Pathways That Harvest Chemical Energy

1. Pathways That Harvest Chemical Energy

2. 7 Pathways That Harvest Chemical Energy • 7.1 How Does Glucose Oxidation Release Chemical Energy? • 7.2 What Are the Aerobic Pathways of Glucose Metabolism? • 7.3 How Is Energy Harvested from Glucose in the Absence of Oxygen? • 7.4 How Does the Oxidation of Glucose Form ATP? • 7.5 Why Does Cellular Respiration Yield So Much More Energy Than Fermentation? • 7.6 How Are Metabolic Pathways Interrelated and Controlled?

3. 7.1 How Does Glucose Oxidation Release Chemical Energy? Fuels: molecules whose stored energy can be released for use. The most common fuel in organisms is glucose. Other molecules are first converted into glucose or other intermediate compounds.

4. 7.1 How Does Glucose Oxidation Release Chemical Energy? Principles governing metabolic pathways: • Complex chemical transformations occur in a series of reactions. • Each reaction is catalyzed by a specific enzyme. • Metabolic pathways are similar in all organisms. • In eukaryotes, metabolic pathways are compartmentalized in organelles. • Each pathway is regulated by key enzymes.

5. 7.1 How Does Glucose Oxidation Release Chemical Energy? Burning or metabolism of glucose: Glucose metabolism pathway traps the free energy in ATP:

6. 7.1 How Does Glucose Oxidation Release Chemical Energy? ΔG from complete combustion of glucose = –686 kcal/mole Highly exergonic; drives endergonic formation of many ATP

7. Figure 7.1 Energy for Life Three metabolic pathways involved in harvesting the energy of glucose

8. 7.1 How Does Glucose Oxidation Release Chemical Energy? If O2 is present, four pathways operate: • Glycolysis, pyruvate oxidation, citric acid cycle, and electron transport chain. If O2 is not present, pyruvate is metabolized in fermentation.

9. Figure 7.2 Energy-Producing Metabolic Pathways

11. 7.1 How Does Glucose Oxidation Release Chemical Energy? Redox reactions: one substance transfers electrons to another substance Reduction: gain of one or more electrons by an atom, ion, or molecule Oxidation: loss of one or more electrons Also applies if hydrogen atoms are gained or lost.

12. 7.1 How Does Glucose Oxidation Release Chemical Energy? Oxidation and reduction always occur together. The reactant that becomes reduced is the oxidizing agent. The reactant that becomes oxidized is the reducing agent.

13. Figure 7.3 Oxidation and Reduction Are Coupled

14. 7.1 How Does Glucose Oxidation Release Chemical Energy? In combustion of glucose, glucose is the reducing agent, O2 is the oxidizing agent. Energy is transferred in a redox reaction. Energy in the reducing agent becomes associated with the reduced product.

15. 7.1 How Does Glucose Oxidation Release Chemical Energy? Coenzyme NAD is an electron carrier in redox reactions. Two forms: NAD+ (oxidized) NADH + H+ (reduced)

16. Figure 7.4 NAD Is an Energy Carrier in Redox Reactions (A)

17. 7.1 How Does Glucose Oxidation Release Chemical Energy? A hydrideion (H–) is transferred, leaving a free H+ H– : a proton with two electrons

18. Figure 7.4 NAD Is an Energy Carrier in Redox Reactions (B)

19. 7.1 How Does Glucose Oxidation Release Chemical Energy? Oxygen accepts electrons from NADH: exergonic—ΔG = –52.4 kcal/mole Oxidizing agent is molecular oxygen—O2

20. 7.2 What Are the Aerobic Pathways of Glucose Metabolism? Glycolysis takes place in the cytosol. Involves 10 enzyme-catalyzed reactions Results in: 2 molecules of pyruvate 4 molecules ATP 2 molecules NADH

21. Figure 7.5 Glycolysis Converts Glucose into Pyruvate (Part 1) probably 4 parts?

22. Figure 7.5 Glycolysis Converts Glucose into Pyruvate (Part 2) probably 4 parts?

23. Figure 7.5 Glycolysis Converts Glucose into Pyruvate (Part 3) probably 4 parts?

24. Figure 7.5 Glycolysis Converts Glucose into Pyruvate (Part 4) probably 4 parts?

25. Figure 7.5 Glycolysis Converts Glucose into Pyruvate (Part 5) probably 4 parts?

26. Figure 7.5 Glycolysis Converts Glucose into Pyruvate (Part 6) probably 4 parts?

27. 7.2 What Are the Aerobic Pathways of Glucose Metabolism? A kinase is an enzyme that catalyzes transfer of a phosphate group from ATP to another substrate. In the first half of glycolysis, the glucose molecule is split into two 3-carbon molecules (G3P).

28. 7.2 What Are the Aerobic Pathways of Glucose Metabolism? Phosphorylation: addition of a phosphate group Enzyme-catalyzed transfer of a phosphate group to ADP is called substrate-level phosphorylation.

29. Figure 7.6 Changes in Free Energy During Glycolysis

30. 7.2 What Are the Aerobic Pathways of Glucose Metabolism? Pyruvate Oxidation: • Links glycolysis and the citric acid cycle • Pyruvate is converted to acetyl CoA • Takes place in the mitochondrial matrix

31. Figure 7.8 Pyruvate Oxidation and the Citric Acid Cycle (Part 1) 1st part – pyruvate oxidation

32. 7.2 What Are the Aerobic Pathways of Glucose Metabolism? Acetyl CoA is the starting point of the citric acid cycle: • Coenzyme A is removed in the first reaction and can be reused • The cycle is in steady state: the concentrations of the intermediates don’t change • Outputs: CO2, reduced electron carriers, and ATP

33. Figure 7.8 Pyruvate Oxidation and the Citric Acid Cycle (Part 2) citric acid cycle

34. Figure 7.7 The Citric Acid Cycle Releases Much More Free Energy Than Glycolysis Does

35. 7.2 What Are the Aerobic Pathways of Glucose Metabolism? The electron carriers that are reduced during the citric acid cycle must be reoxidized to take part in the cycle again. Fermentation—if no O2 is present Oxidative phosphorylation—O2 is present

36. 7.3 How Is Energy Harvested from Glucose in the Absence of Oxygen? Fermentation occurs in the cytosol. Pyruvate is reduced by NADH + H+ and NAD+ is regenerated.

37. 7.3 How Is Energy Harvested from Glucose in the Absence of Oxygen? Lactic acid fermentation: • Occurs in microorganisms, some muscle cells. • Pyruvate is the electron acceptor.

38. Figure 7.9 Lactic Acid Fermentation

39. 7.3 How Is Energy Harvested from Glucose in the Absence of Oxygen? Alcoholic fermentation: • Yeasts and some plant cells • Pyruvate is converted to acetaldehyde, CO2 is released • Acetaldehyde is reduced by NADH + H+, producing NAD+ and ethyl alcohol

40. Figure 7.10 Alcoholic Fermentation

41. 7.4 How Does the Oxidation of Glucose Form ATP? Oxidative phosphorylation: ATP is synthesized as electron carriers are reoxidized in the presence of O2. Two stages: Electron transport chain Chemiosmosis

42. 7.4 How Does the Oxidation of Glucose Form ATP? Why does the electron transport chain have so many steps? Why not in one step?

43. 7.4 How Does the Oxidation of Glucose Form ATP? Too much free energy would be released all at once—it could not be harvested by the cell. In a series of reactions, each releases a small amount of energy that can be captured by an endergonic reaction.

44. 7.4 How Does the Oxidation of Glucose Form ATP? The electron transport chain: • On the inner mitochondrial membrane • 4 protein complexes: I, II, III, IV • Cytochrome c • Ubiquinone (Q)—a lipid

45. Figure 7.11 The Oxidation of NADH + H+ (Part 1)

46. Figure 7.11 The Oxidation of NADH + H+ (Part 2)

47. Figure 7.12 The Complete Electron Transport Chain

48. 7.4 How Does the Oxidation of Glucose Form ATP? The electron transport chain results in the active transport of protons (H+) across the inner mitochondrial membrane. The transmembrane complexes act as proton pumps.

49. Figure 7.13 A Chemiosmotic Mechanism Produces ATP (Part 1)

50. Figure 7.13 A Chemiosmotic Mechanism Produces ATP (Part 2)