Cellular Respiration: How Cells Use Stored Energy Chapter 8 Killer Bees Descendents of African honeybees that were imported to Brazil in the 1950s More aggressive, wider-ranging than other honeybees Africanized bee s muscle cells have large mitochondria AT -- Energy Storage hotosynthesizers get energy from the sun Animals get energy second- or third-hand from plants or other organisms In either case, the energy is converted to the chemical bond energy of AT The phosphate groups (O 3 ) of AT carry (-) charge, so they tend to repel each other Getting rid of a phosphate results in more stable products, lower energy. Adding phosphates requires an energy input and the energy is stored there. 1
Making AT (when we say make AT, we usually mean the attachment of that last hosphate group to AD) lants make AT during photosynthesis (use at least some of it fixing CO into carbohydrate) Cells of all organisms make AT by breaking down carbohydrates, fats, and protein Cells also have ways to construct adenine and ribose and put them together interestingly, those pathways require AT.. Main Types of Energy-Releasing athways Anaerobic pathways Aerobic pathways Evolved first Don t require oxygen Start with glycolysis in cytoplasm Completed in cytoplasm Evolved later Require oxygen Start with glycolysis in cytoplasm Completed in mitochondria Summary Equation for Aerobic Respiration C 6 H 1 0 6 + 6 O 6 CO + 6 H 0 glucose oxygen carbon water dioxide
CYTOLASM glucose AT AT 4 Glycolysis e - + H + ( AT net) pyruvate e Overview - + H CO of + Aerobic e 8 4 CO Krebs Respiration - + H + e - + H + FADH Cycle AT e - Electron Transfer hosphorylation 3 AT H + water e - + oxygen Typical Energy Yield: 36 AT Figure 8.3 age 135 The Role of Coenzymes NAD NAD and FAD accept electrons and hydrogen Become and FADH Deliver electrons and hydrogen to the mitochondrial electron transfer chain FAD First part is Glycolysis Breaking down Glucose A simple sugar (C 6 H 1 O 6 ) Atoms held together by covalent bonds In-text figure age 136 3
Glycolysis Occurs in Two Stages Energy-requiring steps AT energy activates glucose (and its six-carbon derivatives) Energy-releasing steps The products of the first part are split into threecarbon pyruvate molecules AT and form Energy-Requiring Steps Energy-Requiring Steps of Glycolysis AT invested AT glucose AD glucose-6-phosphate AT fructose-6-phosphate AD fructose1,6-bisphosphate GAL GAL Figure 8.4() age 137 NAD + GAL i 1,3-bisphosphoglycerate AD AT 3-phosphoglycerate NAD + GAL i Energy- AD Releasing Steps 1,3-bisphosphoglycerate AT 3-phosphoglycerate -phosphoglycerate H O E AD AT pyruvate -phosphoglycerate H O E AD AT pyruvate Figure 8.4 age 137 4
Glycolysis: Net Energy Yield Energy requiring steps: AT invested Energy releasing steps: formed 4 AT formed Net yield is AT and Second Stage Reactions reparatory reactions yruvate is oxidized into two-carbon acetyl units and carbon dioxide NAD + is reduced Krebs cycle The acetyl units are oxidized to carbon dioxide NAD + and FAD are reduced reparatory Reactions NAD + pyruvate coenzyme A (CoA) O S - CoA acetyl-coa O carbon dioxide 5
Coenzyme A Krebs Cycle oxaloacetate NAD + =CoA acetyl-coa CoA citrate H O H O H O FADH FAD malate fumarate succinate isocitrate NAD + O O α-ketoglutarate NAD + CoA O O succinyl-coa Figure 8.6 age 139 AT AD + phosphate group The Krebs Cycle Overall Reactants Overall roducts Acetyl-CoA 3 NAD + FAD AD and i Coenzyme A CO 3 FADH AT 6
Results of the Second Stage All of the carbon molecules in pyruvate end up in carbon dioxide Coenzymes are reduced (they pick up electrons and hydrogen) One molecule of AT forms Four-carbon oxaloacetate regenerated Coenzyme Reductions during Glycolysis and Krebs cycle: Glycolysis reparatory reactions Krebs cycle FADH + 6 Total FADH + 10 Now, the third stage: Electron Transfer hosphorylation Occurs in the mitochondria Coenzymes deliver electrons to electron transfer chains Electron transfer sets up H + ion gradients (this is why the reduced coenzymes are so important!) Flow of H + down gradients powers AT formation (AT synthase) 7
Creating an H + Gradient OUTER COMARTMENT INNER COMARTMENT Making AT: Chemiosmotic Model AT AD + i INNER COMARTMENT Importance of Oxygen Electron transport phosphorylation requires the presence of oxygen Oxygen withdraws spent electrons from the electron transfer chain, then combines with H + to form water If the electrons are not removed, the chain of transfer stops) 8
Summary of Energy Harvest (per molecule of glucose) Glycolysis - AT formed by substrate-level phosphorylation Krebs cycle - AT formed by substrate-level phosphorylation Electron transport phosphorylation - 3 AT formed 36 AT is typical yield (can be less if intermediates are pulled away to be used elsewhere) 9
Anaerobic athways Do not use oxygen roduce less AT than aerobic pathways Two types Fermentation pathways Anaerobic electron transport Fermentation athways Begin with glycolysis Do not break glucose down completely to carbon dioxide and water Yield only the AT from glycolysis Steps that follow glycolysis serve only to regenerate NAD + GLYCOLYSIS Lactate Fermentation C 6 H 1 O 6 AT energy input AD NAD + 4 AT energy output pyruvate AT net LACTATE FORMATION Muscle pain lactate electrons, hydrogen from 10
Alcoholic Fermentation GLYCOLYSIS AT C 6 H 1 O 6 energy input AD NAD + 4 AT energy output pyruvate AT net ETHANOL FORMATION H O CO acetaldehyde electrons, hydrogen from ethanol Anaerobic Electron Transport Carried out by certain bacteria Electron transfer chain is in bacterial plasma membrane Final electron acceptor is compound from environment (such as nitrate), not oxygen AT yield is low fatty acids FOOD complex fats glycogen Alternative Energy carbohydrates proteins Sources glycerol simple sugars glucose-6-phosphate GLYCOLYSIS GAL pyruvate amino acids NH 3 urea carbon backbones acetyl-coa KREBS CYCLE Figure 8.11 age 145 11
Fats and Amino acids Evolution of Metabolic athways When life originated, atmosphere had little oxygen Earliest organisms used anaerobic pathways Later, noncyclic pathway of photosynthesis increased atmospheric oxygen Cells arose that used oxygen as final acceptor in electron transport rocesses Are Linked: sunlight energy HOTOSYNTHESIS water + carbon dioxide sugar molecules oxygen AEROBIC RESIRATION In-text figure age 146 1