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Chapter 9

Chapter 9. Cellular Respiration: Harvesting Chemical Energy. Light energy. ECOSYSTEM. Photosynthesis in chloroplasts. Organic molecules. CO 2 + H 2 O. + O 2. Cellular respiration in mitochondria. ATP. powers most cellular work. Heat energy. Figure 9.2. Energy

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Chapter 9

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  1. Chapter 9 Cellular Respiration: Harvesting Chemical Energy

  2. Light energy ECOSYSTEM Photosynthesisin chloroplasts Organicmolecules CO2 + H2O + O2 Cellular respirationin mitochondria ATP powers most cellular work Heatenergy Figure 9.2 • Energy • Flows into an ecosystem as sunlight and leaves as heat

  3. Catabolic Pathways and Production of ATP • The breakdown of organic molecules is exergonic • One catabolic process, fermentation • Is a partial degradation of sugars that occurs without oxygen • Concept 9.1: Catabolic pathways yield energy by oxidizing organic fuels • Due to the transfer of electrons

  4. Cellular respiration • Is the most prevalent and efficient catabolic pathway • Consumes oxygen and organic molecules such as glucose • Yields ATP

  5. becomes oxidized(loses electron) Na + Cl Na+ + Cl– becomes reduced(gains electron) The Principle of Redox Reactions • Redox reactions • Transfer electrons from one reactant to another by oxidation and reduction • In oxidation • A substance loses electrons, or is oxidized • In reduction • A substance gains electrons, or is reduced

  6. Products Reactants becomes oxidized + + + Energy 2O2 CO2 2 H2O CH4 becomes reduced H C C O O O O H O H H H H Oxygen(oxidizingagent) Methane(reducingagent) Carbon dioxide Water Figure 9.3 • Some redox reactions • Do not completely exchange electrons • Change the degree of electron sharing in covalent bonds

  7. becomes oxidized C6H12O6 + 6O2 6CO2 + 6H2O + Energy becomes reduced Oxidation of Organic Fuel Molecules During Cellular Respiration • During cellular respiration • Glucose is oxidized and oxygen is reduced in a series of steps

  8. 2 e– + 2 H+ 2 e– + H+ NAD+ NADH H Dehydrogenase O O H H Reduction of NAD+ + + 2[H] C NH2 NH2 C (from food) Oxidation of NADH N N+ Nicotinamide(reduced form) Nicotinamide(oxidized form) CH2 O O O O– P O H H OH O O– HO P NH2 HO CH2 O N N H N H N O H H HO OH Figure 9.4 Stepwise Energy Harvest via NAD+ and the Electron Transport Chain • Electrons from organic compounds • Are usually first transferred to NAD+, a coenzyme • NADH, the reduced form of NAD+ • Passes the electrons to the electron transport chain

  9. H2 + 1/2 O2 Explosiverelease ofheat and lightenergy (a) Uncontrolled reaction Free energy, G Figure 9.5 A H2O • If electron transfer is not stepwise • A large release of energy occurs • As in the reaction of hydrogen and oxygen to form water

  10. The electron transport chain • Passes electrons in a series of steps instead of in one explosive reaction • Uses the energy from the electron transfer to form ATP

  11. 2 H + 1/2 O2 (from food via NADH) Controlled release of energy for synthesis ofATP 2 H+ + 2 e– ATP ATP Free energy, G Electron transport chain ATP 2 e– 1/2 O2 2 H+ H2O Figure 9.5 B (b) Cellular respiration

  12. The Stages of Cellular Respiration: A Preview • Respiration is a cumulative function of three metabolic stages • Glycolysis • Breaks down glucose into two molecules of pyruvate • The citric acid cycle (Krebs) • Completes the breakdown of glucose • Oxidative phosphorylation (ETC) • Is driven by the electron transport chain • Generates ATP

  13. Electrons carried via NADH and FADH2 Electrons carried via NADH Oxidativephosphorylation:electron transport andchemiosmosis Citric acid cycle Glycolsis Pyruvate Glucose Cytosol Mitochondrion ATP ATP ATP Substrate-level phosphorylation Oxidative phosphorylation Substrate-level phosphorylation Figure 9.6 • An overview of cellular respiration

  14. Enzyme Enzyme ADP P Substrate + ATP Product Figure 9.7 • Both glycolysis and the citric acid cycle • Can generate ATP by substrate-level phosphorylation

  15. Concept 9.2: Glycolysis harvests energy by oxidizing glucose to pyruvate • Glycolysis • Means “splitting of sugar” • Breaks down glucose into pyruvate • Occurs in the cytoplasm of the cell

  16. Glycolysis Oxidativephosphorylation Citricacidcycle ATP ATP ATP Energy investment phase Glucose P 2 ATP + 2 used 2 ATP Energy payoff phase formed P 4 ATP 4 ADP + 4 2 NAD+ + 4 e- + 4 H + + 2 H+ 2 NADH 2 Pyruvate + 2 H2O Glucose 2 Pyruvate + 2 H2O 4 ATP formed – 2 ATP used 2 ATP + 2 H+ 2 NADH 2 NAD+ + 4 e– + 4 H + Figure 9.8 • Glycolysis consists of two major phases • Energy investment phase • Energy payoff phase

  17. CH2OH Citric acid cycle H H Oxidative phosphorylation H Glycolysis H HO HO OH H OH Glucose 1 2 3 5 4 ATP Hexokinase ADP CH2OH P O H H H H OH HO H OH Glucose-6-phosphate Phosphoglucoisomerase CH2O P O CH2OH H HO HO H H HO Fructose-6-phosphate ATP Phosphofructokinase ADP CH2 O O CH2 P P O HO H OH H HO Fructose- 1, 6-bisphosphate Aldolase H O CH2 P Isomerase C O O C CHOH CH2OH O CH2 P Dihydroxyacetone phosphate Glyceraldehyde- 3-phosphate Figure 9.9 A A closer look at the energy investment phase of Glycolysis -2 ATP

  18. 2 NAD+ Triose phosphate dehydrogenase P i 2 2 NADH + 2 H+ 10 7 9 8 6 2 O C O P CHOH P CH2 O 1, 3-Bisphosphoglycerate 2 ADP Phosphoglycerokinase 2 ATP O– 2 C CHOH O P CH2 3-Phosphoglycerate Phosphoglyceromutase O– 2 C O P C H O CH2OH 2-Phosphoglycerate Enolase 2 H2O O– 2 C O P C O CH2 Phosphoenolpyruvate 2 ADP Pyruvate kinase 2 ATP O– 2 C O C O CH3 Figure 9.8 B Pyruvate A closer look at the energy payoff phase of Glycolysis +4 ATP = net ?

  19. Concept 9.3: The citric acid cycle completes the energy-yielding oxidation of organic molecules • The citric acid cycle • Takes place in the matrix of the mitochondrion

  20. CYTOSOL MITOCHONDRION + H+ NAD+ NADH O– CoA S 2 C O C O C O CH3 1 3 CH3 Acetyle CoA Pyruvate CO2 Coenzyme A Transport protein Figure 9.10 • Before the citric acid cycle can begin • Pyruvate must first be converted to acetyl CoA, which links the cycle to glycolysis

  21. Pyruvate(from glycolysis,2 molecules per glucose) Oxidativephosphorylation Glycolysis Citricacidcycle ATP ATP ATP CO2 CoA NADH + 3 H+ Acetyle CoA CoA CoA Citricacidcycle 2 CO2 3 NAD+ FADH2 FAD 3 NADH + 3 H+ ADP + Pi ATP Figure 9.11 • An overview of the citric acid cycle

  22. Citric acid cycle Oxidative phosphorylation Glycolysis S CoA C O CH3 Acetyl CoA CoA SH H2O O C COO– NADH 1 COO– CH2 + H+ COO– CH2 COO– NAD+ Oxaloacetate 8 C COO– HO CH2 2 CH2 HC COO– COO– COO– HO CH HO CH Malate Citrate COO– CH2 Isocitrate COO– CO2 Citric acid cycle 3 H2O 7 NAD+ COO– NADH COO– CH + H+ Fumarate CH2 CoA SH HC a-Ketoglutarate CH2 COO– C O 4 6 SH CoA COO– COO– COO– CH2 5 CH2 FADH2 CO2 CH2 CH2 NAD+ FAD C O COO– Succinate NADH CoA S P i + H+ Succinyl CoA GDP GTP ADP ATP Figure 9.12 Figure 9.12

  23. Concept 9.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis • NADH and FADH2 • Donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation • In the electron transport chain, electrons from NADH and FADH2 lose energy in several steps

  24. NADH 50 FADH2 Multiproteincomplexes I 40 FAD FMN II Fe•S Fe•S O III Cyt b 30 Fe•S Cyt c1 IV Free energy (G) relative to O2 (kcl/mol) Cyt c Cyt a Cyt a3 20 10 0 O2 2 H + + 12 Figure 9.13 H2O • At the end of the chain • Electrons are passed to oxygen, forming water

  25. A rotor within the membrane spins clockwise whenH+ flows past it down the H+ gradient. INTERMEMBRANE SPACE H+ H+ H+ H+ H+ H+ H+ A stator anchoredin the membraneholds the knobstationary. A rod (for “stalk”)extending into the knob alsospins, activatingcatalytic sites inthe knob. H+ Three catalytic sites in the stationary knobjoin inorganic Phosphate to ADPto make ATP. ADP + ATP P i MITOCHONDRIAL MATRIX Figure 9.14 Chemiosmosis: The Energy-Coupling Mechanism • ATP synthase • Is the enzyme that actually makes ATP

  26. At certain steps along the electron transport chain • Electron transfer causes protein complexes to pump H+ from the mitochondrial matrix to the intermembrane space

  27. The resulting H+ gradient (ionic and pH gradient) • Stores energy • Drives chemiosmosis in ATP synthase • Is referred to as a proton-motive force • Chemiosmosis • Is an energy-coupling mechanism that uses energy in the form of a H+ gradient across a membrane to drive cellular work

  28. Inner Mitochondrial membrane Oxidative phosphorylation. electron transport and chemiosmosis Glycolysis ATP ATP ATP H+ H+ H+ H+ Cyt c Protein complex of electron carners Intermembrane space Q IV I III ATP synthase Inner mitochondrial membrane II H2O FADH2 2 H+ + 1/2 O2 FAD+ NADH+ NAD+ ATP ADP + P i (Carrying electrons from, food) H+ Mitochondrial matrix Chemiosmosis ATP synthesis powered by the flow Of H+ back across the membrane Electron transport chain Electron transport and pumping of protons (H+), which create an H+ gradient across the membrane Figure 9.15 Oxidative phosphorylation • Chemiosmosis and the electron transport chain

  29. An Accounting of ATP Production by Cellular Respiration • During respiration, most energy flows in this sequence • Glucose to NADH to electron transport chain to proton-motive force to ATP

  30. Electron shuttles span membrane MITOCHONDRION CYTOSOL 2 NADH or 2 FADH2 2 FADH2 2 NADH 2 NADH 6 NADH Glycolysis Oxidative phosphorylation: electron transport and chemiosmosis Citric acid cycle 2 Acetyl CoA 2 Pyruvate Glucose + 2 ATP + 2 ATP + about 32 or 34 ATP by oxidative phosphorylation, depending on which shuttle transports electrons from NADH in cytosol by substrate-level phosphorylation by substrate-level phosphorylation About 36 or 38 ATP Maximum per glucose: Figure 9.16 • There are three main processes in this metabolic enterprise

  31. About 40% of the energy in a glucose molecule • Is transferred to ATP during cellular respiration, making approximately 36 to 38 ATP

  32. Concept 9.5: Fermentation enables some cells to produce ATP without the use of oxygen • Cellular respiration (aerobic) • Relies on oxygen to produce ATP • In the absence of oxygen (anaerobic) • Cells can still produce ATP through fermentation

  33. Glycolysis • Can produce ATP with or without oxygen, in aerobic or anaerobic conditions • Couples with fermentation to produce ATP

  34. Types of Fermentation • Fermentation consists of • Glycolysis plus reactions that regenerate NAD+, which can be reused by glyocolysis

  35. In alcohol fermentation • Pyruvate is converted to ethanol in two steps, one of which releases CO2

  36. During lactic acid fermentation • Pyruvate is reduced directly to NADH to form lactate as a waste product

  37. P1 2 ATP 2 ADP + 2 O – C O C O Glucose Glycolysis CH3 2 Pyruvate 2 NADH 2 NAD+ 2 CO2 H H H C O C OH CH3 CH3 2 Ethanol 2 Acetaldehyde (a) Alcohol fermentation P1 2 ATP 2 ADP + 2 Glucose Glycolysis O– C O C O 2 NADH 2 NAD+ CH3 O C O H OH C CH3 2 Lactate (b) Lactic acid fermentation Figure 9.17

  38. Fermentation and Cellular Respiration Compared • Both fermentation and cellular respiration • Use glycolysis to oxidize glucose and other organic fuels to pyruvate

  39. Fermentation and cellular respiration • Differ in their final electron acceptor • Cellular respiration • Produces many more ATP

  40. Glucose CYTOSOL Pyruvate No O2 present Fermentation O2 present Cellular respiration MITOCHONDRION Ethanol or lactate Acetyl CoA Citric acid cycle Figure 9.18 • Pyruvate is a key juncture in catabolism

  41. The Evolutionary Significance of Glycolysis • Glycolysis • Occurs in nearly all organisms • Probably evolved in ancient prokaryotes before there was oxygen in the atmosphere

  42. Concept 9.6: Glycolysis and the citric acid cycle connect to many other metabolic pathways

  43. The Versatility of Catabolism • Catabolic pathways • Funnel electrons from many kinds of organic molecules into cellular respiration

  44. Fats Carbohydrates Proteins Amino acids Fatty acids Sugars Glycerol Glycolysis Glucose Glyceraldehyde-3- P NH3 Pyruvate Acetyl CoA Citric acid cycle Oxidative phosphorylation Figure 9.19 • The catabolism of various molecules from food

  45. Regulation of Cellular Respiration via Feedback Mechanisms • Cellular respiration • Is controlled by allosteric enzymes at key points in glycolysis and the citric acid cycle

  46. Glucose AMP Glycolysis Stimulates Fructose-6-phosphate + Phosphofructokinase – – Fructose-1,6-bisphosphate Inhibits Inhibits Pyruvate Citrate ATP Acetyl CoA Citric acid cycle Oxidative phosphorylation Figure 9.20 • The control of cellular respiration

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