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Pathways that Harvest and Store Chemical Energy

6. Pathways that Harvest and Store Chemical Energy. Chapter 6 Pathways that Harvest and Store Chemical Energy. Key Concepts 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism

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Pathways that Harvest and Store Chemical Energy

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  1. 6 Pathways that Harvest andStore Chemical Energy

  2. Chapter 6 Pathways that Harvest and Store Chemical Energy • Key Concepts • 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism • 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy • 6.3 Carbohydrate Catabolism in the Absence of Oxygen Releases a Small Amount of Energy

  3. Chapter 6 Pathways that Harvest and Store Chemical Energy 6.4 Catabolic and Anabolic Pathways Are Integrated 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy 6.6 Photosynthetic Organisms Use Chemical Energy to Convert CO2 to Carbohydrates

  4. Chapter 6 Opening Question Why does fresh air inhibit the formation of alcohol by yeast cells?

  5. Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism Energy is stored in chemical bonds and can be released and transformed by metabolic pathways. Chemical energy available to do work is termed free energy (G).

  6. Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism • Five principles govern metabolic pathways: • Chemical transformations occur in a series of intermediate reactions that form a metabolic pathway. • Each reaction is catalyzed by a specific enzyme. • Most metabolic pathways are similar in all organisms.

  7. Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism In eukaryotes, many metabolic pathways occur inside specific organelles. Each metabolic pathway is controlled by enzymes that can be inhibited or activated.

  8. Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism In cells, energy-transforming reactions are often coupled: An energy-releasing (exergonic) reaction is coupled to an energy-requiring (endergonic) reaction. Two coupling molecules are the coenzymes ATP and NADH.

  9. Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism Adenosine triphosphate (ATP) is a kind of “energy currency” in cells. Energy released by exergonic reactions is stored in the bonds of ATP. When ATP is hydrolyzed, free energy is released to drive endergonic reactions.

  10. Figure 6.1 The Concept of Coupling Reactions

  11. Figure 6.2 ATP

  12. Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism Hydrolysis of ATP is exergonic: ATP + H2O ADP + Pi + free energy ΔG is about –7.3 kcal/mol

  13. Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism The free energy of the bond between phosphate groups is much higher than the energy of the O—H bond that forms after hydrolysis.

  14. Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism Energy can also be transferred by the transfer of electrons in reduction–oxidation, or redox reactions. Reduction is the gain of one or more electrons. Oxidation is the loss of one or more electrons.

  15. Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism Oxidation and reduction always occur together.

  16. Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism It is also useful to think of oxidation and reduction in terms of gain or loss of hydrogen atoms: Transfers of hydrogen atoms involve transfers of electrons (H = H+ + e–). When a molecule loses a hydrogen atom, it becomes oxidized.

  17. Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism The more reduced a molecule is, the more energy is stored in its bonds. Energy is transferred in a redox reaction. Energy in the reducing agent is transferred to the reduced product.

  18. Figure 6.3 Oxidation, Reduction, and Energy

  19. Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism Coenzyme NAD is a key electron carrier in redox reactions. NAD+ (oxidized form) NADH (reduced form)

  20. Figure 6.4 NAD+/NADH Is an Electron Carrier in Redox Reactions (Part 1)

  21. Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism Reduction of NAD+ is highly endergonic: NAD+ + H+ + 2 e– NADH Oxidation of NADH is highly exergonic: NADH + H+ + ½ O2 NAD+ + H2O

  22. Figure 6.4 NAD+/NADH Is an Electron Carrier in Redox Reactions (Part 2)

  23. Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism Energy is released in catabolismby oxidation and trapped by reduction of coenzymes such as NADH. Energy for anabolicprocesses is supplied by ATP. Most energy-releasing reactions produce NADH, but most energy-consuming reactions require ATP. Oxidative phosphorylation transfers energy from NADH to ATP.

  24. Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Cellular respiration: the set of metabolic reactions used by cells to harvest energy from food A lot of energy is released when reduced molecules with many C—C and C—H bonds are fully oxidized to CO2. The oxidation occurs in a series of small steps, allowing the cell to harvest about 34% of the energy released.

  25. Figure 6.5 Energy Metabolism Occurs in Small Steps

  26. Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Catabolism of glucose under aerobic conditions (in the presence of O2), occurs in three linked biochemical pathways: Glycolysis—glucose is converted to pyruvate. Pyruvate oxidation—pyruvate is oxidized to acetyl CoA and CO2. Citric acid cycle—acetyl CoA is oxidized to CO2.

  27. Figure 6.6 Energy-Releasing Metabolic Pathways

  28. Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Glycolysis Ten reactions Takes place in the cytosol Final products: 2 molecules of pyruvate (pyruvic acid) 2 molecules of ATP 2 molecules of NADH

  29. Figure 6.7 Glycolysis Converts Glucose into Pyruvate (Part 1)

  30. Figure 6.7 Glycolysis Converts Glucose into Pyruvate (Part 2)

  31. Figure 6.7 Glycolysis Converts Glucose into Pyruvate (Part 3)

  32. Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Steps 6 and 7 are examples of reactions that occur repeatedly in metabolic pathways:

  33. Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Oxidation–reduction (step 6): exergonic; glyceraldehyde 3-phosphate is oxidized and energy is trapped via reduction of NAD+ to NADH. Substrate-level phosphorylation (step 7): also exergonic; energy released transfers a phosphate from 1,3-bisphosphoglycerate to ADP, forming ATP.

  34. Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Pyruvate Oxidation Occurs in mitochondria in eukaryotes. Products: CO2 and acetate; acetate is then bound to coenzyme A (CoA) to form acetyl CoA. NAD+ is reduced to NADH.

  35. Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy

  36. Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Citric Acid Cycle Eight reactions Occurs in mitochondria in eukaryotes Operates twice for every glucose molecule that enters glycolysis Starts with Acetyl CoA; acetyl group is oxidized to two CO2 Oxaloacetate is regenerated in the last step

  37. Figure 6.8 The Citric Acid Cycle

  38. Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Final reaction of citric acid cycle:

  39. Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Cells transfer energy from NADH and FADH2 to ATP by oxidative phosphorylation: NADH oxidation is used to actively transport protons (H+) across the inner mitochondrial membrane, resulting in a proton gradient. Diffusion of protons back across the membrane then drives the synthesis of ATP.

  40. Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy When NADH is reoxidized to NAD+, O2 is reduced to H2O: NADH + H+ + ½ O2 NAD+ + H2O This occurs in a series of redox electron carriers, called the respiratory chain,embedded in the inner membrane of the mitochondrion.

  41. Figure 6.9 Electron Transport and ATP Synthesis in Mitochondria

  42. Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Electron transport: electrons from the oxidation of NADH and FADH2 pass from one carrier to the next in the chain. The oxidation reactions are exergonic, energy released is used to actively transport H+ ions across the membrane.

  43. Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Oxidation is always coupled with reduction. When NADH is oxidized to NAD+, the reduction reaction is the formation of water from O2. 2 H+ + 2 e– + ½ O2 H2O The key role of O2 in cells is to act as an electron acceptor and become reduced.

  44. Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy ATP synthase uses the H+ gradient to drive synthesis of ATP by chemiosmosis: Chemiosmosis:Movement of ions across a semipermeable barrier from a region of higher concentration to a region of lower concentration. ATP synthase converts the potential energy of the proton gradient into chemical energy in ATP.

  45. Figure 6.10 Chemiosmosis

  46. Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy ATP synthase is a molecular motor with two subunits: F0 is a transmembrane domain that functions as the H+ channel. F1 has six subunits. As protons pass through F0, it rotates, causing part of the F1 unit to rotate. ADP and Pi bind to active sites that become exposed on the F1 unit as it rotates, and ATP is made.

  47. Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy ATP synthase structure is similar in all organisms. In prokaryotes, the proton gradient is set up across the cell membrane. In eukaryotes, chemiosmosis occurs in mitochondria and chloroplasts. The mechanism of chemiosmosis is similar in almost all forms of life.

  48. Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Chemiosmosis can be demonstrated experimentally. A proton gradient can be introduced artificially in chloroplasts or mitochondria in a test tube. ATP is synthesized if ATP synthase, ADP, and inorganic phosphate are present.

  49. Figure 6.11 An Experiment Demonstrates the Chemiosmotic Mechanism

  50. Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy About 32 molecules of ATP are produced for each fully oxidized glucose. The role of O2: most of the ATP is formed by oxidative phosphorylation, which is due to the reoxidation of NADH. Some bacteria and archaea use other electron acceptors. Geobacter metallireducens can use iron (Fe3+) or uranium, making it potentially useful in environmental cleanup.

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