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Transcript
CELLULAR RESPIRATION HARVESTING CHEMICAL ENERGY Four Features of Enzymes 3) The same enzyme sometimes works for both the forward and reverse reactions, but not always 4) Each type of enzyme recognizes and binds to only certain substrates Activation Energy For a reaction to occur, an energy barrier must be surmounted Enzymes make the energy barrier smaller activation energy without enzyme starting substance activation energy with enzyme energy released by the reaction products two substrate molecules substrates contacting active site of enzyme Induced-Fit Model active sight TRANSITION STATE (tightest binding but least stable) end product enzyme unchanged by the reaction Substrate molecules are brought together Substrates are oriented in ways that favor reaction Active sites may promote acid-base reactions Active sites may shut out water Factors Influencing Enzyme Activity Temperature pH Salt concentration Allosteric regulators Coenzymes and cofactors Allosteric Activation allosteric activator vacant allosteric binding site active site altered, can bind substrate enzyme active site active site cannot bind substrate Allosteric Inhibition allosteric inhibitor allosteric binding site vacant; active site can bind substrate active site altered, can’t bind substrate Feedback Inhibition enzyme 2 enzyme 1 SUBSTRATE enzyme 3 enzyme 4 enzyme 5 A cellular change, caused by a specific activity, shuts down the activity that brought it about END PRODUCT (tryptophan) Effect of Temperature Small increase in temperature increases molecular collisions, reaction rates High temperatures disrupt bonds and destroy the shape of active site Effect of pH Enzyme Helpers Cofactors Coenzymes NAD+, NADP+, FAD Accept electrons and hydrogen ions; transfer them within cell Derived from vitamins Metal ions Ferrous iron in cytochromes Producing the Universal Currency of Life All energy-releasing pathways require characteristic starting materials yield predictable products and byproducts produce ATP ATP Is Universal Energy Source Photosynthesizers get energy from the sun Animals get energy second- or thirdhand from plants or other organisms Regardless, the energy is converted to the chemical bond energy of ATP A review of how ATP drives cellular work Making ATP Plants make ATP during photosynthesis Cells of all organisms make ATP by breaking down carbohydrates, fats, and protein Redox Reactions The loss of electrons is called oxidation. The addition of electrons is called reduction Ae- + B A + Be- Overview of Aerobic Respiration C6H1206 + 6O2 glucose oxygen 6CO2 + 6H20 carbon dioxide water Electrons “fall” from organic molecules to oxygen during cellular respiration In cellular respiration, glucose and other fuel molecules are oxidized, releasing energy. In the summary equation of cellular respiration: C6H12O6 + 6O2 -> 6CO2 + 6H2O Glucose is oxidized, oxygen is reduced, and electrons loose potential energy. Cellular respiration does not oxidize glucose in a single step that transfers all the hydrogen in the fuel to oxygen at one time. Rather, glucose and other fuels are broken down gradually in a series of steps, each catalyzed by a specific enzyme Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings An overview of cellular respiration (Layer 1) An overview of cellular respiration (Layer 2) An overview of cellular respiration (Layer 3) Glycolysis Occurs in Two Stages Energy-requiring steps ATP energy activates glucose and its six- carbon derivatives Energy-releasing steps The products of the first part are split into three-carbon pyruvate molecules ATP and NADH form Energy-Requiring Steps glucose ATP ADP P glucose-6-phosphate P fructose-6-phosphate ATP ADP P fructose-1,6-bisphosphate 2 ATP invested EnergyReleasing Steps PGAL PGAL NAD+ NADH Pi P P NADH Pi P 1,3-bisphosphoglycerate ADP NAD+ ATP P 1,3-bisphosphoglycerate ADP ATP substrate-level phosphorylation 2 ATP invested P P 3-phosphoglycerate 3-phosphoglycerate P P 2-phosphoglycerate H2 O 2-phosphoglycerate H2 O PEP PEP P ADP ATP P ADP ATP substrate-level phosphorylation 2 ATP invested pyruvate pyruvate Net Energy Yield from Glycolysis Energy requiring steps: 2 ATP invested Energy releasing steps: 2 NADH formed 4 ATP formed Net yield is 2 ATP and 2 NADH Second-Stage Reactions Occur in the mitochondria Pyruvate is broken down to carbon dioxide More ATP is formed More coenzymes are reduced PREPARATORY STEPS pyruvate coenzyme A (CoA) NAD+ (CO2) NADH CoA Acetyl–CoA KREBS CYCLE CoA oxaloacetate citrate H O 2 NADH H2O NAD+ malate NAD+ H2O isocitrate NADH fumarate FADH2 FAD a-ketogluterate CoA NAD+ NADH succinate CoA succinyl–CoA ATP ADP + phosphate group (from GTP) Two Parts of Second Stage Preparatory reactions Pyruvate is oxidized into two-carbon acetyl units and carbon dioxide NAD+ is reduced Krebs cycle The acetyl units are oxidized to carbon dioxide NAD+ and FAD are reduced Preparatory Reactions pyruvate + coenzyme A + NAD+ acetyl-CoA + NADH + CO2 One of the carbons from pyruvate is released in CO2 Two carbons are attached to coenzyme A and continue on to the Krebs cycle What is Acetyl-CoA? A two-carbon acetyl group linked to coenzyme A CH3 Acetyl group C=O Coenzyme A The Krebs Cycle Overall Reactants Acetyl-CoA 3 NAD+ FAD ADP and Pi Overall Products Coenzyme A 2 CO2 3 NADH FADH2 ATP A summary of the Krebs cycle Results of the Second Stage All of the carbon molecules in pyruvate end up in carbon dioxide Coenzymes are reduced (they pick up electrons and hydrogen) One molecule of ATP is formed Four-carbon oxaloacetate is regenerated Coenzyme Reductions During First Two Stages Glycolysis Preparatory reactions Krebs cycle 2 NADH 2 FADH2 + 6 NADH Total 2 FADH2 + 10 NADH 2 NADH Electron Transport Phosphorylation Occurs in the mitochondria Coenzymes deliver electrons to electron transport systems Electron transport sets up H+ ion gradients Flow of H+ down gradients powers ATP formation Electron Transport Electron transport systems are embedded in inner mitochondrial compartment NADH and FADH2 give up electrons that they picked up in earlier stages to electron transport system Electrons are transported through the system The final electron acceptor is oxygen Creating an H+ Gradient OUTER COMPARTMENT NADH INNER COMPARTMENT Making ATP: Chemiosmotic Model ATP INNER COMPARTMENT ADP + Pi Importance of Oxygen Electron transport phosphorylation requires the presence of oxygen Oxygen withdraws spent electrons from the electron transport system, then combines with H+ to form water Summary of Energy Harvest (per molecule of glucose) Glycolysis Krebs cycle and preparatory reactions 2 ATP formed by substrate-level phosphorylation 2 ATP formed by substrate-level phosphorylation Electron transport phosphorylation 32 ATP formed Energy Harvest from Coenzyme Reductions What are the sources of electrons used to generate the 32 ATP in the final stage? 4 ATP - generated using electrons released during glycolysis and carried by NADH 28 ATP - generated using electrons formed during second-stage reactions and carried by NADH and FADH2 Energy Harvest Varies NADH formed in cytoplasm cannot enter mitochondrion It delivers electrons to mitochondrial membrane Membrane proteins shuttle electrons to NAD+ or FAD inside mitochondrion Electrons given to FAD yield less ATP than those given to NAD+ Efficiency of Aerobic Respiration 686 kcal of energy are released 7.5 kcal are conserved in each ATP When 36 ATP form, 270 kcal (36 X 7.5) are captured in ATP Efficiency is 270 / 686 X 100 = 39 percent Most energy is lost as heat Anaerobic Pathways Do not use oxygen Produce less ATP than aerobic pathways Two types Fermentation pathways Anaerobic electron transport Fermentation Pathways Begin with glycolysis Do not break glucose down completely to carbon dioxide and water Yield only the 2 ATP from glycolysis Steps that follow glycolysis serve only to regenerate NAD+ Lactate Fermentation GLYCOLYSIS C6H12O6 2 ATP energy input 2 NAD+ 2 ADP 2 4 NADH ATP energy output 2 pyruvate 2 ATP net LACTATE FORMATION electrons, hydrogen from NADH 2 lactate Alcoholic Fermentation GLYCOLYSIS C6H12O6 2 ATP energy input 2 NAD+ 2 ADP 2 4 NADH ATP 2 pyruvate energy output 2 ATP net ETHANOL FORMATION 2 H2O 2 CO2 2 acetaldehyde electrons, hydrogen from NADH 2 ethanol Yeasts Single-celled fungi Carry out alcoholic fermentation Saccharomyces cerevisiae Baker’s yeast Carbon dioxide makes bread dough rise Saccharomyces ellipsoideus Used to make beer and wine Anaerobic Electron Transport Carried out by certain bacteria Electron transport system is in bacterial plasma membrane Final electron acceptor is compound from environment (such as nitrate), NOT oxygen ATP yield is almost as good as from aerobic respiration Energy from Proteins Proteins are broken down to amino acids Amino acids are broken apart Amino group is removed, ammonia forms, is converted to urea and excreted Carbon backbones can enter the Krebs cycle or its preparatory reactions Energy from Fats Most stored fats are triglycerides Triglycerides are broken down to glycerol and fatty acids Glycerol is converted to PGAL, an intermediate of glycolysis Fatty acids are broken down and converted to acetyl-CoA, which enters Krebs cycle Evolution of Metabolic Pathways When life originated, atmosphere had little oxygen Earliest organisms used anaerobic pathways Later, noncyclic pathway of photosynthesis increased atmospheric oxygen Cells arose that used oxygen as final acceptor in electron transport