening agent, which especially helps young plants grow quickly and be resilient to wind and water. Other types of plant cells include xylem, whose purpose is to transport water throughout the plant, and <em>phloem</em>, whose purpose is to transport <mark class="term" data-term="organic" data-term-def="Originating from a living organism; a compound that contains hydrocarbons." data-term-url="/en/glossary/view/organic/8530">organic</mark> <mark class="term" data-term="nutrient" data-term-def="A chemical substance (e.g., minerals, vitamins, proteins) that is needed by an organism to survive and grow. See also: macronutrient and micronutrient." data-term-url="/en/glossary/view/nutrient/7058">nutrients</mark>.</p><p>The realm of cellular discovery is one that is still alive and well, despite its extensive history. In 2013, a group of European scientists identified a new <mark class="term" data-term="organelle" data-term-def="Structure or compartment within a cell that performs a specialized function such as respiration or photosynthesis. An organelle is analogous to&hellip;" data-term-url="/en/glossary/view/organelle/5281">organelle</mark> inside 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 tannin-producing plants, like grapevines and tea trees (Brillouet et al., 2013). Called <em>tannosomes</em>, the organelles originate within the <mark class="term" data-term="chloroplast" data-term-def="Organelle in plant and algae cells where photosynthesis occurs." data-term-url="/en/glossary/view/chloroplast/6543">chloroplasts</mark> and are responsible for creating the bitter tasting polyphenol that wards off predators and gives wine and tea their familiar “dry” feeling in the mouth. And in the same year, researchers in the United States identified that the types of <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> developed by ribosomes occurred in phases along with the phases of the cell cycle (Stumpf et al., 2013). Identifying <em>which</em> proteins are produced <em>when</em> has implications for cancer <mark class="term" data-term="research" data-term-def="A study or an investigation." data-term-url="/en/glossary/view/research/8257">research</mark>, since <mark class="term" data-term="hypothesis" data-term-url="/en/glossary/view/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;">hypotheses</mark> currently exist suggesting inefficient protein <mark class="term" data-term="synthesis" data-term-def="The production of a chemical compound by combining simpler compounds or elements." data-term-url="/en/glossary/view/synthesis/8756">synthesis</mark> (translation) in cancer cells. While it is easy to think that modern technological advances means that we’ve discovered all the components of cells, we must remember that, like <mark class="term" data-term="Robert Hooke" data-term-def="An English physicist, born on the Isle of Wight (1635-1703). Hooke's studies were extremely diverse, encompassing biology, geology, physics, chemistry, and&hellip;" data-term-url="/en/glossary/view/Hooke%2C+Robert/4460">Robert Hooke</mark>, there are sometimes things preventing us from seeing everything and that new discoveries may still await.</p><p><em>This module is an updated version of </em> <a href="http://www.visionlearning.com/en/library/Biology/2/The-Discovery-and-Structure-of-Cells/208"><em>The Discovery and Structure of Cells</em></a>.</p></section> <footer class="module__main__footer"> <hr class="border-color-dark"> <p class="citation"> <em> Heather MacNeill Falconer, M.A./M.S., Nathan H Lents, Ph.D. “Discovery and Structure of Cells” Visionlearning Vol. BIO-1 (2), 2003. </em> </p>  <div class="title-list" id="refs" name="refs"> <p class="h6 title-list__title"> References </p> <ul class="title-list__list"> <li><p></li> <li>Anderson, D. (2009). Overview: The curious observer. <em>Lens on Leeuwenhoek</em>. Retrieved from: http://lensonleeuwenhoek.net/content/overview-curious-observer.</li> <li>Brillouet, J. M., Romieu, C., Schoefs, B., Solymosi, K., Cheynier, V., Fulcrand, H., . . . Conejero, G. (2013). The tannosome is an organelle forming condensed tannins in the chlorophyllous organs of Tracheophyta. <em>Annals of Botany, 112</em>(6), 1003-1014.</li> <li>Harris, H. (2001). <em>The birth of the cell</em>. New Haven, CT: Yale University Press.</li> <li>Hooke, R. (1664). <em>Micrographia: Some physical descriptions of minute bodies made by magnifying glasses with observations and inquiries thereupon</em>. London: The Royal Society of London.</li> <li>Leeuwenhoek, A. van. (1674). More observations from Mr. Leeuwenhoek in a letter of Sept. 7, 1674 sent to the publisher. <em>Philosophical Transactions of the Royal Society, 9</em>, 178-182.</li> <li>Schwann, T. (1847). <em>Microscopic investigations on the accordance in the structure and growth of plants and animals</em>. (H. Smith, Trans.). London: The Sydenham Society. (Original work published in 1839).</li> <li>Stumpf, C. R., Moreno, M.V., Olshen, A. B., Tayloremail, B. S., Ruggero, D. (2013). The translational landscape of the mammalian cell cycle. <em>Molecular Cell, 52</em>(4), 574-582.</p></li> </ul> </div>  <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/Membranes-I/198"> <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_198-23061209062656.jpeg" alt="Membranes I"> </div> <div class="flex-grow-shrink"> <h2 class="h6 font-weight-normal"> Membranes I: <em>Structure and function of biological membranes</em> </h2> </div> </article> </a> </li> <li> <a class="no-hover-focus height-100" href="/en/library/Biology/2/Membranes-II/204"> <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_204-23061209062709.jpg" alt="Membranes II"> </div> <div class="flex-grow-shrink"> <h2 class="h6 font-weight-normal"> Membranes II: <em>Passive and active transporters</em> </h2> </div> </article> </a> </li> <li> <a class="no-hover-focus height-100" href="/en/library/Biology/2/Cell-Division-I/196"> <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_196-23061209062737.jpg" alt="Cell Division I"> </div> <div class="flex-grow-shrink"> <h2 class="h6 font-weight-normal"> Cell Division I: <em>The cell cycle</em> </h2> </div> </article> </a> </li> <li> <a class="no-hover-focus height-100" href="/en/library/Biology/2/Cellular-Organelles-I/195"> <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_195-23061209062726.jpeg" alt="Cellular Organelles I"> </div> <div class="flex-grow-shrink"> <h2 class="h6 font-weight-normal"> Cellular Organelles I: <em>Endosymbiosis and membrane-bound organelles</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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