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+ Chapter 9: Cellular Respiration: Harvesting Chemical Energy AP Biology + Overview: Life Is Work Living cells require energy from outside sources Herbivore: giant panda, obtain energy by eating plants Carnivore: hyena eats other animals Omnivore: squirrel eats insects and seeds and chicken legs? + Energy flows into ecosystem as sunlight and leaves as heat Photosynthesis O2 &organic molecules (used in cellular respiration) Cells use chemical energy stored in organic molecules to regenerate ATP, which powers work Concept 9.1: Catabolic pathways yield + energy by oxidizing organic fuels MULTIPLE processes central to cellular respiration and related pathways Breakdown of organic molecules is exergonic Cellular respiration: includes both aerobic and anaerobic respiration; often used to refer to aerobic respiration Aerobic respiration: consumes organic molecules and O2; yields ATP Anaerobic respiration: similar to aerobic respiration but consumes compounds other than O2 Fermentation: partial degradation of sugars that occurs without O2 Carbohydrates, fats, and proteins all consumed as fuel, however it is helpful to trace cellular respiration with the sugar glucose: C6H12O6 + 6 O2 6 CO2 + 6 H2O + Energy (ATP + heat) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Redox Reactions: + Oxidation and Reduction Transfer of electrons during chemical reactions releases energy stored in organic molecules used to synthesize ATP Oxidation Is Loss Reduction Is Gain Redox reactions: oxidation-reduction reactions; chemical reactions that transfer electrons between reactants Oxidation: substance loses electrons, or is oxidized Reduction: substance gains electrons, or is reduced (the amount of positive charge is reduced) REMEMBER: OIL-RIG Fig. 9-UN1 + becomes oxidized (loses electron) becomes reduced (gains electron) + Reducing agent: electron donor Oxidizing agent: electron receptor is called the Some redox reactions do not transfer electrons but change electron sharing in covalent bonds Ex: reaction between methane and O2 Oxidation of Organic Fuel Molecules + During Cellular Respiration During cellular respiration, the fuel (such as glucose) is oxidized, and O2 is reduced: Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Stepwise Energy Harvest via NAD+ and the + Electron Transport Chain Cellular respiration: glucose & other organic molecules broken down in a series of steps NAD+: Electrons from organic compounds usually first transferred to NAD+, a coenzyme NADH: reduced form of NAD+ represents stored energy that is tapped to synthesize ATP NADH passes electrons to electron transport chain functions as oxidizing agent during cellular respiration passes electrons in series of steps instead of one EXPLOSIVE reaction O2 pulls electrons down chain in an energy-yielding tumble Energy yielded is used to regenerate ATP Fig. 9-5 + H2 + 1/2 O2 2H (from food via NADH) Controlled release of + – 2H + 2e energy for synthesis of ATP 1/ 2 O2 Explosive release of heat and light energy 1/ (a) Uncontrolled reaction (b) Cellular respiration 2 O2 The Stages of Cellular Respiration: A + Preview Three stages: Glycolysis: breaks down glucose into two molecules of pyruvate Citric acid cycle: completes the breakdown of glucose Oxidative phosphorylation: accounts for most of the ATP synthesis Fig. 9-6-1 + Electrons carried via NADH Glycolysis Pyruvate Glucose Cytosol ATP Substrate-level phosphorylation Fig. 9-6-2 + Electrons carried via NADH and FADH2 Electrons carried via NADH Citric acid cycle Glycolysis Pyruvate Glucose Mitochondrion Cytosol ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation Fig. 9-6-3 + Electrons carried via NADH and FADH2 Electrons carried via NADH Citric acid cycle Glycolysis Pyruvate Glucose Oxidative phosphorylation: electron transport and chemiosmosis Mitochondrion Cytosol ATP ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation Oxidative phosphorylation + ATP Production Oxidative phosphorylation: process that generates most of the ATP powered by redox reactions accounts for almost 90% of the ATP generated by cellular respiration Substrate-level phosphorylation: produces smaller amount of ATP in glycolysis and the citric acid cycle Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings + Concept 9.2: Glycolysis harvests chemical energy by oxidizing glucose to pyruvate Glycolysis or “splitting of sugar” breaks down glucose into two molecules of pyruvate Glycolysis occurs in the cytoplasm and has two major phases: Energy investment phase Energy payoff phase + Glycolysis Consists of series of chemical reactions catalyzed by specific enzymes Takes place in cytosol of cell Four main steps 6-C glucose Two 3-C pyruvic acid one six-carbon molecule of glucose is oxidized to produce TWO three-carbon molecule of pyruvic acid + Step 1 Two phosphate groups attached to glucose forming new 6-C compound Phosphate groups supplied by two ATP molecules 2 ATP used 2 ADP + Step 2 6-C compound split into 2 PGAL molecules + Step 3 Both PGAL molecules are oxidized and receive phosphate group 2 NADH 2 NAD+ Product: New 3-C compound + Step 4 Phosphate groups added in Steps 1&3 removed from 3-C compounds formed in Step 3 THUS! we get 2 pyruvic acid molecules Each phosphate group is added to ADP to give us.......ATP!!!!!!!!! Total: 4 ATP + Let’s talk numbers... Two ATP used in Step 1 Four ATP produced in Step 4 Net Yield: Two ATP Fig. 9-9-1 + Glucose ATP 1 Hexokinase ADP Glucose Glucose-6-phosphate ATP 1 Hexokinase ADP Glucose-6-phosphate Fig. 9-9-2 + Glucose ATP 1 Hexokinase ADP Glucose-6-phosphate 2 Phosphoglucoisomerase Fructose-6-phosphate Glucose-6-phosphate 2 Phosphoglucoisomerase Fructose-6-phosphate Fig. 9-9-3 + Glucose ATP 1 Hexokinase ADP Fructose-6-phosphate Glucose-6-phosphate 2 Phosphoglucoisomerase ATP 3 Phosphofructokinase Fructose-6-phosphate ATP 3 Phosphofructokinase ADP ADP Fructose1, 6-bisphosphate Fructose1, 6-bisphosphate Fig. 9-9-4 + Glucose ATP 1 Hexokinase ADP Glucose-6-phosphate 2 Phosphoglucoisomerase Fructose1, 6-bisphosphate 4 Fructose-6-phosphate ATP Aldolase 3 Phosphofructokinase ADP 5 Isomerase Fructose1, 6-bisphosphate 4 Aldolase 5 Isomerase Dihydroxyacetone phosphate Dihydroxyacetone phosphate Glyceraldehyde3-phosphate Glyceraldehyde3-phosphate Fig. 9-9-5 + 2 NAD+ 2 NADH + 2 H+ 6 Triose phosphate dehydrogenase 2 Pi 2 1, 3-Bisphosphoglycerate Glyceraldehyde3-phosphate 2 NAD+ 2 NADH 6 Triose phosphate dehydrogenase 2 Pi + 2 H+ 2 1, 3-Bisphosphoglycerate Fig. 9-9-6 + 2 NAD+ 2 NADH + 2 H+ 6 Triose phosphate dehydrogenase 2 Pi 2 1, 3-Bisphosphoglycerate 2 ADP 7 Phosphoglycerokinase 2 ATP 2 1, 3-Bisphosphoglycerate 2 ADP 2 3-Phosphoglycerate 2 ATP 2 7 Phosphoglycerokinase 3-Phosphoglycerate Fig. 9-9-7 + 2 NAD+ 2 NADH + 2 H+ 6 Triose phosphate dehydrogenase 2 Pi 2 1, 3-Bisphosphoglycerate 2 ADP 7 Phosphoglycerokinase 2 ATP 2 3-Phosphoglycerate 8 2 3-Phosphoglycerate Phosphoglyceromutase 2 8 Phosphoglyceromutase 2-Phosphoglycerate 2 2-Phosphoglycerate Fig. 9-9-8 + 2 NAD+ 2 NADH + 2 H+ 6 Triose phosphate dehydrogenase 2 Pi 2 1, 3-Bisphosphoglycerate 2 ADP 7 Phosphoglycerokinase 2 ATP 2 3-Phosphoglycerate 2 2-Phosphoglycerate 8 Phosphoglyceromutase 9 2 2 H2O 2-Phosphoglycerate Enolase 9 Enolase 2 H2O 2 Phosphoenolpyruvate 2 Phosphoenolpyruvate Fig. 9-9-9 + 2 NAD+ 6 Triose phosphate dehydrogenase 2 Pi 2 NADH + 2 H+ 2 1, 3-Bisphosphoglycerate 2 ADP 7 Phosphoglycerokinase 2 ATP 2 Phosphoenolpyruvate 2 ADP 2 3-Phosphoglycerate 8 Phosphoglyceromutase 2 ATP 2 10 Pyruvate kinase 2-Phosphoglycerate 9 2 H2O Enolase 2 Phosphoenolpyruvate 2 ADP 10 Pyruvate kinase 2 ATP 2 2 Pyruvate Pyruvate + Energy Yield Kilocalorie- 1,000 calories; often unit used to measure energy Complete oxidation on ONE glucose molecule= 686 kcal One ADP One ATP absorbs 12 kcal Can you calculate the efficiency of glycolysis? 2 ATP produced Percent usually a calculation x100 + Efficiency of Glycolysis + What does that mean?! Two ATP molecules produces during glycolysis only use small percentage of energy that could be released from glucose SO! anaerobic pathways are NOT very efficient Probably evolved before aerobic pathways Concept 9.3: The citric acid cycle completes + energy-yielding oxidation of organic molecules In presence of O2, pyruvate enters the mitochondrion Before citric acid cycle can begin, pyruvate must be converted to acetyl CoA, which links the cycle to glycolysis Citric acid cycle also called the Krebs cycle takes place within the mitochondrial matrix oxidizes organic fuel derived from pyruvate, generating… 1 ATP 3 NADH 1 FADH2 per turn Fig. 9-11 + Pyruvate CO2 NAD+ CoA NADH + H+ Acetyl CoA CoA CoA Citric acid cycle FADH2 2 CO2 3 NAD+ 3 NADH FAD + 3 H+ ADP + P i ATP + • • Eight steps, each catalyzed by a specific enzyme Acetyl group of acetyl CoA joins cycle by combining with oxaloacetate, forming citrate • • • Citric Acid Cycle Acetyl CoA + Oxaloacetate = Citrate Next seven steps decompose the citrate back to oxaloacetate, making the process a cycle NADH and FADH2 produced by cycle relay electrons extracted from food to the electron transport chain Fig. 9-12-1 + Acetyl CoA CoA—SH 1 Oxaloacetate Citrate Citric acid cycle Fig. 9-UN6 + Inputs Outputs S—CoA C 2 ATP 6 NADH O CH3 2 Acetyl CoA O C COO CH2 COO 2 Oxaloacetate Citric acid cycle 2 FADH2 Fig. 9-12-2 + Acetyl CoA CoA—SH H2O 1 Oxaloacetate 2 Citrate Isocitrate Citric acid cycle Fig. 9-12-3 + Acetyl CoA CoA—SH 1 H2O Oxaloacetate 2 Citrate Isocitrate NAD+ Citric acid cycle 3 NADH + H+ CO2 -Ketoglutarate Fig. 9-12-4 + Acetyl CoA CoA—SH 1 H2O Oxaloacetate 2 Citrate Isocitrate NAD+ Citric acid cycle NADH + H+ 3 CO2 CoA—SH -Ketoglutarate 4 NAD+ Succinyl CoA NADH + H+ CO2 Fig. 9-12-5 + Acetyl CoA CoA—SH 1 H2O Oxaloacetate 2 Citrate Isocitrate NAD+ Citric acid cycle NADH + H+ 3 CO2 CoA—SH -Ketoglutarate 4 CoA—SH 5 NAD+ Succinate GTP GDP ADP ATP Pi Succinyl CoA NADH + H+ CO2 Fig. 9-12-6 + Acetyl CoA CoA—SH H2O 1 Oxaloacetate 2 Citrate Isocitrate NAD+ Citric acid cycle NADH + H+ 3 CO2 Fumarate CoA—SH 6 -Ketoglutarate 4 CoA—SH 5 FADH2 NAD+ FAD Succinate GTP GDP ADP ATP Pi Succinyl CoA NADH + H+ CO2 Fig. 9-12-7 + Acetyl CoA CoA—SH H2O 1 Oxaloacetate 2 Malate Citrate Isocitrate NAD+ Citric acid cycle 7 H2O NADH + H+ 3 CO2 Fumarate CoA—SH -Ketoglutarate 4 6 CoA—SH 5 FADH2 NAD+ FAD Succinate GTP GDP ADP ATP Pi Succinyl CoA NADH + H+ CO2 Fig. 9-12-8 + Acetyl CoA CoA—SH NADH +H+ H2O 1 NAD+ 8 Oxaloacetate 2 Malate Citrate Isocitrate NAD+ Citric acid cycle 7 H2O NADH + H+ 3 CO2 Fumarate CoA—SH 6 -Ketoglutarate 4 CoA—SH 5 FADH2 NAD+ FAD Succinate GTP GDP ADP ATP Pi Succinyl CoA NADH + H+ CO2 + Following glycolysis and citric acid cycle, NADH and FADH2 account for most of energy extracted from food Concept 9.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis NADH and FADH2 donate electrons to electron transport chain, which powers ATP synthesis via oxidative phosphorylation Electron transport chain is in cristae of mitochondrion Most of chain’s components are proteins, which exist in multiprotein complexes Carriers alternate reduced and oxidized states as they accept and donate electrons Electrons drop in free energy as they go down the chain and are finally passed to O2, forming H2O Fig. 9-UN7 + INTERMEMBRANE SPACE H+ ATP synthase ADP + P i MITOCHONDRIAL MATRIX ATP H+ + Electrons transferred from NADH or FADH2 to the electron transport chain Electrons passed through a number of proteins including cytochromes (each with an iron atom) to O2 Electron transport chain generates no ATP ETC function: break the large free-energy drop from food to O2 into smaller steps that release energy in manageable amounts Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 9-13 + NADH 50 2 e– NAD+ FADH2 2 e– 40 FMN FAD Multiprotein complexes FAD Fe•S Fe•S Q Cyt b 30 Fe•S Cyt c1 I V Cyt c Cyt a Cyt a3 20 10 2 e– (from NADH or FADH2) 0 2 H+ + 1/2 O2 H2O Chemiosmosis: The Energy-Coupling + Mechanism Electron transfer in ETC causes proteins to pump H+ from mitochondrial matrix to intermembrane space What kind of energy is this creating? H+ then moves back across membrane, passing through channels in ATP synthase ATP synthase uses exergonic flow of H+ to drive phosphorylation of ATP This is example of chemiosmosis use of energy in a H+ gradient to drive cellular work Fig. 9-15 Magnetic bead Electromagnet Sample Internal rod Catalytic knob Nickel plate RESULTS Rotation in one direction Rotation in opposite direction Number of photons detected (103) + EXPERIMENT No rotation 30 25 20 0 Sequential trials Fig. 9-15a + EXPERIMENT Magnetic bead Electromagnet Sample Internal rod Catalytic knob Nickel plate Fig. 9-15b + RESULTS Rotation in one direction Rotation in opposite direction No rotation 30 25 20 0 Sequential trials stored in a H gradient across membrane couples the + Energy redox reactions of the electron transport chain to ATP + synthesis Proton-motive force: H+ gradient, name emphasizes its capacity to do work + Let’s remember… Glycolysis TWO pyruvic acid molecules TWO pyruvic acid TWO acetyl CoA SO, 1 glycolysis cycle = 2 turns of Krebs Cycle 2 Krebs Cycle produces: 6 NADH 2 FADH2 2 ATP 4 CO2 + Energy Yield Glycolysis: 2 ATP Krebs Cycle: 2 ATP Each NADH molecule that supplies ETC can generate 3 ATP Each FADH2 can generate 2 ATP 10 NADH and 2 FADH2 made by aerobic respiration How many ATP from the NADH and FADH2? During cellular respiration, most energy flows in this sequence: glucose NADH electron transport chain proton-motive force ATP About 40% of energy in a glucose molecule is transferred to ATP during cellular respiration + How much energy total?! 10 NADH 30 ATP 2 FADH2 4 ATP Glycolysis 2 ATP Krebs 2 ATP TOTAL: 38 ATP + In reality… Real number of ATP varies cell to cell Eukaryotic cells cannot diffuse NADH through inner mito. membrane SO! Active transport needed Uses ATP 38-2=36 + Efficiency of Aerobic Respiration Almost 20x more efficient Fig. 9-17 + Electron shuttles span membrane CYTOSOL 2 NADH Glycolysis Glucose 2 Pyruvate MITOCHONDRION 2 NADH or 2 FADH2 6 NADH 2 NADH 2 Acetyl CoA + 2 ATP Citric acid cycle + 2 ATP Maximum per glucose: About 36 or 38 ATP 2 FADH2 Oxidative phosphorylation: electron transport and chemiosmosis + about 32 or 34 ATP Concept 9.5: Fermentation and anaerobic +respiration enable cells to produce ATP without the use of oxygen Most cellular respiration requires O2 to produce ATP Glycolysis can produce ATP with or without O2 (in aerobic or anaerobic conditions) In absence of O2, glycolysis couples with fermentation or anaerobic respiration to produce ATP Anaerobic respiration uses ETC with an electron acceptor other than O2 Ex: sulfate Fermentation uses phosphorylation instead of ETC to generate ATP + Types of Fermentation Fermentation consists of glycolysis plus reactions that regenerate NAD+, which can be reused by glycolysis *CYCLE* Two common types: alcohol fermentation lactic acid fermentation + Alcoholic Fermentation Pyruvate converted to ethanol in two steps; first releasing CO2 CO2 released = carbonation in beer Alcohol fermentation by yeast is used in brewing, winemaking, and baking + Lactic Acid Fermentation Pyruvate reduced to NADH, forming lactate as an end product, with no release of CO2 LAF in some fungi and bacteria is used to make cheese and yogurt Human muscle cells use lactic acid fermentation to generate ATP when O2 is scarce Cramp = LA build up when LAF overused when no O2 available Fermentation vs Aerobic Respiration + Both: use glycolysis to oxidize glucose and other organic fuels to pyruvate Different: ATP produced per glucose molecule: Fermentation: 2 ATP Cellular Respiration: 38 ATP Anaerobes: Fermentation: Obligate anaerobes carry out it or; cannot survive in presence of O2 Yeast and many bacteria are facultative anaerobes can survive using either fermentation or cellular respiration pyruvate is a fork in the metabolic road that leads to two alternative catabolic routes final electron acceptors: Fermentation: an organic molecule (such as pyruvate or acetaldehyde) Cellular respiration: O2 in cellular respiration Fig. 9-19 Glucose + CYTOSOL Glycolysis Pyruvate No O2 present: Fermentation O2 present: Aerobic cellular respiration MITOCHONDRION Ethanol or lactate Acetyl CoA Citric acid cycle + Evolutionary Significance of Glycolysis Glycolysis: occurs in nearly all organisms probably evolved in ancient prokaryotes before there was oxygen in atmosphere Gycolysis and citric acid cycle are major intersections to various catabolic and anabolic pathways + Versatility of Catabolism Catabolic pathways funnel e- from many kinds of organic molecules into cellular respiration Glycolysis accepts wide range of carbohydrates Break it down and reuse… Proteins must be digested to amino acids amino groups can feed glycolysis or the citric acid cycle Fats digested to glycerol used in glycolysis; fatty acids used in generating acetyl CoA) Fatty acids broken down by beta oxidation yield acetyl CoA Oxidized gram of fat produces more than twice as much ATP as an oxidized gram of carbohydrate BIG PICTURE: What does this mean in terms of YOUR diet? +Biosynthesis (Anabolic Pathways) Body uses small molecules to build other substances These small molecules may come directly from food, glycolysis, OR citric acid cycle Regulation of Cellular + Respiration via Feedback Mechanisms Feedback inhibition is most common mechanism for control ATP concentration drops respiration speeds up; Plenty of ATP respiration slows down Control of catabolism based mainly on regulating activity of enzymes at strategic points in the catabolic pathway +You should now be able to: 1. Explain in general terms how redox reactions are involved in energy exchanges 2. Name the three stages of cellular respiration; for each, state the region of the eukaryotic cell where it occurs and the products that result 3. In general terms, explain the role of the electron transport chain in cellular respiration Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings + 4. Explain where and how the respiratory electron transport chain creates a proton gradient 5. Distinguish between fermentation and anaerobic respiration 6. Distinguish between obligate and facultative anaerobes Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings