What are Glycolysis, Fermentation, and Aerobic Respiration? Glycolysis: breakdown of glucose (6C) into two moles of pyruvate (3C) –Occurs in the cytoplasm

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  • What are Glycolysis, Fermentation, and Aerobic Respiration?Glycolysis: breakdown of glucose (6C) into two moles of pyruvate (3C) Occurs in the cytoplasm of all cellsConsists of 10 steps, each catalyzed by a different enzymeNet gain of 2 ATPs (2.2% potential energy of glucose); nicotinamide adenine dinucleotide (NAD+) required and NADH produced Fermentations (Anaerobic Conditions) Lactate Fermentation: pyruvate from glycolysis reduced to lactate; occurs in muscles when starved of oxygen; bacteria produce lactate in yogurt and some cheesesAlcohol Fermentation: pyruvate converted to ethanol via ethanal; CO2 byproduct; used in production of wineOxidation of NADH to NAD+ allows continued gylcolysisThe Mitochondrion (Site of Aerobic Respiration in Eukaryotes)Evolved from aerobic bacteria (have ATP synthase in membrane)Aerobic Respiration: oxygen gas allows complete oxidation of glucose and production of 36 ATPs (~40% potential energy of glucose)

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  • What are the Processes Involved in Aerobic Cellular Respiration?The Transition Reaction (pyruvate acetyl CoA)Acetyl Coenzyme-A: central character in metabolism (can be produced from carbohydrates, lipids, and certain amino acids)Pyruvate converted to acetyl group (2C); loss of CO2 moleculeCoenzyme-A (CoA): a large thiol derived from ATP and pantothenic acid (derived from thiamine and riboflavin); binds to acetyl group at the thiol group (-SH) of CoA; complex enters mitochondrionThe Citric Acid Cycle (Krebs Cycle, TCA Cycle)Acetyl group condensed with oxaloacetate (4C) citrate (6C); series of oxidation reactions produce CO2, NADH, and other energy compounds (ex. FADH2); final reaction produces oxaloacetate, completing the cycleTwo turns of the cycle per starting glucose moleculeOxidative Phosphorylation (the payoff)Oxidations of NADH and FADH2 coupled to the production of ATP Series of electron transport reactions produce ATP; final electron acceptor is molecular oxygen, which is used to produce waterInvolves several enzymes, proteins in the mitochondrial inner membrane, H+ pump, and H+ reservoir between the membranes

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  • How are Lipids Used as an Energy Source?Lipid MetabolismFats emulsified by bile in duodenumBile: micelles consisting of bile salts, lecithin, cholesterol, proteins, and inorganic ionsLipases from pancreas hydrolyze triglycerides to monoglycerides and fatty acidsIf energy needed, fatty acids degraded to enter Krebs Cycle, if not, triglycerides re-formed and stored in adipocytesFatty Acid DegradationFatty acids degraded to acetyl-CoA by -oxidation Cycle (involves sequential loss of acetyl groups from carbon chain of fatty acid)Energy yield depends on length of carbon chain (ex. 16C palmitic acid results in 129 ATPs, ~3.5x more than glucose)Ketoacidosis: results if oxaloacetate in short supply; acetyl-CoA converted into ketones, which are weak acids; can occur due to starvation, low-carbohydrate diet, or by uncontrolled diabetesFatty Acid Synthesis (via sequential additions of 2C groups)Excess acetyl-CoA used to synthesize fatty acids, which are then stored as triglycerides

  • Figure 9.20

  • How are Proteins Used as an Energy Source?Digestion of ProteinsProteins can supply energy, but not their primary function (most amino acids used for protein synthesis)Body can burn muscle protein if starvedDegradation of Amino AcidsAmino group transferred to a keto acid acceptor to form new amino acid (-ketoglutarate glutamate, which enters the Krebs Cycle)Aspartate from diet oxaloacetate (needed in Krebs Cycle)Alanine from diet + -ketoglutarate pyruvate and glutamateAmino acid carbon skeletons enter glycolysis or Krebs Cycle after oxidative deamination of amino group (requires NAD+ and H2O)The Urea CycleAmmonium ions (toxic) result from oxidative deamination of amino acids converted into urea, which is excreted in urineOccurs in mitochondria and cytoplasmUnusual amino acids produced as intermediates (ornithine, citrulline)

  • How are Glucose and Glycogen Synthesized?Gluconeogenesis (the synthesis of glucose)Occurs during starvation to keep the brain and red blood cells supplied with glucose, and occurs following exercise (Cori Cycle: lactate converted to glucose, which is re-supplied to muscle tissue)Occurs in the mammalian liver; other starting materials include glycerol, and most amino acidsGlycogenolysis (the degradation of glycogen)Glycogen stored in liver and muscles, but only liver-based glycogen used to supply blood (and brain)Glycogen degraded to supply blood glucose in response to hypoglycemia (via glucagon levels) or threat (via epinephrine)Glycogenesis (the synthesis of glycogen)Stimulated by hyperglycemia (via insulin levels)Insulin acts as an inhibitor of glycogen phosphorylase, and stimulates glycogen synthase and glucokinase

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  • What are the Effects of Insulin and Glucagon on Cellular Metabolism?InsulinProduced by -cells of the islets of Langerhans in the pancreas; secreted when blood glucose levels high (ex. after meals)Increases cellular uptake of glucose from blood Target cells mainly liver, adipose, and muscle cells (with membrane receptors) Activates biosynthesis and inhibits catabolism: stimulates glycogen synthesis, protein synthesis, and inhibits breakdown of glycogen, synthesis of glucose, and breakdown of triglyceridesGlucagon: opposite effects of insulinProduced by -cells of the islets of Langerhans Diabetes mellitus (inadequate production of insulin)Symptoms: odor of acetone on the breath; large amounts of sugar-containing urine; weakness, comaLipid metabolism increases since most glucose excreted in urine; shortage of oxaloacetate can lead to ketoacidosisTreated by insulin injections, pancreas transplants, and more recently, with adult stem cells

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