Chapter 9: Cellular Respiration and Fermentation
Chapter 9: Cellular Respiration and Fermentation Biol 5A
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Date Created: 02/16/16
Chapter 9: Cellular Respiration and Fermentation Catabolic Pathways and Production of ATP Organic compounds possess potential energy that can be broken down by a cell system and enzymes to simpler waste products that have less energy. Some of it can be used to do work, most is lost as heat Fermentation: catabolic process; partial degradation of sugars and other organic fuels that occurs without the use of oxygen Aerobic respiration: most prevalent/efficient catabolic pathway; oxygen is consumed as a react along with the organic fuel o cells of most eukaryotes and many prokaryotes o some prokaryotes use substances other than oxygen as a reactant to harvest chemical energy—anaerobic respiration cellular respiration: includes aerobic and anaerobic processes; but mainly for aerobic processes o organic compounds + oxygen = carbon dioxide + water + energy breakdown of glucose is exergonic, products store less energy than the reactants and reaction can occur spontaneously (without input of energy) catabolic processes don’t directly perform cellular work—linked to ATP o cell must regenerate its supply of ATP from ADP and Pi The Principle of Redox redox reactions: transfer of one or more electrons from one reactant to another o oxidation: loss of electrons o reduction: addition of electrons energy must be added to pull an electron away from an atom the more electronegative the atom, the more energy is required to take an electron away from it electron loses potential energy when it shifts from a less electronegative atom toward a more electronegative one, and energy is released Oxidation of Organic Fuel Molecules During Cellular Respiration cellular respiration is the oxidation of glucose and other molecules in food hydrogen is transferred from glucose to oxygen the oxidation of glucose transfers electrons to a lower energy state, liberating energy that becomes available for ATP synthesis main energy-yielding foods, carbohydrates and fats, are reservoirs of electrons associated with hydrogen enzymes in cell will lower the barrier of activation energy, allowing the sugar to be oxidized in steps Stepwise Energy Harvest via NAD+ and the Electron Transport Chain Each electron travels with a proton (H atom) H atoms are not transferred directly to oxygen, but instead are passed to an electron carrier, a coenzyme called NAD+ o Can easily cycle between oxidized (NAD+) and reduced (NADH) states o Functions as oxidizing agent Enzymes called dehydrogenases remove a pair of hydrogen atoms (2 protons and 2 electrons) from glucose, thereby oxidizing it o Enzyme delivers 2 electrons and 1 proton to NAD+ o Other proton released as H+ ion in solution Each NADH molecule formed during respiration represents stored energy that can be tapped to make ATP when the electrons complete their fall down an energy gradient from NADH to oxygen Electron transport chain: number of molecules, mostly proteins, built in inner membrane of mitochondria of eukaryotic cells and the plasma membrane of aerobically respiring prokaryotes o Electrons removed from glucose are shuttled by NADH to the top (high E) of chain, at the bottom of chain the O2 captures the electrons along with hydrogen nuclei, forming water o Electrons cascade down chain from one molecule to next in series of redox reactions, losing small amount of energy until they reach oxygen o Oxygen pulls electrons down the chain (because of its electronegativity The Stages of Cellular Respiration: A Preview Harvesting of energy from glucose by cellular respiration: o Glycolysis o Pyruvate oxidation and the citric acid cycle o Oxidative phosphorylation: electron transport chain and chemiosmosis Glycolysis: occurs in cytosol, begins degradation process by breaking glucose into two molecules of pyruvate Pyruvate enters mitochondrion and is oxidized to acetyl CoA, which enters citric acid cycle and carbon dioxide is given of ETC accepts electrons from breakdown products of first two stages via NADH and passes electrons from one molecule to another At the end of chain, electrons combined with molecular oxygen and H+ ions to form water Energy released at each step of chain is stored in mitochondria and can be used to make ATP from ADP Oxidative phosphorylation: mode of ATP synthesis, powered by redox reactions of ETC ETC and chemiosmosis occurs in inner membrane of mitochondrion Oxidative phosphorylation accounts for 90% of ATP generated Smaller amount of ATP is formed directly during glycolysis and citric acid cycle by substrate level phosphorylation o Occurs when enzyme transfers phosphate group from substrate molecule (organic molecule generated as an intermediate during catabolism of glucose) to ADP, rather than adding inorganic phosphate to ADP as in oxidative phosphorylation Glycolysis harvests chemical energy by oxidizing glucose to pyruvate “sugar splitting” Glucose (6 carbon sugar) is split into two 3- carbon sugars and then oxidized to form pyruvate Two phases: o Energy investment: cell spends ATP o Energy payof: ATP is produced by substrate level phosphorylation and NAD+ is reduced to NADH by electrons released from oxidation of glucose Net energy yield from glycolysis per glucose molecule: 2 ATP and 2 NADH All of the carbon originally present in glucose is accounted for in pyruvate, no carbon dioxide is released Occurs whether or not oxygen is present o If oxygen is present, chemical energy stored in pyruvate and NADH can be extracted by pyruvate oxidation, citric acid cycle, and oxidative phosphorylation After pyruvate is oxidized, the citric acid cycle completes the energy- yielding oxidation of organic molecules Glycolysis requires less than a quarter of the chemical energy in glucose than can be released by cells; most of the energy remains in the two molecules of pyruvate If oxygen is present, pyruvate enters mitochondrion where oxidation of glucose is completed (in cytosol for prokaryotes) Oxidation of Pyruvate to Acetyl CoA Pyruvate is converted to acetyl CoA upon entering mitochondrion via active transport Process carried out by a multi-enzyme complex that catalyzes three reactions: o Pyruvate’s carboxyl group is removed and given of as a molecule of carbon dioxide o Remaining two carbon fragment is oxidized, forming acetate. Extracted electrons are transferred to NAD+, storing energy in form of NADH o CoA is attached via its sulfur atom to the acetate, forming acetyl CoA (has a high potential energy) The Citric Acid Cycle Cycle functions to oxidize fuel derived from pyruvate Pyruvate is broken down to three carbon dioxide molecules (including the one released during conversion of pyruvate to acetyl CoA) Cycle generates 1 ATP per turn by substrate level phosphorylation but most chemical energy is transferred to NAD+ and coenzyme FAD during redox reactions Reduced coenzymes, NADH and FADH2, shuttle their high energy electrons into the ETC 8 steps, each catalyzed by a specific enzyme o Acetyl group of acetyl CoA joins cycle by combining with oxaloacetate, from citrate Next 7 steps decompose citrate back to oxaloacetate (regeneration) o for each acetyl group entering cycle, 3 NAD+ are reduced to NADH o in step 6, electrons transferred to FAD which accepts 2 electrons and 2 protons to become FADH2 o step 5 produces GTP by substrate level phosphorylation in animals can be used to make ATP molecule or directly be used to power work in a cell in plants/bacteria, step 5 forms ATP molecule directly by substrate level phosphorylation output from step 5 represents only ATP generated directly from cycle most ATP produced from respiration results from oxidative phosphorylation, when NADH and FADH2 made from citric acid cycle relay electrons extracted from food to ETC o supply energy for phosphorylation of ADP to ATP During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis substrate level phosphorylation produces 2 net ATP from glycolysis and 2 ATP from citric acid cycle The Pathway of Electron Transport folding of inner membrane to form cristae increases its surface area, providing for thousands of copies if ETC in each mitochondrion most components are proteins that exist in protein complexes I to IV prosthetic groups: non-protein compounds essential for catalytic functions of certain enzymes are bound to protein complexes electron carriers alternate between reduced and oxidized states as they accept and donate electrons each component of chain becomes reduced when it accepts electrons from uphill; it then returns to oxidized form as it passes electrons to its downhill (more electronegative) neighbor electrons removed from glucose by NAD+ are transferred from NADH to first molecule of ETC in complex I o the molecule is a flavoprotein, FMN in the next redox reaction, flavoprotein returns to oxidized form as it passes electrons to an iron-sulfur protein Fe-S in complex I Fe-S passes electrons to ubiquinone, Q o Only member of ETC that isn’t a protein most of the remaining electron carriers between Q and oxygen are proteins called cytochromes o their prosthetic group (heme group) has an iron atom that accepts and donates electrons o several type in ETC each with a diferent protein and slightly diferent heme group o last cytochrome of chain, cyt a3, passes electrons to oxygen o each oxygen atom also picks up a pair of H+ from aq solution, forming water Another source of electrons for ETC is FADH2 o Adds its electrons to ETC from complex II, at a lower energy level than NADH does o ETC provides about 1/3 less energy for ATP synthesis when electron donor is FADH2 ETC doesn’t make an ATP directly, it simply eases the fall of electrons from food to oxygen Chemiosmosis: The Energy- Coupling Mechanism ATP synthase: enzyme that makes ATP from ADP and inorganic phosphate, inside of inner membrane of mitochondrion or prokaryotic plasma membrane o Uses the energy of an existing ion gradient to power ATP synthesis o Power source is the diference in concentration of H+ on opposite sides of inner mitochondrial membrane Chemiosmosis: energy stored in the form of a hydrogen ion gradient across a membrane used to drive cellular work such as the synthesis of ATP Flow of H+ through synthase powers ATP generation o Synthase is made of four parts, each made of multiple polypeptides Establishing H+ gradient is a major function of ETC o Uses exergonic flow of electrons from NADH and FADH2 to pump H+ across the membrane, from mitochondrial matrix into intermembrane space o Energy stored in an H+ gradient across a membrane couples redox reactions of ETC to ATP synthesis o At certain steps of ETC, electron transfers cause H+ to be taken up and released into the surrounding solution o H+ gradient that results is referred to as a proton motive force; force drives H+ back across the membrane through H+ channels provided by ATP synthases Chemiosmosis is an energy coupling mechanism that uses energy stored in the form of an H+ gradient across a membrane to drive cellular work o Chloroplasts use chemiosmosis to generate ATP during photosynthesis An Accounting of ATP Production by Cellular Respiration Most energy flows in this sequence: o Glucose – NADH – ETC – proton-motive force – ATP Glycolysis: +2 ATP, +2 NADH/FADH2 Pyruvate oxidation: +2 NADH Citric acid cycle: +2 ATP, +6 NADH, +2 FADH2 Oxidative phosphorylation: +26/28 ATP (depending on if NADH or FADH2 shuttled the electrons from glycolysis) Fermentation and anaerobic respiration enable cells to produce ATP without the use of oxygen An electron transport chain is used in anaerobic respiration but not in fermentation Anaerobic respiration: o Takes place in certain prokaryotic organisms that live in environments without oxygen o Final electron acceptor is not oxygen, but some other electronegative substance Ex: marine bacteria use sulfate ion at the end of their respiratory chain Operation of chain builds up proton-motive force used to produce ATP, but hydrogen sulfide is made as a by-product instead of water Fermentation: o Way of harvesting chemical energy without using oxygen or electron transport chain (without cellular respiration) o An extension of glycolysis that allows continuous generation of ATP by substrate level phosphorylation of glycolysis Requires a sufficient supply of NAD+ to accept electrons during the oxidation step of glycolysis Under aerobic conditions NAD+ is recycled from NADH by the transfer of electrons to the electron transport chain but under anaerobic conditions electrons are transferred from NADH to pyruvate, the end product of glycolysis Types of Fermentation Alcohol fermentation o Pyruvate is converted to ethanol in two steps: Carbon dioxide is released from pyruvate, which is converted to the two-carbon compound acetaldehyde Acetaldehyde is reduced by NADH to ethanol which regenerates supply of NAD+ needed for continuation of glycolysis o Many bacteria carry out this fermentation under anaerobic conditions o Yeast (fungus) also carries out alcohol fermentation—used in brewing, winemaking, and baking Lactic acid fermentation o Pyruvate is reduced directly by NADH to form lactate as an end product, with no release of Co2 (lactate is the ionized form of lactic acid) o This fermentation by certain fungi and bacteria is used in the dairy industry to make cheese and yogurt o Human muscle cells make ATP by this fermentation when oxygen is scarce Occurs during strenuous exercise Cells switch from aerobic respiration to fermentation The excess lactate is gradually carried away by the blood to the liver, where it is converted back to pyruvate by liver cells Comparing Fermentation with Anaerobic and Aerobic Respiration Three alternative cellular pathways for producing ATP by harvesting chemical energy for food All use glycolysis to oxidize glucose and other organic fuels to pyruvate, with net production of 2 ATP by substrate level phosphorylation NAD+ is oxidizing agent that accepts electrons from food during glycolysis Process of oxidizing NADH back to NAD+: o Fermentation—final electron acceptor is an organic molecule such as pyruvate or acetaldehyde o Cellular respiration—electrons carried by NADH are transferred to an ETC, which regenerates NAD+ Amount of ATP produced: o Fermentation—2 molecules of ATP from substrate level phosphorylation o Cellular respiration—pyruvate is completely oxidized in the mitochondrion, thereby harvesting more energy from each sugar molecule, about 32 molecules of ATP per glucose molecule Obligate anaerobes: only carry out fermentation or anaerobic respiration o Cannot survive in the presence of oxygen Facultative anaerobes: can make ATP to survive using either fermentation or respiration o Our muscles behave as such The Versatility of Catabolism Aside from glucose, other organic molecules such as fats, proteins, sucrose, starch, etc can be used by cellular respiration to make ATP Biosynthesis (Anabolic Pathways) Not all of the organic molecules of food are destined to be oxidized as fuel to make ATP Food must also provide the carbon skeletons that cells require to make their own molecules Compounds formed as intermediates of glycolysis and the citric acid cycle can be diverted into anabolic pathways as precursors from which the cell can synthesize the molecules it requires Glycolysis and the citric acid cycle function as metabolic interchanges that enable our cells to convert some kinds of molecules to others as we need them Regulation of Cellular Respiration via Feedback Mechanisms Feedback inhibition: the end product of the anabolic pathway inhibits the enzyme that catalyzes an early step in the pathway—prevents the needless diversion of key metabolic intermediates from uses that are more urgent The cell also controls its catabolism—the control is based mainly on regulating the activity of enzymes at strategic points in the catabolic pathway One important switch is phosphofructokinase, the enzyme that catalyzes a step of glycolysis o It’s the first step that commits the substrate irreversibly to the glycolytic pathway o By controlling the rate of this step, the cell can speed up or slow down the entire catabolic process Phosphofructokinase is the “pacemaker” of respiration o Also an allosteric enzyme with receptor sites for specific inhibitors and activators It’s inhibited by ATP and stimulated by AMP, which the cell derives from ADP As ATP accumulates, inhibition of enzyme slows down glycolysis Enzyme becomes active again as cellular work converts ATP to ADP (and AMP) faster than ATP is being regenerated The energy that keeps us alive is released, not produced, by cellular respiration
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