Cellular Respiration: Harvesting Chemical Energy

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Eighth Edition Neil Campbell and Jane Reece Lectures by

1 Chapter 9 Cellular Respiration: Harvesting Chemical Energy PowerPoint Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp

2 Overview: Life Is Work Living cells require energy from outside sources Energy flows into the ecosystem as light and leaves as heat In contrast, the chemical elements essential for life are recycled Photosynthesis generates oxygen and organic molecules Used by mitochondria of eukaryotes as fuel for cellular respiration Respiration breaks this fuel down, generating ATP The waste products, CO 2 and H 2 0 raw materials for photosynthesis

3 Fig. 9-2 Light energy ECOSYSTEM CO 2 + H 2 O Photosynthesis in chloroplasts Cellular respiration in mitochondria Organic molecules + O 2 ATP ATP powers most cellular work Heat energy

4 Fig. 9-1 Some animals, such as the giant panda, obtain energy by eating plants, and some animals feed on other organisms that eat plants

5 Concepts in This Chapter 1. Catabolic pathways yield energy by oxidizing organic fuels 2. Glycolysis harvest chemical energy by oxidizing glucose to pyruvate 3. The citric acid cycle completes the energy-yielding oxidation of organic molecules 4. During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis 5. Fermentation and anaerobic respiration enable some cells to produce ATP without the use of oxygen

6 CONCEPT 9.1: CATABOLIC PATHWAYS YIELD ENERGY BY OXIDIZING ORGANIC FUELS

7 Concept 9.1: Catabolic pathways yield energy by oxidizing organic fuels Several processes are central to cellular respiration and related pathways Organic compounds posses potential energy as a result of the arrangement of their atoms Catabolic pathways release the energy stored in complex organic molecules Enzymes catalyze Some of this energy is used to do work; the rest is dissipated as heat The breakdown of organic molecules is exergonic

8 Catabolic Pathways and Production of ATP Fermentation a partial degradation of sugars that occurs without O 2 Aerobic respiration consumes organic molecules and O 2 and yields ATP More efficient and widespread than fermentation Anaerobic respiration similar to aerobic respiration but consumes compounds other than O 2 Cellular respiration includes both aerobic and anaerobic respiration but is often used to refer to aerobic respiration

9 Catabolic Pathways and Production of ATP Anaerobic respiration is similar in principle to the combustion of gasoline in an automobile engine after oxygen is mixed with fuel Food provides the fuel for respiration The exhaust is carbon dioxide and water The overall process is: Organic compounds+ O 2 CO 2 + H 2 O+ Energy Carbohydrates, fats, and proteins are all consumed as fuel, but it is helpful to trace cellular respiration with the sugar glucose: C 6 H 12 O O 2 6 CO H 2 O + Energy (ATP + heat) This process is exergonic ΔG= -686 kcal per mole of glucose

10 Redox Reactions: Oxidation and Reduction The transfer of electrons during chemical reactions releases energy stored in organic molecules This released energy is ultimately used to synthesize ATP Chemical reactions that transfer electrons between reactants are called oxidation-reduction reactions, or redox reactions

11 The Principle of Redox Oxidation, The loss of electrons from one substance Reduction, The gain of electrons or reduction of positive charge Consider the formation of NaCl from Na and Cl

12 Fig. 9-UN1 The Principle of Redox becomes oxidized (loses electron) becomes reduced (gains electron) Charge on Cl is reduced from 0 to -1

13 Fig. 9-UN2 The Principle of Redox becomes oxidized becomes reduced

14 The Principle of Redox The electron donor is called the reducing agent The electron receptor is called the oxidizing agent

15 Fig. 9-UN2 The Principle of Redox becomes oxidized Reducing agent Oxidizing agent becomes reduced

16 The Principle of Redox Some redox reactions do not transfer electrons but change the electron sharing in covalent bonds An example is the reaction between methane and O 2

Fig. 9-3 When H 2 0 is formed, electrons of the covalent bond stay closer to O 2, so oxygen has gained electrons and is reduced Reactants becomes oxidized Products becomes

17 Fig. 9-3 When H 2 0 is formed, electrons of the covalent bond stay closer to O 2, so oxygen has gained electrons and is reduced Reactants becomes oxidized Products becomes reduced Methane (reducing agent) Oxygen (oxidizing agent) Carbon dioxide Water When CO 2 is formed, electrons end up closer to oxygen; in effect C has lost its electrons and is oxidized

18 The Principle of Redox Because oxygen is so electronegative it is one of the most potent oxidizing agents Energy must be added to pull an electron away from an atom Redox reactions moving electrons closer to oxygen release energy

19 Oxidation of Organic Fuel Molecules During Cellular Respiration Respiration is a redox reaction The fuel (such as glucose) is oxidized, and O 2 is reduced: becomes oxidized becomes reduced

20 Oxidation of Organic Fuel Molecules During Cellular Respiration During cellular respiration, the fuel (such as glucose) is oxidized, and O 2 is reduced: becomes oxidized becomes reduced The electrons lose potential energy along the way and energy is released Organic molecules that have an abundance of hydrogen are excellent fuels Their bonds are a source of hilltop electrons whose energy may be released as they fall down an energy gradient when they are transferred to oxygen

21 Oxidation of Organic Fuel Molecules During Cellular Respiration In respiration, the oxidation of glucose transfers electrons to a lower energy state, releasing energy that becomes available for ATP synthesis The main energy foods, carbohydrates and fats are reservoirs of electrons associated with hydrogen Only the barrier of activation energy holds back the flood of electrons to a lower energy state Glucose would combine instantaneously with O 2, energy would be lost Body temperature is not high enough to initiate this; instead enzymes are involved

22 Stepwise Energy Harvest via NAD + and the Electron Transport Chain If energy is released all at once it cannot be harnessed for work In cellular respiration, glucose and other organic molecules are broken down in a series of steps Each step is catalyzed by an enzyme At key steps electrons are stripped from glucose Each electron travels with a proton These H+ are not transferred directly to oxygen, but first to an electron carrier, NAD+, a coenzyme

23 Fig. 9-UN4 Dehydrogenase

24 Stepwise Energy Harvest via NAD + and the Electron Transport Chain NAD +, is an electron acceptor, Functions as an oxidizing agent during cellular respiration Enzymes called dehydrogenases remove a pair of hydrogen atoms (2e, 2p) from glucose Dehydrogenase delivers the 2e and 1p to NAD+ The other H+ is released into the surrounding solution NAD+ has its charge neutralized and is reduced to NADH Each NADH (the reduced form of NAD + ) represents stored energy that is tapped to synthesize ATP NAD+ is the most versatile electron acceptor in cellular respiration and functions in several of the steps

25 Fig. 9-UN4 Dehydrogenase

Fig. 9-4 2 e + 2 H + 2 e + H + Dehydrogenase NADH H + NAD + + 2[H] Reduction of

26 Fig e + 2 H + 2 e + H + Dehydrogenase NADH H + NAD + + 2[H] Reduction of NAD + Oxidation of NADH + H + Nicotinamide (reduced form) Nicotinamide (oxidized form)

27 Stepwise Energy Harvest via NAD + and the Electron Transport Chain Electrons lose very little of their potential energy when they are transferred from glucose to NAD+ Each NADH represent stored energy that can be tapped to make ATP

28 How do electrons stored as potential energy in NADH finally get to oxygen? The reaction to form H 2 O from H 2 and O 2 is explosive Used to power main engines of space shuttle after launch NADH passes the electrons to the electron transport chain Consists of mostly proteins built into mitochondria of eukaryotic cells; plasma membrane of prokaryotes Breaks down the fall of electrons to oxygen into several energyreleasing steps Electrons released from food shuttled by NADH to the top higher-energy end of the chain At the bottom, O 2 captures these electrons along with H+, forming water This process is exergonic, ΔG= -53 kcal/mol The energy yielded is used to regenerate ATP

Fig. 9-5 H 2 + 1 / 2 O 2 2 H 1 / 2 O 2 (from food via NADH) 2 H + + 2 e Controlled release of energy for

29 Fig. 9-5 H / 2 O 2 2 H 1 / 2 O 2 (from food via NADH) 2 H e Controlled release of energy for synthesis of ATP Explosive release of heat and light energy 1 / 2 O 2 (a) Uncontrolled reaction (b) Cellular respiration

30 The Stages of Cellular Respiration: A Preview Cellular respiration has three stages: Glycolysis Occurs in cytosol breaks down glucose into two molecules of pyruvate The citric acid cycle Occurs in mitochondrial matrix (eukaryotes) completes the breakdown of glucose Oxidative phosphorylation Occurs in inner membrane of mitochondria (eukaryotes) accounts for most of the ATP synthesis

Fig. 9-6-1 Electrons carried via NADH Glycolysis

31 Fig Electrons carried via NADH Glycolysis Glucose Pyruvate Cytosol ATP Substrate-level phosphorylation

Fig. 9-6-2 Electrons carried via NADH Electrons carried via NADH and FADH 2 Glucose Glycolysis Pyruvate

32 Fig Electrons carried via NADH Electrons carried via NADH and FADH 2 Glucose Glycolysis Pyruvate Citric acid cycle Cytosol Mitochondrion ATP Substrate-level phosphorylation ATP Substrate-level phosphorylation

Fig. 9-6-3 Electrons carried via NADH Electrons carried via NADH and FADH 2 Glucose Glycolysis Pyruvate Citric acid cycle Oxidative phosphorylation:

33 Fig Electrons carried via NADH Electrons carried via NADH and FADH 2 Glucose Glycolysis Pyruvate Citric acid cycle Oxidative phosphorylation: electron transport and chemiosmosis Cytosol Mitochondrion ATP Substrate-level phosphorylation ATP Substrate-level phosphorylation ATP Oxidative phosphorylation

34 The Stages of Cellular Respiration: A Preview The process that generates most of the ATP is called oxidative phosphorylation because it is powered by redox reactions The electron transport chain accepts electrons from glycolysis and the citric acid cycle The electrons move from molecule to molecule until they combine with molecular oxygen to form water Energy is released at each step in the chain and stored in a form mitochondria can use to make ATP BioFlix: Cellular Respiration

35 The Stages of Cellular Respiration: A Preview Oxidative phosphorylation accounts for almost 90% of the ATP generated by cellular respiration A smaller amount of ATP is formed in glycolysis and the citric acid cycle by substrate-level phosphorylation An enzyme transfers a phosphate group from an organic substrate molecule to ADP, forming ATP Respiration uses the small steps in the respiratory pathway to break the large denomination of energy contained in glucose to the small change of ATP The quantity of energy is more practical for the energy level of work required in the cell.

36 Fig. 9-7 Enzyme ADP Enzyme P Substrate + ATP Product

37 Summary Fermentation a partial degradation of sugars that occurs without O 2 Aerobic respiration consumes organic molecules and O 2 and yields ATP More efficient and widespread than fermentation Anaerobic respiration similar to aerobic respiration but consumes compounds other than O 2 Cellular respiration includes both aerobic and anaerobic respiration but is often used to refer to aerobic respiration

38 The Stages of Cellular Respiration: A Preview Cellular respiration has three stages: Glycolysis Occurs in cytosol breaks down glucose into two molecules of pyruvate The citric acid cycle Occurs in mitochondrial matrix (eukaryotes) completes the breakdown of glucose Oxidative phosphorylation Occurs in inner membrane of mitochondria (eukaryotes) accounts for most of the ATP synthesis

39 The Principle of Redox Oxidation The loss of electrons from one substance Reduction The gain of electrons or reduction of positive charge

40 The Principle of Redox Na + Cl----- Na + + Cl - Oxidized = Na Reduced = Cl C 6 H 12 O 6 + 6O CO H 2 O + Energy Oxidized= C 6 H 12 O 6 ; Reduced O 2 CH 4 + 2O CO 2 + H 2 O+ Energy Oxidized= CH 4 ; Reduced= O2 C 4 H 6 O 5 + NAD C 4 H 4 O 5 + NADH + H + Oxidized= C 4 H 6 O 5 ; Reduced= NAD +

41 CONCEPT 9.2: GLYCOLYSIS HARVESTS CHEMICAL ENERGY BY OXIDIZING GLUCOSE TO PYRUVATE

42 Concept 9.2: Glycolysis harvests chemical energy by oxidizing glucose to pyruvate During glycolysis glucose is broken down into two molecules of pyruvate Glycolysis means splitting of sugar Glucose, a 6-carbon sugar is split into two three-carbon sugars These small sugars are then oxidized and their remaining atoms rearranged to form two molecules of pyruvate Glycolysis occurs in the cytoplasm and has two major phases: Energy investment phase Energy payoff phase

Fig. 9-8 Energy investment phase Glucose 2 ADP + 2 P 2 ATP used Energy payoff phase 4 ADP + 4 P 4 ATP formed 2 NAD + + 4 e + 4 H + 2

43 Fig. 9-8 Energy investment phase Glucose 2 ADP + 2 P 2 ATP used Energy payoff phase 4 ADP + 4 P 4 ATP formed 2 NAD e + 4 H + 2 NADH + 2 H + 2 Pyruvate + 2 H 2 O Net Glucose 2 Pyruvate + 2 H 2 O 4 ATP formed 2 ATP used 2 ATP 2 NAD e + 4 H + 2 NADH + 2 H +

44 Glycolysis Energy investment phase The cell spends ATP Energy payoff ATP is produced by substrate-level phosphorylation NAD+ is reduced to NADH by electrons released from the oxidation of glucose Net energy yield 2 ATP+ 2 NADH Glycolysis occurs whether or not O 2 is present If O 2 is present then the chemical energy stores in pyruvate and NADH can be extracted by the citric acid cycle and oxidative phosphorylation

45 CONCEPT 9.3: THE CITRIC ACID CYCLE COMPLETES THE ENERGY- YIELDING OXIDATION OF ORGANIC MOLECULES

46 Concept 9.3: The citric acid cycle completes the energy-yielding oxidation of organic molecules Glycolysis releases less than ¼ of the chemical energy stored n glucose Most is stockpiled in the 2 molecules of pyruvate In the presence of O 2, pyruvate enters the mitochondrion The enzymes of the citric acid cycle complete the oxidation of glucose Before the citric acid cycle can begin, pyruvate must be converted to acetyl CoA, which links the cycle to glycolysis This occurs in three steps

Fig. 9-10 CYTOSOL MITOCHONDRION NAD + NADH + H + 2 Pyruvate Transport protein 1 3 CO 2 Coenzyme A Acetyl CoA 1.

47 Fig CYTOSOL MITOCHONDRION NAD + NADH + H + 2 Pyruvate Transport protein 1 3 CO 2 Coenzyme A Acetyl CoA 1. The carboxyl group (fully oxidized, little chemical energy) removed and given off as CO 2 2. The remaining 2-carbon fragment is oxidized, forming acetate; an enzyme transfers the extracted electrons to NAD+ to form NADH 3. Acetate combines with coenzyme A (unstable bond) to form the very reactive molecule acetyl CoA

48 The citric acid cycle Acetyl CoA has a high potential energy Meaning that the reaction to yield lower-energy products is highly exergonic Now ready to feed its acetyl group into the citric acid cycle for further oxidation The citric acid cycle is also called the Krebs cycle or the tricarboxylic acid cycle It takes place within the mitochondrial matrix The cycle oxidizes organic fuel derived from pyruvate generating per turn 1 ATP (substrate-level phosphorylation), 3 NADH and 1 FADH 2 NADH and FADH 2 transfer high energy electrons to the electron transport chain 3 CO 2 are released

Fig. 9-11 Pyruvate NAD + NADH CO 2 CoA + H + Acetyl CoA CoA CoA

49 Fig Pyruvate NAD + NADH CO 2 CoA + H + Acetyl CoA CoA CoA Citric acid cycle 2 CO 2 FADH 2 FAD 3 3 NAD + NADH + 3 H + ADP + P i ATP

50 The citric acid cycle The citric acid cycle has eight steps, each catalyzed by a specific enzyme The acetyl group of acetyl CoA joins the cycle by combining with oxaloacetate, forming citrate The next seven steps decompose the citrate back to oxaloacetate, making the process a cycle The NADH and FADH 2 produced by the cycle relay electrons extracted from food to the electron transport chain

51 Summary The citric acid cycle yields energy used for the phosphorylation of ADP to ATP: Per molecule of Glucose, it generates 2 ATP 8 NADH 2 FADH 2 Releases 6 CO 2

52 CONCEPT 9.4: DURING OXIDATIVE PHOSPHORYLATION, CHEMIOSMOSIS COUPLES ELECTRON TRANSPORT TO ATP SYNTHESIS

53 Concept 9.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis Only 4 of 38 ATP ultimately produced by the respiration are produced by substrate-level phosphorylation 2 ATP are produced during glycolysis 2 ATP are produced during the citric acid cycle NADH and FADH 2 account for most of the energy extracted from food These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation

54 The Pathway of Electron Transport The electron transport chain is a collection of molecules embedded in the cristae of the mitochondrion Folding of inner mitochondrial membrane into cristae increases surface area, providing space for 1000s of copies of the chain in each mitochondrion Plasma membrane in prokaryotes Most of the chain s components are proteins, which exist in multiprotein complexes numbered I-IV Tightly bound to these are prosthetic groups Nonprotein components essential for the catalytic functions of certain enzymes

55 The Pathway of Electron Transport The carriers alternate reduced and oxidized states as they accept and donate electrons Each component of the chain becomes reduced when it accepts electrons from its uphill neighbor It then returns to its oxidized form as it passes electrons to its downhill more electronegative neighbor Electrons drop in free energy as they go down the chain and are finally passed to O 2, forming H 2 O

The Pathway of Electron Transport Electrons carried by NADH are transferred to the first molecule of the electron transport chain, a flavoprotein In the next redox reaction the flavoprotein returns

56 The Pathway of Electron Transport Electrons carried by NADH are transferred to the first molecule of the electron transport chain, a flavoprotein In the next redox reaction the flavoprotein returns to its oxidized form as it passes its electrons to an iron-sulfur protein FeS passes the electrons to ubiquinone A small hydrophobic nonprotein molecule Individually mobile NADH e NAD + Ι FMN Fe S FADH 2 e 2 FAD FAD Fe S ΙΙ Q Cyt b Fe S ΙΙΙ Multiprotein complexes Cyt c 1 Cyt c IV Most of the remaining electron carries are cytochromes Their prosthetic group is a heme group with an iron atom that accepts and donates electrons 20 Cyt a Cyt a 3 The last cytochrome, cyta 3 passes its electrons to O 2 Each oxygen atoms also picks up a pair of hydrogen ions from the aqueous solution, forming water 10 2 (from NADH or FADH 2 ) 0 2 H / 2 O 2 e H 2 O

57 The Pathway of Electron Transport FADH 2 is also a source of electrons These electrons have a lower free energy and are added at a lower energy level than NADH About 1/3 less ATP is provided when the electron donor is FADH 2 rather than NADH The electron transport chain generates no ATP The chain s function is to break the large free-energy drop from food to O 2 into smaller steps that release energy in manageable amounts

58 Chemiosmosis: The Energy-Coupling Mechanism Electron transfer in the electron transport chain causes proteins to pump H + from the mitochondrial matrix to the intermembrane space 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 Osmosis in this case refers to the flow of H+ across a membrane

Fig. 9-14 INTERMEMBRANE SPACE Rotor H + Stator

59 Fig INTERMEMBRANE SPACE Rotor H + Stator Internal rod Catalytic knob ADP + P i ATP MITOCHONDRIAL MATRIX

60 Fig. 9-15a EXPERIMENT Magnetic bead Electromagnet Sample Internal rod Catalytic knob Nickel plate

61 Fig. 9-15b RESULTS 30 Rotation in one direction Rotation in opposite direction No rotation Sequential trials

62 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 protonmotive force, emphasizing its capacity to do work

Fig. 9-16 H + H + Protein complex of electron carriers H + Cyt c H + Ι Q ΙΙ FADH 2 ΙΙΙ FAD ΙV 2 H + + 1 / 2 O 2 H 2 O ATP

63 Fig H + H + Protein complex of electron carriers H + Cyt c H + Ι Q ΙΙ FADH 2 ΙΙΙ FAD ΙV 2 H / 2 O 2 H 2 O ATP synthase NADH (carrying electrons from food) NAD + ADP + P i H + ATP 1 Electron transport chain 2 Chemiosmosis Oxidative phosphorylation

64 An Accounting of ATP Production by Cellular Respiration During cellular respiration, most energy flows in this sequence: glucose NADH electron transport chain proton-motive force ATP About 40% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making about 38 ATP

Fig. 9-17 CYTOSOL Electron shuttles span membrane 2 NADH or MITOCHONDRION 2 FADH 2 2 NADH 2 NADH 6 NADH 2 FADH 2 Glycolysis 2 Glucose Pyruvate 2 Acetyl CoA

65 Fig CYTOSOL Electron shuttles span membrane 2 NADH or MITOCHONDRION 2 FADH 2 2 NADH 2 NADH 6 NADH 2 FADH 2 Glycolysis 2 Glucose Pyruvate 2 Acetyl CoA Citric acid cycle Oxidative phosphorylation: electron transport and chemiosmosis + 2 ATP + 2 ATP + about 32 or 34 ATP Maximum per glucose: About 36 or 38 ATP

66 CONCEPT 9.5: FERMENTATION AND ANAEROBIC RESPIRATION ENABLE CELLS TO PRODUCE ATP WITHOUT THE USE OF OXYGEN

67 Concept 9.5: Fermentation and anaerobic respiration enable cells to produce ATP without the use of oxygen Most cellular respiration requires O 2 to produce ATP The electronegative oxygen is required to pull electrons down the electron transport chain Otherwise oxidative phosphorylation would cease Glycolysis can produce ATP with or without O 2 (in aerobic or anaerobic conditions) There are two general mechanisms by which certain cells oxidize organic fuel and generate ATP without using oxygen Anaerobic respiration Fermentation

68 Fermentation and anaerobic respiration Anaerobic respiration uses an electron transport chain with an electron acceptor other than O 2 Sulfate-reducing marine bacteria use the sulfate ion at the end of their respiratory chain ATP is produced and H 2 S is the by product Fermentation uses phosphorylation instead of an electron transport chain to generate ATP That is without cellular respiration

69 Types of Fermentation Fermentation consists of glycolysis plus reactions that regenerate NAD +, which can be reused by glycolysis Remember that glycolysis occurs with or without oxygen When oxygen is present the additional ATP is made by oxidative phosphorylation When oxygen is absent, substrate level phosphorylation continues to allow additional generation of ATP There must be a sufficient supply of NAD+ When NAD+ runs out glycolysis is shut down There are two common types of fermentation: alcohol fermentation lactic acid fermentation

needed for glycolysis to continue Many bacteria carry out alcohol fermentation under anaerobic conditions Alcohol

70 Alcohol Fermentation In alcohol fermentation, pyruvate is converted to ethanol in two steps, Step 1 releases CO 2 from pyruvate and converts it acetaldehyde In step 2 acetaldehyde is reduced by NADH to alcohol This regenerates NAD+ needed for glycolysis to continue Many bacteria carry out alcohol fermentation under anaerobic conditions Alcohol fermentation by yeast is used in brewing, winemaking, and baking (CO2 allows bread to rise) Animation: Fermentation Overview

71 Fig. 9-18a 2 ADP + 2 P i 2 ATP Glucose Glycolysis 2 Pyruvate 2 NAD + 2 NADH 2 CO H + 2 Ethanol 2 Acetaldehyde (a) Alcohol fermentation

72 Lactic Acid Fermentation In lactic acid fermentation, pyruvate is reduced directly to NADH lactate is formed as an end product no release of CO 2 Lactic acid fermentation by some fungi and bacteria is used to make cheese and yogurt Human muscle cells use lactic acid fermentation to generate ATP when O 2 is scarce May occur in early stages of strenuous exercise

73 Fig. 9-18b 2 ADP + 2 P i 2 ATP Glucose Glycolysis 2 NAD + 2 NADH + 2 H + 2 Pyruvate 2 Lactate (b) Lactic acid fermentation

74 Fermentation and Aerobic Respiration Compared Both processes use glycolysis to oxidize glucose and other organic fuels to pyruvate The processes have different final electron acceptors: an organic molecule (such as pyruvate or acetaldehyde) in fermentation and O 2 in cellular respiration Cellular respiration produces 38 ATP per glucose molecule; fermentation produces 2 ATP per glucose molecule

75 Organisms vary in pathways available to them to break down sugars Obligate anaerobes carry out fermentation or anaerobic respiration cannot survive in the presence of O 2 e.g. vertebrate brain cells facultative anaerobes can survive using either fermentation or cellular respiration E.g. yeast and many bacteria pyruvate is a fork in the metabolic road that leads to two alternative catabolic routes Converted to acetyl CoA in aerobic conditions In anaerobic conditions serves as an electron acceptor to recycle NAD+

Fig. 9-19 Glucose CYTOSOL Glycolysis No O 2 present: Fermentation Pyruvate O 2 present:

76 Fig Glucose CYTOSOL Glycolysis No O 2 present: Fermentation Pyruvate O 2 present: Aerobic cellular respiration Ethanol or lactate Acetyl CoA MITOCHONDRION Citric acid cycle

77 The Evolutionary Significance of Glycolysis Glycolysis occurs in nearly all organisms Glycolysis probably evolved in ancient prokaryotes before there was oxygen in the atmosphere

78 Summary Aerobic respiration Anaerobic respiration Fermentation Uses glycolysis Uses glycolysis Uses glycolysis Electron transport chain Produces 2 ATP per glucose molecule Organic molecules e.g. pyruvate

79 Summary Aerobic respiration Anaerobic respiration Fermentation Uses glycolysis Uses glycolysis Uses glycolysis Electron transport chain Electron transport chain Don t use electron transport chain O 2 final electron acceptor Produces 38 ATP per glucose molecule Something other than O e.g..s Produces 2 ATP per glucose molecule Organic molecules e.g. pyruvate Produces 2 ATP per glucose molecule

80 Summary Alcohol Fermentation Pyruvate converted to ethanol CO2 released Lactic Acid Fermentation Direct Yoghurt, cheese, human muscle cells

81 Summary Alcohol Fermentation Lactic Acid Fermentation Pyruvate converted to ethanol Pyruvate converted to NADH 2 steps Direct CO2 released No CO2 release Yeast- brewing, baking Yoghurt, cheese, human muscle cells

82 Summary: Obligate anaerobes carry out fermentation or anaerobic respiration cannot survive in the presence of O 2 e.g. vertebrate brain cells facultative anaerobes can survive using either fermentation or cellular respiration E.g. yeast and many bacteria pyruvate is a fork in the metabolic road that leads to two alternative catabolic routes

83 CONCEPT 9.6: GLYCOLYSIS AND THE CITRIC ACID CYCLE CONNECT TO MANY OTHER METABOLIC PATHWAYS

84 Concept 9.6: Glycolysis and the citric acid cycle connect to many other metabolic pathways Gycolysis and the citric acid cycle are major intersections to various catabolic and anabolic pathways

85 The Versatility of Catabolism Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration Glycolysis accepts a wide range of carbohydrates Polysaccharides (e.g. starch) can be hydrolyzed to glucose monomers Digestion of disaccharides e.g. sucrose provides glucose Proteins must be digested to amino acids amino groups can feed glycolysis or the citric acid cycle The amino groups must be removed via deamination Nitrogenous waste is excreted as ammonia, urea, etc Carbon skeletons are modified by enzymes and enter glycolysis or citric acid cycle

86 The Versatility of Catabolism Fats must be digested to glycerol (used in glycolysis) and fatty acids (used in generating acetyl CoA) Fatty acids are broken down by beta oxidation and yield acetyl CoA An oxidized gram of fat produces more than twice as much ATP as an oxidized gram of carbohydrate

Fig. 9-20 Proteins Carbohydrates Fats Amino acids Sugars Glycerol Fatty acids Glycolysis

87 Fig Proteins Carbohydrates Fats Amino acids Sugars Glycerol Fatty acids Glycolysis Glucose Glyceraldehyde-3- P NH 3 Pyruvate Acetyl CoA Citric acid cycle Oxidative phosphorylation

88 Biosynthesis (Anabolic Pathways) The body uses small molecules to build other substances These small molecules may come directly from food, from glycolysis, or from the citric acid cycle Anabolic pathways consume rather than generate ATP

89 Regulation of Cellular Respiration via Feedback Mechanisms Feedback inhibition is the most common mechanism for control If ATP concentration begins to drop, respiration speeds up; when there is plenty of ATP, respiration slows down Control of catabolism is based mainly on regulating the activity of enzymes at strategic points in the catabolic pathway

Fig. 9-21 Glucose Inhibits Glycolysis Fructose-6-phosphate Phosphofructokinase

90 Fig Glucose Inhibits Glycolysis Fructose-6-phosphate Phosphofructokinase Fructose-1,6-bisphosphate AMP Stimulates + Inhibits Pyruvate ATP Acetyl CoA Citrate Citric acid cycle Oxidative phosphorylation

91 Fig. 9-UN5 Inputs Outputs Glycolysis + 2 ATP 2 NADH Glucose 2 Pyruvate

92 Fig. 9-UN6 Inputs Outputs S CoA C O 2 ATP CH 3 2 Acetyl CoA 6 NADH O C COO CH 2 Citric acid cycle COO 2 FADH 2 2 Oxaloacetate

93 Fig. 9-UN7 INTER- MEMBRANE SPACE H + ATP synthase ADP + P i ATP MITO- CHONDRIAL MATRIX H +

94 You should now be able to: 1. Explain in general terms how redox reactions are involved in energy exchanges 2. Name the three stages of cellular respiration; for each, state the region of the eukaryotic cell where it occurs and the products that result 3. In general terms, explain the role of the electron transport chain in cellular respiration

95 4. Explain where and how the respiratory electron transport chain creates a proton gradient 5. Distinguish between fermentation and anaerobic respiration 6. Distinguish between obligate and facultative anaerobes

Cellular Respiration: Harvesting Chemical Energy

Chapter 9 Cellular Respiration: Harvesting Chemical Energy You should be able to: 1. Explain how redox reactions are involved in energy exchanges. Name and describe the three stages of cellular respiration;