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Cellular Respiration: Harvesting Chemical Energy Chapter 9 Assemble polymers, pump substances across membranes, move and reproduce The giant panda Obtains energy for its cells by eating plants which get their energy from the sun (sun in, heat out) Figure 9.1 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 Heat energy Catabolic pathways yield energy by oxidizing organic fuels The breakdown of organic molecules is exergonic Release of energy by the transfer of e- Organic molecules possess PE due to atom arrangements Compounds participating in exergonic reactions act as fuels Enzymes help degrade molecules to release PE One catabolic process, fermentation Is a partial degradation of sugars that occurs without oxygen Cellular respiration Is the most prevalent and efficient catabolic pathway Consumes oxygen and organic molecules such as glucose Yields ATP Aerobic Anaerobic (bacteria) To keep working Cells must regenerate ATP C6H12O6 +6O2 6CO2 + 6H2O + Energy (ATP +heat) Catabolic pathways yield energy Due to the transfer of electrons Redox reactions Transfer electrons from one reactant to another by oxidation and reduction In A substance loses electrons, or is oxidized In oxidation reduction A substance gains electrons, or is reduced Examples of redox reactions becomes oxidized (loses electron) Na + Cl Na+ + becomes reduced (gains electron) Cl– Some Products Reactants becomes oxidized + CH4 CO 2O2 + Energy 2 H2O becomes reduced O O O C O H H C + 2 O H H H Do not completely exchange electrons Change the degree of electron sharing in covalent bonds H redox reactions Methane (reducing agent) Figure 9.3 Oxygen (oxidizing agent) 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 Reducing Agent is the electron donor (sugar) Oxidizing Agent is the electron acceptor (oxygen) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Breaking down of Sugar (Redox Reactions) • Key Players – Dehydrogenase (enzyme strips H atoms) – NAD+ (coenzyme used to shuttle e-) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Players • Dehydrogenase removes a pair of H atoms (2 e-, 2 p) from glucose thus oxidizing it. • Dehydrogenase delivers 2e- and 1 p to NAD+ and the other proton is releases as H+ ion. NAD+ is now NADH (stores energy for later use) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings How do you get energy from NADH? • ETC – e- carry molecules in inner membrane of mitochondria – e- move down from one carrier to another through REDOX reactions – At the bottom, oxygen captures the e- with hydrogen to form water 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 – Steps slow the reaction and allow the controlled release of energy that can be used instead of a massive loss Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Electrons from organic compounds Are usually first transferred to NAD+, a coenzyme 2 e– + 2 H+ NAD+ Dehydrogenase O NH2 H C CH2 O O N+ Nicotinamide (oxidized form) Reduction of NAD+ + 2[H] (from food) Oxidation of NADH O– O P O H – O P O HO O H OH HO CH2 NH2 N N H O H HO N H OH N 2 e– + H+ H Figure 9.4 NADH H O C H N NH2 Nicotinamide (reduced form) + NADH, the reduced form of NAD+ Passes the electrons to the electron transport chain 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 If Figure 9.5 A Explosive release of heat and light energy H2O (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 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 (b) Cellular respiration 2 Respiration is a cumulative function of three metabolic stages Glycolysis The citric acid cycle Oxidative phosphorylation Glycolysis Occurs in the cytosol Breaks down glucose into two molecules of pyruvate The citric acid cycle (Krebs) Occurs in the mitochondria Completes the breakdown of glucose Releases CO2 Makes e- available for MASS production of ATP Oxidative phosphorylation Occurs in the mitochondra Is driven by the electron transport chain Generates ATP An overview of cellular respiration Electrons carried via NADH and FADH2 Electrons carried via NADH Glycolsis Pyruvate Glucose Cytosol ATP Figure 9.6 Substrate-level phosphorylation Citric acid cycle Oxidative phosphorylation: electron transport and chemiosmosis Mitochondrion 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 Product ATP Glycolysis pyruvate Glycolysis harvests energy by oxidizing glucose to Means “splitting of sugar” Breaks down glucose into pyruvate Occurs in the cytoplasm of the cell 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 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 H HH HO H HO OH H OH Glycolysis Glucose ATP 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 ATP 3 Phosphofructokinase ADP P O CH2 O CH2 O P HO H OH HO H Fructose1, 6-bisphosphate 4 Aldolase 5 H P O CH2 Isomerase C O C O CHOH CH2OH CH2 O P Figure 9.9 A Dihydroxyacetone phosphate 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 CH2 O P 1, 3-Bisphosphoglycerate 2 ADP 7 Phosphoglycerokinase 2 ATP O– 2 C CHOH CH2 O P 3-Phosphoglycerate 8 Phosphoglyceromutase 2 O– C O H C O P CH2OH 2-Phosphoglycerate 9 Enolase 2H O 2 2 O– C O C O P CH2 Phosphoenolpyruvate 2 ADP 10 Pyruvate kinase 2 ATP 2 O– C O C O Figure 9.8 B CH3 Pyruvate 2 ATP 2NADH The citric acid cycle completes the energy-yielding oxidation of organic molecules The citric acid cycle Takes place in the matrix of the mitochondrion Active transport takes pyruvate into the mitochondria 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 CH3 Acetyle CoA CO2 Coenzyme A C atom of carboxyl group removed and released as CO2 Remaining 2 C compound is oxidized to form acetate and NAD+=NADH Coenzyme A attaches to acetate=acetyl coA which has a high PE MITOCHONDRI ON CYTOSOL NAD H NAD+ O– + H+ S Co A C O 2 C C O O 1 3 CH3 Pyruvate Transport protein Figure 9.10 CO2 Coenzyme A CH3 Acetyle CoA Generation of Acetyl coA makes: 1 NADH for each pyruvate 2 NADH have now been made 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 3 NADH FAD + 3 H+ ADP + P i ATP Figure 9.11 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 8 Oxaloacetate HO C COO– COO– CH2 COO– HO CH H2O CH2 COO– NAD+ CH2 2 HC COO– COO– Malate Figure CH2 HO Citrate 9.12 Isocitrate COO– H2O CO2 Citric acid cycle 7 CH COO– 3 NAD+ COO– COO– CH Fumarate HC CH2 CoA SH 6 CoA SH COO– FAD CH2 CH2 C O COO– Succinate Pi S CoA GTP GDP Succinyl CoA ADP ATP Figure 9.12 C O COO– CH2 5 CH2 FADH2 4 COO– NAD+ NADH + H+ + H+ α-Ketoglutarate CH2 COO– NADH CO2 One cycle produces 3 NADH, 1 FADH2 and 1 ATP GO AROUND TWICE!!!! So that is a total of: 6 NADH, 2 FADH2 and 2 ATP Made of two parts: ETC Chemiosmosis Collection of molecules embedded in inner membrane of mitocondria In the electron transport chain Electrons from NADH and FADH2 lose energy in several steps At Electrons are passed to oxygen, forming water NO ATP is made but e- energy becomes manageable NADH 50 FADH2 Free energy (G) relative to O2 (kcl/mol) the end of the chain 40 I Fe•S 30 20 Multiprotein complexes FAD Fe•S II O III Cyt b Fe•S Cyt c1 IV Cyt c Cyt a Cyt a3 10 0 Figure 9.13 FMN 2 H + + 1⁄2 O2 H2 O Energy coupling mechanism Energy is stored in H+ gradient across a membrane It is used to drive the production of ATP ATPase is needed 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 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 The resulting H+ gradient Stores energy Drives chemiosmosis in ATP synthase Is referred to as a proton-motive force Chemiosmosis Inner Mitochondrial membrane Oxidative phosphorylation. electron transport and chemiosmosis Glycolysis ATP and the electron transport chain ATP ATP H+ H+ H+ Intermembrane space Protein complex of electron carners Q I Inner mitochondrial membrane Figure 9.15 IV III ATP synthase II FADH2 NADH+ Mitochondrial matrix H+ Cyt c NAD+ FAD+ 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 Net production of 36-38 ATP 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 2 Acetyl CoA + 2 ATP by substrate-level phosphorylation Maximum per glucose: Figure 9.16 6 NADH Citric acid cycle + 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 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 a small amount of ATP through fermentation Glycolysis Can produce ATP with or without oxygen, in aerobic or anaerobic conditions It is the ETC that requires oxygen (without it the e- are not pulled down the series of proteins and chemiosmosis fails) Glycolysis can couple with fermentation to produce ATP Fermentation consists of: Glycolysis plus reactions that regenerate NAD+, which can be reused by glyocolysis During lactic acid fermentation Pyruvate is reduced directly to NADH to form lactate as a waste product NAD+ is now free to accept e- and make 2 ATP from remaining sugar molecule (s) Lactate accumulates in muscles=sore and fatigued In alcohol fermentation Pyruvate is converted to ethanol in two steps, one of which releases CO2 2 ADP + 2 P1 2 ATP O– C O Glucose Glycolysis C O CH3 2 Pyruvate 2 NADH 2 NAD+ H 2 CO2 H C O H C OH 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+ C O H C OH CH3 2 Lactate Figure 9.17 (b) Lactic acid fermentation 2 NADH CH3 Both fermentation and cellular respiration Use glycolysis to oxidize glucose and other organic fuels to pyruvate Fermentation Differ in their final electron acceptor Cellular and cellular respiration respiration Produces more ATP 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 Acetyl CoA Citric acid cycle Catabolic pathways Funnel electrons from many kinds of organic molecules into cellular respiration The catabolism of various molecules from food Food isn’t just for energy Biosynthesis-making of cellular molecules for repair and growth Consumption of ATP