7. Pathways That Harvest Chemical Energy

Pathways That Harvest Chemical Energy7 Pathways That Harvest Chemical Energy7.1 How Does Glucose Oxidation Release Chemical Energy?7.2 What Are the Aerobic Pathways of Glucose Metabolism?7.3 How Is Energy Harvested from Glucose in the Absence of Oxygen?7.4 How Does the Oxidation of Glucose Form ATP?7.5 Why Does Cellular Respiration Yield So Much More Energy Than Fermentation?7.6 How Are Metabolic Pathways Interrelated and Controlled?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.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.7.1 How Does Glucose Oxidation Release Chemical Energy?Burning or metabolism of glucose:Glucose metabolism pathway traps the free energy in ATP:7.1 How Does Glucose Oxidation Release Chemical Energy?ΔG from complete combustion of glucose = –686 kcal/moleHighly exergonic; drives endergonic formation of many ATPFigure 7.1 Energy for LifeThree metabolic pathways involved in harvesting the energy of glucose7.1 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.Figure 7.2 Energy-Producing Metabolic Pathways7.1 How Does Glucose Oxidation Release Chemical Energy?Redox reactions: one substance transfers electrons to another substanceReduction: gain of one or more electrons by an atom, ion, or moleculeOxidation: loss of one or more electronsAlso applies if hydrogen atoms are gained or lost.7.1 How Does Glucose Oxidation Release Chemical Energy?Oxidation and reduction always occur together.The reactant that becomes reduced is the oxidizing agent.The reactant that becomes oxidized is the reducing agent.Figure 7.3 Oxidation and Reduction Are Coupled7.1 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.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)Figure 7.4 NAD Is an Energy Carrier in Redox Reactions (A)7.1 How Does Glucose Oxidation Release Chemical Energy? A hydride ion (H–) is transferred, leaving a free H+H– : a proton with two electrons Figure 7.4 NAD Is an Energy Carrier in Redox Reactions (B)7.1 How Does Glucose Oxidation Release Chemical Energy?Oxygen accepts electrons from NADH: exergonic—ΔG = –52.4 kcal/moleOxidizing agent is molecular oxygen—O27.2 What Are the Aerobic Pathways of Glucose Metabolism?Glycolysis takes place in the cytosol.Involves 10 enzyme-catalyzed reactionsResults in: 2 molecules of pyruvate 4 molecules ATP 2 molecules NADHFigure 7.5 Glycolysis Converts Glucose into Pyruvate (Part 1)probably 4 parts?Figure 7.5 Glycolysis Converts Glucose into Pyruvate (Part 2)probably 4 parts?Figure 7.5 Glycolysis Converts Glucose into Pyruvate (Part 3)probably 4 parts?Figure 7.5 Glycolysis Converts Glucose into Pyruvate (Part 4)probably 4 parts?Figure 7.5 Glycolysis Converts Glucose into Pyruvate (Part 5)probably 4 parts?Figure 7.5 Glycolysis Converts Glucose into Pyruvate (Part 6)probably 4 parts?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).7.2 What Are the Aerobic Pathways of Glucose Metabolism?Phosphorylation: addition of a phosphate groupEnzyme-catalyzed transfer of a phosphate group to ADP is called substrate-level phosphorylation.Figure 7.6 Changes in Free Energy During Glycolysis7.2 What Are the Aerobic Pathways of Glucose Metabolism?Pyruvate Oxidation:Links glycolysis and the citric acid cyclePyruvate is converted to acetyl CoATakes place in the mitochondrial matrixFigure 7.8 Pyruvate Oxidation and the Citric Acid Cycle (Part 1)1st part – pyruvate oxidation7.2 What Are the Aerobic Pathways of Glucose Metabolism?Acetyl CoA is the starting point of the citric acid cycle:Coenzyme A is removed in the first reaction and can be reusedThe cycle is in steady state: the concentrations of the intermediates don’t changeOutputs: CO2, reduced electron carriers, and ATPFigure 7.8 Pyruvate Oxidation and the Citric Acid Cycle (Part 2)citric acid cycleFigure 7.7 The Citric Acid Cycle Releases Much More Free Energy Than Glycolysis Does 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 presentOxidative phosphorylation—O2 is present7.3 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.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.Figure 7.9 Lactic Acid Fermentation7.3 How Is Energy Harvested from Glucose in the Absence of Oxygen?Alcoholic fermentation:Yeasts and some plant cellsPyruvate is converted to acetaldehyde, CO2 is releasedAcetaldehyde is reduced by NADH + H+, producing NAD+ and ethyl alcoholFigure 7.10 Alcoholic Fermentation7.4 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 Chemiosmosis7.4 How Does the Oxidation of Glucose Form ATP?Why does the electron transport chain have so many steps?Why not in one step?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.7.4 How Does the Oxidation of Glucose Form ATP?The electron transport chain:On the inner mitochondrial membrane4 protein complexes: I, II, III, IVCytochrome cUbiquinone (Q)—a lipidFigure 7.11 The Oxidation of NADH + H+ (Part 1)Figure 7.11 The Oxidation of NADH + H+ (Part 2)Figure 7.12 The Complete Electron Transport Chain7.4 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.Figure 7.13 A Chemiosmotic Mechanism Produces ATP (Part 1)Figure 7.13 A Chemiosmotic Mechanism Produces ATP (Part 2)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 force7.4 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.7.4 How Does the Oxidation of Glucose Form ATP?ATP synthase allows protons to diffuse back to the mitochondrial matrix, and uses the energy of that diffusion to make ATP from ADP and Pi.Figure 7.14 Two Experiments Demonstrate the Chemiosmotic Mechanism (Part 1)Figure 7.14 Two Experiments Demonstrate the Chemiosmotic Mechanism (Part 2)7.4 How Does the Oxidation of Glucose Form ATP?ATP synthesis can be uncoupled: if a different H+ diffusion channel is inserted into the mitochondrial membrane, the energy of the diffusion is lost as heat.The protein thermogenin occurs in human infants and hibernating animals.7.4 How Does the Oxidation of Glucose Form ATP?ATP synthase:F0 subunit—transmembraneF1 subunit—projects into the mitochondrial matrix, rotates to expose active sites for ATP synthesis7.5 Why Does Cellular Respiration Yield So Much More Energy Than Fermentation?Energy yields:Glycolysis and fermentation: 2 ATPGlycolysis and cellular respiration: 32 ATPFermentation by-products have a lot of energy remaining.Figure 7.15 Cellular Respiration Yields More Energy Than Glycolysis Does (Part 1)Figure 7.15 Cellular Respiration Yields More Energy Than Glycolysis Does (Part 2)7.6 How Are Metabolic Pathways Interrelated and Controlled?Metabolic pathways are interrelated. Interchange of molecules occurs between the pathways.7.6 How Are Metabolic Pathways Interrelated and Controlled?Catabolic interconversions:Polysaccharides → hydrolyzed to glucose, enters glycolysisLipids broken down to glycerol → DAP fatty acids → acetyl CoAProteins → hydrolyzed to amino acids—feed into glycolysis or the citric acid cycle at various pointsFigure 7.16 Relationships among the Major Metabolic Pathways of the Cell7.6 How Are Metabolic Pathways Interrelated and Controlled?Anabolic interconversions:Most catabolic reactions are reversibleGluconeogenesis: glucose from citric acid cycle and glycolysis intermediatesFigure 7.17 Coupling Metabolic Pathways7.6 How Are Metabolic Pathways Interrelated and Controlled?Metabolic homeostasis: concentrations of the biochemical molecules remain constant (e.g., glucose concentration in blood)Allosteric control of enzymes in catabolic pathwaysNegative and positive feedback controlsFigure 7.18 Regulation by Negative and Positive FeedbackFigure 7.19 Allosteric Regulation of Glycolysis and the Citric Acid Cycle (Part 1)Figure 7.19 Allosteric Regulation of Glycolysis and the Citric Acid Cycle (Part 2)7.6 How Are Metabolic Pathways Interrelated and Controlled?The main control point in glycolysis is phosphofructokinase—allosterically inhibited by ATP.The main control point in the citric acid cycle is isocitrate dehydrogenase—inhibited by NADH + H+ and ATP.7.6 How Are Metabolic Pathways Interrelated and Controlled?Acetyl CoA—accumulation of citrate diverts acetyl CoA to fatty acid synthesis.Cell differentiation: Slow twitch muscle cells have many mitochondria that catabolize aerobically, providing a steady supply of ATP.