Chapter 7 How Cells Release Chemical Energy

1. Chapter 7How Cells Release Chemical Energy

2. 7.1 Mighty Mitochondria • More than forty disorders related to defective mitochondria are known (such as Friedreich’s ataxia); many of those afflicted die young

3. A Mitochondrion

4. Two Main Metabolic Pathways • Aerobic metabolic pathways (using oxygen) are used by most eukaryotic cells • Anaerobicmetabolic pathways (which occur in the absence of oxygen) are used by prokaryotes and protists in anaerobic habitats

5. Aerobic Respiration • In modern eukaryotic cells, most of the aerobic respiration pathway takes place inside mitochondria • Like chloroplasts, mitochondria have an internal folded membrane system that allows them to make ATP efficiently • Electron transfer chains in this membrane set up hydrogen ion gradients that power ATP synthesis • At the end of these chains, electrons are transferred to oxygen molecules

6. INTERACTION: Structure of a mitochondrion To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

7. 7.2 Overview of Carbohydrate Breakdown Pathways • Photoautotrophs make ATP during photosynthesis and use it to synthesize glucose and other carbohydrates • Most organisms, including photoautotrophs, make ATP by breaking down glucose and other organic compounds

8. energy Photosynthesis glucose CO2 H2O O2 Aerobic Respiration energy Figure 7-2 p118

9. Overview of Aerobic Respiration • Three stages • Glycolysis • Acetyl-CoA formation and Krebs cycle • Electron transfer phosphorylation (ATP formation) C6H12O6 (glucose) + O2 (oxygen) → CO2 (carbon dioxide) + H2O (water) • Coenzymes NADH and FADH2 carry electrons and hydrogen

10. Aerobic Respiration glucose In the Cytoplasm 4 ATP (2 net) Glycolysis 2 ATP 2 pyruvate 2 NADH Krebs Cycle 6 CO2 2 ATP 8 NADH, 2 FADH2 In the Mitochondrion Electron Transfer Phosphorylation H2O oxygen 32 ATP Figure 7-3 p119

11. ANIMATED FIGURE: Overview of aerobic respiration To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

12. Aerobic Respiration vs. Anaerobic Fermentation • Aerobic respiration and fermentation both begin with glycolysis, which converts one molecule of glucose into two molecules of pyruvate • After glycolysis, the two pathways diverge • Fermentation is completed in the cytoplasm, yielding 2 ATP per glucose molecule • Aerobic respiration is completed in mitochondria, yielding 36 ATP per glucose molecule

13. Carbohydrate breakdown pathways start in the cytoplasm, with glycolysis. Glycolysis Fermentation concludes in cytoplasm. In eukaryotes, aerobic respiration concludes inside mitochondria. Figure 7-4 p119

14. ANIMATED FIGURE: Where pathways start and finish To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

15. Take-Home Message: How do cells access the chemical energy in carbohydrates? • Most cells convert the chemical energy of carbohydrates to chemical energy of ATP by aerobic respiration or fermentation • Aerobic respiration and fermentation pathways start in cytoplasm, with glycolysis • Fermentation is anaerobic and ends in the cytoplasm • Aerobic respiration requires oxygen. In eukaryotes, it ends in mitochondria

16. 3D ANIMATION: Cellular Respiration

17. 7.3 Glycolysis – Glucose Breakdown Starts • The reactions of glycolysis convert one molecule of glucose to two molecules of pyruvate for a net yield of two ATP • An energy investment of ATP is required to start glycolysis

18. Glycolysis • Two ATP are used to split glucose and form 2 PGAL, each with one phosphate group • Enzymes convert 2 PGAL to 2 PGA, forming 2 NADH • Four ATP are formed by substrate-level phosphorylation(net 2 ATP) • Glycolysis ends with the formation of two three-carbon pyruvate molecules

19. ANIMATED FIGURE: Glycolysis To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

20. ATP-Requiring Steps An enzyme (hexokinase) transfers a phosphate group from ATP to glucose, forming glucose-6-phosphate. 1 A phosphate group from a second ATP is transferred to the glucose-6phosphate. The resulting molecule is unstable, and it splits into two three carbon molecules. The molecules are interconvertible, so we will call them both PGAL (phosphoglyceraldehyde). Two ATP have now been invested in the reactions. 2 ATP-Generating Steps Enzymes attach a phosphate to the two PGAL, and transfer two electrons and a hydrogen ion from each PGAL to NAD+. Two PGA (phosphoglycerate) and two NADH are the result. 3 4 Enzymes transfer a phosphate group from each PGA to ADP. Thus, two ATP have formed by substrate-level phosphorylation. The original energy investment of two ATP has now been recovered. Enzymes transfer a phosphate group from each of two intermediates to ADP. Two more ATP have formed by substrate-level phosphorylation. Two molecules of pyruvate form at this last reaction step. 5 6 Summing up, glycolysis yields two NADH, two ATP (net), and two pyruvate for each glucose molecule. Depending on the type of cell and environmental conditions, the pyruvate may enter the second stage of aerobic respiration or it may be used in other ways, such as in fermentation. Stepped Art Figure 7-5 p121

21. Take-Home Message:What is glycolysis? • Glycolysis is the first stage of carbohydrate breakdown in both aerobic respiration and fermentation • The reactions of glycolysis occur in the cytoplasm • Glycolysis converts one molecule of glucose to two molecules of pyruvate, with a net energy yield of two ATP; two NADH also form

22. 3D ANIMATION: Cellular Respiration

23. ANIMATION: Energy inputs and release in glycolosis To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

24. 7.4 Second Stage of Aerobic Respiration • The second stage of aerobic respiration completes the breakdown of glucose that began in glycolysis • Occurs in mitochondria • Includes two sets of reactions: acetyl CoA formation and the Krebs cycle (each occurs twice in the breakdown of one glucose molecule)

25. Acetyl CoA Formation • In the inner compartment of the mitochondrion, enzymes split pyruvate, forming acetyl CoA and CO2 (which diffuses out of the cell) • NADH is formed

26. The Krebs Cycle • Krebs cycle • A sequence of enzyme-mediated reactions that break down 1 acetyl CoA into 2 CO2 • Oxaloacetate is used and regenerated • 3 NADH and 1 FADH2 are formed • 1 ATP is formed

27. Second Stage of Aerobic Respiration cytoplasm outer membrane inner membrane The breakdown of 2 pyruvate to 6 CO2 yields 2 ATP and 10 reduced coenzymes (8 NADH, 2 FADH2). The coenzymes will carry their cargo of electrons and hydrogen ions to the third stage of aerobic respiration. matrix

28. ANIMATED FIGURE: The Krebs Cycle - details To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

29. An enzyme splits a pyruvate coenzyme A NAD+ molecule into a two-carbon acetyl group and CO2. Coenzyme A binds the acetyl group (forming acetyl–CoA). NAD+ combines with released hydrogen ions and electrons, forming NADH. 1 The Krebs cycle starts as one carbon atom is transferred from acetyl–CoA tooxaloacetate. Citrate forms, and coenzyme A is regenerated. 2 The final steps of the Krebs cycle regenerate oxaloacetate. 8 A carbon atom is removed from an intermediate and leaves the cell as CO2. NAD+ combines with released hydrogen ions and electrons, forming NADH. 3 NAD+ combines with hydrogen ions and electrons, forming NADH. 7 The coenzyme FAD combines with hydrogen ions and electrons, forming FADH2. 6 A carbon atom is removed from another intermediate and leaves the cell as CO2, and another NADH forms. 4 One ATP forms by substrate-level phosphorylation. 5 Pyruvate’s three carbon atoms have now exited the cell, in CO2. Acetyl–CoA Formation and the Krebs Cycle Krebs Cycle Stepped Art Figure 7-7 p123

30. Take-Home Message: What happens during the second stage of aerobic respiration? • The second stage of aerobic respiration, acetyl–CoA formation and the Krebs cycle, occurs in the inner compartment (matrix) of mitochondria • The pyruvate that formed in glycolysis is converted to acetyl–CoA and CO2; the acetyl–CoA enters the Krebs cycle, which breaks it down to CO2 • For two pyruvate molecules broken down in the second-stage reactions, two ATP form, and ten coenzymes (eight NAD+; two FAD) are reduced

31. 7.5 Aerobic Respiration’s Big Energy Payoff • Many ATP are formed during the third and final stage of aerobic respiration • Electron transfer phosphorylation • Occurs in mitochondria • Results in attachment of phosphate to ADP to form ATP

32. Electron Transfer Phosphorylation • Coenzymes NADH and FADH2 donate electrons and H+ to electron transfer chains • Active transport forms a H+ concentration gradient in the outer mitochondrial compartment • H+ follows its gradient through ATP synthase, which attaches a phosphate to ADP • Finally, oxygen accepts electrons and combines with H+, forming water

33. Electron Transfer Phosphorylation

34. Summary: The Energy Harvest • Typically, the breakdown of one glucose molecule yields 36 ATP • Glycolysis: 2 ATP • Acetyl CoA formation and Krebs cycle: 2 ATP • Electron transfer phosphorylation: 32 ATP

35. Figure 7-9 p125

36. ANIMATED FIGURE: Third-stage reactions To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

37. Take-Home Message: What happens during the third stage of aerobic respiration? • In electron transfer phosphorylation, energy released by electrons flowing through electron transfer chains is captured in the attachment of phosphate to ADP; a typical net yield of aerobic respiration is thirty-six ATP per glucose • The reactions begin when coenzymes that were reduced in the first and second stages of reactions deliver electrons and hydrogen ions to electron transfer chains in the inner mitochondrial membrane

38. Take-Home Message: (cont.) • Energy released by electrons as they pass through electron transfer chains is used to pump H+ from the mitochondrial matrix to the intermembrane space • The H+ gradient that forms across the inner mitochondrial membrane drives the flow of hydrogen ions through ATP synthases, which results in ATP formation

39. ANIMATION: Mitochondrial chemiosmosis To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

40. 7.6 Fermentation • Fermentation pathways break down carbohydrates without using oxygen • The final steps in these pathways regenerate NAD+ but do not produce ATP

41. Fermentation • Glycolysis is the first stage of fermentation • Forms 2 pyruvate, 2 NADH, and 2 ATP • Pyruvate is converted to other molecules, but is not fully broken down to CO2 and water • Regenerates NAD+ but doesn’t produce ATP • Provides enough energy for some single-celled anaerobic species

42. Two Fermentation Pathways • Alcoholic fermentation • Pyruvate is split into acetaldehyde and CO2 • Acetaldehyde receives electrons and hydrogen from NADH, forming NAD+ and ethanol • Lactate fermentation • Pyruvate receives electrons and hydrogen from NADH, forming NAD+ and lactate

43. Glycolysis glucose 2 NAD+ 2 2 4 pyruvate 2 CO2 Alcoholic Fermentation acetaldehyde 2 2 NAD+ ethanol Figure 7-10a p127

44. Figure 7-10b p127

45. Glycolysis glucose 2 NAD+ 2 2 4 pyruvate 2 CO2 Lactate Fermentation 2 2 NAD+ lactate Figure 7-11a p127

46. ANIMATED FIGURE: Fermentation pathways To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

47. Red and White Muscle Fibers • Red muscle fibers make ATP by aerobic respiration • Have many mitochondria • Myoglobin stores oxygen • Sustain prolonged activity • White muscle fibers make ATP by lactate fermentation • Have few mitochondria and no myoglobin • Sustain short bursts of activity

48. Figure 7-11b p127

49. Figure 7-11c p127

50. Take-Home Message:What is fermentation? • ATP can form by carbohydrate breakdown in fermentation pathways, which are anaerobic • The end product of lactate fermentation is lactate. The end product of alcoholic fermentation is ethanol • Both pathways have a net yield of two ATP per glucose molecule; the ATP forms during glycolysis • Fermentation reactions regenerate the coenzyme NAD+, without which glycolysis (and ATP production) would stop