1. Pathways that Harvest and Store Chemical Energy 6

2. 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).

3. Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism Five principles govern metabolic pathways: 1.Chemical transformations occur in a series of intermediate reactions that form a metabolic pathway. 2.Each reaction is catalyzed by a specific enzyme. 3.Most metabolic pathways are similar in all organisms.

4. Concept 6.1 ATP and Reduced Coenzymes Play Important Roles in Biological Energy Metabolism 4.In eukaryotes, many metabolic pathways occur inside specific organelles. 5.Each metabolic pathway is controlled by enzymes that can be inhibited or activated.

5. 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.

6. Figure 6.1 The Concept of Coupling Reactions

7. Figure 6.2 ATP

8. 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.

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

10. 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. When a molecule loses a hydrogen atom, it becomes oxidized.

11. 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.

12. Figure 6.3 Oxidation, Reduction, and Energy

13. 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)

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

15. 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 O 2 NAD + + H 2 O

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

17. 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 CO 2.

18. Figure 6.5 Energy Metabolism Occurs in Small Steps

19. 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 O 2 ), occurs in three linked biochemical pathways: Glycolysis—glucose is converted to pyruvate. Pyruvate oxidation—pyruvate is oxidized to acetyl CoA and CO 2. Citric acid cycle—acetyl CoA is oxidized to CO 2.

20. Figure 6.6 Energy-Releasing Metabolic Pathways

21. 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

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

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

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

25. Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Pyruvate Oxidation Occurs in mitochondria in eukaryotes. Products: CO 2 and acetate; acetate is then bound to coenzyme A (CoA) to form acetyl CoA. NAD + is reduced to NADH.

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

27. 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 CO 2 2 ATP produced

28. Figure 6.8 The Citric Acid Cycle

29. Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Cells transfer energy from NADH and FADH 2 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 (32 ATP). Substrate level phosphorylation—no membrane needed. Less common!

30. Figure 6.9 Electron Transport and ATP Synthesis in Mitochondria

31. 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 FADH 2 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.

32. 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 O 2. 2 H + + 2 e – + ½ O 2 H 2 O The key role of O 2 in cells is to act as an electron acceptor and become reduced.

33. 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.

34. Figure 6.10 Chemiosmosis

35. 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.

36. 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 O 2 : 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 (Fe 3+ ) or uranium, making it potentially useful in environmental cleanup.

37. Concept 6.3 Carbohydrate Catabolism in the Absence of Oxygen Releases a Small Amount of Energy Under anaerobic conditions, NADH is reoxidized by fermentation. There are many different types of fermentation, but all operate to regenerate NAD +. The overall yield of ATP is only two—the ATP made in glycolysis.

38. Concept 6.3 Carbohydrate Catabolism in the Absence of Oxygen Releases a Small Amount of Energy Lactic acid fermentation: End product is lactic acid (lactate). NADH is used to reduce pyruvate to lactic acid, regenerating NAD +. Occurs in many microorganisms and complex organisms, including vertebrate muscle during exercise when O 2 can not be delivered to the muscle fast enough.

39. Figure 6.12 Fermentation (Part 1)

40. Concept 6.3 Carbohydrate Catabolism in the Absence of Oxygen Releases a Small Amount of Energy Alcoholic fermentation: End product is ethyl alcohol (ethanol). Pyruvate is converted to acetaldehyde, and CO 2 is released. NADH is used to reduce acetaldehyde to ethanol, regenerating NAD +. Occurs in certain yeasts and some plant cells under anaerobic conditions.

41. Figure 6.12 Fermentation (Part 2)

42. Concept 6.4 Catabolic and Anabolic Pathways Are Integrated Catabolism: Polysaccharides are hydrolyzed to glucose, which enters glycolysis. Lipids break down to fatty acids and glycerol. Fatty acids can be converted to acetyl CoA. Proteins are hydrolyzed to amino acids that can feed into glycolysis or the citric acid cycle.

43. Concept 6.4 Catabolic and Anabolic Pathways Are Integrated ATP and reduced coenzymes link catabolism, anabolism, and photosynthesis. Cellular respiration and photosynthesis are linked by their reactants and products and by the energy “currency” of ATP and reduced coenzymes.

44. Concept 6.4 Catabolic and Anabolic Pathways Are Integrated In cellular respiration glucose is oxidized: glucose + 6 O 2 6 CO 2 + 6 H 2 O + chemical energy In photosynthesis, light energy is converted to chemical energy: CO 2 + H 2 O + light energy carbohydrates + O 2

45. Figure 6.14 ATP, Reduced Coenzymes, and Metabolism

46. Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy Photosynthesis (anabolic) involves two pathways: Light reactions convert light energy into chemical energy (in ATP and the reduced electron carrier NADPH). Carbon-fixation reactions use the ATP and NADPH to produce carbohydrates.

47. Figure 6.15 An Overview of Photosynthesis

48. Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy Light is a form of electromagnetic radiation; it is propagated as a wave but also behaves as particles (photons). The amount of energy in the radiation is inversely proportional to its wavelength.

49. Figure 6.16 The Electromagnetic Spectrum

50. Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy Photons can be absorbed by specific receptor molecules, which are raised to an excited state (higher energy).

51. Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy Pigments: molecules that absorb wavelengths in the visible spectrum Chlorophyll absorbs blue and red light; the remaining light is mostly green. Absorption spectrum—plot of light energy absorbed against wavelength. Action spectrum—plot of the biological activity of an organism against wavelength.

52. Figure 6.17 Absorption and Action Spectra

53. Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy In plants, two chlorophylls absorb light energy— chlorophyll a and chlorophyll b. Accessory pigments absorb wavelengths between red and blue and transfer some of that energy to the chlorophylls.

54. Figure 6.18 The Molecular Structure of Chlorophyll

55. Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy The pigments are arranged into light- harvesting complexes, or antenna systems. A photosystem spans the thylakoid membrane in the chloroplast It consists of multiple antenna systems surrounding a reaction center.

56. Figure 6.19 Photosystem Organization

57. Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy When chlorophyll (Chl) absorbs light, it enters an excited state (Chl*), then rapidly returns to ground state, releasing an excited electron. Chl* gives the excited electron to an acceptor and becomes oxidized to Chl +. The acceptor molecule is reduced. Chl* + acceptor Chl + + acceptor – The reaction center has converted light energy into chemical energy.

58. Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy

59. The electron acceptor is the first carrier in an electron transport system in the thylakoid membrane. The final acceptor is NADP +, which gets reduced: NADP + + H + + 2 e – NADPH ATP is produced chemiosmotically during electron transport (photophosphorylation).

60. Figure 6.20 Noncyclic Electron Transport Uses Two Photosystems

61. Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy Two photosystems: Photosystem I absorbs light energy at 700 nm and passes an excited electron to NADP +, reducing it to NADPH. Photosystem II absorbs light energy at 680 nm, oxidizes water, and initiates ATP production.

62. Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy Photosystem II When Chl* gives up an electron, it is unstable and grabs an electron from H 2 O, which splits the H—O—H bonds. 2 Chl* + H 2 O 2 Chl + 2 H + + ½ O 2

63. Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy The excited (energetic) electron is passed through a series of thylakoid membrane-bound carriers to a final acceptor at a lower energy level. A proton gradient is generated and used by ATP synthase to make ATP.

64. Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy Photosystem I When Chl* gives up an electron, it grabs another electron from the last carrier in the transport system of Photosystem II. This electron ends up reducing NADP + to NADPH.

65. Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy ATP is needed for carbon-fixation pathways. The noncyclic light reactions would not provide enough ATP. Cyclic electron transport uses only photosystem I and produces only ATP. An electron is passed from an excited chlorophyll, through the electron transport chain, and recycles back to the same chlorophyll.

66. Figure 6.21 Cyclic Electron Transport Traps Light Energy as ATP

67. Concept 6.6 Photosynthetic Organisms Use Chemical Energy to Convert CO 2 to Carbohydrates Calvin cycle: the energy in ATP and NADPH is used to “fix” CO 2 in reduced form in carbohydrates Occurs in the stroma of the chloroplast. Each reaction is catalyzed by a specific enzyme.

68. Figure 6.22 The Calvin Cycle

69. Concept 6.6 Photosynthetic Organisms Use Chemical Energy to Convert CO 2 to Carbohydrates The C—H bonds generated by the Calvin cycle provide almost all the energy for life on Earth. Photosynthetic organisms (autotrophs) use most of this energy to support their own growth and reproduction. Heterotrophs cannot photosynthesize and depend on autotrophs for chemical energy.