What are Glycolysis, Fermentation, and Aerobic Respiration?

What are Glycolysis, Fermentation, and Aerobic Respiration? • Glycolysis: breakdown of glucose (6C) into two moles of pyruvate (3C) • Occurs in the cytoplasm of all cells • Consists of 10 steps, each catalyzed by a different enzyme • Net 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 cheeses • Alcohol Fermentation: pyruvate converted to ethanol via ethanal; CO2 byproduct; used in production of wine • Oxidation of NADH to NAD+ allows continued gylcolysis • The 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 molecule • Coenzyme-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 mitochondrion • The 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 cycle • Two turns of the cycle per starting glucose molecule • Oxidative 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 water • Involves 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 Metabolism • Fats emulsified by bile in duodenum • Bile: micelles consisting of bile salts, lecithin, cholesterol, proteins, and inorganic ions • Lipases from pancreas hydrolyze triglycerides to monoglycerides and fatty acids • If energy needed, fatty acids degraded to enter Krebs Cycle, if not, triglycerides re-formed and stored in adipocytes • Fatty Acid Degradation • Fatty acids degraded to acetyl-CoA by β-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 diabetes • Fatty 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 Proteins • Proteins can supply energy, but not their primary function (most amino acids used for protein synthesis) • Body can burn muscle protein if starved • Degradation of Amino Acids • Amino group transferred to a keto acid acceptorto form new amino acid (α-ketoglutarate  glutamate, which enters the Krebs Cycle) • Aspartate from diet  oxaloacetate (needed in Krebs Cycle) • Alanine from diet + α-ketoglutarate  pyruvate and glutamate • Amino acid carbon skeletons enter glycolysis or Krebs Cycle after oxidative deamination of amino group (requires NAD+ and H2O) • The Urea Cycle • Ammonium ions (toxic) result from oxidative deamination of amino acids  converted into urea, which is excreted in urine • Occurs in mitochondria and cytoplasm • Unusual 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 (CoriCycle: lactate converted to glucose, which is re-supplied to muscle tissue) • Occurs in the mammalian liver; other starting materials include glycerol, and most amino acids • Glycogenolysis (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

Figure 45.12

What are the Effects of Insulin and Glucagon on Cellular Metabolism? • Insulin • Produced by β-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 triglycerides • Glucagon: opposite effects of insulin • Produced by α-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, coma • Lipid metabolism increases since most glucose excreted in urine; shortage of oxaloacetate can lead to ketoacidosis • Treated by insulin injections, pancreas transplants, and more recently, with adult stem cells

What are Glycolysis, Fermentation, and Aerobic Respiration?