The document summarizes cellular respiration, which consists of three main stages: glycolysis, the citric acid cycle, and oxidative phosphorylation. Glycolysis breaks down glucose into pyruvate and generates a small amount of ATP. The citric acid cycle further oxidizes pyruvate and generates more ATP, NADH, and FADH2. During oxidative phosphorylation, electrons from NADH and FADH2 are passed through an electron transport chain where their energy is used to pump protons across a membrane and generate a proton gradient. ATP synthase uses this proton gradient to generate most of the cell's ATP through chemiosmosis.

1. Chapter 9 Cellular Respiration: Harvesting Chemical Energy

2. Overview: Life Is Work Living cells require energy from outside sources Some animals, such as the giant panda, obtain energy by eating plants; others feed on organisms that eat plants

3. Energy flows into an ecosystem as sunlight and leaves as heat Photosynthesis generates oxygen and organic molecules, which are used in cellular respiration Cells use chemical energy stored in organic molecules to regenerate ATP, which powers work

4. LE 9-2 ECOSYSTEM Light energy Photosynthesis in chloroplasts Cellular respiration in mitochondria Organic molecules + O 2 CO 2 + H 2 O ATP powers most cellular work Heat energy

5. Catabolic pathways yield energy by oxidizing organic fuels Several processes are central to cellular respiration and related pathways

6. Catabolic Pathways and Production of ATP The breakdown of organic molecules is exergonic Fermentation is a partial degradation of sugars that occurs without oxygen Cellular respiration consumes oxygen and organic molecules and yields ATP Although carbohydrates, fats, and proteins are all consumed as fuel, it is helpful to trace cellular respiration with the sugar glucose: C 6 H 12 O 6 + 6O 2  6CO 2 + 6H 2 O + Energy (ATP + heat)

7. Redox Reactions: Oxidation and Reduction The transfer of electrons during chemical reactions releases energy stored in organic molecules This released energy is ultimately used to synthesize ATP

8. The Principle of Redox Chemical reactions that transfer electrons between reactants are called oxidation-reduction reactions, or redox reactions In oxidation, a substance loses electrons, or is oxidized In reduction, a substance gains electrons, or is reduced (the amount of positive charge is reduced) Remember “Oil Rig” “ Oxidation” is loss (of electrons) “ Reduction” is gain (of electrons) X e - + Y X + Y e - becomes oxidized (loses electron) becomes reduced (gains electron)

9. The electron donor is called the reducing agent The electron receptor is called the oxidizing agent

10. Some redox reactions do not transfer electrons but change the electron sharing in covalent bonds An example is the reaction between methane and oxygen

11. LE 9-3 Reactants becomes oxidized becomes reduced Products H Methane (reducing agent) Oxygen (oxidizing agent) Carbon dioxide Water H C H H O O O O C O H H CH 4 2 O 2 + + + CO 2 Energy 2 H 2 O

12. Oxidation of Organic Fuel Molecules During Cellular Respiration During cellular respiration, the fuel (such as glucose) is oxidized and oxygen is reduced: C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O + Energy becomes oxidized becomes reduced

13. Stepwise Energy Harvest via NAD + and the Electron Transport Chain In cellular respiration, glucose and other organic molecules are broken down in a series of steps Electrons from organic compounds are usually first transferred to NAD + , a coenzyme As an electron acceptor, NAD + functions as an oxidizing agent during cellular respiration Each NADH (the reduced form of NAD + ) represents stored energy that is tapped to synthesize ATP

14. LE 9-4 NAD + Nicotinamide (oxidized form) Dehydrogenase 2 e – + 2 H + 2 e – + H + NADH H + H + Nicotinamide (reduced form) + 2[ H ] (from food) +

15. NADH passes the electrons to the electron transport chain Unlike an uncontrolled reaction, the electron transport chain passes electrons in a series of steps instead of one explosive reaction Oxygen pulls electrons down the chain in an energy-yielding tumble The energy yielded is used to regenerate ATP

16. LE 9-5 2 H + + 2 e – 2 H (from food via NADH) Controlled release of energy for synthesis of ATP ATP ATP ATP 2 H + 2 e – H 2 O + 1 / 2 O 2 1 / 2 O 2 H 2 + 1 / 2 O 2 H 2 O Explosive release of heat and light energy Cellular respiration Uncontrolled reaction Free energy, G Free energy, G Electron transport chain

17. The Stages of Cellular Respiration: A Preview Cellular respiration has three stages: Glycolysis (breaks down glucose into two molecules of pyruvate) The citric acid cycle (completes the breakdown of glucose) Oxidative phosphorylation (accounts for most of the ATP synthesis) The process that generates most of the ATP is called oxidative phosphorylation because it is powered by redox reactions

18. LE 9-6_1 Mitochondrion Glycolysis Pyruvate Glucose Cytosol ATP Substrate-level phosphorylation

19. LE 9-6_2 Mitochondrion Glycolysis Pyruvate Glucose Cytosol ATP Substrate-level phosphorylation ATP Substrate-level phosphorylation Citric acid cycle

20. LE 9-6_3 Mitochondrion Glycolysis Pyruvate Glucose Cytosol ATP Substrate-level phosphorylation ATP Substrate-level phosphorylation Citric acid cycle ATP Oxidative phosphorylation Oxidative phosphorylation: electron transport and chemiosmosis Electrons carried via NADH Electrons carried via NADH and FADH 2

21. Oxidative phosphorylation accounts for almost 90% of the ATP generated by cellular respiration A small amount of ATP is formed in glycolysis and the citric acid cycle by substrate-level phosphorylation

22. LE 9-7 Enzyme ADP P Substrate Product Enzyme ATP +

23. Glycolysis harvests energy by oxidizing glucose to pyruvate Glycolysis (“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

24. LE 9-8 Energy investment phase Glucose 2 ATP used 2 ADP + 2 P 4 ADP + 4 P 4 ATP formed 2 NAD + + 4 e – + 4 H + Energy payoff phase + 2 H + 2 NADH 2 Pyruvate + 2 H 2 O 2 Pyruvate + 2 H 2 O 2 ATP 2 NADH + 2 H + Glucose 4 ATP formed – 2 ATP used 2 NAD+ + 4 e – + 4 H + Net Glycolysis Citric acid cycle Oxidative phosphorylation ATP ATP ATP

25. LE 9-9a_1 Glucose ATP ADP Hexokinase ATP ATP ATP Glycolysis Oxidation phosphorylation Citric acid cycle Glucose-6-phosphate

26. LE 9-9a_2 Glucose ATP ADP Hexokinase ATP ATP ATP Glycolysis Oxidation phosphorylation Citric acid cycle Glucose-6-phosphate Phosphoglucoisomerase Phosphofructokinase Fructose-6-phosphate ATP ADP Fructose- 1, 6-bisphosphate Aldolase Isomerase Dihydroxyacetone phosphate Glyceraldehyde- 3-phosphate

27. LE 9-9b_1 2 NAD + Triose phosphate dehydrogenase + 2 H + NADH 2 1, 3-Bisphosphoglycerate 2 ADP 2 ATP Phosphoglycerokinase Phosphoglyceromutase 2-Phosphoglycerate 3-Phosphoglycerate

28. LE 9-9b_2 2 NAD + Triose phosphate dehydrogenase + 2 H + NADH 2 1, 3-Bisphosphoglycerate 2 ADP 2 ATP Phosphoglycerokinase Phosphoglyceromutase 2-Phosphoglycerate 3-Phosphoglycerate 2 ADP 2 ATP Pyruvate kinase 2 H 2 O Enolase Phosphoenolpyruvate Pyruvate The thing to remember about all of these reactions is that in a series of well-controlled chemical reactions Glucose is converted to Pyruvate

31. Glycolysis uses glucose (6C) to produce two molecules of pyruvate (3C) The first step of glycolysis involves phosphorylation ATP is used to add a phosphate group to glucose. This is followed by a second phosphorylation reaction (ATP) to produce hexose biphosphate The hexose biphosphate is still contains 6 carbons. It is now split to to form two triose phosphate molecules (3C) each (glyceraldehyde) The next step is a combined oxidation-phosphorylation reaction. The enzyme first oxidizes the triose phosphate (glyceraldehyde ) into a different compound - glycerate

32. After the oxidation reaction, the enzyme will attach an inorganic phosphate from the cytoplasm to the triose phosphate to form a triose biphosphate. This reaction does not involve ATP. Finally, each triose biphosphate gives up one of its phosphate groups. This phosphate group is taken up by ADP to form ATP. This occurs once more in the last step of glycolysis, again forming one ATP but also producing pyruvate. This process is summarized by the following equation:

34. The citric acid cycle completes the energy-yielding oxidation of organic molecules Before the citric acid cycle can begin, pyruvate must be converted to acetyl CoA , which links the cycle to glycolysis

35. LE 9-10 CYTOSOL Pyruvate NAD + MITOCHONDRION Transport protein NADH + H + Coenzyme A CO 2 Acetyl Co A

36. The citric acid cycle, also called the Krebs cycle, takes place within the mitochondrial matrix The cycle oxidizes organic fuel derived from pyruvate, generating one ATP, 3 NADH, and 1 FADH 2 per turn

37. LE 9-11 Pyruvate (from glycolysis, 2 molecules per glucose) ATP ATP ATP Glycolysis Oxidation phosphorylation Citric acid cycle NAD + NADH + H + CO 2 CoA Acetyl CoA CoA CoA Citric acid cycle CO 2 2 3 NAD + + 3 H + NADH 3 ATP ADP + P i FADH 2 FAD

38. The citric acid cycle has eight steps, each catalyzed by a specific enzyme The acetyl group of acetyl CoA joins the cycle by combining with oxaloacetate, forming citrate The next seven steps decompose the citrate back to oxaloacetate, making the process a cycle The NADH and FADH 2 produced by the cycle relay electrons extracted from food to the electron transport chain

39. LE 9-12_1 ATP ATP ATP Glycolysis Oxidation phosphorylation Citric acid cycle Citric acid cycle Citrate Isocitrate Oxaloacetate Acetyl CoA H 2 O

40. LE 9-12_2 ATP ATP ATP Glycolysis Oxidation phosphorylation Citric acid cycle Citric acid cycle Citrate Isocitrate Oxaloacetate Acetyl CoA H 2 O C O 2 NAD + NADH + H +  -Ketoglutarate C O 2 NAD + NADH + H + Succinyl CoA

41. LE 9-12_3 ATP ATP ATP Glycolysis Oxidation phosphorylation Citric acid cycle Citric acid cycle Citrate Isocitrate Oxaloacetate Acetyl CoA H 2 O C O 2 NAD + NADH + H +  -Ketoglutarate C O 2 NAD + NADH + H + Succinyl CoA Succinate GTP GDP ADP ATP FAD FADH 2 P i Fumarate

42. LE 9-12_4 ATP ATP ATP Glycolysis Oxidation phosphorylation Citric acid cycle Citric acid cycle Citrate Isocitrate Oxaloacetate Acetyl CoA H 2 O C O 2 NAD + NADH + H +  -Ketoglutarate C O 2 NAD + NADH + H + Succinyl CoA Succinate GTP GDP ADP ATP FAD FADH 2 P i Fumarate H 2 O Malate NAD + NADH + H +

43. Concept 9.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis Following glycolysis and the citric acid cycle, NADH and FADH 2 account for most of the energy extracted from food These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation

44. The Pathway of Electron Transport The electron transport chain is in the cristae of the mitochondrion Most of the chain’s components are proteins, which exist in multiprotein complexes The 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 O 2 , forming water

45. LE 9-13 ATP ATP ATP Glycolysis Oxidative phosphorylation: electron transport and chemiosmosis Citric acid cycle NADH 50 FADH 2 40 FMN Fe•S I FAD Fe•S II III Q Fe•S Cyt b 30 20 Cyt c Cyt c 1 Cyt a Cyt a 3 IV 10 0 Multiprotein complexes Free energy ( G ) relative to O2 (kcal/mol) H 2 O O 2 2 H + + 1 / 2

46. The electron transport chain generates no ATP The chain’s function is to break the large free-energy drop from food to O 2 into smaller steps that release energy in manageable amounts

47. Chemiosmosis: The Energy-Coupling Mechanism Electron transfer in the electron transport chain causes proteins to pump H + from the mitochondrial matrix to the intermembrane space H + then moves back across the membrane, passing through channels in ATP synthase ATP synthase uses the exergonic flow of H + to drive phosphorylation of ATP This is an example of chemiosmosis, the use of energy in a H + gradient to drive cellular work

48. LE 9-14 INTERMEMBRANE SPACE H + H + H + H + H + H + H + H + ATP MITOCHONDRAL MATRIX ADP + P i A rotor within the membrane spins as shown when H + flows past it down the H + gradient. A stator anchored in the membrane holds the knob stationary. A rod (or “stalk”) extending into the knob also spins, activating catalytic sites in the knob. Three catalytic sites in the stationary knob join inorganic phosphate to ADP to make ATP. Comparison of the ETC to an electric motor

49. The energy stored in a H + gradient across a membrane couples the redox reactions of the electron transport chain to ATP synthesis The H + gradient is referred to as a proton-motive force, emphasizing its capacity to do work

50. LE 9-15 Protein complex of electron carriers H + ATP ATP ATP Glycolysis Oxidative phosphorylation: electron transport and chemiosmosis Citric acid cycle H + Q III I II FAD FADH 2 + H + NADH NAD + (carrying electrons from food) Inner mitochondrial membrane Inner mitochondrial membrane Mitochondrial matrix Intermembrane space H + H + Cyt c IV 2H + + 1 / 2 O 2 H 2 O ADP + H + ATP ATP synthase ETC: Electron transport and pumping of protons (H + ), Which create an H + gradient across the membrane P i Chemiosmosis ATP synthesis powered by the flow of H + back across the membrane Oxidative phosphorylation

51. An Accounting of ATP Production by Cellular Respiration During cellular respiration, most energy flows in this sequence: glucose  NADH  electron transport chain  proton-motive force  ATP About 40% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making about 38 ATP

52. LE 9-16 CYTOSOL Electron shuttles span membrane 2 NADH or 2 FADH 2 MITOCHONDRION Oxidative phosphorylation: electron transport and chemiosmosis 2 FADH 2 2 NADH 6 NADH Citric acid cycle 2 Acetyl CoA 2 NADH Glycolysis Glucose 2 Pyruvate + 2 ATP by substrate-level phosphorylation + 2 ATP by substrate-level phosphorylation + about 32 or 34 ATP by oxidation phosphorylation, depending on which shuttle transports electrons form NADH in cytosol About 36 or 38 ATP Maximum per glucose:

53. Fermentation enables some cells to produce ATP without the use of oxygen Cellular respiration requires O 2 to produce ATP Glycolysis can produce ATP with or without O 2 (in aerobic or anaerobic conditions) In the absence of O 2 , glycolysis couples with fermentation to produce ATP

54. Types of Fermentation Fermentation consists of glycolysis plus reactions that regenerate NAD + , which can be reused by glycolysis Two common types are alcohol fermentation and lactic acid fermentation

55. In alcohol fermentation, pyruvate is converted to ethanol in two steps, with the first releasing CO 2 Alcohol fermentation by yeast is used in brewing, winemaking, and baking Play

56. LE 9-17a CO 2 + 2 H + 2 NADH 2 NAD + 2 Acetaldehyde 2 ATP 2 ADP + 2 P i 2 Pyruvate 2 2 Ethanol Alcohol fermentation Glucose Glycolysis

57. In lactic acid fermentation, pyruvate is reduced to NADH, forming lactate as an end product, with no release of CO 2 Lactic acid fermentation by some fungi and bacteria is used to make cheese and yogurt Human muscle cells use lactic acid fermentation to generate ATP when O 2 is scarce

58. LE 9-17b CO 2 + 2 H + 2 NADH 2 NAD + 2 ATP 2 ADP + 2 P i 2 Pyruvate 2 2 Lactate Lactic acid fermentation Glucose Glycolysis

59. Fermentation and Cellular Respiration Compared Both processes use glycolysis to oxidize glucose and other organic fuels to pyruvate The processes have different final electron acceptors: an organic molecule (such as pyruvate) in fermentation and O 2 in cellular respiration Cellular respiration produces much more ATP

60. Yeast and many bacteria are facultative anaerobes , meaning that they can survive using either fermentation or cellular respiration In a facultative anaerobe, pyruvate is a fork in the metabolic road that leads to two alternative catabolic routes

61. LE 9-18 Pyruvate Glucose CYTOSOL No O 2 present Fermentation Ethanol or lactate Acetyl CoA MITOCHONDRION O 2 present Cellular respiration Citric acid cycle

62. The Evolutionary Significance of Glycolysis Glycolysis occurs in nearly all organisms Glycolysis probably evolved in ancient prokaryotes before there was oxygen in the atmosphere

63. Glycolysis and the citric acid cycle connect to many other metabolic pathways Gycolysis and the citric acid cycle are major intersections to various catabolic and anabolic pathways

64. The Versatility of Catabolism Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration Glycolysis accepts a wide range of carbohydrates Proteins must be digested to amino acids; amino groups can feed glycolysis or the citric acid cycle Fats are digested to glycerol (used in glycolysis) and fatty acids (used in generating acetyl CoA) An oxidized gram of fat produces more than twice as much ATP as an oxidized gram of carbohydrate

65. LE 9-19 Citric acid cycle Oxidative phosphorylation Proteins NH 3 Amino acids Sugars Carbohydrates Glycolysis Glucose Glyceraldehyde-3- P Pyruvate Acetyl CoA Fatty acids Glycerol Fats

66. Biosynthesis (Anabolic Pathways) The body uses small molecules to build other substances These small molecules may come directly from food, from glycolysis, or from the citric acid cycle

67. Regulation of Cellular Respiration via Feedback Mechanisms Feedback inhibition is the most common mechanism for control If ATP concentration begins to drop, respiration speeds up; when there is plenty of ATP, respiration slows down Control of catabolism is based mainly on regulating the activity of enzymes at strategic points in the catabolic pathway

68. LE 9-20 Citric acid cycle Oxidative phosphorylation Glycolysis Glucose Pyruvate Acetyl CoA Fructose-6-phosphate Phosphofructokinase Fructose-1,6-bisphosphate – Inhibits ATP Citrate Inhibits Stimulates AMP + –