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metabolism and energetics.docx

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University of Toronto Scarborough
Biological Sciences
Kenneth Welch

Metabolism and Energetics An Overview of Metabolism Metabolism •Metabolism is all the chemical reactions that occur in an organism •Cellular metabolism •Cells break down excess carbohydrates first, then lipids •Cells conserve amino acids •40% of the energy released in catabolism is captured in ATP •Rest is released as heat Anabolism •Performance of structural maintenance and repairs •Support of growth •Production of secretions •Building of nutrient reserves Cells and Mitochondria •cells provide small organic molecules for their mitochondria •Mitochondria produce ATP used to perform cellular functions Carbohydrate Metabolism Most cells generate ATP through the breakdown of carbohydrates •Glycolysis •One molecule of glucose = two pyruvate ions, two ATP, two NADH •Aerobic metabolism (cellular respiration) •Two pyruvates = 34 ATP •The chemical formula for this process is C 6 12+ 6 O  62CO + 6 H 2 2 Glycolysis •The breakdown of glucose to pyruvic acid •This process requires: •Glucose molecules •Cytoplasmic enzymes •ATP and ADP •Inorganic phosphate •NAD (nicotinamide adenine dinucleotide) •The overall reaction is: Glucose + 2 NAD + 2 ADP + 2P  i 2 Pyruvic acid + 2 NADH + 2 ATP Mitochondrial ATP Production (cellular respiration) •Pyruvic acid molecules enter mitochondria •Broken down completely in TCA cycle •Decarboxylation •Hydrogen atoms passed to coenzymes •Oxidative phosphorylation Oxidative phosphorylation and the ETS •Requires coenzymes and consumes oxygen •Key reactions take place in the electron transport system (ETS) •Cytochromes of the ETS pass electrons to oxygen, forming water •The basic chemical reaction is: 2 H + O  2 H O 2 2 2 •Per molecule of glucose entering these pathways •Glycolysis – has a net yield of 2 ATP •Electron transport system – yields approximately 28 molecules of ATP •TCA cycle – yields 2 molecules of ATP Synthesis of glucose and glycogen •Gluconeogenesis •Synthesis of glucose from noncarbohydrate precursors •Lactic acid, glycerol, amino acids •Liver cells synthesis glucose when carbohydrates are depleted •Glycogenesis •Formation of glycogen •Glucose stored in liver and skeletal muscle as glycogen •Important energy reserve Lipid catabolism •Lipolysis •Lipids broken down into pieces that can be converted into pyruvate •Triglycerides are split into glycerol and fatty acids •Glycerol enters glycolytic pathways •Fatty acids enter the mitochondrion Lipid catabolism •Beta-oxidation •Breakdown of fatty acid molecules into 2-carbon fragments •Enter the TCA •Irreversible •Lipids and energy production •Cannot provide large amounts in ATP in a short amount of time •Used when glucose reserves are limited •Almost any organic molecule can be used to form glycerol •Essential fatty acids cannot be synthesized and must be included in diet •Linoleic and linolenic acid Lipid transport and distribution •5 types of lipoprotein •Lipid-protein complex that contains large glycerides and cholesterol •Chylomicrons •Largest lipoproteins composed primarily of triglycerides •Very low-density lipoproteins (VLDLs) •contain triglycerides, phospholipids and cholesterol Lipid transport and distribution •5 types of lipoprotein (continued) •Intermediate-density lipoproteins (IDLs) •Contain smaller amounts of triglycerides •Low-density lipoproteins (LDLs) •Contain mostly cholesterol •High-density lipoproteins (HDLs) •Equal amounts of lipid and protein Lipoprotein lipase •Enzyme that breaks down complex lipids •Found in capillary walls of liver, adipose tissue, skeletal and cardiac muscle •Releases fatty acids and monglycerides Protein Metabolism Amino acid catabolism •If other sources inadequate, mitochondria can break down amino acids •TCA cycle •removal of the amino group (-NH ) 2 •Transamination – attaches removed amino group to a keto acid •Deamination – removes amino group generating NH 4+ •Proteins are an impractical source of ATP production Protein synthesis •Essential amino acids •Cannot be synthesized by the body in adequate supply •Nonessential amino acids •Can be synthesized by the body via amination •Addition of the amino group to a carbon framework Nucleic Acid Metabolism Nucleic acid metabolism •Nuclear DNA is never catabolized for energy •RNA catabolism •RNA molecules are routinely broken down and replaced •Generally recycled as nucleic acids •Can be catabolized to simple sugars and nitrogenous bases •Do not contribute significantly to energy reserves Nucleic
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