2. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Section B: The Process of Cellular Respiration 1.Respiration involves glycolysis, the Krebs cycle, and electron transport: an overview 2. Glycolysis harvests chemical energy by oxidizing glucose to pyruvate: a closer look 3. The Krebs cycle completes the energy-yielding oxidation of organic molecules: a closer look 4. The inner mitochondrial membrane couples electron transport to ATP synthesis: a closer look 5. Cellular respiration generates many ATP molecules for each sugar molecule it oxidizes: a review CHAPTER 9 CELLULAR RESPIRATION: HARVESTING CHEMICAL ENERGY

3. 9.3 Section Objectives – page 231 Compare and contrast cellular respiration and fermentation. Compare and contrast cellular respiration and fermentation. Section Objectives: Explain how cells obtain energy from cellular respiration. Explain how cells obtain energy from cellular respiration.

4. Section 9.3 Summary – pages 231-237 Cellular Respiration The process by which mitochondria break down food molecules to produce ATP is called cellular respiration. The process by which mitochondria break down food molecules to produce ATP is called cellular respiration. There are three stages of cellular respiration: glycolysis, the citric acid cycle, and the electron transport chain. There are three stages of cellular respiration: glycolysis, the citric acid cycle, and the electron transport chain.

5. Cellular Respiration Respiration occurs in three metabolic stages: glycolysis, the Krebs cycle, and the electron transport chain and oxidative phosphorylation. Respiration occurs in three metabolic stages: glycolysis, the Krebs cycle, and the electron transport chain and oxidative phosphorylation.

6. Section 9.3 Summary – pages 231-237 Cellular Respiration The first stage, glycolysis, is anaerobic—no oxygen is required. The first stage, glycolysis, is anaerobic—no oxygen is required. The last two stages are aerobic and require oxygen to be completed. The last two stages are aerobic and require oxygen to be completed.

7. Section 9.3 Summary – pages 231-237 Glycolysis Glycolysis is a series of chemical reactions in the cytoplasm of a cell that break down glucose, a six- carbon compound, into two molecules of pyruvic acid, a three-carbon compound. Glycolysis is a series of chemical reactions in the cytoplasm of a cell that break down glucose, a six- carbon compound, into two molecules of pyruvic acid, a three-carbon compound. Glucose 2ATP 2ADP 2PGAL 4ADP + 4P 2NAD+ 2NADH + 2H + 4ATP 2 Pyruvic acid

8. Section 9.3 Summary – pages 231-237 Glycolysis is not very effective, producing only two ATP molecules for each glucose molecule broken down. Glycolysis is not very effective, producing only two ATP molecules for each glucose molecule broken down. Glucose 2ATP 2ADP 2PGAL 4ADP + 4P 2NAD+ 2NADH + 2H + 4ATP 2 Pyruvic acid Glycolysis

9. Section 9.3 Summary – pages 231-237 Before citric acid cycle and electron transport chain can begin, pyruvic acid undergoes a series of reactions in which it gives off a molecule of CO 2 and combines with a molecule called coenzyme A to form acetyl-CoA. Before citric acid cycle and electron transport chain can begin, pyruvic acid undergoes a series of reactions in which it gives off a molecule of CO 2 and combines with a molecule called coenzyme A to form acetyl-CoA. Pyruvic acid Outside the mitochondrion Mitochondrial membrane Inside the mitochondrion Pyruvic acid Intermediate by-product NAD + NADH + H + CO 2 Coenzyme A - CoA Acetyl-CoA Glycolysis

10. Section 9.3 Summary – pages 231-237 Fermentation During heavy exercise, when your cells are without oxygen for a short period of time, an anaerobic process called fermentation follows glycolysis and provides a means to continue producing ATP until oxygen is available again. During heavy exercise, when your cells are without oxygen for a short period of time, an anaerobic process called fermentation follows glycolysis and provides a means to continue producing ATP until oxygen is available again.

11. Section 9.3 Summary – pages 231-237 Alcoholic fermentation Another type of fermentation, alcoholic fermentation, is used by yeast cells and some bacteria to produce CO 2 and ethyl alcohol. Another type of fermentation, alcoholic fermentation, is used by yeast cells and some bacteria to produce CO 2 and ethyl alcohol.

12. In alcohol fermentation, pyruvate is converted to ethanol in two steps. In alcohol fermentation, pyruvate is converted to ethanol in two steps. –First, pyruvate is converted to a two-carbon compound, acetaldehyde by the removal of CO 2. –Second, acetaldehyde is reduced by NADH to ethanol. –Alcohol fermentation by yeast is used in brewing and winemaking. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 9.17a

13. Section 9.3 Summary – pages 231-237 Lactic acid fermentation Lactic acid fermentation is one of the processes that supplies energy when oxygen is scarce. Lactic acid fermentation is one of the processes that supplies energy when oxygen is scarce. In this process, the reactions that produced pyruvic acid are reversed. In this process, the reactions that produced pyruvic acid are reversed. Two molecules of pyruvic acid use NADH to form two molecules of lactic acid. Two molecules of pyruvic acid use NADH to form two molecules of lactic acid.

14. Section 9.3 Summary – pages 231-237 Lactic acid fermentation This releases NAD + to be used in glycolysis, allowing two ATP molecules to be formed for each glucose molecule. This releases NAD + to be used in glycolysis, allowing two ATP molecules to be formed for each glucose molecule. The lactic acid is transferred from muscle cells, to the liver that converts it back to pyruvic acid. The lactic acid is transferred from muscle cells, to the liver that converts it back to pyruvic acid.

15. During lactic acid fermentation, pyruvate is reduced directly by NADH to form lactate (ionized form of lactic acid). During lactic acid fermentation, pyruvate is reduced directly by NADH to form lactate (ionized form of lactic acid). –Lactic acid fermentation by some fungi and bacteria is used to make cheese and yogurt. –Muscle cells switch from aerobic respiration to lactic acid fermentation to generate ATP when O 2 is scarce. The waste product, lactate, may cause muscle fatigue, but ultimately it is converted back to pyruvate in the liver. The waste product, lactate, may cause muscle fatigue, but ultimately it is converted back to pyruvate in the liver. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 9.17b

16. In the energy investment phase, ATP provides activation energy by phosphorylating glucose. In the energy investment phase, ATP provides activation energy by phosphorylating glucose. –This requires 2 ATP per glucose. In the energy payoff phase, ATP is produced by substrate-level phosphorylation and NAD + is reduced to NADH. In the energy payoff phase, ATP is produced by substrate-level phosphorylation and NAD + is reduced to NADH. 4 ATP (net) and 2 NADH are produced per glucose. 4 ATP (net) and 2 NADH are produced per glucose. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 9.8

17. Some ATP is also generated in glycolysis and the Krebs cycle by substrate-level phosphorylation. Some ATP is also generated in glycolysis and the Krebs cycle by substrate-level phosphorylation. –Here an enzyme transfers a phosphate group from an organic molecule (the substrate) to ADP, forming ATP. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 9.7

18. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 9.9a

19. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 9.9b

20. Carbohydrates, fats, and proteins can all be catabolized through the same pathways. Carbohydrates, fats, and proteins can all be catabolized through the same pathways. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 9.19

21. As pyruvate enters the mitochondrion, a multienzyme complex modifies pyruvate to acetyl CoA which enters the Krebs cycle in the matrix. As pyruvate enters the mitochondrion, a multienzyme complex modifies pyruvate to acetyl CoA which enters the Krebs cycle in the matrix. –A carboxyl group is removed as CO 2. –A pair of electrons is transferred from the remaining two-carbon fragment to NAD + to form NADH. –The oxidized fragment, acetate, combines with coenzyme A to form acetyl CoA. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 9.10

22. Section 9.3 Summary – pages 231-237 The citric acid cycle The citric acid cycle, also called the Krebs cycle, is a series of chemical reactions similar to the Calvin cycle in that the molecule used in the first reaction is also one of the end products. The citric acid cycle, also called the Krebs cycle, is a series of chemical reactions similar to the Calvin cycle in that the molecule used in the first reaction is also one of the end products. For every turn of the cycle, one molecule of ATP and two molecules of carbon dioxide are produced. For every turn of the cycle, one molecule of ATP and two molecules of carbon dioxide are produced.

23. Section 9.3 Summary – pages 231-237 The Citric Acid Cycle (Acetyl-CoA) Citric acid NAD + NADH + H + O==O (CO 2 ) NAD + O==O (CO 2 ) ADP + ATP FAD FADH 2 Citric Acid Cycle NAD + NADH + H + Oxaloacetic acid The mitochondria host the citric acid cycle. NADH + H +

24. The Krebs cycle consists of eight steps. The Krebs cycle consists of eight steps. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 9.11

25. Section 9.3 Summary – pages 231-237 The citric acid cycle Citric acid The two- carbon compound acetyl-CoA reacts with a four-carbon compound called oxaloacetic acid to form citric acid, a six-carbon molecule. Citric acid The two- carbon compound acetyl-CoA reacts with a four-carbon compound called oxaloacetic acid to form citric acid, a six-carbon molecule. Acetyl-CoA Citric acid Oxaloacetic acid

26. The conversion of pyruvate and the Krebs cycle produces large quantities of electron carriers. The conversion of pyruvate and the Krebs cycle produces large quantities of electron carriers. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 9.12

27. Section 9.3 Summary – pages 231-237 Formation of CO 2 A molecule of CO 2 is formed, reducing the eventual product to a five- carbon compound. In the process, a molecule of NADH and H + is produced. Formation of CO 2 A molecule of CO 2 is formed, reducing the eventual product to a five- carbon compound. In the process, a molecule of NADH and H + is produced. NAD + NADH + H + O==O (CO 2 ) The citric acid cycle

28. Section 9.3 Summary – pages 231-237 Formation of the second CO 2 Another molecule of CO 2 is released, forming a four- carbon compound. One molecule of ATP and a molecule of NADH are also produced. Formation of the second CO 2 Another molecule of CO 2 is released, forming a four- carbon compound. One molecule of ATP and a molecule of NADH are also produced. NAD + NADH + H + O= =O (CO 2 ) ADP + ATP The citric acid cycle

29. FADH 2 NADH + H + Section 9.3 Summary – pages 231-237 Recycling of oxaloacetic acid The four-carbon molecule goes through a series of reactions in which FADH 2, NADH, and H + are formed. The carbon chain is rearranged, and oxaloacetic acid is again made available for the cycle. Recycling of oxaloacetic acid The four-carbon molecule goes through a series of reactions in which FADH 2, NADH, and H + are formed. The carbon chain is rearranged, and oxaloacetic acid is again made available for the cycle. NAD + FAD The citric acid cycle

30. Redox reactions also occur when the movement of electrons is not complete but involve a change in the degree of electron sharing in covalent bonds. Redox reactions also occur when the movement of electrons is not complete but involve a change in the degree of electron sharing in covalent bonds. In the combustion of methane to form water and carbon dioxide, the nonpolar covalent bonds of methane (C-H) and oxygen (O=O) are converted to polar covalent bonds (C=O and O-H). In the combustion of methane to form water and carbon dioxide, the nonpolar covalent bonds of methane (C-H) and oxygen (O=O) are converted to polar covalent bonds (C=O and O-H). Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 9.3

31. Only 4 of 38 ATP ultimately produced by respiration of glucose are derived from substrate-level phosphorylation. Only 4 of 38 ATP ultimately produced by respiration of glucose are derived from substrate-level phosphorylation. The vast majority of the ATP comes from the energy in the electrons carried by NADH (and FADH 2 ). The vast majority of the ATP comes from the energy in the electrons carried by NADH (and FADH 2 ). The energy in these electrons is used in the electron transport system to power ATP synthesis. The energy in these electrons is used in the electron transport system to power ATP synthesis. The inner mitochondrial membrane couples electron transport to ATP synthesis: a closer look The inner mitochondrial membrane couples electron transport to ATP synthesis: a closer look

32. Thousands of copies of the electron transport chain are found in the extensive surface of the cristae, the inner membrane of the mitochondrion. Thousands of copies of the electron transport chain are found in the extensive surface of the cristae, the inner membrane of the mitochondrion. –Most components of the chain are proteins that are bound with prosthetic groups that can alternate between reduced and oxidized states as they accept and donate electrons. Electrons drop in free energy as they pass down the electron transport chain. Electrons drop in free energy as they pass down the electron transport chain. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

33. Section 9.3 Summary – pages 231-237 The electron transport chain In the electron transport chain, the carrier molecules NADH and FADH 2 gives up electrons that pass through a series of reactions. Oxygen is the final electron acceptor. In the electron transport chain, the carrier molecules NADH and FADH 2 gives up electrons that pass through a series of reactions. Oxygen is the final electron acceptor. Enzyme Electron carrier proteins e - NADH FADH 2 NAD + FAD Electron pathway 4H + + O 2 + 4 electrons H2OH2O H2OH2O ADP + ATP Inner membrane Center of mitochondrion Space between inner and outer membranes

35. Electrons carried by NADH are transferred to the first molecule in the electron transport chain, flavoprotein. Electrons carried by NADH are transferred to the first molecule in the electron transport chain, flavoprotein. –The electrons continue along the chain which includes several cytochrome proteins and one lipid carrier. The electrons carried by FADH 2 have lower free energy and are added to a later point in the chain. The electrons carried by FADH 2 have lower free energy and are added to a later point in the chain. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 9.13

36. Electrons from NADH or FADH 2 ultimately pass to oxygen. Electrons from NADH or FADH 2 ultimately pass to oxygen. –For every two electron carriers (four electrons), one O 2 molecule is reduced to two molecules of water. The electron transport chain generates no ATP directly. The electron transport chain generates no ATP directly. Its function is to break the large free energy drop from food to oxygen into a series of smaller steps that release energy in manageable amounts. Its function is to break the large free energy drop from food to oxygen into a series of smaller steps that release energy in manageable amounts. The movement of electrons along the electron transport chain does contribute to chemiosmosis and ATP synthesis. The movement of electrons along the electron transport chain does contribute to chemiosmosis and ATP synthesis.

37. A protein complex, ATP synthase, in the cristae actually makes ATP from ADP and P i. A protein complex, ATP synthase, in the cristae actually makes ATP from ADP and P i. ATP used the energy of an existing proton gradient to power ATP synthesis. ATP used the energy of an existing proton gradient to power ATP synthesis. –This proton gradient develops between the intermembrane space and the matrix. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 9.14

38. The ATP synthase molecules are the only place that will allow H + to diffuse back to the matrix. The ATP synthase molecules are the only place that will allow H + to diffuse back to the matrix. This exergonic flow of H + is used by the enzyme to generate ATP. This exergonic flow of H + is used by the enzyme to generate ATP. This coupling of the redox reactions of the electron transport chain to ATP synthesis is called chemiosmosis. This coupling of the redox reactions of the electron transport chain to ATP synthesis is called chemiosmosis. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

39. Fig. 9.15

40. The mechanism of ATP generation by ATP synthase is still an area of active investigation. The mechanism of ATP generation by ATP synthase is still an area of active investigation. –As hydrogen ions flow down their gradient, they cause the cylinder portion and attached rod of ATP synthase to rotate. –The spinning rod causes a conformational change in the knob region, activating catalytic sites where ADP and inorganic phosphate combine to make ATP. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 9.14

41. Section 9.3 Summary – pages 231-237 Overall, the electron transport chain adds 32 ATP molecules to the four already produced. Overall, the electron transport chain adds 32 ATP molecules to the four already produced. The electron transport chain

42. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 9.16

43. During respiration, most energy flows from glucose -> NADH -> electron transport chain -> proton-motive force -> ATP. During respiration, most energy flows from glucose -> NADH -> electron transport chain -> proton-motive force -> ATP. Considering the fate of carbon, one six- carbon glucose molecule is oxidized to six CO 2 molecules. Considering the fate of carbon, one six- carbon glucose molecule is oxidized to six CO 2 molecules. Some ATP is produced by substrate-level phosphorylation during glycolysis and the Krebs cycle, but most comes from oxidative phosphorylation. Some ATP is produced by substrate-level phosphorylation during glycolysis and the Krebs cycle, but most comes from oxidative phosphorylation. Cellular respiration generates many ATP molecules for each sugar molecule it oxidizes: a review Cellular respiration generates many ATP molecules for each sugar molecule it oxidizes: a review

44. Each NADH from the Krebs cycle and the conversion of pyruvate contributes enough energy to generate a maximum of 3 ATP (rounding up). Each NADH from the Krebs cycle and the conversion of pyruvate contributes enough energy to generate a maximum of 3 ATP (rounding up). –The NADH from glycolysis may also yield 3 ATP. Each FADH 2 from the Krebs cycle can be used to generate about 2ATP. Each FADH 2 from the Krebs cycle can be used to generate about 2ATP. In some eukaryotic cells, NADH produced in the cytosol by glycolysis may be worth only 2 ATP. In some eukaryotic cells, NADH produced in the cytosol by glycolysis may be worth only 2 ATP. –The electrons must be shuttled to the mitochondrion. –In some shuttle systems, the electrons are passed to NAD +, in others the electrons are passed to FAD.

45. How efficient is respiration in generating ATP? How efficient is respiration in generating ATP? –Complete oxidation of glucose releases 686 kcal per mole. –Formation of each ATP requires at least 7.3 kcal/mole. –Efficiency of respiration is 7.3 kcal/mole x 38 ATP/glucose/686 kcal/mole glucose = 40%. –The other approximately 60% is lost as heat. Cellular respiration is remarkably efficient in energy conversion. Cellular respiration is remarkably efficient in energy conversion.

46. Section 9.3 Summary – pages 231-237 Comparing Photosynthesis and Cellular Respiration Photosynthesis Cellular Respiration Food synthesized Food broken down Energy from sun stored in glucose Energy of glucose released Carbon dioxide taken in Carbon dioxide given off Oxygen given off Oxygen taken in Produces sugars from PGALProduces CO2 and H 2 O Requires light Does not require light Occurs only in presence of chlorophyll Occurs in all living cells Table 9.1 Comparison of Photosynthesis and Cellular Respiration