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Chapter 9

Chapter 9. Cellular Respiration: Harvesting Chemical Energy. Overview: Life Is Work. Living cells require energy from outside sources Some animals, such as the giant panda, obtain energy by eating plants; others feed on organisms that eat plants.

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Chapter 9

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  1. Chapter 9 Cellular Respiration: Harvesting Chemical Energy

  2. Overview: Life Is Work • Living cells require energy from outside sources • Some animals, such as the giant panda, obtain energy by eating plants; others feed on organisms that eat plants

  3. Energy flows into an ecosystem as sunlight and leaves as heat • Photosynthesis generates oxygen and organic molecules, which are used in cellular respiration • Cells use chemical energy stored in organic molecules to regenerate ATP, which powers work

  4. LE 9-2 Light energy ECOSYSTEM Photosynthesis in chloroplasts Organic molecules CO2 + H2O + O2 Cellular respiration in mitochondria ATP powers most cellular work Heat energy

  5. Concept 9.1: Catabolic pathways yield energy by oxidizing organic fuels • Several processes are central to cellular respiration and related pathways

  6. Catabolic Pathways and Production of ATP • The breakdown of organic molecules is exergonic • Fermentation is a partial degradation of sugars that occurs without oxygen • Cellular respiration consumes oxygen and organic molecules and yields ATP • Although carbohydrates, fats, and proteins are all consumed as fuel, it is helpful to trace cellular respiration with the sugar glucose: C6H12O6 + 6O2 6CO2 + 6H2O + Energy (ATP + heat)

  7. 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

  8. becomes oxidized(loses electron) Xe- + Y X + Ye- becomes reduced(gains electron) 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)

  9. The electron donor is called the reducing agent • The electron receptor is called the oxidizing agent

  10. Some redox reactions do not transfer electrons but change the electron sharing in covalent bonds • An example is the reaction between methane and oxygen

  11. LE 9-3 Products Reactants becomes oxidized 2 O2 2 H2O Energy CH4 CO2 + + + becomes reduced H O O H H C O O H H C O H Oxygen (oxidizing agent) Methane (reducing agent) Carbon dioxide Water

  12. becomes oxidized C6H12O6 + 6O2 6CO2 + 6H2O + Energy becomes reduced Oxidation of Organic Fuel Molecules During Cellular Respiration • During cellular respiration, the fuel (such as glucose) is oxidized and oxygen is reduced:

  13. Stepwise Energy Harvest via NAD+ and the Electron 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

  14. LE 9-4 2e–+ 2H+ 2e–+ H+ H+ NADH NAD+ Dehydrogenase + 2[H] (from food) H+ + Nicotinamide (reduced form) Nicotinamide (oxidized form)

  15. 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 • Oxygen pulls electrons down the chain in an energy-yielding tumble • The energy yielded is used to regenerate ATP

  16. LE 9-5 H2 1/2 O2 + + 1/2 O2 2 H (from food via NADH) Controlled release of energy for synthesis of ATP 2 H+ + 2 e– ATP Explosive release of heat and light energy ATP Free energy, G Free energy, G ATP Electron transport chain 2 e– 1/2 O2 2 H+ H2O H2O Uncontrolled reaction Cellular respiration

  17. The Stages of Cellular Respiration: A Preview • Cellular respiration has three stages: • Glycolysis (breaks down glucose into two molecules of pyruvate) • The citric acid cycle (completes the breakdown of glucose) • Oxidative phosphorylation (accounts for most of the ATP synthesis) • The process that generates most of the ATP is called oxidative phosphorylation because it is powered by redox reactions [Animation listed on slide following figure]

  18. LE 9-6_1 Glycolysis Pyruvate Glucose Cytosol Mitochondrion ATP Substrate-level phosphorylation

  19. LE 9-6_2 Glycolysis Citric acid cycle Pyruvate Glucose Cytosol Mitochondrion ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation

  20. LE 9-6_3 Electrons carried via NADH and FADH2 Electrons carried via NADH Oxidative phosphorylation: electron transport and chemiosmosis Glycolysis Citric acid cycle Pyruvate Glucose Cytosol Mitochondrion ATP ATP ATP Substrate-level phosphorylation Oxidative phosphorylation Substrate-level phosphorylation

  21. Animation: Cell Respiration Overview

  22. Oxidative phosphorylation accounts for almost 90% of the ATP generated by cellular respiration • A small amount of ATP is formed in glycolysis and the citric acid cycle by substrate-level phosphorylation

  23. LE 9-7 Enzyme Enzyme ADP P Substrate ATP + Product

  24. Concept 9.2: Glycolysis harvests energy by oxidizing 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 Animation: Glycolysis

  25. LE 9-8 Energy investment phase Glucose 2 ATP 2 ADP + 2 P used Citric acid cycle Glycolysis Oxidative phosphorylation Energy payoff phase formed ATP ATP ATP 4 ADP + 4 P 4 ATP 2 NADH + 2 H+ 2 NAD+ + 4 e– + 4 H+ 2 Pyruvate + 2 H2O Net 2 Pyruvate + 2 H2O Glucose 4 ATP formed – 2 ATP used 2 ATP 2 NAD+ + 4 e– + 4 H+ 2 NADH + 2 H+

  26. LE 9-9a_1 Citric acid cycle Glycolysis Oxidation phosphorylation ATP ATP ATP Glucose ATP Hexokinase ADP Glucose-6-phosphate

  27. LE 9-9a_2 Citric acid cycle Glycolysis Oxidation phosphorylation ATP ATP ATP Glucose ATP Hexokinase ADP Glucose-6-phosphate Phosphoglucoisomerase Fructose-6-phosphate ATP Phosphofructokinase ADP Fructose- 1, 6-bisphosphate Aldolase Isomerase Dihydroxyacetone phosphate Glyceraldehyde- 3-phosphate

  28. LE 9-9b_1 2 NAD+ Triose phosphate dehydrogenase NADH 2 + 2 H+ 1, 3-Bisphosphoglycerate 2 ADP Phosphoglycerokinase 2 ATP 3-Phosphoglycerate Phosphoglyceromutase 2-Phosphoglycerate

  29. LE 9-9b_2 2 NAD+ Triose phosphate dehydrogenase NADH 2 + 2 H+ 1, 3-Bisphosphoglycerate 2 ADP Phosphoglycerokinase 2 ATP 3-Phosphoglycerate Phosphoglyceromutase 2-Phosphoglycerate Enolase 2 H2O Phosphoenolpyruvate 2 ADP Pyruvate kinase 2 ATP Pyruvate

  30. Concept 9.3: The citric acid cycle completes the energy-yielding oxidation of organic molecules • Before the citric acid cycle can begin, pyruvate must be converted to acetyl CoA, which links the cycle to glycolysis

  31. LE 9-10 MITOCHONDRION CYTOSOL NAD+ NADH + H+ Acetyl Co A CO2 Coenzyme A Pyruvate Transport protein

  32. The citric acid cycle, also called the Krebs cycle, takes place within the mitochondrial matrix • The cycle oxidizes organic fuel derived from pyruvate, generating one ATP, 3 NADH, and 1 FADH2 per turn Animation: Electron Transport

  33. LE 9-11 Pyruvate (from glycolysis, 2 molecules per glucose) Citric acid cycle Glycolysis Oxidation phosphorylation CO2 NAD+ CoA NADH ATP ATP ATP + H+ Acetyl CoA CoA CoA Citric acid cycle 2 CO2 FADH2 3 NAD+ 3 NADH FAD + 3 H+ ADP + P i ATP

  34. The citric acid cycle has eight steps, each catalyzed by a specific enzyme • The acetyl group of acetyl CoA joins the cycle by combining with oxaloacetate, forming citrate • The next seven steps decompose the citrate back to oxaloacetate, making the process a cycle • The NADH and FADH2 produced by the cycle relay electrons extracted from food to the electron transport chain

  35. LE 9-12_1 Citric acid cycle Glycolysis Oxidation phosphorylation ATP ATP ATP Acetyl CoA H2O Oxaloacetate Citrate Isocitrate Citric acid cycle

  36. LE 9-12_2 Citric acid cycle Glycolysis Oxidation phosphorylation ATP ATP ATP Acetyl CoA H2O Oxaloacetate Citrate Isocitrate CO2 Citric acid cycle NAD+ NADH + H+ a-Ketoglutarate CO2 NAD+ NADH Succinyl CoA + H+

  37. LE 9-12_3 Citric acid cycle Glycolysis Oxidation phosphorylation ATP ATP ATP Acetyl CoA H2O Oxaloacetate Citrate Isocitrate CO2 Citric acid cycle NAD+ NADH + H+ Fumarate a-Ketoglutarate FADH2 CO2 NAD+ FAD Succinate NADH P i Succinyl CoA + H+ GDP GTP ADP ATP

  38. LE 9-12_4 Citric acid cycle Glycolysis Oxidation phosphorylation ATP ATP ATP Acetyl CoA NADH H2O + H+ NAD+ Oxaloacetate Malate Citrate Isocitrate CO2 Citric acid cycle NAD+ H2O NADH + H+ Fumarate a-Ketoglutarate FADH2 CO2 NAD+ FAD Succinate NADH P i Succinyl CoA + H+ GDP GTP ADP ATP

  39. Concept 9.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP 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. The Pathway of Electron Transport • The electron transport chain is in the 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 water

  41. LE 9-13 NADH 50 FADH2 Multiprotein complexes I FAD 40 FMN II Fe•S Fe•S Q III Cyt b Oxidative phosphorylation: electron transport and chemiosmosis Citric acid cycle Glycolysis Fe•S 30 Cyt c1 IV Free energy (G) relative to O2 (kcal/mol) Cyt c ATP ATP ATP Cyt a Cyt a3 20 10 2 H+ + 1/2 O2 0 H2O

  42. The electron transport chain generates no ATP • The chain’s function is to break the large free-energy drop from food to O2 into smaller steps that release energy in manageable amounts

  43. Chemiosmosis: The Energy-Coupling Mechanism • 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 channels in 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. LE 9-14 INTERMEMBRANE SPACE A rotor within the membrane spins as shown when H+ flows past it down the H+ gradient. H+ H+ H+ H+ H+ H+ H+ A stator anchored in the membrane holds the knob stationary. A rod (or “stalk”) extending into the knob also spins, activating catalytic sites in the knob. H+ Three catalytic sites in the stationary knob join inorganic phosphate to ADP to make ATP. ADP + ATP P i MITOCHONDRAL MATRIX

  45. 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 Animation: Fermentation Overview

  46. LE 9-15 Inner mitochondrial membrane Oxidative phosphorylation: electron transport and chemiosmosis Citric acid cycle Glycolysis ATP ATP ATP H+ H+ H+ H+ Cyt c Protein complex of electron carriers Intermembrane space Q IV III I ATP synthase II Inner mitochondrial membrane H2O 2H+ + 1/2 O2 FADH2 FAD NAD+ NADH + H+ ATP ADP + P i (carrying electrons from food) H+ Mitochondrial matrix Electron transport chain Electron transport and pumping of protons (H+), Which create an H+ gradient across the membrane Chemiosmosis ATP synthesis powered by the flow of H+ back across the membrane Oxidative phosphorylation

  47. An Accounting of ATP Production by Cellular Respiration • During cellular respiration, most energy flows in this sequence: glucose NADH electron transport chain proton-motive force ATP • About 40% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making about 38 ATP

  48. LE 9-16 Electron shuttles span membrane MITOCHONDRION CYTOSOL 2 NADH or 2 FADH2 2 NADH 2 NADH 6 NADH 2 FADH2 Oxidative phosphorylation: electron transport and chemiosmosis Glycolysis 2 Acetyl CoA Citric acid cycle 2 Pyruvate Glucose + 2 ATP + 2 ATP + about 32 or 34 ATP by substrate-level phosphorylation by substrate-level phosphorylation by oxidation phosphorylation, depending on which shuttle transports electrons form NADH in cytosol About 36 or 38 ATP Maximum per glucose:

  49. Concept 9.5: Fermentation enables some cells to produce ATP without the use of oxygen • Cellular respiration requires O2 to produce ATP • Glycolysis can produce ATP with or without O2 (in aerobic or anaerobic conditions) • In the absence of O2, glycolysis couples with fermentation to produce ATP

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