1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Chapter 9 Cellular Respiration: Harvesting Chemical Energy

2. 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 different tasks

3. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The giant panda – Obtains energy for its cells by eating plants Figure 9.1

4. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Energy – Flows into ecosystems as sunlight and leaves as heat Light energy ECOSYSTEM CO 2 + H 2 O Photosynthesis in chloroplasts Cellular respiration in mitochondria Organic molecules + O 2 ATP powers most cellular work Heat energy Figure 9.2

5. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Photosynthetic organisms trap a portion of the sunlight energy and transform it into chemical energy (organic molecules) with O 2 is released. Cells use some of the chemical energy in organic molecules to make ATP; the energy source for cellular work. Energy leaves organisms as it dissipates as heat The products of respiration (CO 2 and H 2 O) are the raw materials for photosynthesis. Photosynthesis produces glucose and oxygen, the raw materials for respiration

6. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 9.1: Catabolic pathways yield energy by oxidizing organic fuels Catabolic Pathways and Production of ATP Organic compounds store energy in their arrangement of atoms With the help of enzymes, a cell systematically degrades complex organic molecules that are rich in potential energy to simpler waste products that have less energy

7. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fermentation a catabolic process – Is a partial degradation of sugars that occurs without oxygen (anaerobic) Cellular respiration – Is the most prevalent and efficient catabolic pathway – Consumes oxygen and organic molecules such as glucose – Yields ATP

8. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Respiration can be summarized: organic + oxygen → carbon + water + Energy compounds dioxide cellular respiration is most often described as the oxidation of glucose: C 6 H 12 O 6 + 6 O 2 → 6 CO 2 + 6H 2 O + Energy (ATP + heat) The breakdown of glucose is exergonic (free energy change) (ΔG= – 686 kcal per mol) – ΔG → the products of the chemical process store less energy than reactants and the reaction can happen spontaneously (without an input of energy)

9. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings To keep working – Cells must regenerate ATP from ADP + Pi – To understand how cellular respiration accomplishes this, let’s examine the fundamental process known as oxidation and reduction

10. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Redox Reactions: Oxidation and Reduction Why do the catabolic pathways that decompose glucose and other organic fuels yield energy? – The answer is based on the transfer of electrons during the chemical reactions. The relocation of electrons releases energy stored in organic molecules, and this energy is used to synthesize ATP

11. 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

12. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Examples of redox reactions Na + Cl Na + + Cl – becomes oxidized (loses electron) becomes reduced (gains electron) Xē + Y X + Y ē becomes oxidized becomes reduced We could generalize a redox reaction this way: X is the electron donor, is called the reducing agent, it reduces Y which accepts the donated electron Y is the electron acceptor, it oxidizes X by removing its electron

13. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Some redox reactions – Do not completely exchange electrons – Change the degree of electron sharing in covalent bonds – An electron loses potential energy when it shifts from a less electronegative atom toward more electronegative one → this energy can be put to work CH 4 H H H H C OO O O O C HH Methane (reducing agent) Oxygen (oxidizing agent) Carbon dioxideWater + 2O 2 CO 2 + Energy + 2 H 2 O becomes oxidized becomes reduced Reactants Products Figure 9.3

14. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Oxidation of Organic Fuel Molecules During Cellular Respiration Oxidation of methane is the main combustion reaction that occurs at the burner of a gas stove During cellular respiration – Glucose is oxidized and oxygen is reduced C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O + Energy becomes oxidized becomes reduced

15. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In general, organic molecules that have an abundance of hydrogen are excellent fuels because their bonds are a source of hilltop electrons whose energy may be released as these electrons fall down an energy gradient when they are transferred to oxygen By oxidizing glucose, respiration liberates stored energy from glucose and makes it available for ATP synthesis

16. 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 each is catalyzed by an enzyme – The hydrogen atoms are not transferred to oxygen, but instead are usually passed first to a coenzyme called NAD+ (nicotinamide adenine dinucleotide). – NAD functions as coenzyme in the redox reactions thus is an oxidizing agent. It is found in all cells and helps in e transfer.

17. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Electrons from organic compounds – Are usually first transferred to NAD +, a coenzyme – NAD+ accept electrons and act as an oxidizing agent during respiration NAD + H O O OO–O– O O O–O– O O O P P CH 2 HO OH H H HOOH HO H H N+N+ C NH 2 H N H N N Nicotinamide (oxidized form) NH 2 + 2[H] (from food) Dehydrogenase Reduction of NAD + Oxidation of NADH 2 e – + 2 H + 2 e – + H + NADH O H H N C + Nicotinamide (reduced form) N Figure 9.4 How does NAD+ trap electrons from glucose and other organic molecules? Enzymes called dehydrogenases removes a pair of hydrogen atoms (2 electrons and 2 protons) from the substrate (a sugar for example) thereby oxidizing it

18. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings NADH, the reduced form of NAD + – Passes the electrons to the electron transport chain – Each NADH molecule formed during respiration represents stored energy that can be tapped to make ATP when the electrons complete their fall down an energy gradient from NADH to oxygen

19. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings How do electrons that are extracted from food and stored by NADH finally reach oxygen? If electron transfer is not stepwise – A large release of energy occurs – As in the reaction of hydrogen and oxygen to form water (a) Uncontrolled reaction Free energy, G H2OH2O Explosive release of heat and light energy Figure 9.5 A H 2 + 1 / 2 O 2

20. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The electron transport chain – Passes electrons in a series of steps instead of one explosive reaction – Uses the energy from the electron transfer to form ATP – The transport chain consists of a number of molecules, mostly proteins, built into the inner membrane of a mitochondrion – ET from NADH to O 2 is an exergonic reaction with a free energy change of – 53 kcal/mol – Food → NADH → ETC → Oxygen

21. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2 H 1 / 2 O 2 (from food via NADH) 2 H + + 2 e – 2 H + 2 e – H2OH2O 1 / 2 O 2 Controlled release of energy for synthesis of ATP ATP Electron transport chain Free energy, G (b) Cellular respiration + Figure 9.5 B

22. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Stages of Cellular Respiration: A Preview Respiration is a cumulative function of three metabolic stages – Glycolysis – The citric acid cycle – Oxidative phosphorylation does not require O 2 require O 2

23. 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

24. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings An overview of cellular respiration Figure 9.6 Electrons carried via NADH Glycolsis Glucose Pyruvate ATP Substrate-level phosphorylation Electrons carried via NADH and FADH 2 Citric acid cycle Oxidative phosphorylation: electron transport and chemiosmosis ATP Substrate-level phosphorylation Oxidative phosphorylation Mitochondrion Cytosol

25. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings An overview of cellular respiration During glycolysis each glucose molecule is broken down into 2 pyruvate molecules Pyruvate inters into mitochondria where it will be oxidized by the citric acid cycle to CO2. NADH and FADH2 transfer electrons from glucose to ETCs in the inner mitochondria membrane During oxidative phosphorylation, ETCs convert chemical energy to a form of energy used for ATP synthesis in a process called chemiosmosis.

26. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Both glycolysis and the citric acid cycle – Can generate ATP by substrate-level phosphorylation – Occurs when an enzyme transfers a phosphate group from a substrate molecule to ADP rather than adding an inorganic phosphate to ADP as in oxidative phosphorylation Figure 9.7 Enzyme ATP ADP Product Substrate P +

27. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 9.2: Glycolysis harvests energy by oxidizing glucose to pyruvate Glycolysis (can occur in the presence or absence of O2) – Means “splitting of sugar” a 6-C sugar (glucose) to 3-C sugar (pyruvate) – Breaks down glucose into pyruvate – Occurs in the cytoplasm of the cell

28. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Glycolysis consists of two major phases – Energy investment phase – Energy payoff phase Glycolysis Citric acid cycle Oxidative phosphorylation ATP 2 ATP 4 ATP used formed Glucose 2 ATP + 2 P 4 ADP + 4 P 2 NAD + + 4 e - + 4 H + 2 NADH + 2 H + 2 Pyruvate + 2 H 2 O Energy investment phase Energy payoff phase Glucose 2 Pyruvate + 2 H 2 O 4 ATP formed – 2 ATP used 2 ATP 2 NAD + + 4 e – + 4 H + 2 NADH + 2 H + Figure 9.8

29. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Dihydroxyacetone phosphate Glyceraldehyde- 3-phosphate H H H H H OH HO CH 2 OH H H H H O H OH HO OH P CH 2 O P H O H HO H CH 2 OH P O CH 2 O O P HO H H OH O P CH 2 C O CH 2 OH H C CHOH CH 2 O O P ATP ADP Hexokinase Glucose Glucose-6-phosphate Fructose-6-phosphate ATP ADP Phosphoglucoisomerase Phosphofructokinase Fructose- 1, 6-bisphosphate Aldolase I somerase Glycolysis 1 2 3 4 5 CH 2 OH Oxidative phosphorylation Citric acid cycle Figure 9.9 A A closer look at the energy investment phase This is the reaction from which glycolysis Gets its name This reaction never reaches equilibrium in the cell

30. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2 NAD + NADH 2 + 2 H + Triose phosphate dehydrogenase 2 P i 2 P C CHOH O P O CH 2 O 2 O–O– 1, 3-Bisphosphoglycerate 2 ADP 2 ATP Phosphoglycerokinase CH 2 OP 2 C CHOH 3-Phosphoglycerate Phosphoglyceromutase O–O– C C CH 2 OH H O P 2-Phosphoglycerate 2 H 2 O 2 O–O– Enolase C C O P O CH 2 Phosphoenolpyruvate 2 ADP 2 ATP Pyruvate kinase O–O– C C O O CH 3 2 6 8 7 9 10 Pyruvate O Figure 9.8 B A closer look at the energy payoff phase

31. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 – completes glucose oxidation by breaking down pyruvate derivitatives into carbon dioxide.

32. 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 NADH + H + NAD + 2 31 CO 2 Coenzyme A Pyruvate Acetyle CoA S CoA C CH 3 O Transport protein O–O– O O C C CH 3 Figure 9.10

33. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings An overview of the citric acid cycle ATP 2 CO 2 3 NAD + 3 NADH + 3 H + ADP + P i FAD FADH 2 Citric acid cycle CoA Acetyle CoA NADH + 3 H + CoA CO 2 Pyruvate (from glycolysis, 2 molecules per glucose) ATP Glycolysis Citric acid cycle Oxidative phosphorylatio n Figure 9.11 For each turn of Krebs cycle, two carbons exit completely as CO2, three NADH and one FADH2 are formed. One ATP is made by substrate-level phosphorylation

34. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 9.12 Acetyl CoA NADH Oxaloacetate Citrate Malate Fumarate Succinate Succinyl CoA  -Ketoglutarate Isocitrate Citric acid cycle SCoA SH NADH FADH 2 FAD GTP GDP NAD + ADP P i NAD + CO 2 CoA SH CoA SH CoA S H2OH2O + H + H2OH2O C CH 3 O OCCOO – CH 2 COO – CH 2 HO C COO – CH 2 COO – CH 2 HCCOO – HOCH COO – CH CH 2 COO – HO COO – CH HC COO – CH 2 COO – CH 2 CO COO – CH 2 CO COO – 1 2 3 4 5 6 7 8 Glycolysis Oxidative phosphorylation NAD + + H + ATP Citric acid cycle Figure 9.12 A closer look at the citric acid cycle NAD is reduced to NADH+ CoA is displaced by a phosphate group Which is transferred to GDP forming GTP And then to ATP.

35. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis NADH and FADH 2 – Donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation Concept 9.4:

36. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Pathway of Electron Transport – Electrons from NADH and FADH 2 lose energy in several steps – Couples this exergonic slide of electrons to ATP synthesis or oxidative phosphorylation – The electron transport chain is made of electron carrier molecules embedded in the inner membrane of mitochondria.

37. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings – Each carrier in the chain has a higher electronegativity than the carrier before it, so electrons are pulled downhill towards oxygen – Most carriers are protein molecules except for ubiquinone (Q) – At the end of the chain, electrons are passed to oxygen, forming water

38. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings H2OH2O O2O2 NADH FADH2 FMN FeS O FAD Cyt b Cyt c 1 Cyt c Cyt a Cyt a 3 2 H + + 1  2 I II III IV Multiprotein complexes 0 10 20 30 40 50 Free energy (G) relative to O 2 (kcl/mol) Figure 9.13 NADH is oxidized and flavoprotein is reduced as high energy electrons from NADH are transferred to FMN ↓ Flavoprotein is oxidized as it passes electrons to an iron sulfur protein, FeS. ↓ FeS is oxidized as it pass electrons to ubiquinone Q ↓ Q passes electrons on to a succession of electron carriers, most of which are cytochromes. ↓ cyt a3, the last cytochrome passes electrons to oxygen.

39. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chemiosmosis: The Energy-Coupling Mechanism ATP synthase – Is the enzyme that actually makes ATP INTERMEMBRANE SPACE H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ P i + ADP ATP 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. 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. MITOCHONDRIAL MATRIX Figure 9.14

40. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings At certain steps along the electron transport chain – Electron transfer causes protein complexes to pump H + from the mitochondrial matrix to the intermembrane space

41. 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 The resulting H + gradient – Stores energy – Drives chemiosmosis in ATP synthase – Is referred to as a proton-motive force

42. 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 H+H+ H+H+ H+H+ H+H+ H+H+ ATP P i Protein complex of electron carriers Cyt c I II III IV (Carrying electrons from, food) NADH + FADH 2 NAD + FAD + 2 H + + 1 / 2 O 2 H2OH2O ADP + Electron transport chain Electron transport and pumping of protons (H + ), which create an H + gradient across the membrane Chemiosmosis ATP synthesis powered by the flow Of H + back across the membrane ATP synthase Q Oxidative phosphorylation Intermembrane space Inner mitochondrial membrane Mitochondrial matrix Figure 9.15

43. 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

44. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings There are three main processes in this metabolic enterprise Electron shuttles span membrane CYTOSOL 2 NADH 2 FADH 2 2 NADH 6 NADH 2 FADH 2 2 NADH Glycolysis Glucose 2 Pyruvate 2 Acetyl CoA Citric acid cycle Oxidative phosphorylation: electron transport and chemiosmosis MITOCHONDRION by substrate-level phosphorylation by substrate-level phosphorylation by oxidative phosphorylation, depending on which shuttle transports electrons from NADH in cytosol Maximum per glucose: About 36 or 38 ATP + 2 ATP + about 32 or 34 ATP or Figure 9.16

45. 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 1 ATP → - 7.3 kcal/mol 38 ATP X 7.3 / 686 = 40%

46. 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

47. 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

48. 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 In alcohol fermentation – Pyruvate is converted to ethanol in two steps, one of which releases CO 2 During lactic acid fermentation – Pyruvate is reduced directly to NADH to form lactate as a waste product

49. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2 ADP + 2 P1P1 2 ATP Glycolysis Glucose 2 NAD + 2 NADH 2 Pyruvate 2 Acetaldehyde 2 Ethanol (a) Alcohol fermentation 2 ADP + 2 P1P1 2 ATP Glycolysis Glucose 2 NAD + 2 NADH 2 Lactate (b) Lactic acid fermentation H H OH CH 3 C O – O C CO CH 3 H CO O–O– CO CO O CO C OHH CH 3 CO 2 2 Figure 9.17

50. 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 Fermentation and cellular respiration – Differ in their final electron acceptor Cellular respiration – Produces more ATP

51. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Pyruvate is a key juncture in catabolism Glucose CYTOSOL Pyruvate No O 2 present Fermentation O 2 present Cellular respiration Ethanol or lactate Acetyl CoA MITOCHONDRION Citric acid cycle Figure 9.18

52. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Evolutionary Significance of Glycolysis Glycolysis – Occurs in nearly all organisms – Probably evolved in ancient prokaryotes before there was oxygen in the atmosphere

53. 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 Concept 9.6: Glycolysis and the citric acid cycle connect to many other metabolic pathways

54. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The catabolism of various molecules from food Amino acids Sugars Glycerol Fatty acids Glycolysis Glucose Glyceraldehyde-3- P Pyruvate Acetyl CoA NH 3 Citric acid cycle Oxidative phosphorylation Fats Proteins Carbohydrates Figure 9.19

55. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

56. 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

57. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The control of cellular respiration Glucose Glycolysis Fructose-6-phosphate Phosphofructokinase Fructose-1,6-bisphosphate Inhibits Pyruvate ATP Acetyl CoA Citric acid cycle Citrate Oxidative phosphorylation Stimulates AMP + – – Figure 9.20