{"id":242,"date":"2019-06-24T12:44:33","date_gmt":"2019-06-24T16:44:33","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/053humanbiology\/chapter\/4-10-cellular-respiration\/"},"modified":"2022-01-25T02:14:27","modified_gmt":"2022-01-25T07:14:27","slug":"4-10-cellular-respiration","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/053humanbiology\/chapter\/4-10-cellular-respiration\/","title":{"raw":"4.10\u00a0Cellular Respiration","rendered":"4.10\u00a0Cellular Respiration"},"content":{"raw":"Created by:\u00a0CK-12\/Adapted by Christine Miller\r\n\r\n[caption id=\"attachment_241\" align=\"aligncenter\" width=\"333\"]\"Image Figure 4.10.1 Ready to make s'mores!<\/em>[\/caption]\r\n\r\n
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Bring on the S'mores!<\/h1>\r\n<\/div>\r\nThis inviting camp fire can be used for both heat and light. Heat and light are two forms of [pb_glossary id=\"1342\"]energy[\/pb_glossary] that are released when a fuel like wood is burned. The [pb_glossary id=\"1298\"]cells [\/pb_glossary]of living things also get energy by \"burning.\" They \"burn\" [pb_glossary id=\"1191\"]glucose[\/pb_glossary] in a process called [pb_glossary id=\"1328\"] cellular respiration[\/pb_glossary].\r\n
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What Is Cellular Respiration?<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"1328\"]Cellular respiration[\/pb_glossary]<\/strong>\u00a0is the process by which living\u00a0cells\u00a0break down [pb_glossary id=\"1191\"]glucose[\/pb_glossary] molecules and release\u00a0[pb_glossary id=\"1342\"]energy[\/pb_glossary]. The process is similar to burning, although it doesn\u2019t produce light or intense\u00a0heat\u00a0as a campfire does. This is because cellular respiration releases the energy in glucose\u00a0slowly\u00a0<\/em>and\u00a0<\/em>in many small steps. It uses the\u00a0energy\u00a0released to form molecules of\u00a0[pb_glossary id=\"1240\"]ATP[\/pb_glossary]<\/strong>, the energy-carrying molecules that\u00a0cells\u00a0use to power biochemical processes. In this way, cellular respiration is an example of energy coupling: glucose is broken down in an exothermic reaction, and then the energy from this reaction powers the endothermic reaction of the formation of ATP.\u00a0 Cellular respiration involves many\u00a0chemical reactions, but they can all be summed up with this chemical equation:\r\n

C6<\/sub>H12<\/sub>O6<\/sub>\u00a0 6O2<\/sub>\u00a0\u2192 6CO2<\/sub>\u00a0 6H2<\/sub>O Chemical Energy (in ATP)<\/strong><\/p>\r\nIn words, the equation shows that glucose (C6<\/sub>H12<\/sub>O6<\/sub>)\u00a0and oxygen (O2<\/sub>)\u00a0react to form carbon dioxide (CO2<\/sub>) and\u00a0water\u00a0(H2<\/sub>O), releasing energy in the process. Because oxygen is required for cellular respiration, it is an\u00a0[pb_glossary id=\"1762\"]aerobic<\/strong>[\/pb_glossary] process.\r\n\r\nCellular respiration occurs in the [pb_glossary id=\"1298\"]cells[\/pb_glossary] of all living things, both [pb_glossary id=\"1800\"]autotrophs[\/pb_glossary] and [pb_glossary id=\"1988\"]heterotrophs[\/pb_glossary]. All of them burn [pb_glossary id=\"1191\"]glucose[\/pb_glossary] to form [pb_glossary id=\"1240\"]ATP[\/pb_glossary]. The reactions of [pb_glossary id=\"1328\"]cellular respiration[\/pb_glossary] can be grouped into three stages: glycolysis, the Krebs cycle (also called the citric acid cycle), and electron transport. Figure 4.10.2 gives an overview of these three stages, which are also described in detail below.\r\n\r\n[caption id=\"attachment_241\" align=\"aligncenter\" width=\"720\"]\"Image Figure 4.10.2 Cellular respiration takes place in the stages shown here. The process begins with a molecule of glucose, which has six carbon atoms. What happens to each of these atoms of carbon?<\/em>[\/caption]\r\n\r\n

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Cellular Respiration Stage I:\u00a0Glycolysis<\/h1>\r\n<\/div>\r\nThe first stage of cellular respiration is\u00a0[pb_glossary id=\"1969\"]glycolysis[\/pb_glossary]<\/strong>,\u00a0which happens\u00a0in the [pb_glossary id=\"1334\"]cytosol[\/pb_glossary] of the [pb_glossary id=\"1198\"]cytoplasm[\/pb_glossary].\r\n

Splitting Glucose<\/h2>\r\nThe word\u00a0glycolysis<\/em>\u00a0literally means \u201cglucose splitting,\u201d which is exactly what happens in this stage.\u00a0[pb_glossary id=\"1345\"]Enzymes[\/pb_glossary]\u00a0split a molecule of glucose into two molecules of pyruvate (also known as pyruvic acid). This occurs in several steps, as summarized in the\u00a0following\u00a0diagram.\r\n\r\n[caption id=\"attachment_241\" align=\"aligncenter\" width=\"784\"]\"\" Figure 4.10.3 Glycolysis is a complex ten-step reaction that ultimately converts glucose into two molecules of pyruvate.\u00a0This releases energy, which is transferred to ATP. How many ATP molecules are made during this stage of cellular respiration?<\/em>[\/caption]\r\n

Results of Glycolysis<\/h2>\r\nEnergy is needed at the start of [pb_glossary id=\"1969\"]glycolysis[\/pb_glossary] to split the glucose molecule into two pyruvate molecules which go on to stage II of cellular respiration. The energy needed to split glucose is provided by two molecules of ATP; this is called the energy investment phase. As glycolysis proceeds, energy is released, and the energy is used to make four molecules of ATP; this is the energy harvesting phase. As a result, there is a net gain<\/em> of two ATP molecules during glycolysis. During this stage, high-energy electrons are also transferred to molecules of NAD \u00a0to produce two molecules of NADH, another energy-carrying molecule. NADH is used in stage III of cellular respiration to make more ATP.\r\n
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Structure of the Mitochondrion<\/h2>\r\n<\/div>\r\n\r\n[caption id=\"attachment_241\" align=\"alignleft\" width=\"496\"]\"Image Figure 4.10.5 Labelled mitochondrion structure. <\/em>[\/caption]\r\n\r\nBefore you read about the remaining stages of cellular respiration, you need to know more about the [pb_glossary id=\"1357\"]mitochondrion[\/pb_glossary], where these stages take place. The structure of the mitochondrion plays an important role in aerobic respiration. A diagram of a mitochondrion is shown in Figure 4.10.5.\r\n
\r\n\r\nAs you can see from the figure, a mitochondrion has an inner and outer membrane. The space between the inner and outer membrane is called the <\/span>[pb_glossary id=\"1280\"]intermembrane space[\/pb_glossary]<\/strong>. The space enclosed by the inner membrane is called the\u00a0<\/span>[pb_glossary id=\"2068\"]matrix[\/pb_glossary]<\/strong>. The second stage of cellular respiration (the citric acid cycle) takes place in the matrix. The third stage (electron transport) happens on the inner membrane.<\/span>\r\n\r\n<\/div>\r\n
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Transition Reaction<\/h2>\r\nBefore pyruvate can enter the next stage of cellular respiration it needs to be modified slightly.\u00a0 The transition reaction is a very short reaction which converts the two molecules of pyruvate that enter the mitochondrion to two molecules of acetyl CoA, and two pairs of high energy electron from this process convert NAD to NADH. Pyruvate is a 3 carbon molecule, and acetyl CoA is a two carbon molecule, so this process also creates two molecules of carbon dioxide.\u00a0 \u00a0The carbon dioxide is released, the acetyl CoA moves to the Citric Acid Cycle (stage II), and the NADH carries the high energy electrons to the Electron Transport System (stage III).\r\n\r\n[caption id=\"attachment_241\" align=\"aligncenter\" width=\"747\"]\"In Figure 4.10.14: During the Transition Reaction, pyruvate is converted to acetyl CoA and carbon dioxide.<\/em>[\/caption]\r\n

Cellular Respiration Stage II: The Citric Acid Cycle<\/h1>\r\n<\/div>\r\nRecall that [pb_glossary id=\"1969\"]glycolysis[\/pb_glossary] produces two molecules of pyruvate (pyruvic acid), which are then converted to acetyl CoA during the short transition reaction. These molecules enter the matrix of a mitochondrion, where they start the [pb_glossary id=\"2037\"]Citric Acid Cycle[\/pb_glossary]\u00a0<\/strong>(also known as the Krebs cycle). The reason this stage is considered a cycle is because a molecule called oxaloacetate is present at both the beginning and end of this reaction and is used to break down the two molecules of acetyl CoA.\u00a0 The reactions that occur next are shown in Figure 4.10.6.\r\n\r\n[caption id=\"attachment_2668\" align=\"alignnone\" width=\"1024\"]\"Image Figure 4.10.6 Reactants and products of the citric acid cycle.<\/em>[\/caption]\r\n\r\n
\r\n\r\nSteps of the citric acid cycle<\/span>\r\n\r\n<\/div>\r\nThis cyclical process is often referred to as the Krebs cycle, for Hans Krebs who first described it and was awarded a Nobel prize in 1953 for doing so. The\u00a0 [pb_glossary id=\"2037\"]Citric acid cycle (Krebs cycle)[\/pb_glossary]itself actually begins when acetyl-CoA combines with a four-carbon molecule called OAA (oxaloacetate) (see Figure 4.10.6). This produces citric acid, which has six carbon atoms.\r\n\r\nAfter citric acid forms, it goes through a series of reactions that release energy. The energy is captured in molecules of NADH, ATP, and FADH2<\/sub>, another energy-carrying\u00a0coenzyme. Carbon dioxide is also released as a waste product of these reactions.\r\n\r\nThe final step of the citric acid cycle regenerates<\/em> OAA, the molecule that began the cycle. This molecule is needed for the next turn through the cycle. Two turns are needed because glycolysis produces two<\/em> pyruvic acid molecules when it splits glucose.\r\n

Results of the Glycolysis, Transition Reaction and Citric Acid Cycle<\/h2>\r\nAfter glycolysis, transition reaction, and the citric acid cycle, the glucose molecule has been broken down completely. All six of its carbon atoms have combined with oxygen to form carbon dioxide. The energy from its chemical bonds has been stored in a total of 16 energy-carrier molecules. These molecules are:\r\n