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CHAPTER 6 - RESPIRATION O2 HEAT + ENERGY Glucose CO2 + H2O The only reason humans need to breathe oxygen is to accept electrons in the final stage of ATP synthesis in the mitochondria. CHAPTER OUTLINE I. OVERVIEW II. GLYCOLYSIS Getting to glucose Mechanisms by which ATP is synthesized Glycolysis – steps in the process Glycolysis - summary III. THE AEROBIC PATHWAY Mitochondrion structure A preliminary step The Krebs cycle Oxidative phosphorylation IV. ANAEROBIC PATHWAYS V. OTHER TYPES OF RESPIRATION OVERVIEW All organisms harvest energy from stored chemicals (starch, sugars, lipids) in the same way The metabolic pathways by which organisms liberate stored energy are referred to as cellular respiration THE OVERALL EQUATION FOR RESPIRATION OF GLUCOSE C6H12O6 + O2 CO2 + H2O + ENERGY Carbon Dioxide Cellular Respiration Cellular Respiration Glucose → CO2 + H2O + energy (ATP) This is the same equation for starting a fire using glucose as a fuel. The difference is that the reaction in living systems is tightly controlled and energy normally lost as heat is captured for other uses. Glucose is used as a source of energy for two kinds of respiration: Aerobic Anaerobic Aerobic Respiration - requires oxygen as the terminal electron acceptor 1) Stages involved a) Krebs cycle b) Oxidative phosphorylation (synthesis of ATP) 2) Disposition of Energy a) Some energy is stored in ATP and in other compounds b) Other energy dissipates as heat Anaerobic Respiration: (without oxygen) Fermentation: Metabolic pathways by which energy is liberated from pyruvic acid, the end product of glycolysis, in the absence of oxygen. GLYCOLYSIS Getting to Glycolysis Glycolysis is the breakdown of glucose to pyruvic acid (pyruvate). • • GLUCOSE IS NOT ABUNDANT IN CELLS CELLS OBTAIN GLUCOSE BY BREAKING DOWN GLUCOSECONTAINING STORAGE MOLECULES, OFTEN SUCROSE OR STARCH Sucrose, Starch, Fructose, etc Fig 6-2 COMMON GLUCOSE STORAGE COMPOUNDS •SUCROSE (TABLE SUGAR), FRUCTOSE (FRUIT SUGAR) AND OTHER SUGARS •STARCH •POLYMERS OF FRUCTOSE GLUCOSE IS RETREIVED FROM SUCROSE BY BY HYDROLYSIS Requires the enzyme “sucrase” STARCH IS A BRANCHED POLYMER MADE UP OF GLUCOSE MOLECULES SEVERAL DIFFERENT KINDS OF ENZYMES ARE REQUIRED TO BREAKDOWN STARCH •Amylases •Starch phosphorylase •Debranching enzymes Amylases hydrolyze alpha 1-4 glucose linkages Starch phosphorylase cleaves glucose at the end of a chain end by adding a phosphate to it starch + H2PO4 → glucose 1-phosphate Debranching enzymes hydrolize starch at branch points GLYCOLYSIS Mechanisms by which ATP is synthesized ATP is synthesized during respiration by 1. Substrate-level phosphorylation 2. ATP synthase complexes in mitochondrial and chloroplast membranes (Oxidative Phosphorylation) PHOSPHOENOLPYRUVIC ACID =Transfer of a phosphate directly from an organic molecule to ADP to make ATP ATP synthase complex Oxidative Phosphorylation = Coupling energy from an electron donor with an electrochemical gradient that spans a membrane to phosphorylate ADP Fig 6-15 GLYCOLYSIS Steps in the process This is glycolysis Fig 6-2 Glycolysis occurs in the cytoplasm!!!! Uses 1 ATP Fig 6-4 Uses 2nd ATP We will follow what happens to glyceraldehyde 3-phosphate only. Note-all products are from this point on are doubled 2 molecules Generates 2 NADH Generates 2 ATP 2 molecules 2 molecules Generates 2 ATP Total yield of energy-transport molecules from glycolysis Fig 6-17 AEROBIC RESPIRATION Mitochondrion structure Pyruvic acid is imported into mitochondria The Krebs cycle occurs in the matrix of the mitochondria AEROBIC RESPIRATION Oxidative decarboxylation of pyruvate Pyruvate is transported into the mitochondria Fig 6-7 Fig 6-17 AEROBIC RESPIRATION Krebs Cycle Fig 6-2 The Krebs cycle is also called the TCA cycle (tricarbocylic acid cycle) because citric acid has three carboxyl groups) or The citric acid cycle The chemical reaction repeatedly recycles, taking in two carbons and producing two CO2 molecules Two carbons enter Fig 6-8 Two CO2 molecules are produced (4/molecule of glucose) Fig 6-8 Three molecules of NADH are produced (6/molecule of glucose) Fig 6-8 One molecule of ATP is produced (2/molecule of glucose) Fig 6-8 One molecules of FADH2 is produced (2/molecule of glucose) Fig 6-8 Fig 6-17 AEROBIC RESPIRATION Oxidative phosphorylation Fig 6-2 NADH and ubiquinol from the Krebs cycle start a series of oxidation reduction reactions that move electrons through a series of carriers. The electron carriers together are called an “electron transport chain” See next slide for oxidationreduction of CoQ Fig 6-10 Fig 6-13 See next slide for cytochrome structure Fig 6-10 Fig 6-11 ELECTRON TRANSPORT Energy from the flow of electrons maintains a proton gradient across the inner mitochondrial membrane This proton gradient drives the synthesis of ATP. This process is called “oxidative phosphorylation” H+ H+ H+ H+ Fig 6-17 ANAROBIC RESPIRATION Glycolysis works in an oxygen free environment and can occur in either anaerobic or aerobic respiration The Krebs cycle and electron transport are inhibited by a lack of oxygen Inhibited Not Inhibited Fig 6-2 If NADH from glycolysis builds up (because it’s not being used in oxidative phosphorylation), NAD+ will become depleted NAD+ is required to oxidize glyceraldehyde-3 phosphate Therefore, glycolysis will stop Excess NADH can be removed by conversion of pyruvic acid to acetaldehyde Fig 6-18 In some animals (you), in some fungi and bacteria, pyruvic acid is reduced to lactic acid instead of alcohol Aerobic respiration Anaerobic respiration Glycolysis Pyruvic acid Krebs cycle Electron transport Alcohol or lactic acid 36ATP 2ATP OTHER TYPES OF RESPIRATION Lipids, proteins, etc Fig 6-19 Lipids are important storage compounds. They can be metabolized to yield acetyl Co-A for aerobic respiration OTHER TYPES OF RESPIRATION Cyanide resistant respiration Cyanide-resistant electron transport Cyanide Fig 6-10 CYANIDE RESISTANT RESPIRATION Aerobic respiration is inhibited when the terminal electron carrier combines with cyanide, azide or certain other negatively charged ions This poisons the enzyme and stops electron transport Some plants, fungi and bacteria This pathway produces heat rather than ATPs but is aerobic (i.e., oxygen is the terminal electron acceptor) Energy is captured from light by Philodendron leaves and used for life processes and growth When it flowers, the Philodendron flower heats to as high as 46 C (115 F). The heat protects the flowers from freezing at night and disperses compound that attract polinators Light energy —> Heat END