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Pathways That Harvest Chemical Energy

Pathways That Harvest Chemical Energy. 7.1 How Does Glucose Oxidation Release Chemical Energy?. Fuels: molecules whose stored energy can be released for use . The most common fuel in organisms is glucose . Other molecules are first converted into glucose or other intermediate compounds.

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Pathways That Harvest Chemical Energy

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  1. Pathways That Harvest Chemical Energy

  2. 7.1 How Does Glucose Oxidation Release Chemical Energy? Fuels: molecules whose stored energy can be released for use. The most common fuel in organisms is glucose. Other molecules are first converted into glucose or other intermediate compounds.

  3. 7.1 How Does Glucose Oxidation Release Chemical Energy? Burning or metabolism of glucose: Glucose metabolism pathway traps the free energy in ATP:

  4. 7.1 How Does Glucose Oxidation Release Chemical Energy? Principles governing metabolic pathways: • Complex chemical transformations occur in a series of reactions. • Each reaction is catalyzed by a specific enzyme. • Metabolic pathways are similar in all organisms. • In eukaryotes, metabolic pathways are compartmentalized in organelles. • Each pathway is regulated by key enzymes.

  5. 7.1 How Does Glucose Oxidation Release Chemical Energy? ΔG from complete combustion of glucose = –686 kcal/mole Highly exergonic; drives endergonic formation of many ATP

  6. Hydrogen Transfer and Carriers • Oxidation-Reduction reaction (redox rxn)- Movement of electrons from one molecule to another. • Oxidation/oxidized: the loss of electrons • Reduction/reduced: the addition of electrons In cellular respiration glucose loses e- while oxygen gains e- (all transported by H) • Examples of e- carriers • NAD+ and NADH (nicotinamide adenine dinucleotide) • FAD+ and FADH2 LEO the lion says GER

  7. 7.1 How Does Glucose Oxidation Release Chemical Energy? In combustion of glucose, glucose is the reducing agent, O2 is the oxidizing agent. Energy is transferred in a redox reaction. Energy in the reducing agent becomes associated with the reduced product.

  8. 7.1 How Does Glucose Oxidation Release Chemical Energy? Coenzyme NAD is an electron carrier in redox reactions. Two forms: NAD+ (oxidized) NADH + H+ (reduced)

  9. Figure 7.4 NAD Is an Energy Carrier in Redox Reactions (A)

  10. Figure 7.1 Energy for Life Three metabolic pathways involved in harvesting the energy of glucose

  11. 7.1 How Does Glucose Oxidation Release Chemical Energy? If O2 is present, four pathways operate: • Glycolysis, pyruvate oxidation, citric acid cycle, and electron transport chain. If O2 is not present, pyruvate is metabolized in fermentation.

  12. Figure 7.2 Energy-Producing Metabolic Pathways

  13. Look up in Table 7.1 in your text!

  14. Three Stages of Cellular Respiration • Glycolysis: breaks down glucose into pyruvate •  releases energy!  • Citric Acid Cycle/Krebs Cycle: converts pyruvate to acetyl CoA and acetyl CoA acts to release more energy •  releases energy!  • Electron Transport Chain (Oxidative Phosphorylation): Transports Electrons generated from the previous cycles across the membrane generating/releasing more energy!

  15. 7.2 What Are the Aerobic Pathways of Glucose Metabolism? Glycolysis takes place in the cytosol. Involves 10 enzyme-catalyzed reactions Requires: 2ATP, 2NAD+, 4ADP Results in: 2 molecules of pyruvate 4 molecules ATP 2 molecules NADH http://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter25/animation__how_glycolysis_works.html

  16. 7.2 What Are the Aerobic Pathways of Glucose Metabolism? A kinase is an enzyme that catalyzes transfer of a phosphate group from ATP to another substrate. In the first half of glycolysis, the glucose molecule is split into two 3-carbon molecules (G3P).

  17. 7.2 What Are the Aerobic Pathways of Glucose Metabolism? Phosphorylation: addition of a phosphate group Enzyme-catalyzed transfer of a phosphate group to ADP is called substrate-level phosphorylation.

  18. Figure 7.6 Changes in Free Energy During Glycolysis

  19. Cellular respiration occurs in the mitochondria. The mitochondria has two membranes. The inner membrane is highly folded- the folds are called christae. The inside of the mitochondria is called the matrix and the space between the two membranes is called the intermembrane space.

  20. 7.2 What Are the Aerobic Pathways of Glucose Metabolism? Pyruvate Oxidation (Prep Step): • Links glycolysis and the citric acid (Krebs) cycle • Pyruvate is converted to acetyl CoA using Coenzyme A • Pyruvate is oxidized (NAD+ = NADH) • A carbon atom is released as CO2 • Coenzyme A joins with the remaining 2 carbon atoms = Acetyl CoA • Takes place in the mitochondrial matrix

  21. Figure 7.8 Pyruvate Oxidation and the Citric Acid Cycle (Part 1) 1st part – pyruvate oxidation

  22. 7.2 What Are the Aerobic Pathways of Glucose Metabolism? Acetyl CoA is the starting point of the citric acid (Krebs) cycle: • Coenzyme A is removed in the first reaction and can be reused • Krebs cycle has 8 steps, each catalyzed by a specific enzyme. Each acetylCoA molecule that enters produces 3 molecules of NADH. Another molecule is also reduced (gains electrons) FADH2. Both of these molecules will donate their electrons in the 3rd step. Carbon dioxide (that you will exhale) is produced as a biproduct of this step. ATP produced does cell work.

  23. Figure 7.8 Pyruvate Oxidation and the Citric Acid Cycle (Part 2) citric acid cycle

  24. Figure 7.7 The Citric Acid Cycle Releases Much More Free Energy Than Glycolysis Does

  25. 7.2 What Are the Aerobic Pathways of Glucose Metabolism? The electron carriers that are reduced during the citric acid cycle must be reoxidized to take part in the cycle again. Fermentation—if no O2 is present Oxidative phosphorylation—O2 is present

  26. 7.4 How Does the Oxidation of Glucose Form ATP? Oxidative phosphorylation: ATP is synthesized as electron carriers are reoxidized in the presence of O2. Two stages: Electron transport chain Chemiosmosis

  27. Oxidative Phosphorylation is the process of extracting ATP form NADH and FADH2. • Occurs on the inner membrane of the mitochondria. • Electrons from NADH and FADH2 pass along an electron transport chain from one protein to another, losing energy at each step • The last electron acceptor is oxygen. When it accepts the two electron, with two H+, it forms water.

  28. Overview of Cellular Respiration • Glycolysis- makes 2 ATP • Krebs Cycle- makes 2 ATP • Electron Transport Chain- makes approx. 34 ATP • Total- 36 ATP made for each glucose molecule

  29. 7.4 How Does the Oxidation of Glucose Form ATP? Why does the electron transport chain have so many steps? Why not in one step?

  30. 7.4 How Does the Oxidation of Glucose Form ATP? Too much free energy would be released all at once—it could not be harvested by the cell. In a series of reactions, each releases a small amount of energy that can be captured by an endergonic reaction.

  31. 7.4 How Does the Oxidation of Glucose Form ATP? The electron transport chain: • On the inner mitochondrial membrane • Cytochrome c • Ubiquinone (Q)—a lipid

  32. Figure 7.11 The Oxidation of NADH + H+ (Part 1)

  33. Figure 7.11 The Oxidation of NADH + H+ (Part 2)

  34. Figure 7.12 The Complete Electron Transport Chain

  35. 7.4 How Does the Oxidation of Glucose Form ATP? The electron transport chain results in the active transport of protons (H+) across the inner mitochondrial membrane. The transmembrane complexes act as proton pumps.

  36. Figure 7.13 A Chemiosmotic Mechanism Produces ATP (Part 2)

  37. 7.4 How Does the Oxidation of Glucose Form ATP? The proton pump results in a proton concentration gradient and an electric charge difference across the inner membrane: potential energy! Proton-motive force

  38. 7.4 How Does the Oxidation of Glucose Form ATP? The protons must pass through a protein channel—ATP synthase—to flow back into the mitochondrial matrix. Chemiosmosis is the coupling of the proton-motive force and ATP synthesis.

  39. FERMENTATION • Does NOT involve oxygen: anaerobic • Occurs in the cytoplasm • LESS ENERGY PRODUCED!

  40. FERMENTATION • Prokaryotic organisms use many types of fermentation pathways: • Lactic Acid Fermentation: • Convert pyruvate into lactic acid • Occurs in humans too (builds up acid in muscle cells causing cramps • Alcoholic fermentation: • Convert pyruvate into ethyl alcohol • Occurs in yeast and some plant cells

  41. 7.5 Why Does Cellular Respiration Yield So Much More Energy Than Fermentation? Energy yields: Glycolysis and fermentation: 2 ATP Glycolysis and cellular respiration: 32 ATP Fermentation by-products have a lot of energy remaining.

  42. Figure 7.15 Cellular Respiration Yields More Energy Than Glycolysis Does (Part 1)

  43. 7.3 How Is Energy Harvested from Glucose in the Absence of Oxygen? Fermentation occurs in the cytosol. Pyruvate is reduced by NADH + H+ and NAD+ is regenerated.

  44. 7.3 How Is Energy Harvested from Glucose in the Absence of Oxygen? Lactic acid fermentation: • Occurs in microorganisms, some muscle cells. • Pyruvate is the electron acceptor.

  45. Figure 7.9 Lactic Acid Fermentation

  46. 7.3 How Is Energy Harvested from Glucose in the Absence of Oxygen? Alcoholic fermentation: • Yeasts and some plant cells • Pyruvate is converted to acetaldehyde, CO2 is released • Acetaldehyde is reduced by NADH + H+, producing NAD+ and ethyl alcohol

  47. Figure 7.10 Alcoholic Fermentation

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