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Chapter 9 – Cellular Respiration • Overview: Life Is Work • Living cells – Require transfusions of energy from outside sources to perform their many tasks • Assemble polymers • Pump substances against the membrane • Move • Reproduce Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cellular Respiration Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Basics of Respiration Biologists define respiration as the aerobic harvesting of energy from food molecules by cells Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Concept 9.1: Catabolic pathways yield energy by oxidizing organic fuels – Cells break down complex organic molecules (rich in potential energy) to simpler waste products (low in potential energy) • To get energy to do work • Rest is given off as heat Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Cells do this in two ways: 1) Fermentation 2) Cellular respiration Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 1) One catabolic process, fermentation – Is a partial degradation of sugars that occurs without oxygen Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2) Cellular respiration – Most prevalent and efficient catabolic pathway – Consumes oxygen and organic molecules such as glucose – Yields more ATP Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The equation for the reaction of cellular respiration: C6H12O6 + 6O2 6CO2 + 6H2O + Energy (ATP & Heat) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Catabolic Pathways and Production of ATP • The breakdown of organic molecules is exergonic – But catabolic pathways don’t do work directly • Just release energy so work can be done Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Redox Reactions: Oxidation and Reduction • Catabolic pathways yield energy – Due to the transfer of electrons – When 1 or more electrons transfers from one reactant to another the process is called oxidation reduction reaction or redox for short Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Principle of Redox • Oxidation – loss of an electron – The substance that gives up its electron is the reducing agent • Reduction the gain of an electron – The substance that gains the electron is the oxidizing agent Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Examples of redox reactions becomes oxidized (loses electron) Reducing agent Na + Cl Na+ becomes reduced (gains electron) Oxidizing agent Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings + Cl– • Specifically in cellular respiration: becomes oxidized C6H12O6 + 6O2 6CO2 + 6H2O + Energy becomes reduced • Glucose is oxidized and oxygen is reduced • Electrons lose potential energy and energy is released Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings To Sum Up • Remember an electron loses potential energy when it shifts from a less electronegative atom to a more electronegative atom (oxygen is very electronegative) • So by oxidizing glucose, respiration frees stored energy (in glucose) and makes it available for work (as ATP) 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 • Because energy can’t be released all at once and still be useful Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Stepwise Energy Harvest • Glucose is broken down in a series of steps with the help of enzymes – First an electron is taken away from a glucose molecule. But a proton goes with the electron, so in effect a Hydrogen atom was removed – Second it is picked up by a coenzyme NAD+ which will act as an oxidizing agent • NAD+ is reduced to NADH Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Stepwise Energy Harvest – Second it is picked up by a coenzyme NAD+ which will act as an oxidizing agent • NAD+ is reduced to NADH • Electrons lose little potential energy going from food to NAD+ • NADH molecule now represents stored energy Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Stepwise Energy Harvest – Third NADH will release their electrons down a series of reactions with oxygen being the final electron acceptor (or receptor) • This is known as the Electron Transport Chain and uses the energy from the electron transfer to form ATP Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings To Sum Up: • The electrons removed from food by NAD+ fall down an energy gradient in the ETC to a far more stable location in the electronegative oxygen atom Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Stages of Cellular Respiration: A Preview – Glycolysis – The citric acid cycle – Oxidative phosphorylation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The first two stages are catabolic pathways that decompose glucose 1) Glycolysis – Occurs in the cytosol – Breaks down glucose into two molecules of pyruvate 2) The citric acid cycle – Occurs in the mitochondrial matrix – Completes the breakdown of glucose Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Remember, why do we do this? • Goal of cellular respiration is ATP production 1) Oxidative phosphorylation - redox rxns of the ETC 2) Substrate level phosphorylation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Substrate level phosphorylation – ATP is formed directly • Enzymes transfer a phosphate group from a substrate molecule to an ADP (adenosine diphosphate) – Generates ATP directly Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • 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 • 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 Glycolysis • Concept 9.2: Glycolysis harvests energy by oxidizing glucose to pyruvate • Glycolysis – Breaks down 1 glucose into 2 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 Citric acid cycle Glycolysis ATP ATP – Energy payoff phase Oxidative phosphorylation 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 – Be thinking: • What do I start with? • What do I add in to this? • What do I get out of this? • Where does this occur? Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • A closer look at the energy payoff phase – Be thinking: • What do I start with? • What do I add in to this? • What do I get out of this? • Where does this occur? Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Citric Acid Cycle • 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 glycolysis to the citric acid cycle CYTOSOL MITOCHONDRION NAD+ NADH + H+ O– S CoA C O 2 This only happens if oxygen is present! 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 1) The carboxyl group is removed (it is fully oxidized so it has little chemical energy left) – First step where CO2 is released 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 2) Electrons are removed and transferred to NAD+ which stores them as NADH 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 3) Coenzyme A attaches via an unstable bond, so compound is now highly reactive 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 – Be thinking: • What do I start with? • What do I add in to this? • What do I get out of this? • Where does this occur? Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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– H2O COO– CH CO2 Citric acid cycle 7 3 NAD+ COO– Fumarate HC CH CH2 CoA SH 6 CoA SH COO– FAD CH2 CH2 COO– C O Succinate Pi S CoA GTP GDP Succinyl CoA ADP ATP Figure 9.12 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 4 C O COO– CH2 5 CH2 FADH2 COO– NAD+ NADH + H+ + H+ a-Ketoglutarate CH2 COO– NADH CO2 Oxidative Phosphorylation . . . ETC • Concept 9.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis • So far, only 4 ATP have been made Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Pathway of Electron Transport • Electron Transport Chain – is a collection of molecules embedded in the inner membrane of the mitochondria – Most components are proteins Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • An NADH molecule begins the process by “dropping off” its electron at the first electron carrier molecule NADH 50 Free energy (G) relative to O2 (kcl/mol) FADH2 40 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 NADH 50 FADH2 Free energy (G) relative to O2 (kcl/mol) • Remember: each component will be reduced when it accepts the electron and oxidized when it passes the electron down to the more electronegative carrier molecule in the chain 40 FMN I Fe•S Multiprotein complexes FAD Fe•S II O III Cyt b 30 Fe•S Cyt c1 IV Cyt c Cyt a Cyt a3 20 10 0 Figure 9.13 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2 H + + 12 O2 H2 O • Finally the electron is passed to oxygen, which is very electronegative. NADH 50 The oxygen also picks up 2 H+ ions from the aqueous solution and forms water Free energy (G) relative to O2 (kcl/mol) FADH2 40 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings I FAD Fe•S Fe•S II Multiprotein complexes O III Cyt b 30 Fe•S Cyt c1 IV Cyt c Cyt a Cyt a3 20 10 0 Figure 9.13 FMN 2 H + + 12 O2 H2 O • FADH goes through mostly the same processes, except it drops off its electron at a lower point on the ETC NADH 50 Free energy (G) relative to O2 (kcl/mol) FADH2 40 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings I FAD Fe•S Fe•S II O 30 Multiprotein complexes III Cyt b Fe•S Cyt c1 IV Cyt c Cyt a Cyt a3 20 10 0 Figure 9.13 FMN 2 H + + 12 O2 H2 O WARNING: The ETC makes no ATP directly! The ETC releases energy in a step-wise series of reactions. It powers ATP synthesis via oxidative phosphorylation. But it needs to be coupled with chemiosmosis to actually make ATP. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chemiosmosis: The Energy-Coupling Mechanism • Inner membrane of mitochondria has many protein complexes called ATP synthase – ATP synthase – enzyme that makes ATP from ADP and inorganic phosphate • It uses the energy of an existing gradient to do this. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The existing gradient is the difference in H+ ion concentration on opposite sides of the inner membrane of the mitochondria Oxidative phosphorylation. electron transport and chemiosmosis Glycolysis ATP Inner Mitochondrial membrane ATP ATP H+ H+ H+ Intermembrane space Q I Inner mitochondrial membrane Mitochondrial matrix Figure 9.15 H+ Cyt c Protein complex of electron carners IV III II FADH2 NADH+ FAD+ NAD+ (Carrying electrons from, food) 2 H+ + 1/2 O2 ATP synthase H2O ADP + ATP Pi H+ Chemiosmosis Electron transport chain Electron transport and pumping of protons (H+), ATP synthesis powered by the flow 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 So what is this process called? • Chemiosmosis – the process in which energy stored in the form of a hydrogen ion gradient across a membrane is used to drive cellular work (like the synthesis of ATP) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • It is the job of the ETC to create this H+ ion gradient Oxidative phosphorylation. electron transport and chemiosmosis Glycolysis ATP ATP ATP H+ H+ H+ Q I Mitochondrial matrix H+ Cyt c Protein complex Intermembrane of electron space carners Inner mitochondrial membrane Inner Mitochondria membrane IV III II FADH2 NADH+ NAD+ FAD+ 2 H+ + 1/2 O2 ATP synthase 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+), Of H+ back across the membrane which create an H+ gradient across the membrane Figure 9.15 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Oxidative phosphorylation • H+ ions are pumped into the intermembrane space by the ETC – The H+ ions want to drift back into the matrix • But they can only come into the matrix easily through ATP synthase channels Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The rushing of H+ ions through ATP synthase acts like water turning a water wheel INTERMEMBRANE SPACE H+ Result is that inorganic phosphate can be joined to ADP to make ATP 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. To Sum Up Chemiosmosis is an energy coupling mechanism that uses energy stored in the form of a H+ gradient across a membrane to drive cellular work In mitochondria, the energy for gradient formation comes from the exergonic redox reactions of the ETC and ATP synthesis is the work performed by ATP synthase 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 • There is a problem though, when we try to account how much ATP is made from glucose (or any organic molecule) We can not state exactly how many ATP come from each glucose because of 3 reasons . . . Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 1) The ETC and ATP synthase work together, but not 100% in synchronicity So there isn’t an exact correlation between NADH (or FADH) and ATP production 1 NADH synthesizes 2.5 – 3.3 ATP 1 FADH synthesizes 1.5 – 2 ATP Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings ( 2) The electron shuttle into the mitochondria can vary (either NAD+ or FAD) at the transition stage Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 3) The H+ ion gradient does other things in the mitochondria, it is not just used for the synthesis of ATP Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Bottom Line • We’ll say there are 38 ATP: 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 An interesting note: • Cellular respiration harvests about 40% of the energy in a glucose molecule – This is perhaps the most efficient energy conversion known Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In comparison: • The most efficient cars only convert about 25% of the energy stored in gas into energy that moves the car 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 – Not as much ATP is made 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 – an extension of glycolysis, 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 glycolysis Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Type 1 •In alcohol fermentation – Pyruvate is converted to ethanol in two steps, one of which releases CO2 – The second regenerates NAD+ so glycolysis can continue Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Type 2 •During lactic acid fermentation – Pyruvate is reduced directly to NADH to form lactate as a waste product – No CO2 is produced Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Contrasted • 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 Some organisms can produce ATP using either cellular respiration or fermentation. They are called facultative anaerobes. Ex: our muscle cells Figure 9.18 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings No O2 present Fermentation O2 present Cellular respiration MITOCHONDRION Ethanol or lactate Acetyl CoA Citric acid cycle • 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