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生物醫學暨環境生物學系 鄭智美 助理教授 第一教學大樓 931 室 電話 3121101- ext 2702

This chapter explores the key concepts of cellular respiration, including catabolic pathways, glycolysis, the citric acid cycle, chemiosmosis, fermentation, and the connection to other metabolic pathways.

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生物醫學暨環境生物學系 鄭智美 助理教授 第一教學大樓 931 室 電話 3121101- ext 2702

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  1. 生物醫學暨環境生物學系 鄭智美 助理教授 第一教學大樓 931室 電話 3121101- ext 2702

  2. Chapter 9 Cellular Respiration: Harvesting Chemical Energy 細胞呼吸: 能量的產生

  3. Key Concepts • Catabolic Pathways yield energy by oxidizing organic fuel • Glycolysis harvests chemical energy by oxidizing glucose to pyruvate • The citric Acid cycle completes the energy yielding oxidation of organic molecules • Chemiosmosis couples electron transport to ATP synthesis • Fermentation enables some cels to produce ATP without the use of oxygen • Glycolysis and citric acid cycle connect to many other metabolic pathways

  4. Figure 9.1 Overview: Life Is Work • Living cells • Require transfusions of energy from outside sources to perform their many tasks • Obtains energy for its cells by eating plants

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

  6. Catabolic pathways yield energy by oxidizingorganic fuels • The breakdown of organic molecules is exergonic Organic compound+ O2→CO2+ H2O+Energy • To keep working • Cells must regenerate ATP

  7. Catabolic Pathways and Production of ATP • fermentation • Is a partial degradation of sugars that occurs without oxygen • Cellular respiration • Is the most prevalent and efficient catabolic pathway • Consumes oxygen and organic molecules such as glucose • Yields ATP

  8. Redox Reactions: Oxidation and Reduction • Catabolic pathways yield energy • Due to the transfer of electrons

  9. The Principle of Redox • 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

  10. becomes oxidized(loses electron) Na + Cl Na+ + Cl– becomes reduced(gains electron) Examples of redox reactions

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

  12. 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

  13. Stepwise Energy Harvest via NAD+ and the Electron Transport Chain • Cellular respiration • Oxidizes glucose in a series of steps

  14. 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 NAD+ as an electron shuttle • 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

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

  16. 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 Cellulae Respiration • 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

  17. 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 Completes the breakdown of glucose • Oxidative phosphorylation Is driven by the electron transport chain Generates ATP

  18. 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

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

  20. 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

  21. 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 H+ 2 NADH 2 NAD+ + 4 e- + 4 H + 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 Energy Input and Output of Glycolysis • Glycolysis consists of two major phases • Energy investment phase • Energy payoff phase

  22. CH2OH Citric acid cycle H H Oxidative phosphorylation H Glycolysis H HO HO OH H OH Glucose 4 1 2 5 3 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

  23. 2 NAD+ Triose phosphate dehydrogenase P i 2 2 NADH + 2 H+ 10 7 9 6 8 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

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

  25. 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 Conversion of pyruvate to Acetyl CoA, the junction beween glycolysis and the citric acid cycle • Before the citric acid cycle can begin • Pyruvate must first be converted to acetyl CoA, which links the cycle to glycolysis

  26. 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

  27. 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 A closer look at the citric acid cycle

  28. 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

  29. The Pathway of Electron Transport • In the electron transport chain • Electrons from NADH and FADH2 lose energy in several steps

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

  31. 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

  32. At certain steps along the electron transport chain Electron transfer causes protein complexes to pump H+ from the mitochondrial matrix to the intermembrane space • The resulting H+ gradient • Stores energy • Drives chemiosmosis in ATP synthase • Is referred to as a proton-motive force

  33. Chemiosmosis • Is an energy-coupling mechanism that uses energy in the form of a H+ gradient across a membrane to drive cellular work

  34. 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

  35. 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

  36. 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

  37. Accounting of ATP production by cellular respiration • About 40% of the energy in a glucose molecule • Is transferred to ATP during cellular respiration, making approximately 38 ATP

  38. Fermentation enables some cells to produce ATP without the use of oxygen • Cellular respiration • Relies on oxygen to produce ATP • In the absence of oxygen • Cells can still produce ATP through fermentation

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

  40. Types of Fermentation • Fermentation consists of • Glycolysis plus reactions that regenerate NAD+, which can be reused by glyocolysis • In alcohol fermentation • Pyruvate is converted to ethanol in two steps, one of which releases CO2 • In lactic acid fermentation • Pyruvate is reduced directly to NADH to form lactate as a waste product

  41. 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 Fermentation

  42. Fermentation and Cellular Respiration Compared • Both fermentation and cellular respiration • Use glycolysis to oxidize glucose and other organic fuels to pyruvate • Fermentation and cellular respiration • Differ in their final electron acceptor • Cellular respiration • Produces more ATP

  43. 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

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

  45. Glycolysis and the citric acid cycle connect to many other metabolic pathways

  46. 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 • Catabolic pathways • Funnel electrons from many kinds of organic molecules into cellular respiration

  47. Biosynthesis (Anabolic Pathways) • The body • Uses small molecules to build other substances • These small molecules • May come directly from food or through glycolysis or the citric acid cycle

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

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