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PATHWAYS THAT HARVEST CHEMICAL ENERGY CHAPTER 9 LECTURE OBJECTIVES How Does Glucose Oxidation Release Chemical Energy? What Are the Aerobic Pathways of Glucose Metabolism? How Does Oxidative Phosphorylation Form ATP? How Is Energy Harvested from Glucose in the Absence of Oxygen? How Are Metabolic Pathways Interrelated and Regulated? CELLULAR RESPIRATION Main fuel of the cells Complex metabolic pathway Enzymatic reactions In organelles (Mitochondrion) Similar in different organisms GENERAL REACTION Glucose oxidation is an exergonic reaction ΔG is the change in free energy ΔG from complete combustion of glucose = –686 kcal/mol Three metabolic pathways are involved in harvesting the energy of glucose: Glycolysis Cellular respiration Fermentation ENERGY FOR LIFE REDOX REACTIONS OXIDATION STATES AND ENERGY Coenzyme NAD+ is a key electron carrier in redox reactions Two forms: NAD+ (oxidized) NADH (reduced) OXYGEN ACCEPTS ELECTRONS FROM NADH Energy Producing Metabolic Pathways Where in the cell do these metabolic pathways occur? GLYCOLYSIS takes place in the cytosol: Converts glucose into pyruvate Produces a small amount of energy Generates no CO2 GLYCOLYSIS: 10 RXNS • Energy-investing reactions one–five require ATP. • Energy-harvesting reactions six–ten yield NADH and ATP. Results in: 2 molecules of pyruvate 2 molecules of ATP 2 molecules of NADH GLUCOSE INTO PYRUVATE PYRUVATE OXIDATION • Links glycolysis and the citric acid cycle; occurs in the mitochondrial matrix • Pyruvate is oxidized to acetate and CO2 is released • NAD+ is reduced to NADH, capturing energy • Some energy is stored by combining acetate and Coenzyme A (CoA) to form acetyl CoA CHANGES IN FREE ENERGY DURING THE PATHWAY AEROBIC PATHWAY • Acetyl CoA is the starting point of the eight –reaction citric acid cycle: • Inputs: acetyl CoA, water and electron carriers NAD+, FAD, and GDP • Energy released is captured by ADP and electron carriers NAD+, FAD, and GDP • Outputs: CO2, reduced electron carriers, and GTP, which converts ADP to ATP Pyruvate Oxidation and Citric Acid Cycle • Fermentation—if no O2 is present • Oxidative phosphorylation—O2 is present OXIDATIVE PHOSPHORYLATION Electron Transport (ETC) • Electrons from NADH and FADH2 pass through the respiratory chain • Electron flow results in a proton concentration gradient in mitochondria. Chemiosmosis • Protons diffuse back into the mitochondria through ATP synthase, a channel protein. • Diffusion is coupled to ATP synthesis ELECTRON TRANSPORT CHAIN CHEMIOSMOSIS • • ATP synthesis can be uncoupled: If a different H+ diffusion channel is inserted into the mitochondrial membrane, the energy is lost as heat. The uncoupling protein thermogenin occurs in human infants and hibernating animals—H+ is released as heat instead of coupled to ATP synthesis FERMENTATION (ETHANOL) FINAL PRODUCT Cellular respiration yields more energy than fermentation per glucose molecule. • Glycolysis plus fermentation = 2 ATP • Glycolysis plus cellular respiration = 32 ATP • In some cells NADH must be shuttled using ATP and net result = 30 ATP USING OTHER NUTRIENTS FOR CELLULAR RESPIRATION CATABOLIC CONVERSIONS • Polysaccharides → hydrolyzed to glucose, enters glycolysis and cellular respiration • Lipids → broken down to: glycerol → DAP fatty acids → acetyl CoA • Proteins → hydrolyzed to amino acids—feed into glycolysis or the citric acid cycle ANABOLIC CONVERSIONS • Most catabolic reactions are reversible • Gluconeogenesis: Glucose formed from citric acid cycle and glycolysis intermediates REGULATING METABOLIC PATHWAYS ALLOSTERIC REGULATION CONTROLS OF CELLULAR RESPIRATION • Glycolysis: phosphofructokinase—allosterically inhibited by ATP. • Citric acid cycle: isocitrate dehydrogenase—inhibited by NADH + H+ and ATP. • If ATP levels are high: 1. Accumulation of citrate diverts acetyl CoA to fatty acid synthesis, for storage. 2. Fatty acids may be metabolized later to produce more acetyl CoA.