BIO 281- Week 7 Notes
BIO 281- Week 7 Notes BIO 281
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Week 7 Chapter 7: Cellular Respiration Saturday, September 24, 2016 3:38 PM Complex Carbohydrates are made up of simple sugars Sugars belong to a class of moleculescalled carbohydrates, distinctive molecules composedof C, H, and O atoms usually in the ratio 1:2:1. The simplest carbohydrates are sugars (saccharides).Most sugars are linear containing five or six carbon atoms. A simple sugar is also called a monosaccharide and two simple sugars is called a disaccharide. Simple sugars combine to form polymerscalled polysaccharides. Long, branched chains of monosaccharides are called complex carbohydrates. All monosaccharidesin cells are in ring form, not linear structures. Cyclic glucose if formed when the oxygen atom of the hydroxyl group on carbon 5 forms a covalentbond with carbon 1, which is part of an aldehyde group. The presence of the polar hydroxyl groups through the sugar rind makes these moleculeshighly soluble in water. Monosaccharides are attached to each other by covalentbonds called glycosidic bonds. 7.1 An Overview of Cellular Respiration Cellular respiration is one of the major sets of catabolic reactions in a cell. During cellular respiration, fuel moleculesare catabolized into smaller units, releasing the energy stored in their chemical bonds to power the work of the cell. Cellular respiration uses chemical energy stored in molecules such as carbohydrates and lipids to produce ATP. Cellular respiration can occur in the presence of oxygen or in the absence of oxygen. Most organisms are capable of aerobic respiration; some bacteria respire anaerobically. For aerobic respiration, oxygen is consumed, and carbon dioxide and water are produced. C6H 12+66O ->26CO + 6H2O + e2ergy Carbohydrates and lipids have a large amount of potential energy in their chemical bonds, while molecules like carbon dioxide and water have less potential energy. Cellular respiration releases a large amount of energy because the sum of the potential energy in all of the chemical bonds of the reactants (glucose and oxygen) is higher than that of the products. The maximum amount of free energy- energy available to do work- released during cellular respiration is -686 kcal per mole of glucose. During this process, energy is released gradually in a series of chemical reactions. This allows some of this energy to be used to form ATP. On average, 32 moleculesof ATP are produced from the aerobic respiration of a single moleculeof glucose. Bio 281 Lecture Page 1 cellular respiration is -686 kcal per mole of glucose. During this process, energy is released gradually in a series of chemical reactions. This allows some of this energy to be used to form ATP. On average, 32 moleculesof ATP are produced from the aerobic respiration of a single moleculeof glucose. The energy needed to form one mole of ATP from ADP and P is at leasi 7.3 kcal. Thus cellular respiration harnesses at least 32 * 7.3 =233.6 kcal of energy in ATP for every mole of glucose that is broken down in the presence of oxygen. About 34% of the total energy released by aerobic respiration is harnessed in the form of ATP (233.6/686=34%),with the remainder of energy given off as heat. This degree of efficiency compares favorablywith that of a gasoline engine, which operates with an efficiency of about 25%. ATP is generated by substrate-level phosphorylation and oxidative phosphorylation. The hydrolysis reaction of a phosphorylated organic moleculeand the addition of a phosphate group to ADP releases enough free energy to drive the synthesis of ATP. This way of generating ATP is called substrate-level phosphorylation because a phosphate group is transferred to ADP from an enzyme-substrate,in this case an organic molecule. This only produces a small amount of the total ATP generated in cellular respiration, about 12% if the fuel moleculeis glucose. Most of the ATP is produced differently. In this case, the chemical energy is transferred first to electron carriers. These carry electrons from one reaction to another. In cellular respiration, electron carriers transport electronsreleased during the catabolism of organic moleculesto the respiratory electron transport chain. These chains transfer electrons along a series of membrane-associatedproteins to a final electron acceptor and in the process harness the energy released to produce ATP. Which in the case of aerobic respiration, oxygen is the final electron acceptor, resulting in the formation of water. This way of generating ATP is called oxidative phosphorylation. Redox reactions play a central role in cellular respiration. Chemical reactions in which electronsare transferred from one atom or molecule to another are referred to as oxidation-reduction reactions. Oxidation is the loss of electrons and reduction is the gain of electrons. The molecule that gains electronsis reduced, the molecule that loses electrons is oxidized. Two important electron carriers are nicotinamide adenine dinucleotide and flavin adenine dinucleotide. These carriers exist in two forms -- an oxidized form (NAD and + FAD) and a reduced form (NADH and FADH ). When 2uel moleculesare catabolized, oxidation reactions occur. These oxidation reactions are coupled with the reduction of electron carrier molecules.The reduction reactions can be written as: + - + NAD + 2e + H -> NADH FAD + 2e + 2H -> FADH 2 Here, these carriers accept electrons, resulting in the production of their reduced forms. In redox reactions involving organic moleculessuch as these carriers, the gain (or loss) of electronsis often accompanied by the gain (or loss) of protons (H ). As a result, reduced moleculescan easily be recognized by an increase in C-H bonds, and the corresponding oxidized moleculesby a decrease in C-H bonds. In these reduced forms, NADH and FADH can don2te electrons.The oxidation of these moleculesallows electrons (and energy) to be transferred to the electron transport chain. NADH -> NAD + 2e + H - + - + FADH -2 FAD + 2e + 2H These reactions produce NAD and FAD, which can then accept electrons from the Bio 281 Lecture Page 2 These reactions produce NAD and FAD, which can then accept electrons from the breakdown of fuel molecules. Cellular respiration can be understood as a redox reaction, even though it consists of many steps. In aerobic respiration, glucose is oxidized, releasing carbon dioxide, and at the same time oxygen is reduced, forming water: Glucose has many shared covalentbonds, which electrons are shared equally. This turns to carbon dioxide which does not share electrons equally. As a result, carbon has partially lost electrons to oxygen and is oxidized. For the reduction reaction, oxygen gas shares electrons equally between the two atoms. In water, the electrons that are shared are not equally distributed because oxygen ins more electronegativeand as a result, oxygen has partially gained electrons and is reduced. Glucose is a good electron donor because its oxidation to carbon dioxide releases a lot of energy, whereas oxygen can readily accept electrons. During cellular respiration, this energy is released in a controlled manner. Some of this energy can be used to synthesize ATP directly and some of it is stored temporarilyin reduced electron carriers and then used to generate ATP by oxidative phosphorylation. Cellular Respiration occurs in four stages Stage 1: Glucose is partially broken down to make pyruvate and energy is transferred to ATP and reduced electron carriers, a process known as glycolysis. Stage 2: pyruvate is oxidized to another moleculecalled acetyl-coenzymeA, producing reduced electron carriers and releasing carbon dioxide. Stage 3: Acetyl-CoA enters the citric acid cycle, also called the tricarboxylic(TCA) cycle or the Krebs cycle. In this series of chemical reactions, the acetyl group is completely oxidized to carbon dioxide and energy is transferred to ATP and reduced to electron carriers. The amount of energy transferred to ATP and reduced electron carriers in this stage is nearly twice that of stages 1 and 2 combined. Stage 4: oxidative phosphorylation- reduced electron carriers generated in stages 1-3 donate electrons to the ETC and a large amount of ATP is produced. In eukaryotes,glycolysis takes place in the cytoplasm, and pyruvate oxidation, the citric acid cycle and oxidative phosphorylationall take place in mitochondria.The ETC is made up of proteins and small moleculesassociated with the inner mitochondrial membrane. In some bacteria, these reactions take place in the cytoplasmand the ETC is located in the plasma membrane. The change in free energy is much greater for the steps that generate reduced electron carriers compared to those that produce ATP directly. 7.5 The Electron Transport Chain and Oxidative Phosphorylation The electron transport chain transfers electrons and pumps protons Electrons donated by NADH and FADH are 2ransported along a series of four large protein complexes that form the electron transport chain (complexes I to IV). These membrane proteins are embedded in the mitochondrialinner membrane. This membrane contains one of the highest concentrationsof proteins found in Bio 281 Lecture Page 3 Electrons donated by NADH and FADH are t2ansported along a series of four large protein complexesthat form the electron transport chain (complexesI to IV). These membrane proteins are embedded in the mitochondrialinner membrane. This membrane contains one of the highest concentrationsof proteins found in eukaryoticmembranes. Electrons enter the ETC at either complex I or II. Electrons donated by NADH enter through complex I, and electronsdonated by FADH enter 2hrough complexII. (ComplexII is the same enzyme that catalyzes step 6 in the citric acid cycle.) These electrons are transported through either complex to complexIII and through complex IV. Within each complex,electrons are passed from electron donors to electron acceptors. Each donor and acceptor is a redox couple, consisting of an oxidized and a reduced form of a molecule. The ETC contains many of these couples. When oxygen accepts electrons at the end of the ETC, it is reduced to form water: O2+ 4e + 4H -> 2H O 2 This reaction is catalyzed by complex IV. Electrons also must be transported between these four complexes. Coenzyme Q, (CoQ), also called ubiquinone, accepts electrons from both complexesI and II. Two electrons and two protons are transferred to CoQ from the mitochondrial matrix, forming CoQH . 2nce CoQH is f2rmed,it diffuses in the inner membrane to complex III. In complexIII, electrons are transferred from CoQH to 2ytochrome c and protons are released into the intermembranespace. When it accepts an electron, cytochromec is reduced, diffuses in the intermembrane space and passes the electron to complexIV. Each of these transfer steps are associated with the release of energy as electrons are passed. The proton gradient is a source of potential energy The inner mitochondrialmembrane is selectivelypermeable: Protonscannot passively diffuse across this membrane, and the movementof other moleculesis controlled by transporters and channels. Since the pumping of protons is coupled with the movementof electrons,the consequence is a proton gradient, a difference in proton concentration across the membrane. The proton gradient has two components:a chemical gradient due to the difference in concentrationand an electrical gradient due to the difference in charge between the two sides of the membrane. The proton gradient is also called an electrochemicalgradient. In summary, the citric acid cycle leads to the generation of a proton electrochemical gradient, which is a source of potential energy. This is used to synthesize ATP. ATP synthase converts the energy of the proton gradient into the energy of ATP In 1961,Peter Mitchell proposed a hypothesis to explain how the energy stored in the proton electrochemicalgradient is used to synthesize ATP. In 1978 he was awarded the Nobel Prize for work that changed how we understand energy is harnessed. For the potential energy of the gradient to be released, there must be an opening in the membrane. Protonsdiffuse down their electrical and concentrationgradients through a transmembraneprotein channel into the mitochondrialmatrix. The movementmust be coupled with the synthesis of ATP. This is made possible by ATP synthase. ATP synthase is composedof two subunits called F and 0 . F f1rms0the channel in the inner mitochondrialmembrane. F is the catalytic unit that synthesizes ATP. 1 Bio 281 Lecture Page 4 The movementmust be coupled with the synthesis of ATP. This is made possible by ATP synthase. ATP synthase is composedof two subunits called F and F 0 F for1s t0e channel in the inner mitochondrialmembrane. F is the1catalytic unit that synthesizes ATP. Proton flow through the channel causes it to rotate, converting the energy of the proton gradient into mechanical rotational energy, a form of kinetic energy. This leads to a rotation of the F 1ubunit in the mitochondrial matrix, which then causes conformationalchanges that allow it to catalyze the synthesis of ATP from ADP and P. ihis way the energy is convertedinto the chemical energy of ATP. Evidence for this idea is called the chemiosmotic hypothesis, did not come for over a decade. Overall in cellular respiration, Glucose hold potential energy in the covalent bonds. Energy is released in a series of reactions and captured in chemical form. These reactions generate ATP directly by substrate-levelphosphorylation. Other redox reactions transfer energy to electron carriers. These carriers donate electrons to the ETC, which uses the energy stored in the electron carriers to pump protons across the membrane. Energy of the reduced carriers is transformedinto energy stored in a proton gradient. ATP synthase converts the energy of the gradient to kinetic energy, which drives the synthesis of ATP. Bio 281 Lecture Page 5 7.2 Glycolysis: The Splitting of Sugar Glycolysisliterally means splitting sugar, because a 6-carbon sugar is split into two, yielding two 3-carbon molecules. The process is anaerobic. Glycolysisis the partial breakdown of glucose This process begins with a molecule of glucose and produces two 3-carbon molecules of pyruvate and a net total of two moleculesof ATP and two moleculesof the electron carrier NADH. ATP is produced directly by substrate-level phosphorylation. Glycolysisis a series of 10 chemical reactions. These reactions form three phases. Phase 1: Prepares glucose for the next two phases by the addition of two phosphate groups to glucose, requiring an input of energy. To supply this energy, two molecules of ATP are hydrolyzed per moleculeof glucose. The first phase is endergonic. The phosphorylation of glucose causes this form of glucose to be trapped in the cell and the presence of two negatively charged phosphate groups in proximity destabilizes the molecule so that it can be broken apart in the second phase of glycolysis. Phase 2: The 6-carbon molecule is split into two 3-carbon molecules. For each moleculeof glucose, two 3-carbon moleculesenter the third phase of glycolysis. Phase 3: ATP and the electron carrier NADH are produced. This phase ends with the production of two moleculesof pyruvate. Summary of this process: single moleculeof glucose produces two moleculesof pyruvate. In turn this produces four molecules of ATP and two moleculesof NADH. Two molecules of ATP are consumed during the initial phase of glycolysis, resulting of a net gain of two ATP and two NADH. 7.3 Pyruvate Oxidation Pyruvatecontains a good deal of chemical potential energy in its bonds. In the presence of oxygen, pyruvate can be oxidized further to release more energy. Pyruvateoxidation is a key step that links glycolysis to the critic acid cycle. This is the first step that takes inside the mitochondria. The oxidation of pyruvate connects glycolysisto the citric acid cycle The end product of glycolysis is pyruvate, which can be transported into the mitochondria. Mitochondria are made up of membranes that define two spaces, the intermembrane space, and the space enclosed by the inner membrane is called the mitochondrial matrix. Pyruvateis transported into the mitochondrial matrix, where it is convertedinto acetyl-CoA. First, part of the pyruvate moleculeis oxidized and splits off to form carbon dioxide, the most oxidized (and least energetic) form of carbon. The electrons lost in this moleculeare donated to NAD , which is reduced to NADH. The remaining part of the pyruvate molecule -an acetyl group (COCH )- 3till contains a large amount of potential energy that can be harnessed. It is transferred to coenzymeA (CoA), a moleculethat carries the acetyl group to the next set of reactions. Overall, the synthesis of one moleculeof acetyl-CoA from pyruvate results in the formationof one molecule of carbon dioxide and one molecule of NADH. A single molecule of glucose forms two moleculesof pyruvate during glycolysis. Bio 281 Lecture Page 6 Overall, the synthesis of one moleculeof acetyl-CoA from pyruvate results in the formationof one molecule of carbon dioxide and one molecule of NADH. A single molecule of glucose forms two moleculesof pyruvate during glycolysis. Two molecules of carbon dioxide, two moleculesof NADH and two moleculeof acetyl-CoA are produced from a starting glucose molecule. Acetyl-CoA is the substrate of the first step in the citric acid cycle. 7.4 The Citric Acid Cycle This is the stage where fuel molecules are completely oxidized. The acetyl group of acetyl-CoAis completely oxidized to carbon dioxide and the chemical energy is transferred to ATP by substrate-levelphosphorylation and to the reduced electron carriers NADH and FADH . 2 This supplies electrons to the electron transport chain, leading to the production of much more energy in the form of ATP than is obtained by glycolysis alone. The citric acid cycle produces ATP and reduced electron carriers The citric acid cycle takes place in the mitochondrialmatrix. It is composedof eight reactions and is called a cycle because the starting molecule,oxaloacetateis regenerated at the end. In the first reaction, the 2-carbon acetyl group of acetyl-CoA is transferred to a 4- carbon moleculeof oxaloacetateto form the 6-carbon moleculecitric acid or tricarboxylic acid. The molecule of citric acid is then oxidized in a series of reactions. The last reaction of the cycle regenerates a moleculeof oxaloacetate,joining to a new group and allowing the cycle to continue. The citric acid cycle results in the complete oxidation of the acetyl group of acetyl- CoA. Since the first reaction creates a moleculewith six carbons and the last reaction regenerates a 4-carbon molecule,two carbons are eliminated during the cycle. Carbons are released as carbon dioxide and combined with the carbon dioxide during pyruvate oxidation, these reactions are the sources of carbon dioxide that we exhale when we breathe. These reactions that produce carbon dioxide are coupled with the reduction of the electron carrier NAD to NADH. Energy released in the oxidation reactions is transferred to NADH. More reduced electron carriers are produced in two additional redox reactions. The citric acid cycle produces a large quantity of reduced electron carriers: three moleculesof NADH and one moleculeof FADH per tur2 of the cycle. These donate Bio 281 Lecture Page 7 moleculesof NADH and one moleculeof FADH per turn 2f the cycle. These donate electrons to the ETC, which produces ATP through oxidativephosphorylation. One of the reactions of the citric acid cycle is a substrate-levelphosphorylation reaction that generates a molecule of GTP. GTP transfers its terminal phosphate to a moleculeof ADP to form ATP. This is the only substrate-levelphosphorylation in the citric acid cycle. Overall, two molecules of acetyl-CoAproduced from a single molecule of glucose yield two molecules of ATP, six molecules of NADH and two molecules of FADH in 2 the citric acid cycle. 7.6 Anaerobic metabolism and the evolution of cellular respiration One of the major forks in the metabolic road occurs at pyruvate, the end product of glycolysis. When oxygen is present, it is convertedto acetyl-CoA,which then enters the citric acid cycle, resulting in the production of ATP and reduced electron carriers to fuel the electron transport chain. When oxygen is not present, pyruvate is metabolized along a number of different pathways. Fermentation extracts energy from glucose in the absence of oxygen In the absence of oxygen, pyruvate can be broken down by fermentation, which does not rely on oxygen or any other electron acceptor. Some organisms such as yeast favor fermentationover oxidative phosphorylation, even in the presence of oxygen. This is sometimesused in aerobic organisms, for example when exercising muscle. + During glycolysis, glucose is oxidized to form pyruvate, and NAD is reduced to form NADH. To continue, NADH must be oxidized in the ETC to NAD . If that did not happen, glycolysiswould stop. With oxygen, NAD is regenerated when NADH donates its electrons to the ETC. + Without oxygen during fermentation,NADH is oxidized to NAD when pyruvate or a derivative of pyruvate is reduced. There are many fermentation pathways, two major pathways are lactic acid fermentation and ethanol fermentation. Lactic acid fermentationoccurs in animals and bacteria. During lactic acid fermentation,electrons from NADH are transferred to pyruvate to produce lactic acid and NAD .+ Glucose + 2 ADP + 2 P -i 2 lactic acid + 2 ATP + 2H O 2 Ethanol formationoccurs in plants and fungi. During ethanol fermentation,pyruvate releases carbon dioxide to form acetaldehyde, and electronsfrom NADH are + transferred to acetaldehyde to produce ethanol and NAD . Glucose + 2 ADP + 2 P -> 2 ethanol + 2 ATP + 2H O i 2 In both pathways, NADH is oxidized to NAD . + These do not appear in the overall equation because there is no net production or + loss of either. NAD molecules that are reduced during glycolysisare oxidized when lactic acid or ethanol is formed. The breakdown of a molecule of glucose by fermentationyields only two molecules of ATP. The energetic gain is always small compared with aerobic respiration because the products are not fully oxidized and still contain a large amount of chemical energy in the bonds. Bio 281 Lecture Page 8 Bio 281 Lecture Page 9