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- 1. CELLULAR RESPIRATION: AEROBIC HARVEST OF FOOD ENERGY • Cellular respiration is: – The main way that chemical energy is harvested from food and converted to ATP – An aerobic process—it requires oxygen © 2010 Pearson Education, Inc.
- 2. • Cellular respiration and breathing are closely related. – Cellular respiration requires a cell to exchange gases with its surroundings. – Cells take in oxygen gas. – Cells release waste carbon dioxide gas. – Breathing exchanges these same gases between the blood and outside air. © 2010 Pearson Education, Inc.
- 5. C6H12O6 CO2 O2 H2O Glucose Oxygen Carbon dioxide Water + 6 + 66 Reduction Oxidation Oxygen gains electrons (and hydrogens) Glucose loses electrons (and hydrogens) Figure 6.UN02
- 6. © 2010 Pearson Education, Inc. The Role of Oxygen in Cellular Respiration • Cellular respiration can produce up to 38 ATP molecules for each glucose molecule consumed. • During cellular respiration, hydrogen and its bonding electrons change partners. – Hydrogen and its electrons go from sugar to oxygen, forming water. – This hydrogen transfer is why oxygen is so vital to cellular respiration.
- 7. © 2010 Pearson Education, Inc. Redox Reactions • Chemical reactions that transfer electrons from one substance to another are called: – Oxidation-reduction reactions or – Redox reactions for short
- 8. © 2010 Pearson Education, Inc. • The loss of electrons during a redox reaction is called oxidation. • The acceptance of electrons during a redox reaction is called reduction.
- 9. © 2010 Pearson Education, Inc. • During cellular respiration glucose is oxidized while oxygen is reduced.
- 10. © 2010 Pearson Education, Inc. • Why does electron transfer to oxygen release energy? – When electrons move from glucose to oxygen, it is as though the electrons were falling. – This “fall” of electrons releases energy during cellular respiration.
- 11. © 2010 Pearson Education, Inc. • Cellular respiration is: – A controlled fall of electrons – A stepwise cascade much like going down a staircase
- 12. © 2010 Pearson Education, Inc. NADH and Electron Transport Chains • The path that electrons take on their way down from glucose to oxygen involves many steps.
- 13. © 2010 Pearson Education, Inc. • The first step is an electron acceptor called NAD+ . – The transfer of electrons from organic fuel to NAD+ reduces it to NADH. • The rest of the path consists of an electron transport chain, which: – Involves a series of redox reactions – Ultimately leads to the production of large amounts of ATP
- 14. Electrons from food Stepwise release of energy used to make Hydrogen, electrons, and oxygen combine to produce water Electron transportchain NADHNAD+ H+ H+ ATP H2O O2 2 2 2 2 2 1 e− e− e− e− e− e− Figure 6.5
- 15. © 2010 Pearson Education, Inc. An Overview of Cellular Respiration • Cellular respiration: – Is an example of a metabolic pathway, which is a series of chemical reactions in cells • All of the reactions involved in cellular respiration can be grouped into three main stages: – Glycolysis – The citric acid cycle – Electron transport
- 16. Cytoplasm Cytoplasm Cytoplasm Animal cell Plant cell Mitochondrion Mitochondrion High-energy electrons carried by NADH High-energy electrons carried mainly by NADH Citric Acid Cycle Electron Transport Glycolysis Glucose 2 Pyruvic acid ATP ATP ATP Figure 6.6
- 17. Cytoplasm Mitochondrion High-energy electrons carried by NADH High-energy electrons carried mainly by NADH Citric Acid Cycle Electron Transport Glycolysis Glucose 2 Pyruvic acid ATP ATP ATP Figure 6.6a
- 18. © 2010 Pearson Education, Inc. The Three Stages of Cellular Respiration • With the big-picture view of cellular respiration in mind, let’s examine the process in more detail.
- 19. © 2010 Pearson Education, Inc. Stage 1: Glycolysis • A six-carbon glucose molecule is split in half to form two molecules of pyruvic acid. • These two molecules then donate high energy electrons to NAD+ , forming NADH.
- 20. Energy investment phase Carbon atom Phosphate group High-energy electron Key Glucose 2 ATP 2 ADP INPUT OUTPUT Figure 6.7-1
- 21. Energy investment phase Carbon atom Phosphate group High-energy electron Key Glucose 2 ATP 2 ADP INPUT OUTPUT Energy harvest phase NADH NADH NAD+ NAD+ Figure 6.7-2
- 22. Energy investment phase Carbon atom Phosphate group High-energy electron Key Glucose 2 ATP 2 ADP INPUT OUTPUT Energy harvest phase NADH NADH NAD+ NAD+ 2 ATP 2 ATP 2 ADP 2 ADP 2 Pyruvic acid Figure 6.7-3
- 23. © 2010 Pearson Education, Inc. • Glycolysis: – Uses two ATP molecules per glucose to split the six-carbon glucose – Makes four additional ATP directly when enzymes transfer phosphate groups from fuel molecules to ADP • Thus, glycolysis produces a net of two molecules of ATP per glucose molecule.
- 25. © 2010 Pearson Education, Inc. Stage 2: The Citric Acid Cycle • The citric acid cycle completes the breakdown of sugar.
- 26. © 2010 Pearson Education, Inc. • In the citric acid cycle, pyruvic acid from glycolysis is first “prepped.”
- 27. (from glycolysis) (to citric acid cycle) Oxidation of the fuel generates NADH Pyruvic acid loses a carbon as CO2 Acetic acid attaches to coenzyme APyruvic acid Acetic acid Acetyl CoA Coenzyme A CoA CO2 NAD+ NADH INPUT OUTPUT Figure 6.9
- 28. © 2010 Pearson Education, Inc. • The citric acid cycle: – Extracts the energy of sugar by breaking the acetic acid molecules all the way down to CO2 – Uses some of this energy to make ATP – Forms NADH and FADH2 Blast Animation: Harvesting Energy: Krebs Cycle
- 29. 3 NAD+ ADP + P 3 NADH FADH2 FAD Acetic acid Citric acid Acceptor molecule Citric Acid Cycle ATP 2 CO2 INPUT OUTPUT Figure 6.10
- 30. © 2010 Pearson Education, Inc. Stage 3: Electron Transport • Electron transport releases the energy your cells need to make the most of their ATP.
- 31. © 2010 Pearson Education, Inc. • The molecules of the electron transport chain are built into the inner membranes of mitochondria. – The chain functions as a chemical machine that uses energy released by the “fall” of electrons to pump hydrogen ions across the inner mitochondrial membrane. – These ions store potential energy.
- 32. © 2010 Pearson Education, Inc. • When the hydrogen ions flow back through the membrane, they release energy. – The hydrogen ions flow through ATP synthase. – ATP synthase: – Takes the energy from this flow – Synthesizes ATP
- 33. Space between membranes Inner mitochondrial membrane Electron carrier Protein complex Electron flow Matrix Electron transport chain ATP synthase NADH NAD+ FADH2 FAD ATPADP + H2OO2 H+ 1 2 H+H+H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+H+ H++ 2 P Figure 6.11
- 34. 1 2 Space between membranes Inner mitochondrial membrane Electron carrier Protein complex Electron flow Matrix Electron transport chain ATP synthase NADH NAD+ FADH2 FAD ATPADP H2OO2 H+ H+H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+H+ H++ 2 P+ Figure 6.11a
- 35. © 2010 Pearson Education, Inc. The Versatility of Cellular Respiration • In addition to glucose, cellular respiration can “burn”: – Diverse types of carbohydrates – Fats – Proteins
- 36. Food Polysaccharides Fats Proteins Sugars Glycerol Fatty acids Amino acids Glycolysis Acetyl CoA Citric Acid Cycle Electron Transport ATP Figure 6.12
- 37. © 2010 Pearson Education, Inc. Adding Up the ATP from Cellular Respiration • Cellular respiration can generate up to 38 molecules of ATP per molecule of glucose.
- 38. Cytoplasm Mitochondrion NADH Citric Acid Cycle Electron Transport Glycolysis Glucose 2 Pyruvic acid 2 ATP 2 ATP NADH NADH FADH2 Maximum per glucose: 2 Acetyl CoA About 34 ATP by direct synthesis by direct synthesis by ATP synthase 2 2 2 6 About 38 ATP Figure 6.13
- 39. © 2010 Pearson Education, Inc. FERMENTATION: ANAEROBIC HARVEST OF FOOD ENERGY • Some of your cells can actually work for short periods without oxygen. • Fermentation is the anaerobic (without oxygen) harvest of food energy.
- 40. © 2010 Pearson Education, Inc. Fermentation in Human Muscle Cells • After functioning anaerobically for about 15 seconds: – Muscle cells will begin to generate ATP by the process of fermentation • Fermentation relies on glycolysis to produce ATP.
- 41. © 2010 Pearson Education, Inc. • Glycolysis: – Does not require oxygen – Produces two ATP molecules for each glucose broken down to pyruvic acid
- 42. © 2010 Pearson Education, Inc. • Pyruvic acid, produced by glycolysis, is – Reduced by NADH, producing NAD+ , which keeps glycolysis going. • In human muscle cells, lactic acid is a by-product. Animation: Fermentation Overview
- 43. Glucose 2 ATP 2 NAD+ 2 NADH 2 NADH 2 NAD+ + 2 2 ADP 2 Pyruvic acid 2 Lactic acid Glycolysis INPUT OUTPUT + 2 P H+ Figure 6.14a
- 44. © 2010 Pearson Education, Inc. • Observation: Muscles produce lactic acid under anaerobic conditions. • Question: Does the buildup of lactic acid cause muscle fatigue? • Hypothesis: The buildup of lactic acid would cause muscle activity to stop. • Experiment: Tested frog muscles under conditions when lactic acid could and could not diffuse away. The Process of Science: Does Lactic Acid Buildup Cause Muscle Burn?
- 45. Battery Force measured Battery Force measured Frog muscle stimulated by electric current Solution prevents diffusion of lactic acid Solution allows diffusion of lactic acid; muscle can work for twice as long Figure 6.15
- 46. © 2010 Pearson Education, Inc. • Results: When lactic acid could diffuse away, performance improved greatly. • Conclusion: Lactic acid accumulation is the primary cause of failure in muscle tissue. • However, recent evidence suggests that the role of lactic acid in muscle function remains unclear.
- 47. © 2010 Pearson Education, Inc. Fermentation in Microorganisms • Fermentation alone is able to sustain many types of microorganisms. • The lactic acid produced by microbes using fermentation is used to produce: – Cheese, sour cream, and yogurt dairy products – Soy sauce, pickles, olives – Sausage meat products
- 48. © 2010 Pearson Education, Inc. • Yeast are a type of microscopic fungus that: – Use a different type of fermentation – Produce CO2 and ethyl alcohol instead of lactic acid • This type of fermentation, called alcoholic fermentation, is used to produce: – Beer – Wine – Breads
- 49. Glucose 2 ATP 2 NAD+ 2 NADH 2 NADH 2 NAD+ + 2 + 2 P 2 Pyruvic acid 2 Ethyl alcohol Glycolysis INPUT OUTPUT 2 CO2 released Bread with air bubbles produced by fermenting yeast Beer fermentation 2 ADP H+ Figure 6.16
- 50. Glucose 2 ATP 2 NAD+ 2 NADH 2 NADH 2 NAD+ + 2 2 ADP 2 Pyruvic acid 2 Ethyl alcohol Glycolysis INPUT OUTPUT 2 CO2 released + 2 P H+ Figure 6.16a
- 51. C6H12O6 CO2 ATPO2 H2O Glucose Oxygen Carbon dioxide Water Energy + 6 + 66 + Figure 6.UN01
- 52. C6H12O6 CO2 O2 H2O Glucose Oxygen Carbon dioxide Water + 6 + 66 Reduction Oxidation Oxygen gains electrons (and hydrogens) Glucose loses electrons (and hydrogens) Figure 6.UN02
- 57. C6H12O6 CO2 ATPO2 H2O+ 6 + 66 + Approx. 38 Figure 6.UN07
- 58. C6H12O6 CO2 ATP O2 H2O Oxidation Glucose loses electrons (and hydrogens) Reduction Oxygen gains electrons (and hydrogens) Electrons (and hydrogens) Figure 6.UN08
- 59. NADH Citric Acid Cycle Electron Transport Glycolysis Glucose 2 Pyruvic acid 2 ATP 2 ATP NADH NADH FADH2 2 Acetyl CoA About 34 ATPby direct synthesis by direct synthesis by ATP synthase 2 2 2 6 About 38 ATP 2 CO2 CO2 O2 H2O 4 Mitochondrion Figure 6.UN09
- 60. © 2010 Pearson Education, Inc. Sunlight energy enters ecosystem Photosynthesis Cellular respiration C6H12O6 Glucose O2 Oxygen CO2 Carbon dioxide H2O Water drives cellular work Heat energy exits ecosystem ATP Figure 6.2
Notas del editor
- Figure 6.3 How breathing is related to cellular respiration.
- Figure 6.3a How breathing is related to cellular respiration.
- Figure 6.UN2 Redox reaction
- Figure 6.5 The role of oxygen in harvesting food energy.
- Figure 6.6 A road map for cellular respiration
- Figure 6.6a A road map for cellular respiration
- Figure 6.7 Glycolysis (Step 1)
- Figure 6.7 Glycolysis (Step 2)
- Figure 6.7 Glycolysis (Step 3)
- Figure 6.8 ATP synthesis by direct phosphate transfer
- Student Misconceptions and Concerns 1. Perhaps more than anywhere else in general biology, students studying aerobic metabolism fail to see the forest for the trees. Students often focus on the details of each stage of aerobic metabolism and devote little attention to the overall process and products. Consider emphasizing the products, locations, and energy yields associated with glycolysis, the citric acid cycle, and electron transport before detailing the specifics of each reaction. 2. Students often fail to realize that aerobic metabolism is a process generally similar to the burning of wood in a fireplace or campfire. Pointing out the general similarities can help students comprehend the overall reaction and heat generation associated with both processes. 3. The advantage of the gradual degradation of glucose may not be obvious to some students. Many analogies exist that reveal the advantages of short and steady steps. Fuel in an automobile is burned slowly to best utilize the energy released from the fuel. A few fireplace logs release gradual heat to keep a room temperature steady. In both situations, excessive use of fuel becomes wasteful, reducing the efficiencies of the systems. Teaching Tips 1. During cellular respiration, our cells convert about 40% of our food energy to useful work. The other 60% of the energy is released as heat. We use this heat to maintain a relatively steady body temperature near 37°C (98–99°F). This is about the same amount of heat generated by a 100-watt incandescent light bulb. If you choose to include a discussion of heat generated by aerobic metabolism, consider the following: a. Ask your students why they feel warm when it is 30°C (86°F) outside if their core body temperature is 37°C (98.6°F). Shouldn’t they feel cold? The answer is, our bodies are always producing heat. At these higher temperatures, we are producing more heat than we need to maintain a body temperature around 37°C. Thus, we sweat and behave in ways that helps us get rid of the extra heat from cellular respiration. b. Share this calculation with your students. Depending upon the size and activity of a person, a human might burn 2,000 dietary Calories (kilocalories) a day. This is enough energy to raise the temperature of 20 liters of liquid water from 0 to 100°C. This is something to think about the next time you heat water on the stove! (Note: Consider bringing a 2-liter bottle as a visual aid, or ten 2-liter bottles to make the point above; 100 Calories raises 1 liter of water 100C; Note: it takes much more energy to melt ice or evaporate water as steam.) 2. The location within a cell of each of the following reactions is often lost in the details of the processes. Yet, the locations are important. The Evolution Connection section at the end of this chapter discusses the significance of glycolysis occurring in the cytosol. Consider pointing to a diagram of a cell, with mitochondrial detail, as you lecture on cellular respiration to emphasize the location of each stage. 3. As you relate the structure of the inner mitochondrial membrane to its functions, challenge the students to suggest an adaptive advantage of the many folds of this inner membrane. These folds greatly increase the membrane region available for the associated reactions. 4. The production of NADH by glycolysis and the citric acid cycle, instead of just the direct production of ATP, can get confusing for students. Help students understand that NADH molecules have energy value, to be cashed in by the electron transport chain. The NADH can therefore be thought of as casino chips, accumulated along the way to be cashed in at the electron transport cashier. 5. The authors developed an analogy between the function of the inner mitochondrial membrane and a dam. A reservoir of hydrogen ions is built up between the two mitochondrial membranes, like a dam holding back water. As the hydrogen ions move down their concentration gradient, they spin the ATP synthase, which helps generate ATP. In a dam, water rushing downhill turns giant turbines, which generate electricity. 6. Students should be reminded that the ATP yield per glucose molecule of up to 38 ATP is only a potential. The complex chemistry of aerobic metabolism can only yield this amount under ideal conditions, when every substrate and enzyme is immediately available. Such circumstances may only rarely occur in a working cell.
- Figure 6.9 The link between glycolysis and the citric acid cycle: the conversion of pyruvic acid to acetyl coA.
- Figure 6.10 The citric acid cycle
- Figure 6.11 How electron transport drives ATP synthase machines
- Figure 6.11a How electron transport drives ATP synthase machines
- Figure 6.12 Energy from food
- Figure 6.13 A summary of ATP yield during cellular respiration
- Figure 6.14a Fermentation: Producing lactic acid
- Figure 6.15 A.V. Hill's apparatus for measuring muscle fatigue
- Figure 6.16 Fermentation: Producing ethyl alcohol
- Figure 6.16a Fermentation: Producing ethyl alcohol
- Figure 6.UN1 Overall equation of cellular respiration
- Figure 6.UN2 Redox reaction
- Figure 6.UN3 Glycosis orientation diagram
- Figure 6.UN4 Citric acid cycle orientation diagram
- Figure 6.UN5 Electron transport orientation diagram
- Figure 6.UN6 Summary: Chemical cycling
- Figure 6.UN7 Summary: Overall equation for cellular respiration
- Figure 6.UN8 Summary: Role of oxygen in cellular respiration
- Figure 6.UN9 Summary: Metabolic pathway of cellular respiration
- Figure 6.2 Energy flow and chemical cycling in ecosystems