Pathways That Harvest Chemical Energy

1. Pathways That Harvest Chemical Energy

2. 9 Pathways That Harvest Chemical Energy • 9.1 How Does Glucose Oxidation Release Chemical Energy? • 9.2 What Are the Aerobic Pathways of Glucose Metabolism? • 9.3 How Does Oxidative Phosphorylation Form ATP? • 9.4 How Is Energy Harvested from Glucose in the Absence of Oxygen? • 9.5 How Are Metabolic Pathways Interrelated and Regulated?

3. 9 Pathways that Harvest Chemical Energy Human infants are born with a lot of “brown fat”—it has many mitochondria with iron-containing pigments. When brown fat is catabolized, its energy is released as heat, which helps keep the baby warm. Opening Question: Can brown fat in adults be a target for weight loss?

4. 9.1 How Does Glucose Oxidation Release Chemical Energy? • Fuels: molecules whose stored energy can be released for use. • In cells, energy from fuel molecules is used to make ATP. • Glucose is the most common fuel in cells.

5. 9.1 How Does Glucose Oxidation Release Chemical Energy? • Cells get energy from glucose by chemical oxidation in a series of metabolic pathways. • Five principles of metabolic pathways: • Complex transformations occur in a series of separate reactions. • Each reaction is catalyzed by a specific enzyme.

6. 9.1 How Does Glucose Oxidation Release Chemical Energy? • Many metabolic pathways are similar in all organisms. • In eukaryotes, metabolic pathways are compartmentalized in specific organelles. • Key enzymes in each pathway can be inhibited or activated to alter the rate of the pathway.

7. 9.1 How Does Glucose Oxidation Release Chemical Energy? • Burning or metabolism of glucose: • C6H12O6 + 6 O2 → • 6 CO2 + 6 H2O + free energy • An oxidation–reduction reaction: glucose loses electrons (becomes oxidized) and oxygen gains them (becomes reduced).

8. 9.1 How Does Glucose Oxidation Release Chemical Energy? • Glucose catabolism pathway stores the free energy in ATP: • ADP + Pi + free energy → ATP • The ATP can be used to do cellular work.

9. 9.1 How Does Glucose Oxidation Release Chemical Energy? • ΔG (change in free energy) from complete combustion of glucose is–686 kcal/mol. • Highly exergonic; it drives the endergonic formation of many ATP molecules.

10. 9.1 How Does Glucose Oxidation Release Chemical Energy? • Three catabolic processes harvest the energy from glucose: • Glycolysis: glucose is converted to pyruvate; it is anaerobic. • Cellular Respiration: pyruvate is converted to three molecules of CO2; it is aerobic.

11. 9.1 How Does Glucose Oxidation Release Chemical Energy? • Fermentation: converts pyruvate into lactic acid or ethyl alcohol (anaerobic). • The breakdown of glucose is incomplete and lactic acid and ethyl alcohol still have a lot of energy.

12. Figure 9.1 Energy for Life

13. 9.1 How Does Glucose Oxidation Release Chemical Energy? • Oxidation–Reduction (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.

14. 9.1 How Does Glucose Oxidation Release Chemical Energy? • Oxidation and reduction always occur together. • The compound that is reduced is the oxidizing agent. • The compound that is oxidized is the reducing agent.

15. In-Text Art, Ch. 9, p. 167 (1)

16. 9.1 How Does Glucose Oxidation Release Chemical Energy? • In glucose metabolism, glucose is the reducing agent, O2 is the oxidizing agent. • Transfer of electrons is often associated with the transfer of hydrogen ions • H = H+ + e– • When a molecule loses H atoms it becomes oxidized.

17. 9.1 How Does Glucose Oxidation Release Chemical Energy? • The more reduced a molecule is, the more energy it has. • In a redox reaction some energy is transferred from the reducing agent (glucose) to the reduced product.

18. Figure 9.2 Oxidation, Reduction, and Energy

19. 9.1 How Does Glucose Oxidation Release Chemical Energy? • Coenzyme NAD+ is a key electron carrier in redox reactions.

20. Figure 9.3 NAD+/NADH Is an Electron Carrier in Redox Reactions

21. 9.1 How Does Glucose Oxidation Release Chemical Energy? • Oxygen accepts electrons from NADH: • NADH + H+ + ½ O2→ NAD+ + H2O • The reaction is exergonic: • ΔG = –52.4 kcal/mol • Oxidizing agent is molecular oxygen (O2).

22. 9.1 How Does Glucose Oxidation Release Chemical Energy? • Eukaryotic and prokaryotic cells harvest energy from glucose using different combinations of metabolic pathways. • Some prokaryotes can harvest energy by anaerobic respiration. • The five metabolic pathways occur in different parts of the cell.

23. Figure 9.4 Energy-Yielding Metabolic Pathways

24. Table 9.1

25. 9.2 What Are the Aerobic Pathways of Glucose Metabolism? • Glycolysis • Takes place in the cytosol • Converts glucose into 2 molecules of pyruvate • Produces 2 ATP and 2 NADH • Occurs in 10 steps.

26. 9.2 What Are the Aerobic Pathways of Glucose Metabolism? • Steps 1–5 require ATP (energy-investing reactions). • Steps 6–10 yield NADH and ATP (energy-harvesting reactions).

27. Figure 9.5 Glycolysis Converts Glucose into Pyruvate (Part 1)

28. Figure 9.5 Glycolysis Converts Glucose into Pyruvate (Part 2)

29. Figure 9.5 Glycolysis Converts Glucose into Pyruvate (Part 3)

30. 9.2 What Are the Aerobic Pathways of Glucose Metabolism? • Steps 6 and 7 of glycolysis:

31. 9.2 What Are the Aerobic Pathways of Glucose Metabolism? • Step 6 is an oxidation–reduction. • Exergonic; the energy is used to reduce NAD+ to NADH. • Step 7 is substrate-level phosphoryation. • Exergonic; the energy is used to transfer a phosphate to ADP and form ATP.

32. 9.2 What Are the Aerobic Pathways of Glucose Metabolism? • Pyruvate Oxidation: • Occurs in the mitochondrial matrix • Produces acetate and CO2 • Acetate binds to coenzyme A to form acetyl CoA • Is a multistep reaction catalyzed by the pyruvate dehydrogenase complex.

33. 9.2 What Are the Aerobic Pathways of Glucose Metabolism? • Pyruvate oxidation: • Exergonic; one NAD+ is reduced to NADH.

34. 9.2 What Are the Aerobic Pathways of Glucose Metabolism? • Citric acid cycle • Acetyl CoA is the starting point. • The acetyl group is completely oxidized to 2 molecules of CO2. • Energy released is captured by ADP, NAD+, FAD, and GDP.

35. Figure 9.6 The Citric Acid Cycle

36. 9.2 What Are the Aerobic Pathways of Glucose Metabolism? • Step 8 of the citric acid cycle: • This oxidation reaction is exergonic.

37. 9.2 What Are the Aerobic Pathways of Glucose Metabolism? • Overall, the oxidation of one glucose molecule yields: • Six CO2 • Ten NADH • Two FADH2 • Four ATP

38. 9.2 What Are the Aerobic Pathways of Glucose Metabolism? • Pyruvate oxidation and the citric acid cycle are regulated by concentrations of starting materials. • The starting molecules (acetyl CoA and oxidized electron carriers) must be replenished. • The electron carriers are reduced and they must be reoxidized.

39. 9.2 What Are the Aerobic Pathways of Glucose Metabolism? • If O2 is present, it accepts the electrons and H2O is formed.

40. 9.3 How Does Oxidative Phosphorylation Form ATP? • Oxidative phosphorylation: ATP is synthesized by reoxidation of electron carriers in the presence of O2. • Two stages: • Electron transport • Chemiosmosis

41. 9.3 How Does Oxidative Phosphorylation Form ATP? • Electron transport: • Electrons from NADH and FADH2 pass through the respiratory chain of membrane-associated carriers. • Electron flow results in a proton concentration gradient across the inner mitochondrial membrane.

42. 9.3 How Does Oxidative Phosphorylation Form ATP? • Chemiosmosis: • Electrons flow back across the membrane through a channel protein, ATP synthase, which couples the diffusion with ATP synthesis.

43. 9.3 How Does Oxidative Phosphorylation Form ATP? • Why does the electron transport chain have so many steps? • Why not in one step? • 2 NADH + 2 H+ + O2 → 2 NAD+ + 2 H2O

44. 9.3 How Does Oxidative Phosphorylation Form ATP? • This reaction is extremely exergonic; too much free energy would be released all at once and 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.

45. 9.3 How Does Oxidative Phosphorylation Form ATP? • The respiratory chain is located in the folded inner mitochondrial membrane. • Energy is released as electrons are passed between carriers.

46. Figure 9.7 The Oxidation of NADH and FADH2 in the Respiratory Chain

47. 9.3 How Does Oxidative Phosphorylation Form ATP? • Protons are also actively transported. • Protons accumulate in the intermembrane space and create a concentration gradient and charge difference. This potential energy is called the proton-motive force. • Diffusion of protons back across the membrane is coupled to ATP synthesis (chemiosmosis).

48. Figure 9.8 The Respiratory Chain and ATP Synthase Produce ATP by a Chemiosmotic Mechanism (Part 1)

49. Figure 9.8 The Respiratory Chain and ATP Synthase Produce ATP by a Chemiosmotic Mechanism (Part 2)

50. 9.3 How Does Oxidative Phosphorylation Form ATP? • ATP synthesis is reversible. • ATP synthase can also act as an ATPase, hydrolyzing ATP to ADP and Pi. • ATP  ADP + Pi + free energy • Why is ATP synthesis favored?