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Chapter 9 Cellular Respiration: Harvesting Chemical Energy PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The giant panda – Obtains energy for its cells by eating plants Figure 9.1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Energy – Flows into an ecosystem as sunlight and leaves as heat Light energy ECOSYSTEM Photosynthesis in chloroplasts Organic CO2 + H2O + O2 Cellular molecules respiration in mitochondria ATP powers most cellular work Figure 9.2 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Heat energy • Concept 9.1: Catabolic pathways yield energy by oxidizing organic fuels Catabolic Pathways and Production of ATP • The breakdown of organic molecules is exergonic Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • One catabolic process, fermentation – Is a partial degradation of sugars that occurs without oxygen Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Cellular respiration – Is the most prevalent and efficient catabolic pathway – Consumes oxygen and organic molecules such as glucose – Yields ATP Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • To keep working – Cells must regenerate ATP Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Redox Reactions: Oxidation and Reduction • How do the catabolic pathways that decompose fuels yield energy? • the transfer of electrons – the relocation of electrons releases energy stored in organic molecules, which is ultimately used to synthesize ATP. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Principle of Redox • Redox reactions – Transfer electrons from one reactant to another by oxidation and reduction • In oxidation – A substance loses electrons, or is oxidized • In reduction – A substance gains electrons, or is reduced Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Examples of redox reactions becomes oxidized (loses electron) Na + Reducing agent Cl Oxidizing agent Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Na+ + becomes reduced (gains electron) Cl– • Some redox reactions – Do not completely exchange electrons – Change the degree of electron sharing in covalent bonds Products Reactants becomes oxidized + CH4 CO 2O2 + Energy 2 H2O becomes reduced O O C O H O O H H H C + 2 H H Methane (reducing agent) Oxygen (oxidizing agent) Figure 9.3 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Carbon dioxide Water Oxidation of Organic Fuel Molecules During Cellular Respiration • During cellular respiration – Glucose is oxidized and oxygen is reduced becomes oxidized C6H12O6 + 6O2 6CO2 + 6H2O + Energy becomes reduced • Oxygen is so electronegative, one of the most potent oxidizing agents • Hydrogen is transferred from glucose to oxygen - electrons are transferred to a lower energy state. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Stepwise Energy Harvest via NAD+ and the Electron Transport Chain • Cellular respiration – Oxidizes glucose in a series of steps, not a single explosive step – Electrons travels with a proton as a hydrogen atom – The hydrogen atoms are not transferred directly to oxygen, but instead are passed first to an electron carrier, a coenzyme called NAD (nicotinamide adenine dinucleotide) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Electrons from organic compounds – Are usually first transferred to NAD+, a coenzyme, an electron acceptor 2 e– + 2 H+ NAD+ Dehydrogenase O NH2 H C CH2 O O– O O P O H – O P O HO O N+ Nicotinamide (oxidized form) H OH HO CH2 H HO H OH (Nicotinamide adenine dinucleotide) N H N Reduction of NAD+ + 2[H] (from food) Oxidation of NADH N H Figure 9.4 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings N H+ NADH H O C H NH2 N O 2 e– + H+ NH2 Nicotinamide (reduced form) + H+ • How NAD+ trap electrons from glucose - Dehydrogenase remove 2 hydrogen atoms (2 electrons and 2 protons) from substrate, but delivers the 2 electrons along with 1 proton to its coenzyme, NAD+ • H-C-OH + NAD+ Dehydrogenase C=O + NADH + H+ • NADH, the reduced form of NAD+ – Passes the electrons to the electron transport chain Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • If electron transfer is not stepwise – A large release of energy occurs – As in the reaction of hydrogen and oxygen to form water Free energy, G H2 + 1/2 O2 Figure 9.5 A Explosive release of heat and light energy H2O Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (a) Uncontrolled reaction • The electron transport chain – Passes electrons in a series of steps instead of in one explosive reaction – Uses the energy from the electron transfer to form ATP Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2H 1/ + 2 O2 1/ O2 (from food via NADH) Free energy, G 2 H+ + 2 e– Controlled release of energy for synthesis of ATP ATP ATP ATP 2 e– 2 H+ H2O Figure 9.5 B Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (b) Cellular respiration 2 The Stages of Cellular Respiration: A Preview • Respiration is a cumulative function of three metabolic stages – Glycolysis – The citric acid cycle – Oxidative phosphorylation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Glycolysis – Breaks down glucose into two molecules of pyruvate • The citric acid cycle – Completes the breakdown of glucose • Oxidative phosphorylation – Is driven by the electron transport chain – Generates ATP Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • An overview of cellular respiration Electrons carried via NADH and FADH2 Electrons carried via NADH Citric acid cycle Glycolsis Pyruvate Glucose Cytosol Mitochondrion ATP Figure 9.6 Oxidative phosphorylation: electron transport and chemiosmosis Substrate-level phosphorylation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings ATP Substrate-level phosphorylation ATP Oxidative phosphorylation • Both glycolysis and the citric acid cycle – Can generate ATP by substrate-level phosphorylation Enzyme Enzyme ADP P Substrate + Figure 9.7 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Product ATP • Concept 9.2: 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Glycolysis consists of two major phases – Energy investment phase – Energy payoff phase Citric acid cycle Glycolysis Oxidative phosphorylation ATP ATP ATP Energy investment phase Glucose 2 ATP + 2 P 2 ATP used Energy payoff phase 4 ADP + 4 P 2 NAD+ + 4 e- + 4 H + 4 ATP formed 2 NADH + 2 H+ 2 Pyruvate + 2 H2O Glucose 4 ATP formed – 2 ATP used Figure 9.8 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2 NAD+ + 4 e– + 4 H + 2 Pyruvate + 2 H2O 2 ATP + 2 H+ 2 NADH • A closer look at the energy investment phase CH2OH HH H HO H HO OH H OH Glycolysis Glucose ATP Phosphorylation trap sugar in the cell and make glucose more reactive 1 Hexokinase ADP CH2OH P HH OH OH H HO H OH Glucose-6-phosphate 2 Phosphoglucoisomerase CH2O P O CH2OH H HO HO H HO H Fructose-6-phosphate Key step for regulation; allosterically regulated by ATP and its product ATP 3 Phosphofructokinase ADP P O CH2 O CH2 O P HO H OH HO H Fructose1, 6-bisphosphate 4 Aldolase Only glyceraldehyde-3-phosphate are used. Cleavage of six-carbon sugar into two G3P Figure 9.9 A 5 H P O CH2 Isomerase C O C O CHOH CH2OH CH2 O P Dihydroxyacetone phosphate Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Glyceraldehyde3-phosphate Citric Oxidative acid cycle phosphorylation • A closer look at the energy payoff phase 6 Triose phosphate dehydrogenase 2 NAD+ 2 Pi 2 NADH + 2 H+ 2 P O C O CHOH Carbonyl group of a sugar is oxidized to a carboxyl group. Substrate-level phosphorylation CH2 O P 1, 3-Bisphosphoglycerate 2 ADP 7 Phosphoglycerokinase 2 ATP O– 2 C CHOH CH2 O P 3-Phosphoglycerate 8 Phosphoglyceromutase Relocation of the remaining phosphate group 2 O– C O H C O Make a double bond by extracting a water molecule P CH2OH 2-Phosphoglycerate 9 Enolase 2H O 2 2 O– C O C O P CH2 Phosphoenolpyruvate 2 ADP 10 Pyruvate kinase Second substrate-level phosphorylation Transfer of phosphate from PEP to ADP. 2NADH : oxidative phosphorylation Pyruvate : citric acid cycle with O2, fermentation without O2 2 ATP 2 O– C O C O Figure 9.8 B Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings CH3 Pyruvate Phosphate ion in cytosol • Concept 9.3: The citric acid cycle completes the energy-yielding oxidation of organic molecules • The citric acid cycle – Takes place in the matrix of the mitochondrion – In prokaryotic cells, In the cytosol Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Before the citric acid cycle can begin – Pyruvate must first be converted to acetyl CoA, which links the cycle to glycolysis – Fully oxidized carboxyl group is removed as a CO2 – The remaining two-carbon fragment is oxidized to acetate. The extracted electrons are transferred to NAD+, forming NADH. – Coenzyme A derived from vitamin B is attached to the actetate by an unstable bond that make the acetyl group very reactive. High potential energy. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings CYTOSOL MITOCHONDRION NAD+ NADH + H+ O– S CoA C O 2 C C O O 1 3 CH3 Pyruvate Transport protein Figure 9.10 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings CH3 Acetyle CoA CO2 Coenzyme A • An overview of the citric acid cycle Pyruvate (from glycolysis, 2 molecules per glucose) Glycolysis Citric acid cycle ATP ATP Oxidative phosphorylatio n ATP CO2 CoA NADH + 3 H+ Acetyle CoA CoA CoA Citric acid cycle 2 CO2 3 NAD+ FADH2 FAD 3 NADH + 3 H+ ADP + P i ATP Figure 9.11 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • A closer look at the citric acid cycle Glycolysis Citric Oxidative acid phosphorylation cycle S CoA C O CH3 Acetyl CoA CoA SH O NADH + H+ C COO– COO– 1 CH2 COO– NAD+ 8 Oxaloacetate HO C COO– COO– CH2 COO– HO CH H2O CH2 CH2 2 HC COO– COO– Malate Figure CH2 HO Citrate 9.12 COO– Isocitrate COO– COO– CH CO2 Citric acid cycle 7 H2O 3 NAD+ COO– Fumarate HC CH2 CoA SH 6 CoA SH COO– FAD CH2 CH2 COO– C O Succinate 4 NADH + H+ C O COO– CH2 5 CH2 FADH2 COO– Isocitrate is oxidized, reducing NAD+ to NADH. a-Ketoglutarate CH2 COO– Two hydrogens are transferred to FAD, forming fADH2 and oxidizing succinate. CH Pi S CoA GTP GDP Succinyl NAD+ CO2 NADH + H+ CoA ADP ATP Figure 9.12 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings CoA is displaced by a phosphate group, which transferred to GDP, forming GTP. • Concept 9.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis • Glycolysis and the citric acid cycle produce only 4 ATP molecules per glucose, all by substrate-level phosphorylation: 2 net ATP from glycolysis and 2 ATP from the citric acid cycle. • NADH and FADH2 – Donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Pathway of Electron Transport • In the electron transport chain – • Electrons from NADH and FADH2 lose energy in several steps At the end of the chain – Electrons are passed to oxygen molecule not oxygen atoms, forming water • Electron transport chain is embedded in the inner membrane of the mitochondrion forming multiprotein complexes (I – IV). • Flavin mononucleotide (FMN), iron-sulfur protein, ubiquinone (Q, coenzyme Q, mobile within the membrane) • Cytochromes contain a heme group which has an iron atom that accepts and donates electrons, similar to the heme group in hemoglobin. Depending on heme groups, different cytochromes! Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • One third less ATP synthesis 5 0 NAD H 2 e– NAD+ FADH 2 4 0 FM N Fe• S Q 2 e– One third less ATP synthesis FA D Multiprotein complexes FA D Fe• S 3 0 2 0 1 0 0 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cyt b Fe• S Cyt c1 IV Cyt c Cyt a Cyt a3 2 e– (from NADH or FADH2) 2 H+ + 1/ 2 O2 H2 O Chemiosmosis: The Energy-Coupling Mechanism • ATP synthase is the enzyme that actually makes ATP • Driving power is a difference in the concentration of H+ Four parts; Rotor, stator, internal rod, catalytic knob INTERMEMBRANE SPACE H+ Stator In mitochondrial and chloroplast membranes, plasma membranes of prokaryotes Rotor Internal rod Catalytic knob ADP + Pi Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings ATP MITOCHONDRIAL MATRIX Is the rotation of the internal rod in ATP synthase reponsible for ATP synthesis? EXPERIMENT Magnetic bead Electromagnet Sample Internal rod Catalytic knob Nickel plate RESULTS Number of photons detected (103) Rotation in one direction Rotation in opposite direction No rotation 30 25 The direction of rotation of one part of the protein complex is responsible for either ATP synthesis or ATP hydrolysis 20 The smallest rotatory motor known in nature 0 Sequential trials Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings How does the inner membrane generate the H+ gradient ? • Along the electron transport chain – Electron transfer causes protein complexes to pump H+ from the mitochondrial matrix to the intermembrane space • The resulting H+ gradient – Stores energy – Drives chemiosmosis in ATP synthase – Is referred to as a proton-motive force Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Chemiosmosis and the electron transport chain Oxidative phosphorylation. electron transport and chemiosmosis Glycolysis ATP Inner Mitochondrial membrane ATP ATP H+ H+ H+ Intermembrane space Protein complex of electron carners Q I Inner mitochondrial membrane IV III ATP synthase II FADH2 NADH+ Mitochondrial matrix H+ Cyt c FAD+ NAD+ 2 H+ + 1/2 O2 H2O ADP + (Carrying electrons from, food) ATP Pi H+ Chemiosmosis Electron transport chain + ATP synthesis powered by the flow Electron transport and pumping of protons (H ), + + which create an H gradient across the membrane Of H back across the membrane Figure 9.15 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Oxidative phosphorylation • The chain is an energy converter that uses the exergonic flow of electrons from NADH and FADH2 to pump H+ from the mitochondrial matrix to the intermembrane space • The proton-motive force drives H+ back across the membrane through the H+ channels provided by ATP synthases. • Chloroplast in photosysntesis, light drive electron flow • Prokaryotes generate H+ gradient acrosss their plasma membrane • Chemiosmosis (peter Mitchell, Nobel prize in 1978) – Is an energy-coupling mechanism that uses energy in the form of a H+ gradient across a membrane to drive cellular work Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings An Accounting of ATP Production by Cellular Respiration • During respiration, most energy flows in this sequence – Glucose to NADH to electron transport chain to proton-motive force to ATP Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • There are three main processes in this metabolic enterprise Electron shuttles span membrane CYTOSOL MITOCHONDRION 2 NADH or 2 FADH2 2 NADH 2 NADH Glycolysis Glucose 2 Pyruvate 6 NADH Citric acid cycle 2 Acetyl CoA + 2 ATP by substrate-level phosphorylation Maximum per glucose: + 2 ATP 2 FADH2 Oxidative phosphorylation: electron transport and chemiosmosis + about 32 or 34 ATP by substrate-level by oxidative phosphorylation, depending on which shuttle transports electrons phosphorylation from NADH in cytosol About 36 or 38 ATP Figure 9.16 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • About 40% of the energy in a glucose molecule – Is transferred to ATP during cellular respiration, making approximately 38 ATP Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Concept 9.5: Fermentation enables some cells to produce ATP without the use of oxygen • Cellular respiration – Relies on oxygen to produce ATP • In the absence of oxygen – Cells can still produce ATP through fermentation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Glycolysis – Can produce ATP with or without oxygen, in aerobic or anaerobic conditions – Couples with fermentation to produce ATP Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Types of Fermentation • Fermentation consists of – Glycolysis plus reactions that regenerate NAD+, which can be reused by glyocolysis Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In alcohol fermentation – Pyruvate is converted to ethanol in two steps, one of which releases CO2 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • During lactic acid fermentation – Pyruvate is reduced directly to NADH to form lactate as a waste product Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2 ADP + 2 P1 2 ATP O– C O Glucose Glycolysis C O CH3 2 Pyruvate 2 NADH 2 NAD+ H 2 CO2 H H C OH C O CH3 CH3 2 Ethanol 2 Acetaldehyde (a) Alcohol fermentation 2 ADP + 2 Glucose P1 2 ATP Glycolysis O– C O C O O 2 NAD+ 2 NADH C O H C OH CH3 2 Lactate Figure 9.17 (b) Lactic acid fermentation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings CH3 Fermentation and Cellular Respiration Compared • Both fermentation and cellular respiration – Use glycolysis to oxidize glucose and other organic fuels to pyruvate Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Fermentation and cellular respiration – Differ in their final electron acceptor • Cellular respiration – Produces more ATP Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Pyruvate is a key juncture in catabolism Glucose CYTOSOL Pyruvate No O2 present Fermentation O2 present Cellular respiration MITOCHONDRION Ethanol or lactate Figure 9.18 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Acetyl CoA Citric acid cycle The Evolutionary Significance of Glycolysis • Glycolysis – Occurs in nearly all organisms – Probably evolved in ancient prokaryotes before there was oxygen in the atmosphere Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Concept 9.6: Glycolysis and the citric acid cycle connect to many other metabolic pathways Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Versatility of Catabolism • Catabolic pathways – Funnel electrons from many kinds of organic molecules into cellular respiration Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The catabolism of various molecules from food Proteins Carbohydrates Amino acids Sugars Fats Glycerol Glycolysis Glucose Glyceraldehyde-3- P NH3 Pyruvate Acetyl CoA Citric acid cycle Figure 9.19 Oxidative phosphorylation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fatty acids Biosynthesis (Anabolic Pathways) • The body – Uses small molecules to build other substances • These small molecules – May come directly from food or through glycolysis or the citric acid cycle Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Regulation of Cellular Respiration via Feedback Mechanisms • Cellular respiration – Is controlled by allosteric enzymes at key points in glycolysis and the citric acid cycle Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The control of cellular respiration Glucose Glycolysis Fructose-6-phosphate – Inhibits AMP Stimulates + Phosphofructokinase – Fructose-1,6-bisphosphate Inhibits Pyruvate Citrate ATP Acetyl CoA Citric acid cycle Figure 9.20 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Oxidative phosphorylation