How Cells Harvest Chemical Energy Chapter 6 Cellular Respiration

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  • How Cells Harvest Chemical EnergyChapter 6Cellular Respiration

  • Energy Flow and Chemical Cyclingin the BiosphereFuel molecules in foodrepresent solar energytraced back to the sunAnimals depend on plants:to convert solar energy to chemical energyIn form of sugars and other organic molecules

  • Gas Exchange in the BodyCellular respiration and breathing are closely relatedCellular respiration requires a cell to exchange gases with its surroundingsBreathing exchanges these gases between blood and outside air

  • Cellular respirationCellular respiration is an exergonic process that transfers energy from the bonds in glucose to ATPproduces 38 ATP molecules from each glucose moleculeOther foods (organic molecules) can be used as a source of energy as well

  • Cellular RespirationRelease of energy from molecules accompanied by the use of this energy to synthesize ATP moleculesMetabolic pathway Main method that chemical energy is harvested from food and converted to ATPAerobicRequires oxygen and gives off carbon dioxide

  • Where Is the Energy in Food?The process of aerobic respiration requires oxygen and carbohydrates

    C6H12O6 + 6 O2 6 CO2 + 6 H2O + energy

    The products are carbon dioxide, water, and energy (heat or ATP)

  • How do we get the energy??Energy contained in the arrangement of electrons in chemical bonds in organic moleculesCells tap energy from electrons falling from organic fuels to oxygenWhen the carbon-hydrogen bonds of glucose are broken, electrons are transferred to oxygenOxygen has a strong tendency to attract electrons

  • ATPAdenosine triphosphate (ATP)Nucleotide with the base adenine and the sugar ribose Main energy carrier in cellsFormed during reactions that breakdown organic compounds to CO2 and waterRequires ample oxygenOccurs within the mitochondrionHydrolyzes phosphates to release energy form adenosine diphosphate (ADP)

  • Redox Reaction (O-R)Chemical reaction that transfers electrons from one substance to another electrons retain their potential energyGlucose loses its hydrogen atoms and is ultimately converted to CO2O2 gains hydrogen atoms and is converted to H2O

    Oxidation Reduction

  • Redox ReactionElectrons pass from atoms or molecules to one another as part of many energy reactionsOxidation When an atom or molecule loses an electronGlucose is oxidizedReduction When an atom or molecule gains an electionsOxygen is reduced

  • Other important playersEnzymes are necessary to oxidize glucose and other foodsDehydrogenaseenzyme that removes hydrogen from an organic molecule requires a coenzyme called NAD+ (nicotinamide adenine dinucleotide)shuttle electronsNAD+ can become reduced when it accepts electrons and oxidized when it gives them upReduced to NADH

  • The Finale.First step is transfer of electrons from organic molecule to NAD+Other electron carrier molecules represent the electron transport chainUndergoes series of redox reactionsRelease energy to make ATP

  • ATPNAD+NADHH+H+2e2eElectron transportchainControlledrelease ofenergy forsynthesisof ATP+O2H2O12

  • Stages of Cellular Respiration:GlycolysisCitric Acid Cycle Oxidative Phosphorylation

  • MitochondrionCO2CO2NADHATPHigh-energy electronscarried by NADHNADHCITRIC ACIDCYCLEGLYCOLYSISPyruvateGlucoseandFADH2Substrate-levelphosphorylationSubstrate-levelphosphorylationOXIDATIVEPHOSPHORYLATION(Electron Transportand Chemiosmosis)OxidativephosphorylationATPATPCytoplasmInnermitochondrialmembrane

  • 1. Glycolysis Occurs in the cytoplasmDoes not require oxygen to generate ATPThen, enters aerobic or anaerobic reactions

  • Glycolysis6 Carbon oxidizes glucose into 2 molecules Pyruvate3 Carbonbreaking of the bond yields energy that is used to phosphorylate ADP to ATPin addition, electrons and hydrogen are donated to NAD+ to form NADH GlucoseNAD++22 ADPNADH2P22ATP2+H+2 Pyruvate

  • +ADPATPSubstrateEnzymeProductEnzymePPP

  • Anaerobic verse aerobic?Absence of oxygenFermentationMake lactate or ethanolPresence of oxygenOxidative respirationPyruvate transported to mitochondriaOxidize pyruvate to form acetyl-coA

  • When pyruvate is oxidized:A single carbon cleaved off by the enzyme pyruvate dehydrogenaseThis carbon leaves as part of a CO2 moleculehydrogen and electrons are removed from pyruvate donated to NAD+ to form NADHRemaining two-carbon fragment of pyruvate is joined to a cofactor called coenzyme A (CoA)Final compound called acetyl-CoA

  • Acetyl-CoAThe fate of acetyl-CoA depends on the availability of ATP in the cellInsufficient ATPThe acetyl-CoA heads to the Krebs cyclePlentiful ATPThe acetyl-CoA is diverted to fat synthesis for energy storage

  • 2. Citric Acid CycleKrebs Cycleoccurs within the mitochondrionBreaks down pyruvate into carbon dioxideelectrons passed to an electron transport chain in order to power the production of ATP

  • Stages of Citric Acid Cycleacetyl (two-carbon) compound enters the citric acid cycleAcetyl-CoA enters the cycle and binds to a four-carbon molecule, forming a six-carbon moleculeTwo carbons are removed as CO2 and their electrons donated to NAD+ In addition, an ATP is producedThe four-carbon molecule is recycled and more electrons are extracted, forming NADH and FADH2

  • The Krebs cycleNote: a single glucose molecule produces two turns of the cycle, one for each of the two pyruvate molecules generated by glycolysisCITRIC ACID CYCLENAD+NADH3 H+CO2332CoACoAAcetyl CoAPADP +ATPFADH2FAD

  • CytoplasmGlucoseFADH2MitochondrionMaximum per glucose:OXIDATIVEPHOSPHORYLATION(Electron Transportand Chemiosmosis)CITRIC ACIDCYCLEElectron shuttleacross membrane2NADH2NADH2NADH6NADH2(or 2 FADH2)2 AcetylCoAGLYCOLYSIS2PyruvateAbout38 ATP about 34 ATPby substrate-levelphosphorylationby oxidative phosphorylation 2 ATPby substrate-levelphosphorylation 2 ATP

  • 3. Oxidative PhosphorylationElectron transport chainShuttle molecules NADH and FADH take electrons to oxygen Final acceptorForms H2OCarriers bind and release electrons in redox reactionsPass electrons down the energy staircaseUse energy released from the transfers to transport H+

  • ATPH+Intermembranespace

    O2H2O12InnermitochondrialmembraneH+NAD+H+H+H+H+H+H+H+H+H+H+H+H+MitochondrialmatrixElectronflowElectroncarrierProteincomplexof electroncarriersNADHFADH2FADATPsynthasePADP +Chemiosmosis+ 2OXIDATIVE PHOSPHORYLATIONElectron Transport Chain

  • 3. Oxidative PhosphorylationChemiosmosisUses energy stored in a hydrogen ion gradient to drive ATP synthesisH+ concentration gradient stores potential energyATP synthase drives hydrogen ions through Generates ATP

  • CytoplasmGlucoseFADH2MitochondrionMaximum per glucose:OXIDATIVEPHOSPHORYLATION(Electron Transportand Chemiosmosis)CITRIC ACIDCYCLEElectron shuttleacross membrane2NADH2NADH2NADH6NADH2(or 2 FADH2)2 AcetylCoAGLYCOLYSIS2PyruvateAbout38 ATP about 34 ATPby substrate-levelphosphorylationby oxidative phosphorylation 2 ATPby substrate-levelphosphorylation 2 ATP

  • Fermentation Occurs when O2 is not availableAnimal cells and bacteria convert pyruvate to lactateOther organisms convert pyruvate to alcohol and CO2

  • Glucose Is Not the Only Food MoleculeCells also get energy from foods other than sugars

    The other organic building blocks undergo chemical modifications that permit them to enter cellular respiration

  • Food, such aspeanutsProteinsFatsCarbohydratesGlucoseOXIDATIVEPHOSPHORYLATION(Electron Transportand Chemiosmosis)CITRICACIDCYCLEAcetylCoAGLYCOLYSISPyruvateAmino acidsGlycerolSugarsFatty acidsAmino groupsG3PATP

  • Do plants perform cellular respiration??

    *Respiration only retrieves 40% of the energy in a glucose molecule. The other 60% of the energy is released as heat. We use this heat to maintain a relatively steady body temperature near 37C (9899F). This is about the same amount of heat generated by a 75-watt incandescent light bulb. Organic compounds possess potential energy as a result of their arrangement of atoms.Compounds that can participate in exergonic reactions can act as food.Actually, cellular respiration includes both aerobic and anaerobic processes. However, it is generally used to refer to the aerobic process.It takes about 10 million ATP molecules per second to power one active muscle cell.

    Student Misconceptions and Concerns1. Students should be cautioned against the assumption that energy is created when it is converted from one form to another. This might be a good time to review the principle of conservation of energy (the first law of thermodynamics addressed in Module 5.11).2. Students often fail to realize that aerobic metabolism is a process generally similar to the burning of wood in a fireplace or campfire or the burning of gasoline in an automobile engine. Noting these general similarities can help students comprehend the overall reaction and heat generation associated with these processes.

    Teaching Tips1. Energy coupling at the cellular level may be new to many students, but it is a familiar concept when related to the use of money in our society. Students might be discouraged if the only benefit of work was the ability to make purchases from the employer. (We all might soon tire of a fast-food job that only paid its employees in food!) Money permits the coupling of a generation of value (a paycheck, analogous to an energy-releasing reaction) to an energy-consuming reaction (money, which allows us to make purchases in distant locations). This idea of earning and spending is a common concept we all know well.2. During cellular respiration, our cells convert about 40% of our food energy to useful work. The other 60% of the energy is released as heat. We use this heat to maintain a relatively steady body temperature near 37C (9899F). This is about the same amount of heat generated by a 75-watt incandescent lightbulb. If you choose to include a discussion of heat generation from aerobic metabolism, consider the following. A. Ask your students why they feel warm when it is 30C (86F) outside, if their core body temperature is 37C (98.6F). Shouldnt they feel cold? The answer is, our bodies are always producing heat. At these higher temperatures, we are producing more heat than we need to maintain a body temperature around 37C. Thus, we sweat and behave in ways that helps us get rid of the extra heat from cellular respiration.B. Share this calculation with your students. Depending upon a persons size and level of activity, a human might burn 2,000 dietary calories (kilocalories) a day. This is enough energy to raise the temperature of 20 liters of liquid water from 0 to 100C. This is something to think about the next time you heat water on the stove! (Notes: Consider bringing a 2-liter bottle as a visual aid, or ten 2-liter bottles to make the point above. It takes 100 calories to raise 1 liter of water 100C; it takes much more energy to melt ice or evaporate water as steam.)

    ****Figure 6.6 An overview of cellular respiration.****Figure 6.12 An estimated tally of the ATP produced by substrate-level and oxidative phosphorylation in cellular respiration.*Figure 6.10 Oxidative phosphorylation, using electron transport and chemiosmosis in the mitochondrion.*Figure 6.12 An estimated tally of the ATP produced by substrate-level and oxidative phosphorylation in cellular respiration.**Figure 6.15 Pathways that break down various food molecules.

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