ASIM'S WEBWORLD View: Main School Main AP Biology Chapter Guides RELATED LINKS Review Questions and Answers		AP BIOLOGY: Chapter Nine Outline INTRODUCTION All Living Organisms Require Energy Autotrophs are organisms that convert energy into chemical energy Heterotrophs live on the energy produced by autotrophs Conversion of Chemical Energy to ATP Related to Photosynthesis USING CHEMICAL ENERGY TO DRIVE METABOLISM All Organisms Must Harvest Chemical Energy to Live fig 9.1 Extracting this energy is done in stages First stage is digestion Catabolism is the next stage where energy is obtained from C-H bonds Cellular Respiration C-H bond energy is carried by electrons in the covalent bond Electrons used to produce ATP Energy depleted electron donated to another molecule In oxidative respiration oxygen accepts H+, water formed In fermentation an organic molecule is the H+ recipient Basic reaction of carbohydrate catabolism Reactants are carbohydrates and oxygen Products are carbon dioxide, water and energy Change in free energy is -720 kilocalories, energy released Energy used to produce ATP The ATP Molecule ATP molecule transfers energy from respiration to other cellular sites Structure of ATP fig 9.2 Ribose sugar bound to adenine base and chain of three phosphate groups Linked phosphates store energy of their electrostatic repulsion Phosphate transfer (phosphorylation) charges that molecule HARVESTING ENERGY BY EXTRACTING ELECTRONS Reduction Doesn't Always Involve Complete Transfer of Electrons A Closer Look at Oxidation/Reduction Covalent bond electronegativity Covalent electrons in glucose C-H bond are shared equally Oxygen atoms also share their electrons equally in O2 In CO2 sharing is unequal, C atom electrons shift towards O C atoms of glucose are oxidized (loose electrons) O atoms are reduced (gain electrons) In H2O sharing is similarly unequal H atoms of glucose are oxidized (loose electrons) O atoms are reduced (gain electrons) Energy is released when electron shifts from less electronegative glucose closer to more electronegative oxygen NAD+ Harvests the Energy in Stages In large energy releases more energy is lost as heat Explosion of gas v.s. combustion in engine More useful energy available if done in small steps Six H+ in glucose C-H bonds stripped away in stages Enzymes catalyzed reactions called glycolysis and Krebs cycle H+ transferred to NAD+, a coenzyme carrier fig 9.3 NAD+ becomes NADH NADH energy not harvested all at once either Two electrons from NADH pass to electron transport chain Chain is a series of molecules in the inner mitochondrial membranes Electrons delivered by NADH, captured at end by oxygen Electrons move down the energy gradient, releasing 53 kcal/mole AN OVERVIEW OF OXIDATIVE RESPIRATION Cells Make ATP in Two Ways Substrate-level phosphorylation Chemical bonds of glucose shifted around Reactions release more energy than needed to form ATP fig 9.4 Electron transport chain fig 9.5 Most Organisms Combine the Two in a Four Stage Process fig 9.6 The first stage is glycolysis fig 9.7 Glycolytic enzymes are present in the cytoplasm of the cell Enzymes are not bound to any membrane or organelle Two ATP formed by substrate-level phosphorylation Two ATP required to prepare glucose for the reaction Four ATP are produced in the phosphorylation Four electrons harvested as NADH Process is not a highly efficient, most energy remains in pyruvate The second stage is oxidation of pyruvate fig 9.8 Pyruvate converted to CO2 and two-carbon acetyl-coA One molecule NADH made per pyruvate (two NADH per glucose) The third stage is the Krebs cycle fig 9.9 Alternately called the citric acid cycle or tricarboxylic acid cycle Two more ATP made by substrate-level phosphorylation Large number of NADH made Fourth stage occurs in the electron transport chain fig 9.10 Electrons carried by NADH Large number of ATP molecules formed Organisms Do Things Differently Glycolysis Nearly all organisms do glycolysis Occurs in present or absence of oxygen Electron transport chain operation requires presence of oxygen Oxygen is final electron acceptor Without oxygen animals restricted to substrate-level phosphorylation Other organisms may use other compounds as final electron acceptors In eukaryotes the second, third and fourth stages occur in mitochondria Photosynthetic plants exhibit oxidative respiration like other organisms The Fate of a Candy Bar fig 9.11 A complete mixture of sugars, lipids, proteins and other molecules Complex molecules degraded to simple ones with no energy yield Assume degradation produces only six-carbon glucose molecules Molecules then go through glycolysis and oxidative respiration STAGE ONE: GLYCOLYSIS An Overview of Glycolysis First half of glycolysis One glucose converted into two glyceraldehyde-3-phosphates (G3P) Processes expend energy Second half of glycolysis G3P converted into pyruvate Energy producing processes Ten reactions comprise four main steps Step A, glucose priming Changes arrangement of glucose molecule Uses two ATP Step B, cleavage and rearrangement Six-carbon molecule split into two three carbon molecules Ultimately end up with two G3P molecules Step C, oxidation Two electrons and one proton transferred from G3P to NAD+ Forms NADH, one per G3P, two per glucose Step D, ATP generation G3P converted into pyruvate (two per glucose) Two ATP made per G3P, four ATP per glucose Net energy is 24 k/cal per mole of glucose (3.5% of what's available) Even though amount is small, life survived on it for a billion years Evolution of glycolysis was backwards like most biochemical reactions ATP-producing breakdown of G3P evolved first Synthesis of G3P developed later when original G3P used up All Cells Use Glycolysis Glycolysis was among the earliest pathways to evolve Does not require oxygen, occurs readily in anaerobic environment Reactions occur freely in the cytoplasm All organisms, but a few bacteria, exhibit glycolysis Glycolysis has been added to, but not replaced by other processes Evolution is an incremental process Change occurs by improving upon past success Closing the Metabolic Circle: The Regeneration of NAD+ Three changes occur during glycolysis Glucose is converted to two pyruvates Two ADP's are converted to ATP's Two NAD+ molecules are converted to NADH's Glycolytic processes cannot continue ad infinitum Cell will ultimately accumulate NADH and run out of NAD+ NADH must be recycled back to NAD+ for glycolysis to continue Recycling occurs in one of two ways Oxidative respiration Oxygen is the final electron acceptor, water is final product This process is aerobic Fermentation fig 9.12 Organic molecules serve as the final electron acceptor This process is anaerobic STAGE TWO: THE OXIDATION OF PYRUVATE Oxidation of Pyruvate Occurs in Two Stages Oxidation of pyruvate into acetyl-CoA Oxidation of acetyl-CoA The Oxidation of Pyruvate fig 9.8 One carbon of the three-carbon pyruvate is cleaved, leaves as CO2 This is a decarboxylation reaction that leaves A pair of electrons and associated H+ reduces NAD+ to NADH A two-carbon fragment called an acetyl group Complex reaction involves three intermediate steps Catalyzed within the mitochondria by a multienzyme complex Pyruvate dehydrogenase: enzyme that removes a CO2 from pyruvate Acetyl group added to cofactor (co-enzyme A) makes acetyl-CoA Reaction produces one molecule of NADH Remaining acetyl-CoA is a more important consequence fig 9.13 Formed by many catabolic processes Currency of oxidative metabolism Most acetyl-CoA is directed toward energy storage The rest is oxidized to produce ATP STAGE THREE: THE KREBS CYCLE The Oxidation of Acetyl-CoA Acetyl-CoA is oxidized by binding it to four-carbon oxaloacetate The resulting six-carbon molecule passes through series of reactions Electron-yielding reactions split off two molecules of CO2 The four-carbon molecule is regenerated Single glucose molecule (two G3P) goes through cycle twice Overview of the Krebs Cycle Consists of nine reactions in two stages Step A, priming Acetyl-CoA first joins the cycle Chemical groups are rearranged Step B, energy extraction Four of the six reactions are oxidations, electrons are removed One reaction generates an ATP equivalent via substrate-level phosphorylation Summary box: reactionsof the Krebs Cycle fig 9.A The Products of the Krebs Cycle Stripped electrons are stored in NADH molecules One reaction is not energetic enough to produce NADH Instead makes flavin adenine dinucleotide (FAD+) Carries electrons when reduced to FADH2 Total amount of ATP and electron carriers produced tbl 9.1 Four ATP molecules Twelve reduced electron carriers STAGE FOUR: THE ELECTRON TRANSPORT CHAIN NADH and FADH2 Contain Electrons Gathered From Glucose Breakdown NADH molecules carry their electrons to mitochondrial membrane FADH2 is already attached to the membrane Transfer electrons to NADH dehydrogenase, membrane-embedded protein Electrons passed on to a series of cytochromes, carrier molecules fig 9.14 Lose energy by driving a series of transmembrane proton pumps Series collectively called the electron transport chain fig 9.10 Terminal step is cytochrome c oxidase complex Four electrons reduce one molecule of oxygen gas to form water Final products of oxidative metabolism are CO2 and water The plentiful electron acceptor makes oxidative respiration possible Process cannot occur in the absence of the molecule Electron transport chain is similar to the one in aerobic photosynthesis Chemiosmosis The enzymes of the Krebs cycle are in the matrix Electrons used to pump protons from matrix to outer compartment Proton pumps located on inner membrane Electron flow induces shape change in pump proteins Electrons from NADH activate three pumps Electrons from FADH2 activate two pumps Concentration of protons in outer compartment increases The protons pass back inward through special channels ATP is synthesized when protons diffuse through them fig 9.15 ATP leaves the mitochondrion via facilitated diffusion SUMMARIZING AEROBIC RESPIRATION Chemiosmotic Generation of ATP Each NADH activates three pumps, generates three ATP molecules Each FADH2 activates two pumps, generates two molecules of ATP Each NADH from glycolysis generates only two ATP's Glycolysis occurs in cytoplasm Electron transport chain is in the mitochondria Costs one ATP to get NADH across mitochondrial membrane Overall, 32 ATP's are produced by chemiosmotic phosphorylation Only four ATP's are produced by substrate-level phosphorylation Theoretical Total Energy Is 36 ATP Molecules Actual total in eukaryotes is lower Inner membrane is leaky, some protons reenter without generating ATP Mitochondria use proton gradient for other purposes Truer values are 2.5 ATP per NADH and 1.5 ATP per NADH2 Net total closer to 26 molecules of ATP by chemiosmotic phosphorylation Electron transport chain in prokaryotes not sequestered behind membranes NADH from glycolysis don't loose ATP getting across membrane Prokaryotes produce 32 to 38 ATP per glucose Energy Efficiency (12 x 30)/686 = 52% efficiency of aerobic oxidation of glucose Efficiency of car engine is 25% High efficiency fostered evolution of heterotrophs LIVING WITHOUT OXYGEN: FERMENTATION Bacteria Carry Out Many Different Types of Fermentations An organic molecule serves as the electron acceptor NADH is returned to NAD+ The organic molecule is reduced Bacteria produce reduced acids including acetic, butyric, propionic, lactic acid Eukaryotes Exhibit Fewer Fermentative Processes Than Bacteria Yeasts decarboxylate pyruvate to produce acetaldehyde and CO2 NADH and acetaldehyde are converted to ethyl alcohol and NAD+ Ethanol is another name for ethyl alcohol This is a commercially important process that makes wine and beer fig 9.16 Most multicellular animals regenerate NAD+ without decarboxylation Muscle cells convert NADH + pyruvate to NAD+ + lactic acid Utilizes the enzyme lactate dehydrogenase Blood circulation removes lactic acid from muscle cells With great exertion lactic acid is not removed fast enough Limits physical performance without special training CATABOLISM OF PROTEINS AND FATS Fats and Proteins Are Also Important Sources of Energy fig 9.17 Cellular Respiration of Protein Must first break proteins into constituent amino acids Nitrogen-containing amino group removed from each amino acid Remaining carbon chain converted to substance in Krebs cycle Alanine to pyruvate Glutamate to alpha-ketoglutarate Aspartate to oxaloacetate Cellular Respiration of Fat Fats first degraded to individual fatty acids and glycerol Long carbon chains with many hydrogens hold much energy Fats oxidized in the matrix of the mitochondrion Four enzymes attack the long fatty acid chains Remove carbons in chunks of 2C acetyl groups Entire chain converted into acetyl-CoA Process called beta-oxidation Efficiency of metabolizing fats Each beta-oxidation cycle uses one ATP to prime the process Produces 1 acetyl-CoA + 1 NADH + 1 FADH2 NADH produces 2.5 ATP's FADH2 produces 1.5 ATP's Acetyl-CoA produces 10 ATP's Total number of ATP's from a six carbon fatty acid Two cuts = 2 NADH + 2 FADH2 = 2(2.5+1.5) -2 = 6 ATP's Three acetyl-CoA molecules = 3(10) = 30 ATP's Total = 36 ATP's Overall actual yield is 20% more than glucose Regulation of Cellular Respiration Carbohydrates, proteins, fats, nucleic acids are potential energy sources fig 9.18 When a cell contains sufficient ATP the process is slows down When ATP levels are low, ADP activates enzymes in pathway Regulation called feedback inhibition Pathways regulated by sensitivity of key enzymes to certain metabolites