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Chapter 8 • Overview: The Energy of Life • The living cell – Is a miniature factory where thousands of reactions occur – Converts energy in many ways Figure 8.1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Metabolism – Is the totality of an organism’s chemical reactions – Arises from interactions between molecules Metabolism= Catabolism + Anabolism Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • At maximum stability – The system is at equilibrium Not Stable • More free energy (higher G) • Less stable • Greater work capacity In a spontaneously change • The free energy of the system decreases (∆G<0) • The system becomes more stable • The released free energy can be harnessed to do work Yeah! . More STABLE • Less free energy (lower G) • More stable • Less work capacity (a) Gravitational motion. Objects move spontaneously from a higher altitude to a lower one. Figure 8.5 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (b) Diffusion. Molecules in a drop of dye diffuse until they are randomly dispersed. (c) Chemical reaction. In a cell, a sugar molecule is broken down into simpler molecules. • An analogy for cellular respiration Not stable ∆G < 0 ∆G < 0 ∆G < 0 Stable Figure 8.7 (c) A multistep open hydroelectric system. Cellular respiration is analogous to this system: Glucose is broken down in a series of exergonic reactions that power the work of the cell. The product of each reaction becomes the reactant for the next, so no reaction reaches equilibrium. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Structure and Hydrolysis of ATP • ATP (adenosine triphosphate) – Is the cell’s energy shuttle – Provides energy for cellular functions Adenine N O O -O O - O - O O C C N HC O O O NH2 - Phosphate groups Figure 8.8 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings N CH2 O H N H H H OH CH C OH Ribose • Energy is released from ATP – When the terminal phosphate bond is broken P P P Adenosine triphosphate (ATP) H2O P i + Figure 8.9 Inorganic phosphate P P Adenosine diphosphate (ADP) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Energy • 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 Figure 8.10 + H2O ADP + Coupled reactions: Overall ∆G is negative; together, reactions are spontaneous Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings P ∆G = + 7.3 kcal/mol ∆G = –3.9 kcal/mol • The three types of cellular work – Are powered by the hydrolysis of ATP P i P Motor protein Protein moved (a) Mechanical work: ATP phosphorylates motor proteins Membrane protein ADP + ATP P P Solute P i Solute transported (b) Transport work: ATP phosphorylates transport proteins P Glu + NH3 Reactants: Glutamic acid and ammonia Figure 8.11 NH2 Glu + P i Product (glutamine) made (c) Chemical work: ATP phosphorylates key reactants Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings i Organization of the Chemistry of Life into Metabolic Pathways • A metabolic pathway has many steps – That begin with a specific molecule and end with a product – That are each catalyzed by a specific enzyme Enzyme 1 A Enzyme 2 D C B Reaction 1 Enzyme 3 Reaction 2 Starting molecule Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Reaction 3 Product • Catabolic pathways – Break down complex molecules into simpler compounds – Exergonic reaction - Release energy Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Anabolic pathways – Build complicated molecules from simpler ones – Consume energy Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Forms of Energy • Energy – Is the capacity to cause change – Exists in various forms, of which some can perform work Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Kinetic energy – Is the energy associated with motion • Potential energy – Is stored in the location of matter – Includes chemical energy stored in molecular structure Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chapter 9 Cellular Respiration: Harvesting Chemical Energy Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Overview: Life Is Work • Living cells – Require transfusions of energy from outside sources to perform their many tasks 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 heatLight energy ECOSYSTEM Photosynthesis in chloroplasts Organic CO2 + H2O + O2 Cellular molecules respiration in mitochondria ATP powers most cellular work Figure 9.2 Heat energy Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Concept 9.1: Catabolic pathways yield energy by oxidizing organic fuels Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Catabolic Pathways and Production of ATP • The breakdown of organic molecules is exergonic- releases energy 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 Redox Reactions: Oxidation and Reduction • Catabolic pathways yield energy – Due to the transfer of electrons from high potential energy (i.e. in animal cell from Glucose) to – Low potential energy (electron acceptor usually an electron carrier, but ultimately delivered to oxygen in the mitochondria) 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • 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 + Cl Na+ + becomes reduced (gains electron) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cl– 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 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Electrons from organic compounds – Are usually first transferred to NAD+, a coenzyme- electron carrier 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 N H O H HO N H OH Reduction of NAD+ + 2[H] (from food) Oxidation of NADH NH2 N N 2 e– + H+ H Figure 9.4 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings NADH H O C H N NH2 Nicotinamide (reduced form) + • NADH, the reduced form of NAD+ – Passes the electrons to the electron transport chain or system (ETC also known as the ETS) in the Mitochondria Intermembrane space MATRIX 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 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Oxidative phosphorylation – Is driven by the electron transport chain that occurs in the Mitochondria – Generates ATP Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • A animal cell Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Mitochondria are enclosed by two membranes – A smooth outer membrane – An inner membrane folded into cristae Mitochondrion Intermembrane space Outer membrane Free ribosomes in the mitochondrial matrix Inner membrane Cristae Matrix Figure 6.17 Mitochondrial DNA Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 100 µm • 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings CH2OH H H H HO HO Glycolysis H OH H OH Glucose ATP 1 Hexokinase ADP CH2OH H H HO P H O H OH H OH Glucose-6-phosphate 2 Phosphoglucoisomerase CH2O H H P O CH2OH HO energy investment phase HO H HO Fructose-6-phosphate 3 ATP Phosphofructokinase ADP CH2 O P O CH2 P O HO H OH H HO Fructose1, 6-bisphosphate 4 Aldolase 5 P O CH2 C Isomerase O CH2OH Dihydroxyacetone phosphate Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings H C O CHOH CH2 O Glyceraldehyde3-phosphate P Citric acid cycle 6 2 NAD+ 2 Triose phosphate dehydrogenase 2 NADH + 2 H+ Pi payoff phase 2 P O C O CHOH CH2 1, 3-Bisphosphoglycerate 2 ADP P O 7 Phosphoglycerokinase 2 ATP O– 2 C CHOH O CH2 3-Phosphoglycerate P 8 Phosphoglyceromutase O– 2 C C H O P O CH2OH 2-Phosphoglycerate 9 Enolase 2 H2 O O– 2 C O C P O CH2 Phosphoenolpyruvate 2 ADP 10 Pyruvate kinase 2 ATP 2 O– C C Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings CH3 Pyruvate O O • 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 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 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 Citric Acid cycle Citric acid cycle Glycolysis Oxidative phosphorylation S CoA C O CH3 Acetyl CoA CoA O NADH + H+ SH COO– C COO– 1 CH2 COO– NAD+ Oxaloacetate 8 CH2 2 CH2 CH Malate HO CH Citrate COO– Isocitrate Figure 9.12 COO– CO2 Citric acid cycle 7 COO– HC COO– CH2 H2O COO– COO– C HO COO– HO H2O CH2 3 NAD+ COO– NADH COO– CH Fumarate HC CoA + H+ CH2 SH a-Ketoglutarate CH2 COO– 6 CoA COO– FAD 4 SH CH2 CH2 CH2 COO– C Succinate S Pi GTP ADP ATP 9.12 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings O COO– COO– 5 CH2 FADH2 C GDP NAD+ O CoA Succinyl CoA NADH + H+ CO2 • Concept 9.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis • 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • At the end of the chain – Electrons are passed to oxygen, forming water NADH 50 Free energy (G) relative to O2 (kcl/mol) FADH2 40 FMN I Fe•S Fe•S II O 30 Multiprotein complexes FAD III Cyt b Fe•S 20 Cyt c1 IV Cyt c Cyt a Cyt a3 10 0 Figure 9.13 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2 H + + 12 O2 H2 O Chemiosmosis: The Energy-Coupling Mechanism • ATP synthase – Is the enzyme that actually makes ATP INTERMEMBRANE SPACE H+ H+ H+ H+ H+ H+ H+ A rotor within the membrane spins clockwise when H+ flows past it down the H+ gradient. A stator anchored in the membrane holds the knob stationary. H+ ADP + Pi Figure 9.14 MITOCHONDRIAL MATRIX Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings ATP A rod (for “stalk”) extending into the knob also spins, activating catalytic sites in the knob. Three catalytic sites in the stationary knob join inorganic Phosphate to ADP to make ATP. • At certain steps along the electron transport chain – Electron transfer causes protein complexes to pump H+ from the mitochondrial matrix to the intermembrane space Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • 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 – 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 • 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 Oxidative phosphorylation 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 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 P1 2 ADP + 2 Glucose 2 ATP Glycolysis O– C O C O CH3 2 Pyruvate 2 NADH 2 NAD+ H H 2 H C C OH CH3 O CH3 2 Ethanol 2 Acetaldehyde (a) Alcohol fermentation P1 2 ADP + 2 Glucose O– Glycolysis 2 NAD+ O C H 2 ATP O OH C CH3 2 Lactate (b) Lactic acid fermentation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2 NADH C O C O CH3 CO2 • 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Acetyl CoA Citric acid cycle • 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 Oxidative phosphorylation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fatty acids • 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 Oxidative phosphorylation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings