1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Overview: Life Is Work Living cells require energy from outside sources Some animals, such as the giant panda, obtain energy by eating plants, and some animals feed on other organisms that eat plants

2. 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

3. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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)

4. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

5. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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) Xe - + Y X + Ye - becomes oxidized (loses electron) becomes reduced (gains electron)

6. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The electron donor is called the reducing agent The electron receptor is called the oxidizing agent

7. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Some redox reactions do not transfer electrons but change the electron sharing in covalent bonds An example is the reaction between methane and oxygen

8. LE 9-3 Reactants becomes oxidized becomes reduced Products H Methane (reducing agent) Oxygen (oxidizing agent) Carbon dioxideWater HC H H OO O OCO H H CH 4 2 O 2 + + + CO 2 Energy 2 H 2 O

9. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

10. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 + (nicotinamide adenine dinucleotide), 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

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

12. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fig. 9-UN4 Dehydrogenase

13. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

14. LE 9-5 2 H + + 2 e – 2 H (from food via NADH) Controlled release of energy for synthesis of ATP 2 H + 2 e – H2OH2O + 1 / 2 O 2 H2H2 + H2OH2O Explosive release of heat and light energy Cellular respiration Uncontrolled reaction Free energy, G Electron transport chain

15. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Stages of Cellular Respiration: A Preview Cellular respiration has four stages: – Glycolysis (breaks down glucose into two molecules of pyruvate) – Pyruvate conversion (pyruvate is changed into acetyl CoA for entry into citric acid cycle) – 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

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

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

18. 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

19. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Oxidative phosphorylation (indirect ATP synthesis) 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 (direct ATP synthesis)

20. LE 9-7 Enzyme ADP P Substrate Product Enzyme ATP + Substrate level phosphorylation - direct synthesis of ATP

21. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Step 1: Glycolysis 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

22. 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 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

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

24. LE 9-9a_2 Glucose ATP ADP Hexokinase 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

25. 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

26. 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

27. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Step 2: Pyruvate Conversion Before the citric acid cycle can begin, pyruvate must be converted to acetyl CoA, which links the cycle to glycolysis

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

29. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Step 3: The Citrate Cycle (a.k.a. Krebs Cycle)

30. LE 9-11 Pyruvate (from glycolysis, 2 molecules per glucose) ATP Glycolysis Oxidation phosphorylation Citric acid cycle NAD + NADH + H + CO 2 CoA Acetyl CoA CoA Citric acid cycle CO 2 2 3 NAD + + 3 H + NADH3 ATP ADP + P i FADH 2 FAD

31. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

32. LE 9-12_1 ATP Glycolysis Oxidation phosphorylation Citric acid cycle Citric acid cycle Citrate Isocitrate Oxaloacetate Acetyl CoA H2OH2O

33. LE 9-12_2 ATP Glycolysis Oxidation phosphorylation Citric acid cycle Citric acid cycle Citrate Isocitrate Oxaloacetate Acetyl CoA H2OH2O CO2CO2 NAD + NADH + H +  -Ketoglutarate CO2CO2 NAD + NADH + H + Succinyl CoA

34. LE 9-12_3 ATP Glycolysis Oxidation phosphorylation Citric acid cycle Citric acid cycle Citrate Isocitrate Oxaloacetate Acetyl CoA H2OH2O CO2CO2 NAD + NADH + H +  -Ketoglutarate CO2CO2 NAD + NADH + H + Succinyl CoA Succinate GTP GDP ADP ATP FAD FADH 2 P i Fumarate

35. LE 9-12_4 ATP Glycolysis Oxidation phosphorylation Citric acid cycle Citric acid cycle Citrate Isocitrate Oxaloacetate Acetyl CoA H2OH2O CO2CO2 NAD + NADH + H +  -Ketoglutarate CO2CO2 NAD + NADH + H + Succinyl CoA Succinate GTP GDP ADP ATP FAD FADH 2 P i Fumarate H2OH2O Malate NAD + NADH + H +

36. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Step 4: Oxidative Phosphorylation Following glycolysis and the citric acid cycle, NADH and FADH 2 account for most of the energy (potential energy) extracted from food These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation ( powered by redox reactions)

37. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Pathway of Electron Transport The electron transport chain is in the inner mitochondrial membrane 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 are passed to protein complexes (that increase in electronegativity (increased desire to possess electrons)) as they go down the chain and are finally passed to the ultimate electron acceptor O 2 (most electronegative), forming H 2 O

38. LE 9-13 ATP Glycolysis Oxidative phosphorylation: electron transport and chemiosmosis Citric acid cycle NADH 50 FADH 2 40 FMN FeS I FAD FeS II III Q FeS 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) H2OH2O O2O2 2 H + + 1 / 2

39. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

40. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

41. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

42. LE 9-14 INTERMEMBRANE SPACE H+H+ H+H+ H+H+ H+H+ 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.

43. LE 9-15 Protein complex of electron carriers H+H+ ATP Glycolysis Oxidative phosphorylation: electron transport and chemiosmosis Citric acid cycle H+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+ H+H+ Cyt c IV 2H + + 1 / 2 O 2 H2OH2O ADP + H+H+ ATP synthase Electron transport chain 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

44. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 32 ATP The variability in whether 30 or 32 total ATP are generated per 1 molecule of glucose depends on which of two shuttle pathways is used to transfer electrons from NADH in the cytosol, generated during glycolysis, to either NAD+ or FAD (one shuttle passes electrons to NAD+ to generate NADH and the other shuttle passes electrons to FAD to generate FADH 2 ) - see following Figure 9.16*

45. 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 NADH6 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 26 or 28 ATP by oxidation phosphorylation, depending on which shuttle transports electrons form NADH in cytosol About 30 or 32 ATP Maximum per glucose: *

46. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 9.5: 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 Additionally, anaerobic respiration uses an electron transport chain with an electron acceptor other than O 2, for example sulfate Fermentation uses phosphorylation instead of an electron transport chain to generate ATP

47. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

48. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

49. LE 9-17a CO 2 + 2 H + 2 NADH2 NAD + 2 Acetaldehyde 2 ATP 2 ADP + 2 P i 2 Pyruvate 2 2 Ethanol Alcohol fermentation Glucose Glycolysis

50. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

51. LE 9-17b + 2 H + 2 NADH2 NAD + 2 ATP 2 ADP + 2 P i 2 Pyruvate 2 Lactate Lactic acid fermentation Glucose Glycolysis

52. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

53. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

54. 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

55. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 9.6: 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

56. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

57. 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

58. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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