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Chapter 9

Chapter 9. Cellular Respiration: Harvesting Chemical Energy. Overview: Life Is Work Living cells Require transfusions of energy from outside sources to perform their different tasks. Figure 9.1. The giant panda Obtains energy for its cells by eating plants. Light energy. ECOSYSTEM.

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Chapter 9

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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 different 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 ecosystems as sunlight and leaves as heat

  5. Photosynthetic organisms trap a portion of the sunlight energy and transform it into chemical energy (organic molecules) with O2 is released. • Cells use some of the chemical energy in organic molecules to make ATP; the energy source for cellular work. • Energy leaves organisms as it dissipates as heat • The products of respiration (CO2 and H2O) are the raw materials for photosynthesis. • Photosynthesis produces glucose and oxygen, the raw materials for respiration

  6. Catabolic Pathways and Production of ATP • Concept 9.1: Catabolic pathways yield energy by oxidizing organic fuels • Organic compounds store energy in their arrangement of atoms • With the help of enzymes, a cell systematically degrades complex organic molecules that are rich in potential energy to simpler waste products that have less energy

  7. Fermentation a catabolic process • Is a partial degradation of sugars that occurs without oxygen (anaerobic) • Cellular respiration • Is the most prevalent and efficient catabolic pathway • Consumes oxygen and organic molecules such as glucose • Yields ATP

  8. Respiration can be summarized: organic + oxygen → carbon + water + Energy compounds dioxide cellular respiration is most often described as the oxidation of glucose: C6H12O6 + 6 O2 → 6 CO2 + 6H2O + Energy (ATP + heat) The breakdown of glucose is exergonic (free energy change) (ΔG= – 686 kcal per mol) –ΔG → the products of the chemical process store less energy than reactants and the reaction can happen spontaneously (without an input of energy)

  9. To keep working • Cells must regenerate ATP from ADP + Pi • To understand how cellular respiration accomplishes this, let’s examine the fundamental process known as oxidationand reduction

  10. Redox Reactions: Oxidation and Reduction • Why do the catabolic pathways that decompose glucose and other organic fuels yield energy? • The answer is based on the transfer of electrons during the chemical reactions. The relocation of electrons releases energy stored in organic molecules, and this energy is used to synthesize ATP

  11. The Principle of Redox • Redox reactions • Transfer electrons from one reactant to another by oxidation and reduction • Inoxidation • A substance loses electrons, or is oxidized • In reduction • A substance gains electrons, or is reduced

  12. becomes oxidized(loses electron) becomes oxidized Na + Cl Na+ + Cl– becomes reduced(gains electron) Xē + Y X + Yē becomes reduced • Examples of redox reactions We could generalize a redox reaction this way: X is the electron donor, is called the reducing agent, it reduces Y which accepts the donated electron Y is the electron acceptor, it oxidizes X by removing its electron

  13. 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 • An electron loses potential energy when it shifts from a less electronegative atom toward more electronegative one → this energy can be put to work

  14. becomes oxidized C6H12O6 + 6O2 6CO2 + 6H2O + Energy becomes reduced Oxidation of Organic Fuel Molecules During Cellular Respiration • Oxidation of methane is the main combustion reaction that occurs at the burner of a gas stove • During cellular respiration • Glucose is oxidized and oxygen is reduced

  15. In general, organic molecules that have an abundance of hydrogen are excellent fuels because their bonds are a source of hilltop electrons whose energy may be released as these electrons fall down an energy gradient when they are transferred to oxygen • By oxidizing glucose, respiration liberatesstored energy from glucose and makes it available for ATP synthesis

  16. Stepwise Energy Harvest via NAD+ and the Electron Transport Chain • Cellular respiration • Oxidizes glucose in a series of steps each is catalyzed by an enzyme • The hydrogen atoms are not transferred to oxygen, but instead are usually passed first to a coenzyme called NAD+ (nicotinamide adenine dinucleotide). • NAD functions as coenzyme in the redox reactions thus is an oxidizing agent. It is found in all cells and helps in e transfer.

  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 • NAD+ accept electrons and act as an oxidizing agent during respiration How does NAD+ trap electrons from glucose and other organic molecules? Enzymes called dehydrogenases removes a pair of hydrogen atoms (2 electrons and 2 protons) from the substrate (a sugar for example) thereby oxidizing it

  18. NADH, the reduced form of NAD+ • Passes the electrons to the electron transport chain • Each NADH molecule formed during respiration represents stored energy that can be tapped to make ATP when the electrons complete their fall down an energy gradient from NADH to oxygen

  19. H2 + 1/2 O2 Explosiverelease ofheat and lightenergy (a) Uncontrolled reaction Free energy, G Figure 9.5 A H2O How do electrons that are extracted from food and stored by NADH finally reach oxygen? • 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 one explosive reaction • Uses the energy from the electron transfer to form ATP • The transport chain consists of a number of molecules, mostly proteins, built into the inner membrane of a mitochondrion • ET from NADH to O2 is an exergonic reaction with a free energy change of – 53 kcal/mol • Food → NADH → ETC → Oxygen

  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 does not require O2 require O2

  23. 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

  24. 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

  25. An overview of cellular respiration • During glycolysis each glucose molecule is broken down into 2 pyruvate molecules • Pyruvate inters into mitochondria where it will be oxidized by the citric acid cycle to CO2. • NADH and FADH2 transfer electrons from glucose to ETCs in the inner mitochondria membrane • During oxidative phosphorylation, ETCs convert chemical energy to a form of energy used for ATP synthesis in a process called chemiosmosis.

  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 • Occurs when an enzyme transfers a phosphate group from a substrate molecule to ADP rather than adding an inorganic phosphate to ADP as in oxidative phosphorylation

  27. Concept 9.2: Glycolysis harvests energy by oxidizing glucose to pyruvate • Glycolysis (can occur in the presence or absence of O2) • Means “splitting of sugar” a 6-C sugar (glucose) to 3-C sugar (pyruvate) • Breaks down glucose into pyruvate • Occurs in the cytoplasm of the cell

  28. Glycolysis Oxidativephosphorylation Citricacidcycle ATP ATP ATP Energy investment phase Glucose used P 2 ATP + 2 2 ATP Energy payoff phase formed 4 ADP + 4 P 4 ATP 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. 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 This is the reaction from which glycolysis Gets its name This reaction never reaches equilibrium in the cell

  30. 10 9 7 8 6 • A closer look at the energy payoff phase 2 NAD+ Triose phosphate dehydrogenase P i 2 2 NADH + 2 H+ 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

  31. 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 • completes glucose oxidation by breaking down pyruvate derivitatives into carbon dioxide.

  32. Before the citric acid cycle can begin • Pyruvate must first be converted to acetyl CoA, which links the cycle to glycolysis 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

  33. 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 For each turn of Krebs cycle, two carbons exit completely as CO2, three NADH and one FADH2 are formed. One ATP is made by substrate-level phosphorylation

  34. 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 Figure 9.12 NAD is reduced to NADH+ CoA is displaced by a phosphate group Which is transferred to GDP forming GTP And then to ATP.

  35. 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

  36. The Pathway of Electron Transport • Electrons from NADH and FADH2 lose energy in several steps • Couples this exergonic slide of electrons to ATP synthesis or oxidative phosphorylation • The electron transport chain is made of electron carrier molecules embedded in the inner membrane of mitochondria.

  37. Each carrier in the chain has a higher electronegativity than the carrier before it, so electrons are pulled downhill towards oxygen • Most carriers are protein molecules except for ubiquinone (Q) • At the end of the chain, electrons are passed to oxygen, forming water

  38. NADH 50 FADH2 Multiproteincomplexes I 40 FAD FMN II Fe•S Fe•S O III Cyt b Fe•S 30 Cyt c1 IV Free energy (G) relative to O2 (kcl/mol) Cyt c Cyt a Cyt a3 20 10 O2 0 2 H + + 12 Figure 9.13 H2O NADH is oxidized and flavoprotein is reduced as high energy electrons from NADH are transferred to FMN ↓ Flavoprotein is oxidized as it passes electrons to an iron sulfur protein, FeS. ↓ FeS is oxidized as it pass electrons to ubiquinone Q ↓ Q passes electrons on to a succession of electron carriers, most of which are cytochromes. ↓ cyt a3 , the last cytochrome passes electrons to oxygen.

  39. 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

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

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

  42. Inner Mitochondrial membrane Oxidative phosphorylation. electron transport and chemiosmosis Glycolysis ATP ATP ATP H+ H+ H+ H+ Cyt c Protein complex of electron carriers 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

  43. 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

  44. 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

  45. About 40% of the energy in a glucose molecule • Is transferred to ATP during cellular respiration, making approximately 38 ATP 1 ATP → - 7.3 kcal/mol 38 ATP X 7.3 / 686 = 40%

  46. 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

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

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

  49. 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

  50. 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

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