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Cellular Respiration: Harvesting Energy Pathways in Living Cells

Explore the process of cellular respiration, the efficient catabolic pathway in living cells that oxidizes organic fuels. Learn about the production of ATP, redox reactions, and the stages of respiration, including glycolysis, the citric acid cycle, and oxidative phosphorylation. Discover how energy is harvested and transformed into ATP through the electron transport chain and chemiosmosis.

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Cellular Respiration: Harvesting Energy Pathways in Living Cells

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  1. Chapter 9 Cellular Respiration: Harvesting Chemical Energy

  2. Overview: Life Is Work • Living cells • Require transfusions of energy from outside sources to perform their many tasks

  3. Figure 9.1 • The giant panda • Obtains energy for its cells by eating plants

  4. 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

  5. Concept 9.1: Catabolic pathways yield energy by oxidizing organic fuels

  6. Catabolic Pathways and Production of ATP • The breakdown of organic molecules is exergonic

  7. One catabolic process, fermentation • Is a partial degradation of sugars that occurs without oxygen

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

  9. To keep working • Cells must regenerate ATP

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

  11. The Principle of Redox • Redox reactions • Transfer electrons from one reactant to another by oxidation and reduction

  12. In oxidation • A substance loses electrons, or is oxidized • In reduction • A substance gains electrons, or is reduced

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

  14. 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

  15. 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

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

  17. 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 • Electrons from organic compounds • Are usually first transferred to NAD+, a coenzyme

  18. NADH, the reduced form of NAD+ • Passes the electrons to the electron transport chain

  19. 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

  20. 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

  21. 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

  22. The Stages of Cellular Respiration: A Preview • Respiration is a cumulative function of three metabolic stages • Glycolysis • The citric acid cycle • Oxidative phosphorylation

  23. Glycolysis • Breaks down glucose into two molecules of pyruvate • The citric acid cycle • Completes the breakdown of glucose

  24. Oxidative phosphorylation • Is driven by the electron transport chain • Generates ATP

  25. 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

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

  27. 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

  28. 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

  29. A closer look at the energy investment phase

  30. 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

  31. A closer look at the energy payoff phase

  32. 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

  33. 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

  34. 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

  35. 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

  36. A closer look at the citric acid cycle

  37. 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

  38. 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

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

  40. 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

  41. 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

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

  43. The resulting H+ gradient • Stores energy • Drives chemiosmosis in ATP synthase • Is referred to as a proton-motive force

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

  45. 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

  46. 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

  47. 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

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

  49. Concept 9.5: 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

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

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