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  1. Splash

  2. Chapter Introduction An Overview of Respiration 5.1 Metabolism and Cell Respiration 5.2 The Stages of Aerobic Respiration The Reactions of Respiration 5.3 Glycolysis 5.4 Mitochondria and Respiration 5.5 The Krebs Cycle 5.6 The Electron Transport System 5.7 Oxygen, Respiration, and Photosynthesis Respiration and Cellular Activities 5.8 The Krebs Cycle in Fat and Protein Metabolism 5.9 Respiration and Heat Production 5.10 Control of Respiration Chapter Highlights Chapter Animations Chapter Menu Contents

  3. Learning Outcomes By the end of this chapter you will be able to: A Relate metabolism to cellular respiration. BDistinguish among the three hydrogen-carrier molecules and their functions. C Describe how the three stages of cellular respiration are related. D Relate glycolysis and the Krebs cycle to compartmentalization in the cell. E Explain the significance of the electron transport system to respiration. F Discuss the importance of oxygen to respiration and photosynthesis. G Explain the central role of the Krebs cycle in metabolism. Learning Outcomes

  4. A robin (Erithacus rubecula) perched on a pine tree Cell Respiration: Releasing Chemical Energy • What problem do birds and other small animals face in winter? • What adaptations help them overcome this problem? Chapter Introduction 1

  5. A robin (Erithacus rubecula) perched on a pine tree Cell Respiration: Releasing Chemical Energy • All organisms require energy to carry out their life functions. • Evolution has produced a number of biochemical processes that enable organisms to obtain the energy stored in nutrients such as carbohydrates, fats, and proteins. Chapter Introduction 2

  6. End of the Introduction

  7. An Overview of Respiration 5.1 Metabolism and Cell Respiration • The chemical reactions in an organism, has two complementary parts—synthesis and decomposition. Synthesis reactions form compounds that aid in cell growth and maintenance. Decomposition reactions release energy. ATP is a major energy carrier. Because ATP is made during decomposition and used during biosynthesis, it links these two processes. 5.1 Metabolism and Cell Respiration 1

  8. An Overview of Respiration 5.1 Metabolism and Cell Respiration (cont.) Metabolism includes all decomposition and synthesis reactions in an organism. Cell respiration is part of decomposition. ATP mediates the transfer of energy in metabolism. 5.1 Metabolism and Cell Respiration 2

  9. An Overview of Respiration 5.1 Metabolism and Cell Respiration (cont.) • Cell respiration is a decomposition pathway that provides the energy cells need to function. Respiration releases free energy by oxidizing sugars or other organic substrates. Some of this energy is conserved in ATP which in turn provides the energy to power most life processes. 5.1 Metabolism and Cell Respiration 3

  10. An Overview of Respiration 5.1 Metabolism and Cell Respiration (cont.) • Cell respiration can occur in the presence or absence of oxygen. • In aerobic respiration—occurring in the presence of oxygen—oxygen is the oxidizing agent that receives electrons from the decomposed substrates. • In anaerobic respiration—occurring without oxygen—the substrate may be only partly decomposed, releasing less energy, or a nitrogen or sulfur compound may substitute for oxygen. 5.1 Metabolism and Cell Respiration 4

  11. An Overview of Respiration 5.1 Metabolism and Cell Respiration (cont.) • The raw materials for aerobic respiration are carbohydrates, fats, and proteins. Glucose (C6H12O6) and glucose-phosphate (C6H11O6—H3PO3) are important substrates for respiration. 5.1 Metabolism and Cell Respiration 5

  12. An Overview of Respiration 5.1 Metabolism and Cell Respiration (cont.) • During aerobic respiration, a great deal of energy released as glucose gradually oxidizes and breaks down to carbon dioxide. The overall reaction is summarized in the following equation: 5.1 Metabolism and Cell Respiration 6

  13. The letters a through g represent intermediate compounds in the decomposition of glucose to carbon dioxide and water. An Overview of Respiration 5.1 Metabolism and Cell Respiration (cont.) • Oxidizing one molecule of glucose releases much more energy than a single reaction needs. Cell respiration releases energy by oxidizing glucose in a series of small steps that lead to the production of one molecule of ATP. 5.1 Metabolism and Cell Respiration 7

  14. An Overview of Respiration 5.1 Metabolism and Cell Respiration (cont.) • Cell respiration provides both ATP and the carbon skeletons needed for biosynthesis. 5.1 Metabolism and Cell Respiration 8

  15. Products of cell respiration Click the image to view an animated version. 5.1 Metabolism and Cell Respiration 9

  16. An Overview of Respiration 5.2 The Stages of Aerobic Respiration • The respiration of a simple carbohydrate such as glucose can be divided into three main stages: • glycolysis • the Krebs cycle • the electron transport system Each stage involves a series of chemical reactions catalyzed by enzymes. 5.2 The Stages of Aerobic Respiration 1

  17. An Overview of Respiration 5.2 The Stages of Aerobic Respiration (cont.) Aerobic respiration occurs in three stages—glycolysis, the Krebs cycle, and the electron transport system. As glucose and other substrates are oxidized to carbon dioxide and water, NAD+ is reduced to NADH, and FAD is reduced to FADH2. These reduced compounds carry hydrogen ions (H+) and electrons (e–) to the electron transport system. 5.2 The Stages of Aerobic Respiration 2

  18. An Overview of Respiration 5.2 The Stages of Aerobic Respiration (cont.) • Glycolysisis the initial breakdown of a carbohydrate, usually glucose, into smaller molecules at the beginning of cell respiration or fermentation. The Krebs cyclecompletes the breakdown of the intermediate products of glycolysis, releasing energy; also, a source of carbon skeletons for use in biosynthesis reactions. • The electron transport system is the process in which electrons transfer from one carrier molecule to another in photosynthesis and in cell respiration. It results in storage of some of the energy in ATP molecules. 5.2 The Stages of Aerobic Respiration 3

  19. An Overview of Respiration 5.2 The Stages of Aerobic Respiration (cont.) • During glycolysis, enzymes partially oxidize glucose and split it into two 3-carbon molecules releasing enough energy to form a small amount of ATP. An enzyme releases a molecule of carbon dioxide from each 3-carbon molecule that was produced in glycolysis. The resulting 2-carbon molecules are oxidized completely to carbon dioxide in the second stage, called the Krebs cycle, producing additional ATP molecules. 5.2 The Stages of Aerobic Respiration 4

  20. An Overview of Respiration 5.2 The Stages of Aerobic Respiration (cont.) • Whenever one substance is oxidized, another must be reduced. As glucose is oxidized, electrons and protons, are passed to NAD+(nicotinamide adenine dinucleotide), reducing it to form NADH. 5.2 The Stages of Aerobic Respiration 5

  21. An Overview of Respiration 5.2 The Stages of Aerobic Respiration (cont.) • In the electron transport system, NADH is oxidized as it donates protons and electrons, regenerating the supply of NAD+. The protons and electrons release energy to form ATP as the electron transport system transfers them to oxygen, forming water. 5.2 The Stages of Aerobic Respiration 6

  22. An Overview of Respiration 5.2 The Stages of Aerobic Respiration (cont.) • Most of the ATP is synthesized by the electron transport system. In one step in the Krebs cycle, two hydrogen atoms derived from glucose reduce a second hydrogen-carrier molecule, FAD (flavin adenine dinucleotide), instead of NAD+. NADH, NADPH, and FADH2 all carry hydrogen in cells. 5.2 The Stages of Aerobic Respiration 7

  23. An Overview of Respiration 5.2 The Stages of Aerobic Respiration (cont.) • This process is called aerobic respiration because oxygen must accept the electrons at the end of the electron transport system. The energy released in this reaction is used to synthesize ATP. 5.2 The Stages of Aerobic Respiration 8

  24. End of Section 1

  25. The Reactions of Respiration 5.3 Glycolysis • Both aerobic and anaerobic respiration begin with glycolysis. Three important things happen during glycolysis: 1. the glucose molecule breaks into two pieces 2. some ATP forms 3. some NAD+ is reduced to form NADH 5.3 Glycolysis 1

  26. The Reactions of Respiration 5.3 Glycolysis (cont.) Step A: Glycolysis begins when an enzyme converts a molecule of glucose to glucose-6-phosphate. 5.3 Glycolysis 2

  27. The Reactions of Respiration 5.3 Glycolysis (cont.) Step B: A molecule of ATP provides the phosphate and the energy to power the reaction. Another enzyme rearranges the glucose-6-phosphate, and a second ATP molecule donates another phosphate group. 5.3 Glycolysis 3

  28. The Reactions of Respiration 5.3 Glycolysis (cont.) Step C: The resulting molecule splits into two 3-carbon sugar-phosphates. 5.3 Glycolysis 4

  29. The Reactions of Respiration 5.3 Glycolysis (cont.) Step D: Other enzymes catalyze the rearrangement and partial oxidation of these molecules to form the 3-carbon compound pyruvic acid. 5.3 Glycolysis 5

  30. The Reactions of Respiration 5.3 Glycolysis (cont.) 5.3 Glycolysis 6

  31. The Reactions of Respiration 5.3 Glycolysis (cont.) • In plant cells, starch and sucrose break down to glucose or glucose-1-phosphate, which can enter glycolysis directly at step a. Three-carbon sugar-phosphates formed in photosynthesis can enter the process at step c. 5.3 Glycolysis 7

  32. The Reactions of Respiration 5.3 Glycolysis (cont.) • At the end of glycolysis, the fate of pyruvate depends on whether oxygen is present. • If insufficient oxygen is present, animal cells convert NADH and pyruvate into NAD+ and lactate. • NAD+ cycles back to glycolysis, in an anaerobic pathway known as lactic-acid fermentation. • If sufficient oxygen is present, pyruvate enters the Krebs cycle. 5.3 Glycolysis 8

  33. The Reactions of Respiration 5.4 Mitochondria and Respiration • Mitochondria are the organelles in prokaryotes in which the Krebs cycle and the electron transport system occur. Most ATP is synthesized in the mitochondria. 5.4 Mitochondria and Respiration 1

  34. This transmission electron micrograph shows a mitochondrion in a human liver cell (x80,000; color added). The Reactions of Respiration 5.4 Mitochondria and Respiration (cont.) • A cell may contain anywhere from ten to several thousand mitochondria depending on its energy needs. Each mitochondrion is usually only 2–3 µm long and about 1 µm thick. 5.4 Mitochondria and Respiration 2

  35. The Reactions of Respiration 5.4 Mitochondria and Respiration (cont.) • A mitochondrion has outer and inner membranes. The inner membrane holds the enzymes of the electron transport system and the enzymes for ATP formation. Most of the enzymes of the Krebs cycle are within the fluid-filled interior matrix. The outer membrane regulates the movement of molecules into and out of the mitochondrion. 5.4 Mitochondria and Respiration 3

  36. The Reactions of Respiration 5.5 The Krebs Cycle • The Krebs cycle completes the decomposition and oxidation of glucose to carbon dioxide. As the breakdown products of glucose are oxidized, NAD+ and FAD are reduced, and a small amount of energy is saved as ATP. 5.5 The Krebs Cycle 1

  37. The Reactions of Respiration 5.5 The Krebs Cycle (cont.) Step A: pyruvate is transported into the mitochondria where enzymes release a molecule of carbon dioxide from each pyruvate molecule, leaving a molecule of acetate. A carrier molecule, coenzyme A (CoA), binds to the acetate and delivers the acetate to the Krebs cycle. 5.5 The Krebs Cycle 2

  38. The Reactions of Respiration 5.5 The Krebs Cycle (cont.) Step B: Acetate enters the Krebs cycle, as an enzyme combined the acetate group of acetyl CoA with a 4-carbon acid (oxaloacetate) to form a 6-carbon acid (citrate). Coenzyme A is released and recycled to deliver more acetate. 5.5 The Krebs Cycle 3

  39. The Reactions of Respiration 5.5 The Krebs Cycle (cont.) Steps C and D: Other enzymes catalyze the rearrangement and oxidation of citrate. Two of the carbon atoms in citrate are oxidized to carbon dioxide. The hydrogen atoms that these carbon atoms lose reduce two molecules of NAD+. 5.5 The Krebs Cycle 4

  40. The Reactions of Respiration 5.5 The Krebs Cycle (cont.) Steps E and F: A 4-carbon organic acid rearranged and further oxidized. The result is a new molecule of oxaloacetate that begins another round of the cycle. 5.5 The Krebs Cycle 5

  41. The Reactions of Respiration 5.5 The Krebs Cycle (cont.) 5.5 The Krebs Cycle 6

  42. The Reactions of Respiration 5.6 The Electron Transport System • The oxidation of glucose in glycolysis and the Krebs cycle reduces NAD+ to NADH and FAD to FADH2 which carry hydrogen atoms to the electron transport system. The electron transport system consists of a series of enzymes and other proteins known as cytochromes that are embedded in the inner membranes of mitochondria. 5.6 The Electron Transport System 1

  43. The Reactions of Respiration 5.6 The Electron Transport System (cont.) • The electron transport system separates hydrogen atoms into electrons and protons. The cytochromes transfer the electrons step by step through the system. The last, or terminal, cytochrome is an enzyme that combines the electrons and protons with oxygen, forming water. 5.6 The Electron Transport System 2

  44. The mitochondrial electron transport system Click the image to view an animated version. 5.6 The Electron Transport System 3

  45. The Reactions of Respiration 5.6 The Electron Transport System (cont.) • At each transfer in the electron transport chain, the electrons release free energy that enables enzymes in the inner mitochondrial membrane to actively transport protons from the matrix to the intermembrane space. As protons become highly concentrated they tend to diffuse back into the matrix of the mitochondrion passing through the ATP-synthetase enzyme complex, where ATP is synthesized. 5.6 The Electron Transport System 4

  46. As glucose is oxidized in glycolysis and the Krebs cycle, NAD+ and FAD are reduced to NADH and FADH2. These carriers pass electrons to the electron transport system. ATP forms as these electrons lose energy in reducing oxygen. Each molecule of NADH generates three ATP molecules, and each molecule of FADH2 generates two ATP molecules. The resulting oxidized NAD+ and FAD are recycled as more glucose is oxidized. 5.6 The Electron Transport System 5

  47. The Reactions of Respiration 5.6 The Electron Transport System (cont.) • Bacteria do not have mitochondria. Their cell membranes contain their electron transport systems. In some bacteria, in a process called anaerobic respiration, electrons flow through the system to oxidizers other than oxygen, such as sulfate (SO4–2) or nitrate (NO3–) . 5.6 The Electron Transport System 6

  48. The Reactions of Respiration 5.6 The Electron Transport System (cont.) • Bacteria that can survive for long periods with or without oxygen, switching between fermentation and aerobic respiration, are called facultative aerobes. Bacteria that are poisoned by oxygen and generate ATP entirely from fermentation or anaerobic respiration are called obligate anaerobes. Most organisms, such as animals and plants, are obligate aerobes; they cannot survive for long without oxygen. 5.6 The Electron Transport System 7

  49. The Reactions of Respiration 5.6 The Electron Transport System (cont.) 5.6 The Electron Transport System 8

  50. The Reactions of Respiration 5.7 Oxygen, Respiration, and Photosynthesis • Oxygen is needed to oxidize glucose. • Without oxygen, cells must ferment glucose, forming only two ATP molecules per glucose molecule. • With oxygen present, organisms gain much more energy from their food. 5.7 Oxygen, Respiration, and Photosynthesis 1