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apLectureNotes09 - CHAPTER 9 CELLULAR RESPIRATION...

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Unformatted text preview: CHAPTER 9 CELLULAR RESPIRATION: HARVESTING CHEMICAL ENERGY M OUTLINE I. II. III. Principles of Energy Conservation A. B. C. D. E. Cellular respiration and fermentation are catabolic (energy-yielding) pathways Cells must recycle the ATP they use for work Redox reactions release energy when electrons move closer to electronegative atoms Electrons “fall” from organic molecules to oxygen during cellular respiration The “fall” of electrons during respiration is stepwise, via NAD+ and an electron transport chain The Process of Cellular Respiration A. B. C. D. E. Respiration involves glycolysis, the Krebs cycle, and electron transport: an overview Glycolysis harvests chemical energy by oxidizing glucose to pyruvate: a closer look The Krebs cycle completes the energy-yielding oxidation of organic molecules: a closer look The inner mitochondrial membrane couples electron transport to ATP synthesis: a closer look Cellular respiration generates many ATP molecules for each sugar molecule it oxidizes: a review Related Metabolic Processes A. B. C. Fermentation enables some cells to produce ATP without the help of oxygen Glycolysis and the Krebs cycle connect to many other metabolic pathways Feedback mechanisms control cellular respiration OBJECTIVES After reading this chapter and attending lecture, the student should be able to: Diagram energy flow through the biosphere. Describe the overall summary equation for cellular respiration. Distinguish between substrate-level phosphorylation and oxidative phosphorylation. Explain how exergonic oxidation of glucose is coupled to endergonic synthesis of ATP. 1. 2 3 4. 5. 6 7 8 Define oxidation and reduction. Explain how redox reactions are involved in energy exchanges. Define coenzyme and list those involved in respiration. Describe the structure of coenzymes and explain how they function in redox reactions. 104 Unit 11 The Cell 9. Describe the role of ATP in coupled reactions. 10. Explain why ATP is required for the preparatory steps of glycolysis. l 1. Describe how the carbon skeleton of glucose changes as it proceeds through glycolysis. 12. Identify where in glycolysis that sugar oxidation, substrate-level phosphorylation and reduction of coenzymes occur. 13. Write a summary equation for glycolysis and describe where it occurs in the cell. 14. Describe where pyruvate is oxidized to acetyl CoA, what molecules are produced and how it links glycolysis to the Krebs cycle. 15. Describe the location, molecules in and molecules out for the Krebs cycle. l6. Explain at what point during cellular respiration glucose is completely oxidized. l7. Explain how the exergonic “slide” of electrons down the electron transport chain is coupled to the endergonic production of ATP by chemiosmosis. 18. Describe the process of chemiosmosis. l9. Explain how membrane structure is related to membrane function in chemiosmosis. 20. Summarize the net ATP yield from the oxidation of a glucose molecule by constructing an ATP ledger which includes coenzyme production during the different stages of glycolysis and cellular respiration. 21. Describe the fate of pyruvate in the absence of oxygen. 22. Explain why fermentation is necessary. 23. Distinguish between aerobic and anaerobic metabolism. 24. Describe how food molecules other than glucose can be oxidized to make ATP. 25. Describe evidence that the first prokaryotes produced ATP by glycolysis. 26. Explain how ATP production is controlled by the cell and what role the allosteric enzyme, phosphofructokinase, plays in this process. KEY TERMS fermentation Krebs cycle anaerobic cellular respiration oxidative phosphorylation alcohol fermentation redox reactions substrate-level phosphorylation lactic acid fermentation oxidation acetyl CoA facultative anaerobe reduction cytochrome (cyt) beta oxidation reducing agent ATP synthase oxidizing agent chemiosmosis NAD+ proton-motive force glycolysis aerobic LECTURE NOTES 1. Principles of Energy Conservation As open systems, cells require outside energy sources to perform cellular work (e.g., chemical, transport, and mechanical). Chapter 9 Cellular Respiration: Harvesting Chemical Energy Energy flows into most ecosystems as sunlight. Photosynthetic organisms trap a portion of the light energy and transform it into chemical bond energy of organic molecules. 02 is released as a byproduct. Cells use some of the chemical bond energy in organic molecules to make ATP—the energy source for cellular work. Energy leaves living organisms as it dissipates as heat. The products of respiration (C02 and H20) are the raw materials for photosynthesis. Photosynthesis produces glucose and oxygen, the raw materials for respiration. Chemical elements essential for life are recycled, but energy is not. How do cells harvest chemical energy? Complex organic molecules Catabolic pathwa s ight Ener \gy Metabolic reactions involve energy exchanges l "w- Photosynthesi (anabolic pathways) Organic + molecules 02 Mitochondrio Respiration figs—r—MRX (catabolic Kiri)?» gm pathways) -' .. .b Energy used in enzyme mediated reactions of metabolism (Powers most cellular work) Energy dissipated to the environment Heat energy Simpler waste products with less energy energy Some energy used to do work & some energy dissipated as heat A. Cellular respiration and fermentation are catabolic (energy-yielding) pathways 105 Fermentation = An ATP—producing catabolic pathway in which both electron donors and acceptors are organic compounds. 0 Can be an anaerobic process - Results in a partial degradation of sugars Cellular respiration = An ATP—producing catabolic process in which the ultimate electron acceptor is an inorganic molecule, such as oxygen. 106 Unit 11 The Cell Most prevalent and efficient catabolic pathway Is an exergonic process (AG = — 2870 kJ/mol or — 686 kcal/mol) Can be summarized as: Organic compounds + Oxygen—F Carbon dioxide + Water+ Energy (food) Carbohydrates, proteins, and fats can all be metabolized as fuel, but cellular respiration is most often described as the oxidation of glucose: CGHnO6+ 6 02—» 6C02 + 6H20 + Energy (ATP + Heat) B. Cells recycle the ATP they use for work The catabolic process of cellular respiration transfers the energy stored in food molecules to ATP. ATP (adenosine triphosphate) = Nucleotide with high energy phosphate bonds that the cell hydrolyzes for energy to drive endergonic reactions. C. Redox atoms The cell taps energy stored in ATP by enzymatically transferring terminal phosphate groups from ATP to other compounds. (Recall that direct hydrolysis of ATP would release energy as heat, a form unavailable for cellular work. See Chapter 6.) The compound receiving the phosphate group from ATP is said to be phosphorylated and becomes more reactive in the process. The phosphorylated compound loses its phosphate group as cellular work is performed; inorganic phosphate and ADP are formed in the process (see Campbell, Figure 9.2). Cells must replenish the ATP supply to continue cellular work. Cellular respiration provides the energy to regenerate ATP from ADP and inorganic phosphate. reactions release energy when electrons move closer to electronegative 1. An introduction to redox reactions Oxidation-reduction reactions = Chemical reactions which involve a partial or complete transfer of electrons from one reactant to another; called redox reactions for short. Oxidation = Partial or complete loss of electrons Reduction = Partial or complete gain of electrons Generalized redox reaction: Electron transfer requires both a donor and acceptor, so when one reactant is oxidized the other is reduced. oxidation Where: _ . . I X = Substance being oxrdized; acts 6, + Y . X + Ye’ as a reducing agent because it reduces Y. reduction I Y = Substance being reduced; as an oxidizing agent because it oxidizes X. Chapter 9 Cellular Respiration: Harvesting Chemical Energy 107 Not all redox reactions involve a complete transfer of electrons, but, instead, may just change the degree of sharing in covalent bonds (see Campbell, Figure 9.3). 0 Example: Covalent electrons oxidation of methane are equally l shared, because carbon and hydrogen have similar CH4 + 2 02—} C02 + 2H20 + energy electronegativities. methane oxy en carbon water dioxide 0 As methane reacts with oxygen to form carbon reduction dioxide, electrons shift away from carbon and hydrogen to the more electronegative oxygen. - Since electrons lose potential energy when they shift toward more electronegative atoms, redox reactions that move electrons closer to oxygen release energy. 0 Oxygen is a powerful oxidizing agent because it is so electronegative. D. Electrons “fall” from organic molecules to oxygen during cellular respiration Cellular respiration is a redox process that transfers hydrogen, including electrons with high potential energy, from sugar to oxygen. [—— oxidation ——1 C6H1206 + 6 Oz————> 6 C02 + 6 H20 + energy (used to make ATP) L— reduction ____1 ° Valence electrons of carbon and hydrogen lose potential energy as they shift toward electronegative oxygen. - Released energy is used by cells to produce ATP. - Carbohydrates and fats are excellent energy stores because they are rich in C to H bonds. Without the activation barrier, glucose would combine spontaneously with oxygen. 0 Igniting glucose provides the activation energy for the reaction to proceed; a mole of glucose yields 686 kcal (2870 kl) of heat when burned in air. 0 Cellular respiration does not oxidize glucose in one explosive step, as the energy could not be efficiently harnessed in a form available to perform cellular work. - Enzymes lower the activation energy in cells, so glucose can be slowly oxidized in a stepwise fashion during glycolysis and Krebs cycle. E. The “fall” of electrons during respiration is stepwise, via NAD+ and an electron transport chain Hydrogens stripped from glucose are not transferred directly to oxygen, but are first passed to a special electron acceptor—NAB". Nicotinamide adenine dinucleotide (NADU = A dinucleotide that functions as a coenzyme in the redox reactions of metabolism (see Campbell, Figure 9.4). 0 Found in all cells ' Assists enzymes in electron transfer during redox reactions of metabolism Coenzyme = Small nonprotein organic molecule that is required for certain enzymes to function. Dinucleotide = A molecule consisting of two nucleotides. 108 Unit 11 The Cell During the oxidation of glucose, NAD+ functions as an oxidizing agent by trapping energy-rich electrons from glucose or food. These reactions are catalyzed by enzymes called dehydrogenases, which: - Remove a pair of hydrogen atoms (two electrons and two protons) from substrate - Deliver the two electrons and one proton to NAD+ - Release the remaining proton into the surrounding solution 1" oxidation H l dehydrogenase R—C—Ri’ + NAD+——> R—C—Ri’ + NADH + H+ l H T OH O reductio Where: H l R— C — Rf = Substrate that is oxidized by enzymatic transfer of electrons I to NAD+ OH NAD+ = Oxidized coenzyme (net positive charge) NADH = Reduced coenzyme (electrically neutral) The high energy electrons transferred from substrate to NAD+ are then passed down the electron transport chain to oxygen, powering ATP synthesis (oxidative phosphorylation). Some instructors find it difficult to drive this point home. Surprisingly, some students can recall the intermediate steps of glycolysis or the Krebs cycle, but cannot explain in general terms how energy from food is transferred to ATP. Campbell, Figure 9.16 can be used to give students an overview when respiration is introduced; it is useful to refer to it here so students can place the process you are describing in context. It can be used again later as a summary to bring closure to the topic. Electron transport chains convert some of the chemical energy extracted from food to a form that can be used to make ATP (see Campbell, Figure 9.5). These transport chains: 0 Are composed of electron-carrier molecules built into the inner mitochondrial membrane. Structure of this membrane correlates with its functional role (form fits function). - Accept energy-rich electrons from reduced coenzymes (NADH and FADHz); and duringa series of redox reactions, pass these electrons down the chain to oxygen, the final electron acceptor. The electronegative oxygen accepts these electrons, alongwith hydrogen nuclei, to form water. - Releaw energy from energy-rich electrons in a controlled stepwise fashion; a form that can be harnessed by the cell to power ATP production. If the reaction between hydrogen and oxygen during respiration occurred in a single explosive step, much of the energy released wouldbe lost as heat, a form unavailable to do cellular work. Electron transfer from NADH to oxygen is exergonic, having a free energy change of —222 kJ/mole (—53 kcal/mol). Chapter 9 Cellular Respiration: Harvesting Chemical Energy 109 - Since electrons lose potential energy when they shift toward a more electronegative atom, this series of redox reactions releases energy. - Each successive carrier in the chain has a higher electronegativity than the carrier before it, so the electrons are pulled downhill towards oxygen, the final electron acceptor and the molecule with the highest electronegativity. The Process of Cellular Respiration A. Respiration involves glycolysis, the Krebs cycle, and electron transport: an overview There are three metabolic stages of cellular respiration (see Campbell, Figure 9.6): 1 . Glycolysis 2. Krebs cycle 3. Electron transport chain (ETC) and oxidative phosphorylation Glycolysis is a catabolic pathway that: - Occurs in the cytosol - Partially oxidizes glucose (6C) into two pyruvate (3C) molecules The Krebs cycle is a catabolic pathway that: ' Occurs in the mitochondrial matrix ' Completes glucose oxidation by breaking down a pyruvate derivative (acetyl CoA) into carbon dioxide Glycolysis and the Krebs cycle produce: 0 A small amount of ATP by substrate-level phosphorylation 0 NADH by transferring electrons from substrate to NAD+ (Krebs cycle also produces FADH2 by transferring electrons to FAD) The electron transport chain: - Is located at the inner membrane of the mitochondrion - Accepts energized electrons from reduced coenzymes (NADH and FADHz) that are harvested during glycolysis and Krebs cycle. Oxygen pulls these electrons down the electron transport chain to a lower energy state. - Couples this exergonic slide of electrons to ATP synthesis or oxidative phosphorylation. This process produces most (90%) of the ATP. Oxidative phosphorylation = ATP production that is coupled to the exergonic transfer of electrons from food to oxygen. A small amount of ATP is produced directly by the reactions of glycolysis and Krebs cycle. This mechanism of producing ATP is called substrate-level phosphorylation. Substrate-level phosphorylation = ATP production by direct enzymatic transfer of phosphate from an intermediate substrate in catabolism to ADP. . Glycolysis harvests chemical energy by oxidizing glucose to pyruvate: a closer look Glycolysis = (Glyco = sweet, sugar; lysis = to split); catabolic pathway during which six- carbon glucose is split into two three—carbon sugars, which are then oxidized and rearranged by a step-wise process that produces two pyruvate molecules. - Each reaction is catalyzed by specific enzymes dissolved in the cytosol. - No C02 is released as glucose is oxidized to pyruvate; all carbon in glucose can be accounted for in the two molecules of pyruvate. - Occurs whether or not oxygen is present. The reactions of glycolysis occur in two phases: 110 Unit 11 The Cell Energy-investment phase. Energy-Investment Phase The cell uses ATP to H o phosphorylate the \C” intermediates of glycolysis. I H — lC — OH HO — |C — H H—F—OH H — lC — OH H — C — OH H Energy-yielding phase. Two three-carbon intermediates are oxidized. For each glucose molecule entering glycolysis: l. A net gain of two ATPs is produced by substrate-level phosphorylation. 2. Two molecules of NAD+ are reduced to NADH. Energy conserved in the high- energy electrons of NADH can be used to make ATP by oxidative phosphorylation. You may not want students to memorize the structures or steps of glycolysis, but you should expect them to understand the process, where it occurs, and the major molecules required and produced. It may be helpful to summarize a lecture with an overhead transparency. Energy-investment phase: The energy investment phase includes five preparatory steps that split glucose in two. This process actually consumes ATP. ............................................................................................................... Step 1: Glucose enters the cell, and H2 o—® carbon six is phosphorylated. This H H H o ATP-coupled reaction: : heXOKinase H - Is catalyzed by hexokinase EH0 OH HO H H OH (kinase is an enzyme involvedé H OH fl ADP H OH 1n phosphate transfer) Glucose Glucose6—Phosphate§ , Requires an initial investment Of .............................................................................................................. ATP ' Makes glucose more chemically reactive - Produces glucose-6-phosphate; since the plasma membrane is relatively impermeable to ions, addition of an electrically charged phosphate group traps the sugar in the cell. Step 2: An isomerase catalyzes the H o-® rearrangement of glucose-6-phosphate to 2 phosphog'uco_ CH20-® CH2 0H its isomer, fructose-6-phosphate. H H H isomerase O 5 H H H H ”O OH H OH H OH HO H Glucose 6 —Phosphate Fructose 6 —Phosphate Chapter 9 Cellular Respiration: Harvesting Chemical Energy 1 l 1 Step 3: Carbon one Of ........................................................................................................................................ fructose-6-phosphate is CH2 CH20H _o_ 5 phosphorylated. This : 063) © H HO . hos hofructokinase reaction: H HO ‘ Requires an H H H H H H H investment of ADP Fructose hos hate Fructose another ATP. 6’9 p . 1, 6—di phosphate 5 - Is catalyzed by ........................................................................................................................................ phosphofructokinase, an allosteric enzyme that controls the rate of glycolysis. This step commits the carbon skeleton to glycolysis, a catabolic process, as opposed to being used to synthesize glycogen, an anabolic process. Step 42 Aldolase cleaves the :. ...................................................................................................................................... ._ six—carbon sugar into two @—0—CH2 isomeric three—carbon sugars. __._ |c=o 0 This is the reaction cHZOH for which glycolysis ®-O—CH2 o CH2—0—® Dihydroxyacewne is named. H HO abblase phosphate - For each glucose H H H molecule that begins H H d=o glycolysis, there are Fructose _—" hHOH two product ‘ 1, 6—diphosphate dH2.o_® molecules for this and Glyceraldehyde each succeeding step. phosphate Step 51 A11 isomerase catalyzes the reversible conversion between the tWO three-carbon .................................................................................................... sugars. This reaction: H . . . @O-CHz ' Never reaches equilibrium because | isomerase hZO only one isomer, glyceraldehyde i=0 _——- HOH phosphateais used in the next step Hz 0H 33H2_O_® Of glycolySIS. Dihydroxyyacetone Glyceraldehyde - 15 thus pulled towards the direction phosphate phosphate ‘ of glyceraldehyde phosphate, which is removed as fast as it forms. - Results in the net effect that, for each glucose molecule, two molecules of glyceraldehyde phosphate progress through glycolysis. Energy-yielding phase: The energy—yielding phase occurs after glucose is split into two three-carbon sugars. During these reactions, sugar is oxidized, and ATP and NADH are produced. triose phosphate 2 ® @~O—fi= Step 6: An enzyme dehydrogenase 'i p—o , HOH catalyzes two sequential HOH CH2_O_® reactions: CH2—0—© _ 1- Glyceraldehyde Glyceraldehyde 2 NAD + 1,3 —Diphosphog|ycerate phosph...
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