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How Cells Release Chemical Energy

How Cells Release Chemical Energy. Chapter 7. Impacts, Issues When Mitochondria Spin Their Wheels. Mitochondria are the organelles responsible for releasing the energy stored in foods

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How Cells Release Chemical Energy

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  1. How Cells Release Chemical Energy Chapter 7

  2. Impacts, IssuesWhen Mitochondria Spin Their Wheels • Mitochondria are the organelles responsible for releasing the energy stored in foods • In Luft’s syndrome, the mitochondria are active in oxygen consumption, but with little ATP formation to show for it • In Friedreich’s ataxia, too much iron in the mitochondria causes an accumulation of free radicals that attack valuable molecules of life

  3. The Impact • Proper, or improper, functioning of mitochondria is the difference between health and disease

  4. Section 7.1 Overview of Energy-Releasing Pathways

  5. Producing the Universal Currency of Life All energy-releasing pathways: • require characteristic starting materials • yield predictable products and by-products • produce ATP

  6. ATP Is Universal Energy Source • Photosynthesizers get energy from the sun • Animals get energy second- or third-hand from plants or other organisms • Regardless, the energy is converted to the chemical bond energy of ATP

  7. Making ATP • Plants make ATP during photosynthesis • Cells of all organisms make ATP by breaking down carbohydrates, fats, and protein

  8. Aerobic pathways Evolved later Require oxygen Start with glycolysis in cytoplasm Completed in mitochondria Anaerobic pathways Evolved first Don’t require oxygen Start with glycolysis in cytoplasm Completed in cytoplasm Main Types of Energy-Releasing Pathways

  9. Energy-Releasing Pathways

  10. Overview of Aerobic Respiration C6H1206 + 6O2 6CO2 + 6H20 glucose oxygen carbon dioxide water

  11. Overview of Aerobic Respiration

  12. Main Pathways Start with Glycolysis • Glycolysis occurs in cytoplasm • Reactions are catalyzed by enzymes Glucose 2 Pyruvate (six carbons) (three carbons)

  13. p.106a

  14. Three Series of Reactions Are Required for Aerobic Respiration • Glycolysis is the breakdown of glucose to pyruvate • Small amounts of ATP are generated

  15. Three Series of Reactions Are Required for Aerobic Respiration • The Krebs cycle degrades pyruvate to CO2 and water; • NAD and FAD accept H+ ions and electrons to be carried to the electron transfer chain • Small amounts of ATP are generated

  16. Three Series of Reactions Are Required for Aerobic Respiration • Electron transfer phosphorylationprocesses the H+ ions and electrons to generate lots of ATP • Oxygen is the final electron acceptor

  17. The Role of Coenzymes • NAD+ and FAD accept electrons and hydrogen from intermediates during the first two stages • When reduced, they are NADH and FADH2 • In the third stage, these coenzymes deliver the electrons and hydrogen to the transfer chain

  18. Overview of Aerobic Respiration

  19. Section 7.2 The First Stage: Glycolysis

  20. Glucose • A simple sugar (C6H12O6) • Atoms held together by covalent bonds

  21. Glycolysis Occurs in Two Stages • Energy-requiring steps • ATP energy activates glucose and its six-carbon derivatives

  22. Glycolysis Occurs in Two Stages • Energy-releasing steps • The products of the first part are split into 3-carbon pyruvate molecules • ATP and NADH form

  23. Energy-Requiring Steps

  24. ENERGY - RELEASING STEPS OF GLYCOLYSIS PGAL PGAL NAD NAD + + NADH NADH P P i i P P P P 1,3 - bisphosphoglycerate 1,3 - bisphosphoglycerate substrate - level ADP ADP phosphorylation ATP ATP 2 ATP produced P P 3 - phosphoglycerate 3 - phosphoglycerate P P 2 - phosphoglycerate 2 - phosphoglycerate H O H O 2 2 P P PEP PEP substrate - level ADP ADP phosphorylation ATP ATP 2 ATP produced pyruvate pyruvate to second set of reactions Energy-Releasing Steps

  25. Glycolysis in a Nutshell • Glucose is first phosphorylated in energy-requiring steps, then split to form two molecules of PGAL • Enzymes remove H+ and electrons from PGAL to change NAD+ to NADH (which is used later in electron transfer • PGAL is converted eventually to pyruvate • By substrate-level phosphorylation, four ATP are produced

  26. Substrate-level What? • Substrate-level phosphorylation means that there is a direct transfer of a phosphate group from the substrate of a reaction to some other molecule – in this case, ADP • Substrate is a reactant in a reaction – the substance being acted upon, for example, by an enzyme

  27. Net Energy Yield from Glycolysis Energy requiring steps: 2 ATP invested Energy releasing steps: 2 NADH formed 4 ATP formed Net yield is 2 ATP and 2 NADH

  28. Section 7.3 Second Stage of Aerobic Respiration

  29. Second-Stage Reactions • Occur in the mitochondria • Pyruvate is broken down to carbon dioxide • More ATP is formed • More coenzymes are reduced

  30. Fig. 7-5b, p.112

  31. Second Stage of Aerobic Respiration

  32. Two Parts of Second Stage • Preparatory reactions • Pyruvate is oxidized into two-carbon acetyl units and carbon dioxide • NAD+ is reduced • Krebs cycle • The acetyl units are oxidized to carbon dioxide • NAD+ and FAD are reduced

  33. Preparatory Reactions pyruvate + coenzyme A + NAD+ acetyl-CoA + NADH + CO2 • One of the carbons from pyruvate is released in CO2 • Two carbons are attached to coenzyme A and continue on to the Krebs cycle

  34. What Is Acetyl-CoA? • A two-carbon acetyl group linked to coenzyme A CH3 C=O S Coenzyme A Acetyl group

  35. Second Stage of Aerobic Respiration

  36. H2O NADH ATP NAD+ H2O O O O O malate NAD+ H2O NADH fumarate FADH2 FAD NAD+ CoA NADH succinate succinyl-CoA ADP + phosphate group =CoA KREBS CYCLE acetyl-CoA CoA oxaloacetate citrate isocitrate a-ketoglutarate Stepped Art Fig. 7-7a, p.113

  37. Overall Products Coenzyme A 2 CO2 3 NADH FADH2 ATP Overall Reactants Acetyl-CoA 3 NAD+ FAD ADP and Pi The Krebs Cycle

  38. Results of the Second Stage • All of the carbon molecules in pyruvate end up in carbon dioxide • Coenzymes are reduced (they pick up electrons and hydrogen) • One molecule of ATP is formed • Four-carbon oxaloacetate is regenerated

  39. Two pyruvates cross the inner mitochondrial membrane. outer mitochondrial compartment inner mitochondrial compartment NADH 2 NADH 6 Krebs Cycle Eight NADH, two FADH 2, and two ATP are the payoff from the complete break-down of two pyruvates in the second-stage reactions. FADH2 2 ATP 2 The six carbon atoms from two pyruvates diffuse out of the mitochondrion, then out of the cell, in six CO 6 CO2 Fig. 7-6, p.112

  40. Coenzyme Reductions during First Two Stages • Glycolysis 2 NADH • Preparatory 2 NADH reactions • Krebs cycle 2 FADH2 + 6 NADH • Total 2 FADH2 + 10 NADH

  41. Section 7.4 Third Stage of Aerobic Respiration – The Big Energy Payoff

  42. Electron Transfer Phosphorylation • Occurs in the mitochondria • Coenzymes deliver electrons to electron transfer chains • Electron transfer sets up H+ ion gradients • Flow of H+ down gradients powers ATP formation

  43. Electron Transfer Phosphorylation • Electron transfer chains are embedded in inner mitochondrial compartment • NADH and FADH2 give up electrons that they picked up in earlier stages to electron transfer chain • Electrons are transferred through the chain • The final electron acceptor is oxygen

  44. Creating an H+ Gradient OUTER COMPARTMENT NADH INNER COMPARTMENT

  45. ATP Formation ATP INNER COMPARTMENT ADP+Pi

  46. glucose ATP 2 PGAL ATP 2 NADH 2 pyruvate glycolysis 2 FADH 2 2 CO e – 2 2 acetyl - CoA 2 NADH + H + H 6 NADH 2 ATP Krebs + H ATP Cycle 2 FADH 2 + H ATP 4 CO 36 2 ATP + H + H ADP electron + P i transfer + H + H phosphorylation + H Summary of Transfers

  47. Importance of Oxygen • Electron transfer phosphorylation requires the presence of oxygen • Oxygen withdraws spent electrons from the electron transfer chain, then combines with H+ to form water

  48. Summary of Energy Harvest(per molecule of glucose) • Glycolysis • 2 ATP formed by substrate-level phosphorylation • Krebs cycle and preparatory reactions • 2 ATP formed by substrate-level phosphorylation • Electron transfer phosphorylation • 32 ATP formed

  49. Energy Harvest from Coenzyme Reductions • What are the sources of electrons used to generate the 32 ATP in the final stage? • 4 ATP - generated using electrons released during glycolysis and carried by NADH • 28 ATP - generated using electrons formed during second-stage reactions and carried by NADH and FADH2

  50. Energy Harvest Varies • NADH formed in cytoplasm cannot enter mitochondrion • It delivers electrons to mitochondrial membrane • Membrane proteins shuttle electrons to NAD+ or FAD inside mitochondrion • Electrons given to FAD yield less ATP than those given to NAD+

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