Week 9


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Cellular Respiration

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  • Figure 8.8 The structure and hydrolysis of adenosine triphosphate (ATP).
  • Figure 8.8 The structure and hydrolysis of adenosine triphosphate (ATP).
  • Figure 8.10 How ATP drives transport and mechanical work.
  • Figure 8.11 The ATP cycle.
  • For the Discovery Video Space Plants, go to Animation and Video Files.
  • Figure 9.2 Energy flow and chemical recycling in ecosystems.
  • Figure 9.UN01 In-text figure, p. 164
  • Figure 9.UN03 In-text figure, p. 165
  • Figure 9.4 NAD  as an electron shuttle.
  • Figure 9.UN05 In-text figure, p. 167
  • Figure 9.6 An overview of cellular respiration.
  • Figure 9.6 An overview of cellular respiration.
  • Figure 9.6 An overview of cellular respiration.
  • Figure 9.8 The energy input and output of glycolysis.
  • Figure 9.10 Oxidation of pyruvate to acetyl CoA, the step before the citric acid cycle.
  • Figure 9.11 An overview of pyruvate oxidation and the citric acid cycle.
  • Figure 9.13 Free-energy change during electron transport.
  • Figure 9.14 ATP synthase, a molecular mill.
  • Figure 9.15 Chemiosmosis couples the electron transport chain to ATP synthesis.
  • Figure 9.16 ATP yield per molecule of glucose at each stage of cellular respiration.
  • Figure 9.17 Fermentation.
  • Figure 9.17 Fermentation.
  • Figure 9.18 Pyruvate as a key juncture in catabolism.
  • Figure 9.19 The catabolism of various molecules from food.
  • Week 9

    1. 1. Electron shuttles MITOCHONDRION span membrane 2 NADH or 2 FADH2 2 NADH 2 NADH 6 NADH 2 FADH2 Glycolysis Pyruvate oxidation Oxidative Citric phosphorylation:Glucose 2 Pyruvate 2 Acetyl CoA acid electron transport cycle and chemiosmosis + 2 ATP + 2 ATP + about 26 or 28 ATP About Maximum per glucose: 30 or 32 ATPCYTOSOL Cellular Respiration Dr. Kristen Walker
    2. 2. The Structure and Hydrolysis of ATP • ATP (adenosine triphosphate) is cell’s energy shuttle • ATP is composed of ribose (a sugar), adenine (a nitrogenous base), and three phosphate groups
    3. 3. Figure 8.8a Adenine Phosphate groups Ribose (a) The structure of ATP
    4. 4. • The bonds between the phosphate groups of ATP’s tail can be broken by hydrolysis• Energy is released from ATP when the terminal phosphate bond is broken• This release of energy comes from the chemical change to a state of lower free energy, not from the phosphate bonds themselves
    5. 5. Figure 8.8b Adenosine triphosphate (ATP) Energy Inorganic Adenosine diphosphate (ADP) phosphate (b) The hydrolysis of ATP
    6. 6. How the Hydrolysis of ATP Performs Work • The three types of cellular work (mechanical, transport, and chemical) are powered by the hydrolysis of ATP • In the cell, the energy from the exergonic reaction (release of free energy) of ATP hydrolysis can be used to drive an endergonic reaction (absorbs free energy) • Overall, the coupled reactions are exergonic
    7. 7. • ATP drives endergonic reactions by phosphorylation, transferring a phosphate group to some other molecule, such as a reactant• The recipient molecule is now called a phosphorylated intermediate
    8. 8. Figure 8.10 Transport protein Solute ATP ADP Pi P Pi Solute transported (a) Transport work: ATP phosphorylates transport proteins. Vesicle Cytoskeletal track ATP ADP Pi ATP Motor protein Protein and vesicle moved (b) Mechanical work: ATP binds noncovalently to motor proteins and then is hydrolyzed.
    9. 9. The Regeneration of ATP • ATP is a renewable resource that is regenerated by addition of a phosphate group to adenosine diphosphate (ADP) • Energy to phosphorylate ADP comes from catabolic reactions in the cell • Catabolic - release energy • Anabolic - require energy • ATP cycle is a revolving door through which energy passes during its transfer from catabolic to anabolic pathways
    10. 10. Figure 8.11 ATP H2O Energy from Energy for cellular catabolism (exergonic, work (endergonic, energy-releasing ADP Pi energy-consuming processes) processes)
    11. 11. Life Is Work • Living cells require energy from outside sources • Some animals, such as the chimpanzee, obtain energy by eating plants, and some animals feed on other organisms that eat plants
    12. 12. • Energy flows into an ecosystem as sunlight and leaves as heat• Photosynthesis generates O2 and organic molecules, which are used in cellular respiration• Cells use chemical energy stored in organic molecules to regenerate ATP, which powers work
    13. 13. Figure 9.2 Light energy ECOSYSTEM Photosynthesis in chloroplasts CO2 + H2O Organic + O2 molecules Cellular respiration in mitochondria ATP powers ATP most cellular work Heat energy
    14. 14. Catabolic pathways yield energy byoxidizing organic fuels • Several processes are central to cellular respiration and related pathways
    15. 15. Catabolic Pathways and Production of ATP • Fermentation is a partial degradation of sugars that occurs without O2 • Aerobic respiration consumes organic molecules and O2 and yields ATP • Anaerobic respiration is similar to aerobic respiration but consumes compounds other than O2
    16. 16. • Cellular respiration includes both aerobic and anaerobic respiration but is often used to refer to aerobic respiration• Although carbohydrates, fats, and proteins are all consumed as fuel, it is helpful to trace cellular respiration with the sugar glucose C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + Energy (ATP + heat)
    17. 17. Redox Reactions: Oxidation and Reduction • The transfer of electrons during chemical reactions releases energy stored in organic molecules • This released energy is ultimately used to synthesize ATP
    18. 18. The Principle of Redox • Chemical reactions that transfer electrons between reactants are called oxidation-reduction reactions, or redox reactions • In oxidation, a substance loses electrons, or is oxidized • In reduction, a substance gains electrons, or is reduced (the amount of positive charge is reduced)
    19. 19. Figure 9.UN01 becomes oxidized (loses electron) becomes reduced (gains electron)
    20. 20. Oxidation of Organic Fuel Molecules DuringCellular Respiration • During cellular respiration, the fuel (such as glucose) is oxidized, and O2 is reduced
    21. 21. Figure 9.UN03 becomes oxidized becomes reduced
    22. 22. Stepwise Energy Harvest via NAD+ and theElectron Transport Chain • In cellular respiration, glucose and other organic molecules are broken down in a series of steps • Electrons from organic compounds are usually first transferred to NAD+, a coenzyme • As an electron acceptor, NAD+ functions as an oxidizing agent during cellular respiration • Each NADH (the reduced form of NAD+) represents stored energy that is tapped to synthesize ATP
    23. 23. Figure 9.4 NAD+ NADH Dehydrogenase Reduction of NAD+ (from food) Oxidation of NADH Nicotinamide Nicotinamide (oxidized form) (reduced form)
    24. 24. • NADH passes the electrons to the electron transport chain• Unlike an uncontrolled reaction, the electron transport chain passes electrons in a series of steps instead of one explosive reaction• O2 pulls electrons down the chain in an energy- yielding tumble• The energy yielded is used to regenerate ATP
    25. 25. The Stages of Cellular Respiration:A Preview • Harvesting of energy from glucose has three stages – Glycolysis (breaks down glucose into two molecules of pyruvate) – The citric acid cycle (completes the breakdown of glucose) also called Krebs cycle – Oxidative phosphorylation (accounts for most of the ATP synthesis)
    26. 26. Figure 9.UN05 1. Glycolysis (color-coded teal throughout the chapter) 2. Pyruvate oxidation and the citric acid cycle (color-coded salmon) 3. Oxidative phosphorylation: electron transport and chemiosmosis (color-coded violet)
    27. 27. Figure 9.6-1 Electrons carried via NADH Glycolysis Glucose Pyruvate CYTOSOL MITOCHONDRION ATP Substrate-level phosphorylation
    28. 28. Figure 9.6-2 Electrons Electrons carried carried via NADH and via NADH FADH2 Pyruvate Glycolysis Citric oxidation acid Glucose Pyruvate Acetyl CoA cycle CYTOSOL MITOCHONDRION ATP ATP Substrate-level Substrate-level phosphorylation phosphorylation
    29. 29. Figure 9.6-3 Electrons Electrons carried carried via NADH and via NADH FADH2 Oxidative Pyruvate Glycolysis Citric phosphorylation: oxidation acid electron transport Glucose Pyruvate Acetyl CoA cycle and chemiosmosis CYTOSOL MITOCHONDRION ATP ATP ATP Substrate-level Substrate-level Oxidative phosphorylation phosphorylation phosphorylation
    30. 30. • The process that generates most of the ATP is called oxidative phosphorylation because it is powered by redox reactions BioFlix: Cellular Respiration
    31. 31. • Oxidative phosphorylation accounts for almost 90% of the ATP generated by cellular respiration• A smaller amount of ATP is formed in glycolysis and the citric acid cycle• For each molecule of glucose degraded to CO2 and water by respiration, the cell makes up to 32 molecules of ATP
    32. 32. Glycolysis harvests chemical energy byoxidizing glucose to pyruvate • Glycolysis (“splitting of sugar”) breaks down glucose into two molecules of pyruvate • Glycolysis occurs in the cytoplasm and has two major phases – Energy investment phase – Energy payoff phase • Glycolysis occurs whether or not O2 is present
    33. 33. Figure 9.8 Energy Investment Phase Glucose 2 ADP + 2 P 2 ATP used Energy Payoff Phase 4 ADP + 4 P 4 ATP formed 2 NAD+ + 4 e− + 4 H+ 2 NADH + 2 H+ 2 Pyruvate + 2 H2O Net Glucose 2 Pyruvate + 2 H2O 4 ATP formed − 2 ATP used 2 ATP 2 NAD+ + 4 e− + 4 H+ 2 NADH + 2 H+
    34. 34. After pyruvate is oxidized, the citric acidcycle completes the energy-yielding oxidationof organic molecules • In the presence of O2, pyruvate enters the mitochondrion (in eukaryotic cells) where the oxidation of glucose is completed
    35. 35. Oxidation of Pyruvate to Acetyl CoA • Before the citric acid cycle can begin, pyruvate must be converted to acetyl Coenzyme A (acetyl CoA), which links glycolysis to the citric acid cycle • This step is carried out by a multi-enzyme complex that catalyses three reactions
    36. 36. Figure 9.10 MITOCHONDRION CYTOSOL CO2 Coenzyme A 1 3 2 NAD+ NADH + H+ Acetyl CoA Pyruvate Transport protein
    37. 37. The Citric Acid Cycle • The citric acid cycle, also called the Krebs cycle, completes the break down of pyruvate to CO2 • The cycle oxidizes organic fuel derived from pyruvate, generating 1 ATP, 3 NADH, and 1 FADH2 per turn • The citric acid cycle has eight steps, each catalyzed by a specific enzyme
    38. 38. Figure 9.11 Pyruvate CO2 NAD + CoA NADH + H+ Acetyl CoA CoA CoA Citric acid cycle 2 CO2 FADH2 3 NAD+ FAD 3 NADH + 3 H+ ADP + P i ATP
    39. 39. During oxidative phosphorylation,chemiosmosis couples electron transport toATP synthesis • Following glycolysis and the citric acid cycle, NADH and FADH2 account for most of the energy extracted from food • These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation
    40. 40. The Pathway of Electron Transport • The electron transport chain is in the inner membrane (cristae) of the mitochondrion • Most of the chain’s components are proteins, which exist in multiprotein complexes • The carriers alternate reduced and oxidized states as they accept and donate electrons • Electrons drop in free energy as they go down the chain and are finally passed to O2, forming H2O
    41. 41. Figure 9.13 NADH 50 2 e− NAD+ FADH2 2 e− FAD Multiprotein Free energy (G) relative to O2 (kcal/mol) 40 FMN I complexes Fe• S II Fe• S Q III Cyt b Fe• S 30 Cyt c1 IV Cyt c Cyt a Cyt a3 20 2 e− 10 (originally from NADH or FADH2) 0 2 H+ + 1/2 O2 H2O
    42. 42. • Electrons are transferred from NADH or FADH2 to the electron transport chain• Electrons are passed through a number of proteins including cytochromes (each with an iron atom) to O2• The electron transport chain generates no ATP directly• It breaks the large free-energy drop from food to O2 into smaller steps that release energy in manageable amounts
    43. 43. Chemiosmosis: The Energy-CouplingMechanism • Electron transfer in the electron transport chain causes proteins to pump H+ from the mitochondrial matrix to the intermembrane space • H+ then moves back across the membrane, passing through the proton, ATP synthase • ATP synthase uses the exergonic flow of H+ to drive phosphorylation of ATP • This is an example of chemiosmosis, the use of energy in a H+ gradient to drive cellular work
    44. 44. Figure 9.14 INTERMEMBRANE SPACE H+ Stator Rotor Internal rod Catalytic knob ADP + Pi ATP MITOCHONDRIAL MATRIX
    45. 45. Figure 9.15 H+ H + Protein H + complex H+ Cyt c of electron carriers IV Q III I ATP II synth- 2 H + + 1 /2 O2 H2O ase FADH2 FAD NADH NAD+ ADP + P i ATP (carrying electrons from food) H+ 1 Electron transport chain 2 Chemiosmosis Oxidative phosphorylation
    46. 46. • The energy stored in a H+ gradient across a membrane couples the redox reactions of the electron transport chain to ATP synthesis• The H+ gradient is referred to as a proton- motive force, emphasizing its capacity to do work
    47. 47. An Accounting of ATP Production byCellular Respiration • During cellular respiration, most energy flows in this sequence: glucose → NADH → electron transport chain → proton-motive force → ATP • About 34% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making about 32 ATP
    48. 48. Figure 9.16 Electron shuttles MITOCHONDRION span membrane 2 NADH or 2 FADH2 2 NADH 2 NADH 6 NADH 2 FADH2 Glycolysis Pyruvate oxidation Oxidative Citric phosphorylation: Glucose 2 Pyruvate 2 Acetyl CoA acid electron transport cycle and chemiosmosis + 2 ATP + 2 ATP + about 26 or 28 ATP About Maximum per glucose: 30 or 32 ATP CYTOSOL
    49. 49. Fermentation and anaerobicrespiration enable cells to produce ATPwithout the use of oxygen • Most cellular respiration requires O2 to produce ATP • Without O2, the electron transport chain will cease to operate • In that case, glycolysis couples with fermentation or anaerobic respiration to produce ATP
    50. 50. • Anaerobic respiration uses an electron transport chain with a final electron acceptor other than O2, for example sulfate• Fermentation uses substrate-level phosphorylation instead of an electron transport chain to generate ATP
    51. 51. Types of Fermentation • Fermentation consists of glycolysis plus reactions that regenerate NAD+, which can be reused by glycolysis • Two common types are alcohol fermentation and lactic acid fermentation
    52. 52. • In alcohol fermentation, pyruvate is converted to ethanol in two steps, with the first releasing CO2• Alcohol fermentation by yeast is used in brewing, winemaking, and baking
    53. 53. Figure 9.17a 2 ADP + 2 P i 2 ATP Glucose Glycolysis 2 Pyruvate 2 NAD + 2 NADH 2 CO2 + 2 H+ 2 Ethanol 2 Acetaldehyde (a) Alcohol fermentation
    54. 54. • In lactic acid fermentation, pyruvate is reduced to NADH, forming lactate as an end product, with no release of CO2• Lactic acid fermentation by some fungi and bacteria is used to make cheese and yogurt• Human muscle cells use lactic acid fermentation to generate ATP when O2 is scarce
    55. 55. Figure 9.17b 2 ADP + 2 P i 2 ATP Glucose Glycolysis 2 NAD + 2 NADH + 2 H+ 2 Pyruvate 2 Lactate (b) Lactic acid fermentation
    56. 56. Comparing Fermentation with Anaerobicand Aerobic Respiration • All use glycolysis (net ATP = 2) to oxidize glucose and harvest chemical energy of food • In all three, NAD+ is the oxidizing agent that accepts electrons during glycolysis • The processes have different final electron acceptors: an organic molecule (such as pyruvate or acetaldehyde) in fermentation and O2 in cellular respiration • Cellular respiration produces 32 ATP per glucose molecule; fermentation produces 2 ATP per glucose molecule
    57. 57. • Obligate anaerobes carry out fermentation or anaerobic respiration and cannot survive in the presence of O2• Yeast and many bacteria are facultative anaerobes, meaning that they can survive using either fermentation or cellular respiration• In a facultative anaerobe, pyruvate is a fork in the metabolic road that leads to two alternative catabolic routes
    58. 58. Figure 9.18 Glucose Glycolysis CYTOSOL Pyruvate No O2 present: O2 present: Fermentation Aerobic cellular respiration MITOCHONDRION Ethanol, Acetyl CoA lactate, or other products Citric acid cycle
    59. 59. The Evolutionary Significance of Glycolysis • Ancient prokaryotes are thought to have used glycolysis long before there was oxygen in the atmosphere • Very little O2 was available in the atmosphere until about 2.7 billion years ago, so early prokaryotes likely used only glycolysis to generate ATP • Glycolysis is a very ancient process
    60. 60. Glycolysis and the citric acid cycle connectto many other metabolic pathways • Gycolysis and the citric acid cycle are major intersections to various catabolic and anabolic pathways
    61. 61. The Versatility of Catabolism • Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration • Glycolysis accepts a wide range of carbohydrates • Proteins must be digested to amino acids; amino groups can feed glycolysis or the citric acid cycle
    62. 62. • Fats are digested to glycerol (used in glycolysis) and fatty acids (used in generating acetyl CoA)• Fatty acids are broken down by beta oxidation and yield acetyl CoA• An oxidized gram of fat produces more than twice as much ATP as an oxidized gram of carbohydrate
    63. 63. Figure 9.19 Proteins Carbohydrates Fats Amino Sugars Glycerol Fatty acids acids Glycolysis Glucose Glyceraldehyde 3- P NH3 Pyruvate Acetyl CoA Citric acid cycle Oxidative phosphorylation
    64. 64. Biosynthesis (Anabolic Pathways) • The body uses small molecules to build other substances • These small molecules may come directly from food, from glycolysis, or from the citric acid cycle
    65. 65. Regulation of Cellular Respiration viaFeedback Mechanisms • Feedback inhibition is the most common mechanism for control • If ATP concentration begins to drop, respiration speeds up; when there is plenty of ATP, respiration slows down • Control of catabolism is based mainly on regulating the activity of enzymes at strategic points in the catabolic pathway