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Cellular Respiration: Harvesting Chemical Energy 1 Cellular Respiration • Energy flows into an ecosystem as sunlight and leaves as heat • Photosynthesis generates oxygen and organic molecules, which are used in cellular respiration • All cells can harvest energy from organic molecules to power work • To do this, they break down the organic molecules and use the energy that is released to make ATP from ADP and phosphate Light energy ECOSYSTEM Photosynthesis in chloroplasts Organic + O molecules 2 CO2 + H2O Cellular respiration in mitochondria ATP powers most cellular work Heat energy 2 Catabolic Pathways and Production of ATP • • Heterotrophs live off the energy produced by autotrophs - extracting energy from food via digestion and catabolism There are different catabolic pathways used in ATP production: • Fermentation - the partial degradation of sugars in the absence of oxygen. • Cellular respiration - A more efficient and widespread catabolic process that consumes oxygen as a reactant to complete the breakdown of a variety of organic molecules. 3 Catabolic Pathways and Production of ATP • Although carbohydrates, fats, and proteins are all consumed as fuel, it is helpful to trace cellular respiration with the sugar glucose: • The catabolism of glucose is exergonic with a G of −686 kcal per mole of glucose. • Some of this energy is used to produce ATP, which can perform cellular work C6H12O6 + 6O2 6CO2 + 6H2O + Energy (ATP + heat) 4 Redox Reactions • • Catabolic pathways yield energy through the transfer electrons from one reactant to another by oxidation and reduction Redox reactions • In oxidation - A substance loses electrons, or is oxidized • In reduction - A substance gains electrons, or is reduced becomes oxidized (loses electron) Na + Cl Na+ + Cl– becomes reduced (gains electron) 5 Oxidation of Organic Fuel Molecules During Cellular Respiration • Cellular respiration provides the energy for the cell using the exergonic reaction: becomes oxidized C6H12O6 + 6O2 6CO2 + 6H2O + Energy ~686kcal/mole becomes reduced • During cellular respiration glucose is oxidized and oxygen is reduced • Glucose oxidation is accomplished in a series of steps 6 Glucose Oxidation If electron transfer is not stepwise • A large release of energy occurs • As in the reaction of hydrogen and oxygen to form water H2 + 1/2 O2 Free energy, G • Figure 9.5 A Explosive release of heat and light energy (a) Uncontrolled reaction H2O 7 Glucose Catabolism • Glucose catabolism is a series of redox reactions that release energy by repositioning electrons closer to oxygen atoms. The high energy electrons are stripped from glucose and picked up by NAD+ and FAD. 2 e – + 2 H+ NAD+ H • Dehydrogenase O NH2 C N+ O CH2 O O P O– O H O P O– HO O CH2 H OH HO Nicotinamide (oxidized form) H HO H OH Oxidation of NADH H O C H N NH2 + H Nicotinamide (reduced form) N H N Reduction of NAD+ NADH NH2 N O + 2[H] (from food) 2 e– + H+ N H Figure 9.4 8 The Electron Transport Chain • 2H 1/ + 2 O2 NADH (from food via NADH) 2 H+ + 2 e– 50 Free energy (G) relative to O2 (kcal/mol) • Passes electrons in a series of steps instead of in one explosive reaction Uses the energy from the electron transfer to form ATP Eventually, the electrons, along with H+, are passed to a final acceptor. Controlled release of energy for synthesis of ATP ATP Free energy, G • ATP ATP FADH2 40 FMN I Fe•S Multiprotein complexes FAD Fe•S II Q III Cyt b 30 Fe•S Cyt c1 IV Cyt c Cyt a 20 Cyt a3 10 2 e– 1/ 2 H+ 2 O2 0 H2O 2 H+ + 1/2 O2 H2O9 Glucose Catabolism • • • If molecular oxygen (O2) is the final electron acceptor, the process is called aerobic respiration. If some other inorganic molecule is the final electron acceptor, the process is called anaerobic respiration. If an organic molecule is the final electron acceptor, the process is called fermentation. 10 The Stages of Cellular Respiration • Respiration is a cumulative function of three metabolic stages • Glycolysis - breaks down glucose into two molecules of pyruvate • The Citric Acid Cycle (Kreb’s) - completes the breakdown of glucose • Oxidative phosphorylation - driven by the electron transport chain and Generates ATP 11 Cellular Respiration Electrons carried via NADH and FADH2 Electrons carried via NADH Glycolsis Pyruvate Glucose Cytosol ATP Substrate-level phosphorylation Citric acid cycle Oxidative phosphorylation: electron transport and chemiosmosis Mitochondrion ATP Substrate-level phosphorylation ATP Oxidative phosphorylation 12 Substrate Phosphorylation • Both glycolysis and the citric acid cycle can generate ATP by substrate-level phosphorylation Enzyme Enzyme ADP Substrate P Product + ATP PEP P P Enzyme P ADP Adenosine 13 Glycolysis • • Glycolysis harvests energy by oxidizing glucose to pyruvate Glycolysis • Means “splitting of sugar” • Breaks down glucose into pyruvate • Occurs in the cytoplasm of the cell 14 Glycolysis • Occurs in the cytoplasm of the cell • Results in the partial breakdown of glucose • Anaerobic – no oxygen is used during glycolysis • For each molecule of glucose that passes through glycolysis, the cell nets two ATP molecules. Glycolysis Citric acid cycle ATP ATP Oxidative phosphorylation ATP 15 Glycolysis Glucose Glycolysis Citric acid cycle ATP ATP Oxidation phosphorylation ATP ATP • Energy investment phase Hexokinase ADP ATP/NADH Ledger - 1 ATP Glucose-6-phosphate 16 Glycolysis Glucose Glycolysis Citric acid cycle ATP ATP Oxidation phosphorylation ATP ATP • Total of 2 ATP invested Hexokinase ADP ATP/NADH Ledger - 2 ATP Glucose-6-phosphate Phosphoglucoisomerase Fructose-6-phosphate ATP Phosphofructokinase ADP Fructose1, 6-bisphosphate Aldolase Isomerase Dihydroxyacetone phosphate Glyceraldehyde3-phosphate 17 NAD+ Glycolysis • Energy payoff phase NAD+ Triose phosphate dehydrogenase NADH + H+ Triose phosphate dehydrogenase NADH + H+ 1, 3-Bisphosphoglycerate 1, 3-Bisphosphoglycerate ADP ADP Phosphoglycerokinase ATP Phosphoglycerokinase ATP ATP/NADH Ledger - 2 ATP + 2 ATP + 2 NADH 3-Phosphoglycerate Phosphoglyceromutase 2-Phosphoglycerate 3-Phosphoglycerate Phosphoglyceromutase 2-Phosphoglycerate 18 NAD+ Glycolysis • End-products of glycolysis are 2 pyruvate molecules NAD+ Triose phosphate dehydrogenase NADH + H+ Triose phosphate dehydrogenase NADH + H+ 1, 3-Bisphosphoglycerate 1, 3-Bisphosphoglycerate ADP ADP Phosphoglycerokinase ATP Phosphoglycerokinase ATP ATP/NADH Ledger - 2 ATP + 4 ATP + 2 NADH 3-Phosphoglycerate 3-Phosphoglycerate Phosphoglyceromutase 2-Phosphoglycerate H2O Enolase Phosphoenolpyruvate ADP Phosphoglyceromutase 2-Phosphoglycerate H2O Phosphoenolpyruvate ADP Pyruvate kinase ATP Enolase Pyruvate kinase ATP 19 Pyruvate Pyruvate Glycolysis Summary • • • • • • • • Occurs in the cytoplasm Glucose converted to two 3-C chains Anaerobic - no oxygen 2 ATP used, 4 ATP produced Inefficient - net yield only 2 ATPs Not discarded by evolution but used as starting point for energy production If no O2 - Fermentation occurs End products: • 2 ATP • Pyruvate (3 C) • 2 x CO2 • 2 x NADH Energy investment phase Glucose 2 ADP + 2 P 2 ATP used 4 ATP formed 2 NADH + 2 H+ Energy payoff phase 4 ADP + 4 P 2 NAD+ + 4 e– + 4 H+ 2 Pyruvate + 2 H2O Net Glucose 4 ATP formed – 2 ATP used 2 NAD+ + 4 e– + 4 H+ 2 Pyruvate + 2 H2O 2 ATP 2 NADH + 2 H+ 20 The Citric Acid (Krebs) Cycle The Krebs cycle is named after Hans Krebs and is a metabolic event that follows glycolysis. This process occurs in the fluid matrix of the mitochondrion, uses the pyruvic acid from glycolysis and is aerobic. To begin the Krebs cycle, pyruvic acid is converted to acetyl CoA. 21 Oxidation of Pyruvate • • More energy can be extracted if oxygen is present Within mitochondria, pyruvate is decarboxylated, yielding acetyl-CoA, NADH, and CO2 MITOCHONDRION CYTOSOL NAD+ NADH + H+ Acetyl Co A Pyruvate Transport protein CO2 Coenzyme A 22 The Citric Acid (Krebs) Cycle • • • • Occurs in the mitochondrial matrix Aerobic – although O2 is not used directly in this pathway, it will not occur unless enough is present in the cell. Main catabolic pathway Acetyl-CoA is oxidized in a series of nine reactions 23 Krebs Cycle • • AcetylCoA reacts with oxaloacetate using an enzyme called citrate synthase producing citric acid. Because of this, the Krebs cycle is sometimes called the citric acid cycle. Glycolysis Citric acid cycle ATP ATP Oxidation phosphorylation ATP Acetyl CoA H2O Oxaloacetate Citrate Isocitrate Citric acid cycle 24 Krebs Cycle • • • The next 7 steps decompose the citrate back to oxaloacetate, Citric acid is systematically decarboxylated and dehyrogenated in order to use up the acetyl groups that were attached to the oxaloacetate. This allows oxaloacetate and CoA to be used in the next cycle. Glycolysis Citric acid cycle ATP ATP Oxidation phosphorylation ATP Acetyl CoA H2O Oxaloacetate Citrate Isocitrate CO2 Citric acid cycle NAD+ NADH + H+ Fumarate a-Ketoglutarate FADH2 NAD+ FAD Succinate GTP GDP Pi Succinyl CoA CO2 NADH + H+ ADP ATP 25 Krebs Cycle • The NADH and FADH2 produced by the cycle relay electrons extracted from food to the electron transport chain Glycolysis Citric acid cycle ATP ATP Oxidation phosphorylation ATP Acetyl CoA NADH + H+ H2O NAD+ Oxaloacetate Malate Citrate Isocitrate CO2 Citric acid cycle H2O NAD+ NADH + H+ Fumarate ATP/NADH Ledger + 2 ATP + 6 NADH + 2 FADH2 a-Ketoglutarate FADH2 NAD+ FAD Succinate GTP GDP Pi Succinyl CoA CO2 NADH + H+ ADP ATP 26 Krebs Cycle 27 ETC and Oxidative Phosphorylation • • Occurs along the inner mitochondrial membrane (IMM) in the cristae of the mitochondrion NADH/FADH2 molecules carry electrons from glycolysis and the citric acid cycle to the inner mitochondrial membrane, where they transfer electrons to a series of membrane-associated proteins. 28 The Pathway of Electron Transport • • Most of the chain’s components are proteins, which exist in multiprotein complexes The carriers alternate reduced and oxidized states as they accept and donate electrons Electrons drop in free energy as they go down the chain and are finally passed to O2, forming water NADH 50 FADH2 Free energy (G) relative to O2 (kcal/mol) • 40 FMN I Multiprotein complexes FAD Fe•S II Fe•S Q III Cyt b 30 Fe•S Cyt c1 IV Cyt c Cyt a Cyt a3 20 10 0 2 H+ + 1/2 O2 29 H2O The Pathway of Electron Transport • The electron transport chain generates no ATP The chain’s function is to break the large freeenergy drop from food to O2 into smaller steps that release energy in manageable amounts NADH 50 FADH2 Free energy (G) relative to O2 (kcal/mol) • 40 FMN I Multiprotein complexes FAD Fe•S II Fe•S Q III Cyt b 30 Fe•S Cyt c1 IV Cyt c Cyt a Cyt a3 20 10 0 2 H+ + 1/2 O2 30 H2O Electron Transport Phosphorylation • • • • • Electron transfer in the electron transport chain causes proteins to pump H+ from the mitochondrial matrix to the intermembrane space The ETC uses energy from electrons to pump H+ across a membrane against their concentration gradient - potential energy. H+ then moves back across the membrane, passing through channels in ATP synthase ATP synthase uses the exergonic flow of H+ to drive phosphorylation of ATP This is an example of chemiosmosis, the use of energy in a H+ gradient to drive cellular work 31 LE 9-15 Inner mitochondrial membrane Glycolysis Citric acid cycle ATP ATP Oxidative phosphorylation: electron transport and chemiosmosis ATP H+ H+ H+ H+ Intermembrane space Cyt c Protein complex of electron carriers Q IV III I ATP synthase II Inner mitochondrial membrane FADH2 NADH + H+ 2H+ + 1/2 O2 H2O FAD NAD+ Mitochondrial matrix ATP ADP + P i (carrying electrons from food) H+ Electron transport chain Electron transport and pumping of protons (H+), Which create an H+ gradient across the membrane Chemiosmosis ATP synthesis powered by the flow of H+ back across the membrane Oxidative phosphorylation 32 ATP • • • The energy stored in a H+ gradient across a membrane couples the redox reactions of the electron transport chain to ATP synthesis The H+ gradient is referred to as a proton-motive force, emphasizing its capacity to do work Intermembrane space H+ H+ H+ H+ H+ H+ H+ H+ Rotor Most of the ATP produced in cells is made by the enzyme ATP synthase Rod • • The enzyme is embedded in the membrane and provides a channel through which protons can cross the membrane down their concentration gradient The energy released causes the rotor and the rod structures to rotate. This mechanical energy is converted to chemical energy with the formation of ATP Catalytic head ADP + Pi ATP H+ Mitochondrial matrix 33 LE 9-14 INTERMEMBRANE SPACE H+ H+ H+ H+ H+ H+ A rotor within the membrane spins as shown when H+ flows past it down the H+ gradient. H+ A stator anchored in the membrane holds the knob stationary. A rod (or “stalk”) extending into the knob also spins, activating catalytic sites in the knob. H+ ADP + P ATP i MITOCHONDRAL MATRIX Three catalytic sites in the stationary knob join inorganic phosphate to ADP to make ATP. 34 Summary of Glucose Catabolism 35 Theoretical ATP Yield of Aerobic Respiration 36 Catabolism of Proteins and Fats • Proteins are utilized by deaminating their amino acids, and then metabolizing the product. • Fats are utilized by beta-oxidation. 37 Regulating Aerobic Respiration • Control of glucose catabolism occurs at two key points in the catabolic pathway. • Glycolysis - phosphofructokinase • Pyruvate Oxidation – pyruvate decarboxylase 38 Recycling NADH • • • As long as food molecules are available to be converted into glucose, a cell can produce ATP. Continual production creates NADH accumulation and NAD+ depletion. NADH must be recycled into NAD+. • Aerobic respiration - oxygen as electron acceptor • Fermentation - organic molecule 39 Anaerobic Respiration • • • • • • Final electron acceptor is an inorganic molecule other than oxygen Some use NO3 -: E. coli Some use SO42Important in nitrogen and sulfur cycles ATP varies, less than 38 Only part of Krebs cycle & ETC used 40 Fermentation • In some cases, the high energy electrons picked up by NAD+ during glycolysis are not donated to an ETC. • Instead, NADH donates its extra electrons and H+ directly to an organic molecule, which serves as the final electron acceptor. 41 Fermentation • • • • • • Pyruvate converted to organic product NAD+ regenerated Doesn’t require oxygen Does not use Krebs cycle or ETC Organic molecule is final electron acceptor Produces 2 ATP max 42 Alcohol Fermentation • Occurs in single-celled fungi called yeast • A terminal CO2 is removed from the pyruvic acid (3C) produced during glycolysis, producing acetaldehyde (2C) • Acetaldehyde accepts 2 e- and a H+ from NADH, producing ethanol and NAD+ 2Glucose ADP + 2 P i 2 ATP H G H C OH L CH3 Glucose Y Glycolysis 2 Ethanol 2 NAD+ C 2 ATP O 2 Pyruvate L 22 NADH Y + 2 NAD NADH 2 CO2 H + O H S +2H I C O O C CO2 S CH3 O C 2 Acetaldehyde CH3 2 Pyruvic Acid 2 ADP 2 Ethanol 2 Acetaldehyde 43 Alcohol fermentation Lactic Acid Fermentation • • Used by most animal cells when O2 is not available NADH donates 2 e- and a H+ directly to the pyruvate (3C) produced during glycolysis, producing lactate (3C) and NAD+ 2 ADP + 2 P i Glucose Glucose 2 ADP 2 ATP O - C O C O CH3 2 ATP O– G C O Glycolysis L H C OH Y CH3 C 2 Lactate 2 NAD+ O 2 NAD+ 2 NADH 2 CO2 L + +2H Y 2 NADH 2 Pyruvate S I S 2 Pyruvate 2 Lactate Lactic acid fermentation 44 Fermentation Alcoholic fermentation and lactic acid fermentation each generate 2 ATP / glucose molecule compared to the theoretical maximum of 36 ATP per glucose during aerobic respiration. 45 46 47 Extra Slides 48 Glycolysis ATP/NADH Ledger - 2 ATP The energy investment phase carbons Energy coupling ATP ADP + P: exergonic Glu Glu-6-P :endergonic 49 Glycolysis ATP/NADH Ledger -2ATP +2 NADH +2ATP The energy payoff phase Redox reactions Energy coupling 50 ATP/NADH Ledger Glycolysis -2ATP +2ATP +2ATP +2 NADH More energy coupling End-products of glycolysis are 2 pyruvate molecules 51 52 Flow of Energy in Living Things • • • Oxidation – Reduction • Oxidation occurs when an atom or molecule loses an electron. • Reduction occurs when an atom or molecule gains an electron. Redox reactions occur because every electron that is lost by an atom through oxidation is gained by some other atom through reduction. During redox reactions, H+ are often transferred along with the electrons. Loss of electron (oxidation) o A o B A e– B – + A* B* Gain of electron (reduction) Low energy High energy 53 Electron carriers • • • • • • Molecules that pick up electrons from substances being oxidized and donate them to substances being reduced. For example, during the breakdown of glucose : • Enzymes remove 2 H atoms (2p and 2e) from glucose • Both electrons and one proton are picked up by NAD+ to form NADH • The other proton is released as a hydrogen ion (H+) Oxidized Form NAD+ FAD NADP+ 2e- and 2H+ 2e- and 2H+ 2e- and 2H+ Reduced Form NADH + H+ FADH2 NADPH + H+ 54 Use of chemical cofactor (NAD+) Energy-rich molecule Enzyme H H NAD+ NAD+ 1. Enzymes that harvest hydrogen atoms have a binding site for NAD+ located near another binding site. NAD+ and an energy-rich molecule bind to the enzyme. H NAD+ 2. In an oxidationreduction reaction, a hydrogen atom is transferred to NAD+, forming NADH. Product NAD H NAD H 3. NADH then diffuses away and is available to other molecules. 55 Electron Transport • An electron transport chain (ETC) is a series of electron carriers that are embedded in a membrane and that pass electrons from one carrier to the next in a specific sequence: H+ H+ H+ C Q e– e– FADH2 NADH + H+ NAD+ FAD 2H+ + ½O2 H 2O 56 Electron Flow 57 Electron Transport • As electrons are passed from one carrier to the next in the ETC, some of their energy is released. This energy can be used to make ATP: Electrons from food e– High energy Energy for synthesis of ATP Electron transport chain Low energy e– Formation of water 58 ATP • Adenosine Triphosphate (ATP) is the energy currency of the cell. • Drives movement • Used in endergonic reactions 59 ATP - Energy Coupling • ATP hydrolysis can be coupled to other reactions Endergonic reaction: ∆G is positive, reaction is not spontaneous NH2 Glu + Glutamic acid NH3 Glu Ammonia Glutamine ∆G = +3.4 kcal/mol Exergonic reaction: ∆ G is negative, reaction is spontaneous ATP + H2O ADP + Coupled reactions: Overall ∆G is negative; together, reactions are spontaneous P ∆G = - 7.3 kcal/mol ∆G = –3.9 kcal/mol 60