Chapter 7 How Cells Release Chemical Energy (Sections 7.1 - 7.4)

1. Chapter 7 How Cells Release Chemical Energy(Sections 7.1 - 7.4)

2. 7.1 When Mitochondria Spin Their Wheels • Mitochondria produce ATP by aerobic respiration • Electron transfer chains in mitochondrial membranes set up H+ gradients that power ATP formation • Many disorders related to defective mitochondria are known: • Example: Friedreich’s ataxia, an enzyme-building protein (frataxin) does not work properly, causing loss of coordination (ataxia), weak muscles, and heart problems

3. The Mitochondrion • Mitochondria have an internal folded membrane system that allows them to make ATP

4. 7.2 Extracting Energy From Carbohydrates • Most organisms convert chemical energy of carbohydrates to chemical energy of ATP • Anaerobic and aerobic pathways of carbohydrate breakdown start in the cytoplasm with glycolysis, which converts glucose and other sugars to pyruvate • Anaerobic fermentation pathways end in cytoplasm and yield two ATP per molecule of glucose

5. Key Terms • anaerobic • Occurring in the absence of oxygen • aerobic • Involving or occurring in the presence of oxygen

6. Key Terms • glycolysis • Set of reactions in which glucose or another sugar is broken down to 2 pyruvate for a net yield of 2 ATP • pyruvate • Three-carbon end product of glycolysis • fermentation • An anaerobic pathway by which cells harvest energy from carbohydrates to produce ATP

7. Evolution of Earth’s Atmosphere The first cells on Earth did not use sunlight for energy Ancient organisms extracted energy and carbon from simple molecules such as methane and hydrogen sulfide—gases that were plentiful in Earth’s early atmosphere

8. Evolution of Earth’s Atmosphere • Aerobic respiration, which uses oxygen and yields much more ATP than fermentation, evolved after O2 released by early photoautotrophs changed Earth’s atmosphere • aerobic respiration • Oxygen-requiring pathway that breaks down carbohydrates to produce ATP • photoautotroph • Photosynthetic autotroph

9. Then and Now • Artist’s conception of how Earth was permanently altered by the evolution of photosynthesis and aerobic respiration

10. Aerobic Respiration • This equation summarizes aerobic respiration: C6H12O6 (glucose) + O2 (oxygen) → CO2 (carbon dioxide) + H2O (water) • Note that aerobic respiration requires oxygen (a product of photosynthesis), and produces carbon dioxide and water (the raw materials of photosynthesis)

11. The connection between photosynthesis and aerobic respiration Note the cycling of materials, and one-way flow of energy Energy, Photosynthesis, and Respiration

12. Energy, Photosynthesis, and Respiration energy Photosynthesis glucose CO2 H2O O2 Aerobic Respiration energy Fig, 7.3, p. 108

13. Animation: Links with Photosynthesis

14. Carbohydrate Breakdown Pathways • Both fermentation and aerobic respiration begin in the cytoplasm with glycolysis • After glycolysis, pathways of fermentation and aerobic respiration diverge: • Aerobic respiration continues inside mitochondria, and ends when oxygen accepts electrons at the end of electron transfer chains • Fermentation ends in the cytoplasm, where a molecule other than oxygen accepts electrons

15. Overview: Aerobic Respiration

16. Aerobic Respiration In the Cytoplasm A The first stage, glycolysis, occurs in the cell’s cytoplasm. Enzymes convert a glucose molecule to 2 pyruvate for a net yield of 2 ATP. 2 NAD+ combine with electrons and hydrogen ions during the reactions, so 2 NADH also form. In the Mitochondrion B The second stage occurs in mitochondria. The 2 pyruvate are converted to a molecule that enters the Krebs cycle. CO2 forms and leaves the cell. 2 ATP, 8 NADH, and 2 FADH2 form during the reactions. C The third and final stage, electron transfer phosphorylation, occurs inside mitochondria. 10 NADH and 2 FADH2 give up electrons and hydrogen ions to electron transfer chains. Electron flow through the chains sets up hydrogen oxygen ion gradients that drive ATP formation. Oxygen accepts electrons at the end of the chains. Fig, 7.4, p. 109

17. Animation: Overview of Aerobic Respiration

18. Key Concepts • Energy From Carbohydrates • Various pathways convert chemical energy of glucose and other organic compounds to chemical energy of ATP • Aerobic respiration yields the most ATP from each glucose molecule • In eukaryotes, this pathway ends in mitochondria

19. Animation: Where Pathways Start and Finish

20. 7.3 Glycolysis: Glucose Breakdown Starts • Glycolysis, the first stage of aerobic respiration and fermentation, occurs in cytoplasm • Enzymes use 2 ATP to convert 1 molecule of glucose or another six-carbon sugar to 2 molecules of pyruvate

21. Energy Transfer Electrons and hydrogen ions are transferred to 2 NAD+, which are reduced to NADH 4 ATP form by substrate-level phosphorylation substrate-level phosphorylation A reaction that transfers a phosphate group from a substrate directly to ADP, thus forming ATP

22. 5 Steps of Glycolysis 1. An enzyme transfers a phosphate group from ATP to glucose, forming glucose-6-phosphate 2. A phosphate group from a second ATP is transferred to glucose-6-phosphate • The resulting unstable molecule splits into 2 three-carbon molecules of PGAL (phosphoglyceraldehyde) • 2 ATP have been invested in the reactions

23. 5 Steps of Glycolysis 3. Enzymes attach a phosphate to the 2 PGAL, and transfer 2 electrons and 1 hydrogen ion from each PGAL to NAD+ 2 PGA (phosphoglycerate) and 2 NADH form 4. Enzymes transfer a phosphate group from each PGA to ADP 2 ATP have formed by substrate-level phosphorylation The original investment of 2 ATP has been recovered

24. 5 Steps of Glycolysis 5.Enzymes transfer a phosphate group from each of 2 intermediates to ADP 2 molecules of pyruvate form at this last step 2 more ATP have formed by substrate-level phosphorylation

25. Steps of Glycolysis

26. 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. Fig, 7.5, p. 110

27. 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 Fig, 7.5, p. 110

28. Animation: 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

29. Products of Glycolysis Glycolysis yields 2 NADH, 2 ATP (net), and 2 pyruvate for each glucose molecule Depending on the type of cell and environmental conditions, pyruvate may enter the second stage of aerobic respiration or may be used in other ways, such as in fermentation

30. Key Concepts • Glycolysis • Glycolysis, the first stage of aerobic respiration and of anaerobic fermentation pathways, occurs in cytoplasm • Enzymes of glycolysis convert one molecule of glucose to two molecules of pyruvate for a net yield of two ATP

31. Animation: Cellular Respiration–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

32. 7.4 Second Stage of Aerobic Respiration • The second stage of aerobic respiration, Acetyl–CoA formation and the Krebs cycle, takes place in the mitrochondrion • A mitochondrion’s inner membrane divides its interior into two fluid-filled spaces: the inner compartment (matrix) and the intermembrane space

33. From Pyruvate to CO2 2 pyruvate from glycolysis are converted to 2 acetyl–CoA and 2 CO2 Acetyl–CoA enters the Krebs cycle – it takes two cycles of Krebs reactions to dismantle 2 acetyl–CoA At the end of the Krebs cycle, all carbon atoms in the glucose molecule that entered glycolysis have left the cell in CO2

34. Acetyl-CoA Formation and the Krebs Cycle

35. Acetyl-CoA Formation and the Krebs Cycle cytoplasm outer membrane inner membrane matrix a mitochondrion Fig, 7.6, p. 112

36. 8 Steps of the Second Stage 1. An enzyme splits a pyruvate 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 2. The Krebs cycle starts as one carbon atom is transferred from acetyl–CoA to oxaloacetate • Citrate forms, and coenzyme A is regenerated

37. 8 Steps of the Second Stage 3. A carbon atom is removed and leaves the cell as CO2 NAD+ combines with H+ and electrons, forming NADH 4. Another carbon atom is removed and leaves the cell as CO2, and another NADH forms Pyruvate’s 3 carbon atoms have exited the cell, in CO2 5. 1 ATP forms by substrate-level phosphorylation

38. 8 Steps of the Second Stage 6. The coenzyme FAD combines with hydrogen ions and electrons, forming FADH2 7. NAD+ combines with H+ and electrons, forming NADH 8. The final steps of the Krebs cycle regenerate oxaloacetate

39. Steps of the Second Stage

40. Acetyl–CoA Formation and the Krebs Cycle 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 Krebs Cycle 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. Fig, 7.7, p. 112

41. 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 Fig, 7.7, p. 112

42. Animation: 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

43. Products of the Second Stage Electrons and hydrogen ions are transferred to NAD+ and FAD, which are reduced to NADH and FADH2 ATP forms by substrate-level phosphorylation TOTAL: Breakdown of 2 pyruvate molecules yields 10 reduced coenzymes and 2 ATP

44. Animation: Functional Zones in Mitochondria 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

45. Animation: Cellular Respiration–Krebs Cycle 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

46. Animation: Krebs Cycle Overview