Cellular Respiration: Harvesting Chemical Energy

1. 9-1 Cellular Respiration: Harvesting Chemical Energy

2. 9-2 Energy flow and chemical recycling in ecosystems Chemical elements important to life are recycled by respiration and photosynthesis. Energy is not recycled. Flows into an ecosystem as sunlight, and leaves as heat. The mitochondria of eukaryotes (incl. Plants and algae) use the organic products of PS as fuel for CR. Respiration harvests the energy stored in organic molecules to generate ATP, which powers cellular work. Waste products of respiration (CO2 and H2O) used as raw materials for PS. Chemical elements recycled, energy is not. Chemical elements important to life are recycled by respiration and photosynthesis. Energy is not recycled. Flows into an ecosystem as sunlight, and leaves as heat. The mitochondria of eukaryotes (incl. Plants and algae) use the organic products of PS as fuel for CR. Respiration harvests the energy stored in organic molecules to generate ATP, which powers cellular work. Waste products of respiration (CO2 and H2O) used as raw materials for PS. Chemical elements recycled, energy is not.

3. 9-3 The Principles of Energy Harvest

4. 9-4 The Principles of Energy Harvest Cells harvest chemical energy stored in organic molecules and use it to regenerate ATP ATP drives most cellular work To harvest chemical energy, cells employ metabolic pathways with many steps A few general principles of energy harvesting will help us to understand the complexity of cellular respiration We first must understand the general principles of cellular respiration (building upon what we learned about in the metabolism chapter). Then, we can use these principles to understand the complex steps of cellular respiration. We first must understand the general principles of cellular respiration (building upon what we learned about in the metabolism chapter). Then, we can use these principles to understand the complex steps of cellular respiration.

5. 9-5 Cellular respiration and fermentation are catabolic, energy-yielding pathways The breakdown of glucose and other organic fuels to simpler products is exergonic, yields energy for ATP synthesis Fermentation is the partial breakdown of sugars, without oxygen Cellular respiration is the most common and efficient catabolic pathway, oxygen is consumed as a reactant Organic compounds store energy in their arrangement of atoms With help from enzymes, cells degrade complex organic molecules rich in potential energy to simpler waste products that have less energy Some energy used for work, some lost as heat Metabolic pathways that release stored energy by breaking down complex molecules is called catabolic pathwaysOrganic compounds store energy in their arrangement of atoms With help from enzymes, cells degrade complex organic molecules rich in potential energy to simpler waste products that have less energy Some energy used for work, some lost as heat Metabolic pathways that release stored energy by breaking down complex molecules is called catabolic pathways

6. 9-6 Cellular respiration takes place in mitochondria CR is analogous to gasoline combustion: Food is fuel for respiration CO2 and H2O are exhaust Degradation of glucose is written as: C6H12O6 + 6O2 ? 6CO2 + 6H2O + Energy breakdown is exergonic, ?G = -686 kcal/mol Our main objective is to discover how cells use energy stored in food molecules to make ATP Energy (ATP and heat) What does -?G indicate? Products of chemical process store less energy that the reactants Energy (ATP and heat) What does -?G indicate? Products of chemical process store less energy that the reactants

7. 9-7 Cells recycle the energy they use for work ATP is adenosine triphosphate three negatively charged phosphate groups is an unstable, energy-strong arrangement tends to lose terminal phosphate to become more stable

8. 9-8 Cell taps this energy source by using enzymes to transfer phosphate groups from ATP to other compounds, they are phosphorylated Phosphorylation primes a molecule to undergo a change to perform work and then molecule uses phosphate group in process

9. 9-9 A review of how ATP drives cellular work Phosphate group transfer is the mechanism responsible for most types of cellular work Enzymes shift a phosphate group from ATP to some other molecule, and this phosphorylated molecule undergoes a change that performs work. Examples how ATP drives: Active transport by phosphorylating certain membrane proteins Mechanical work by phosphorylating motor proteins (ones that move organelles along cytoskeletal tracks) Cellular work by phosphorylating key reactants Phosphorylated molecules lose the phosphate groups as work is performed, leaving ADP and inorganic phosphate as products Cellular respiration replenishes the ATP supply by powering the phosphorylation of ADP Phosphate group transfer is the mechanism responsible for most types of cellular work Enzymes shift a phosphate group from ATP to some other molecule, and this phosphorylated molecule undergoes a change that performs work. Examples how ATP drives: Active transport by phosphorylating certain membrane proteins Mechanical work by phosphorylating motor proteins (ones that move organelles along cytoskeletal tracks) Cellular work by phosphorylating key reactants Phosphorylated molecules lose the phosphate groups as work is performed, leaving ADP and inorganic phosphate as products Cellular respiration replenishes the ATP supply by powering the phosphorylation of ADP

10. 9-10

11. 9-11 Redox reactions Release energy when electrons move closer to electronegative atoms You must be asking yourself: What happens when catabolic pathways decompose glucose and other organic fuels? Why do these metabolic pathways yield energy? The answers are based on the transfer of electrons during the chemical reactions

12. 9-12 The relocation of electrons releases the energy stored in food molecules, and this energy is used to synthesize ATP The transfer of one or more electrons (e-) from one reactant to another are called oxidation-reduction reactions, or redox reactions oxidation - loss of electrons from one substance reduction - addition of electrons from another substance Use as nonbiological example, reaction between sodium and chlorine to form table salt: Na + Cl ? Na+ + Cl- Sodium is oxidized Chlorine is reduced We could generalize a redox reaction this way: Xe- + Y ? X + Ye- Substance X, electron donor, is called the reducing agent; it reduced Y Substance Y, electron acceptor, is the oxidizing agent; it oxidizes X Because an electron transfer requires both a donor and an acceptor, oxidation and reduction always go together - Redox ReactionUse as nonbiological example, reaction between sodium and chlorine to form table salt: Na + Cl ? Na+ + Cl- Sodium is oxidized Chlorine is reduced We could generalize a redox reaction this way: Xe- + Y ? X + Ye- Substance X, electron donor, is called the reducing agent; it reduced Y Substance Y, electron acceptor, is the oxidizing agent; it oxidizes X Because an electron transfer requires both a donor and an acceptor, oxidation and reduction always go together - Redox Reaction

13. 9-13 Not all redox reactions involve the complete transfer of electrons from one to another Some change the degree of electron sharing in covalent bonds Methane combustion is a good example Methane is oxidized. When CH4 reacts O2 to form CO2, electrons are shifted away from C atom to O. Oxygen is reduced. O reacts with H atoms of CH4 to form H2O, the electrons are drawn closer to O.

14. 9-14 Methane combustion as an energy-yielding redox reaction During the reaction, covalently shared electrons move away from carbon and hydrogen atoms and closer to oxygen, which is very electronegative. The reaction releases energy to the surroundings, because the electrons lose potential energy as they move closer to electronegative atoms.During the reaction, covalently shared electrons move away from carbon and hydrogen atoms and closer to oxygen, which is very electronegative. The reaction releases energy to the surroundings, because the electrons lose potential energy as they move closer to electronegative atoms.

15. 9-15 Oxygen is very electronegative Energy must be added to pull an electron away from an atom The more electronegative the atom (stronger its pull on electrons), the more energy is required to keep the electron away from it An electron loses potential energy when it shifts from a less electronegative atom toward a more electronegative one A redox reaction that relocates electrons closer to oxygen (such as burning of methane), releases chemical energy that can be put to work Oxygen is one of the most potent of all oxidizing agents.Oxygen is one of the most potent of all oxidizing agents.

16. 9-16 Electrons “fall” from organic molecules to oxygen during cellular respiration Respiration is the the energy-yielding redox process of greatest interest: C6H12O6 + 6O2 ? 6CO2 + 6H2O Glucose is oxidized to CO2, O2 reduced to H2O. Electrons lose potential energy along the way Oxidation of methane by oxygen is main combustion rxn that occurs at burner of gas stove. Combustion of gasoline in auto engine is also a redox rxn; the energy pushes the pistons. Oxidation of methane by oxygen is main combustion rxn that occurs at burner of gas stove. Combustion of gasoline in auto engine is also a redox rxn; the energy pushes the pistons.

17. 9-17 Organic molecules with a lot of hydrogen (carbohydrates, fat) are excellent fuels Their bonds are a source of “hilltop” electrons, have potential to “fall” closer to oxygen Status of the electrons changes as hydrogen is transferred to oxygen, liberates energy By oxidizing glucose, respiration takes energy out of storage and makes it available for ATP synthesis

18. 9-18 The “fall” of electrons during respiration is stepwise, occurs via NAD+ and electron transport chain Glucose and other organic fuels are broken down gradually in a series of steps, each one catalyzed by an enzyme As hydrogen atoms are stripped from glucose they are passed first to a coenzyme called NAD+ (nicotanamide adenine dinucleotide) NAD+ functions as an oxidizing agent during respiration. Cellular respiration does not oxidize glucose in a single explosive step that transfers all the hydrogen from the fuel to the oxygen at one time. Respiration breaks down glucose in a series of steps: At key steps, hydrogen atoms are stripped from the glucose. Hydrogen atoms are not transferred directly to oxygen, but are passed first to a coenzyme NAD+ (nicotinamide adenine dinucleotide). Thus, NAD+ functions as an oxidizing agent during respiration. Cellular respiration does not oxidize glucose in a single explosive step that transfers all the hydrogen from the fuel to the oxygen at one time. Respiration breaks down glucose in a series of steps: At key steps, hydrogen atoms are stripped from the glucose. Hydrogen atoms are not transferred directly to oxygen, but are passed first to a coenzyme NAD+ (nicotinamide adenine dinucleotide). Thus, NAD+ functions as an oxidizing agent during respiration.

19. 9-19 How does NAD+ trap electrons? Enzymes called dehydrogenases remove a pair of hydrogen atoms from substrate Think of it as the removal of 2 electrons and 2 protons (the nuclei of hydrogen atoms) But the enzyme delivers two electrons and only one proton to its coenzyme (NAD+) Other proton is released as a hydrogen ion (H+) into surrounding solution: HCOH + NAD+ ? CO + NADH + H+

20. 9-20 NAD+ and NADH: The oxidized form NAD+ has a positive charge, but NADH is neutral: NAD+ has its charge neutralized by receiving two electrons, but only one proton NADH for the reduced form shows the hydrogen received

21. 9-21 NAD+ as an electron shuttle NAD+ gains electrons: it is an electron acceptor or oxidizing agent NAD+ is the most versatile electron acceptor in cellular respiration, functions in many redox step NAD+ gains electrons: it is an electron acceptor or oxidizing agent NAD+ is the most versatile electron acceptor in cellular respiration, functions in many redox step

22. 9-22 NADH molecule represents “stored energy” that can be tapped to make ATP when the electrons complete their “fall” from NADH to oxygen An electron transport chain is used to break the fall of electrons to oxygen using several energy-releasing steps Consists mostly of proteins built into inner membrane of a mitochondrion Electrons removed from food are “shuttled” by NADH to the “top” end of the chain At “bottom” end, oxygen captures these electrons along with H+, forming water

23. 9-23 An intro to electron transport chains

24. 9-24 Electron transfer from NADH to oxygen is an exergonic reaction ?G = -53 kcal/mol Electrons cascade down chain from one carrier to next, losing small amount of energy at each step, until they reach oxygen Each carrier is more electronegative than the its “uphill” neighbor During cellular respiration, most electrons travel a “downhill” route: food ? NADH ? electron transport chain ? oxygen The energy released from this exergonic electron fall is used to regenerate its supply of ATP. Remember oxygen has a high affinity for electrons! Remember oxygen has a high affinity for electrons!

25. 9-25 The Process of Cellular Respiration

26. 9-26 An Overview of Respiration Respiration involves: Glycolysis Krebs Cycle Electron Transport Chain and Oxidative Phosphorylation Glycolysis occurs in cytoplasm Krebs Cycle, Electron Transport Chain and Oxidative Phosphorylation occur in mitochondria

27. 9-27

28. 9-28 Glycolysis and Krebs Cycle are catabolic pathways that decompose glucose and other organic fuels Electron Transport Chain accepts electrons from breakdown products of first two stages (via NADH) and passes these from one molecule to another at end of chain, the electrons are combined with H+ and O2 to form H20 the energy released at each step is stored in a form the mitochondria can use to make ATP

29. 9-29 Oxidative phosphorylation is used to synthesize ATP and is powered by redox reactions that transfer electrons from food to oxygen accounts for 90% of the ATP Substrate-level phosphorylation generates small amount of ATP during Glycolysis and Krebs Cycle enzyme transfers a phosphate group from substrate molecule to ADP “Substrate molecule” refers to an organic molecule generated during catabolism of glucose“Substrate molecule” refers to an organic molecule generated during catabolism of glucose

30. 9-30 Substrate-level phosphorylation Some ATP is made by direct enzymatic transfer of a phosphate group from a substrate to ADP The phosphate donor in this case is phosphoenolpyruvate (PEP), which is formed by the breakdown of sugar during glycolysis Some ATP is made by direct enzymatic transfer of a phosphate group from a substrate to ADP The phosphate donor in this case is phosphoenolpyruvate (PEP), which is formed by the breakdown of sugar during glycolysis

31. 9-31

32. 9-32 For each molecule of glucose degraded to carbon dioxide and water by respiration, the cell makes up to about 38 molecules of ATP.

33. 9-33 Glycolysis Harvests chemical energy by oxidizing glucose to pyruvate splits one 6-carbon sugar into two 3-carbon sugars the 3-carbon sugars are oxidized and rearranged to form two pyruvate molecules Consists of ten steps, each catalyzed by a specific enzyme, divided into two phases: energy investment phase energy payoff phase Glycolysis means “splitting of sugar” Pyruvate is the ionized form of a three-carbon acid, pyruvic acid Glycolysis means “splitting of sugar” Pyruvate is the ionized form of a three-carbon acid, pyruvic acid

34. 9-34 Glycolysis:Energy input and output Summary of energy input and output of glycolysis: During energy investment phase, cell spends ATP to phosphorylate fuel molecules. Repaid in energy payoff phase when: ATP is produced by substrate phosphorylation, and NAD+ is reduced to NADH by oxidation of the food Net energy yield from glycolysis, per glucose molecule is: 2 ATP + 2 NADH Summary of energy input and output of glycolysis: During energy investment phase, cell spends ATP to phosphorylate fuel molecules. Repaid in energy payoff phase when: ATP is produced by substrate phosphorylation, and NAD+ is reduced to NADH by oxidation of the food Net energy yield from glycolysis, per glucose molecule is: 2 ATP + 2 NADH

35. 9-35

36. 9-36 Energy Payoff Phase

37. 9-37 Carbon in glucose is accounted for in two pyruvate molecules No CO2 is released Occurs with or without O2 If O2 present: energy stored in NADH can be converted to ATP energy by the electron transport chain and oxidative phosphorylation chemical energy left in pyruvate can be extracted by the Krebs cycle

38. 9-38 Krebs Cycle When O2 is present, pyruvate enters mitochondrion, enzymes of the Krebs cycle complete the oxidation of the organic fuel Upon entering, pyruvate is converted to acetyl coenzyme A (acetyl CoA) This step is the transition between glycolysis and Krebs cycle It is accomplished by a multi-enzyme complex that catalyzes three reactions Glycolysis releases less than a quarter of the chemical energy stored in glucose Most of the energy remains stocked in the two molecules of pyruvateGlycolysis releases less than a quarter of the chemical energy stored in glucose Most of the energy remains stocked in the two molecules of pyruvate

39. 9-39 Step 1: carboxyl group removed, CO2 diffuses out Step 2: remaining 2-carbon fragment is oxidized, forms acetate enzyme transfers extracted electrons to NAD+, storing energy in form of NADH Step 3: coenzyme A attaches to acetate by unstable bond, makes acetyl group very reactive The final product is acetyl Co-A, which enters Krebs cycle for further oxidizing

40. 9-40 Conversion of pyruvate to acetyl CoA Conversion of pyruvate to acetyl CoA, the junction between glycolysis and the Krebs cycle, involves three reactions: Step 1: carboxyl group removed, CO2 diffuses out Step 2: remaining 2-carbon fragment is oxidized, forms acetate (ionized form of acetic acid) enzyme transfers extracted electrons to NAD+, storing energy in form of NADH Step 3: coenzyme A attaches to acetate by unstable bond, makes acetyl group very reactive The final product is acetyl Co-A, which enters Krebs cycle for further oxidizingConversion of pyruvate to acetyl CoA, the junction between glycolysis and the Krebs cycle, involves three reactions: Step 1: carboxyl group removed, CO2 diffuses out Step 2: remaining 2-carbon fragment is oxidized, forms acetate (ionized form of acetic acid) enzyme transfers extracted electrons to NAD+, storing energy in form of NADH Step 3: coenzyme A attaches to acetate by unstable bond, makes acetyl group very reactive The final product is acetyl Co-A, which enters Krebs cycle for further oxidizing

41. 9-41 Krebs cycle has eight steps, each catalyzed by a specific enzyme For each turn, two carbons enter as acetate, and two different carbons leave in oxidized form of CO2 Acetate joins the cycle by its enzymatic addition to oxaloacetate, forming citrate Subsequent steps decompose oxaloacetate, giving off CO2 It is the regeneration of oxaloacetate that accounts for the “cycle” in the Krebs cycle

42. 9-42 Most of energy harvested by oxidative steps is conserved in NADH. for each acetate that enters the cycle, three NAD+ are reduced to NADH. In one oxidative step, electrons are transferred to FAD (flavin adenine dinucleotide). reduced form FADH2 donates its electrons to the electron transport chain Another step forms an ATP by substrate-level phosphorylation

43. 9-43 NADH and FADH2 relay extracted electrons to electron transport chain to make ATP by oxidative-phosphorylation Krebs cycle turns twice, once for each of the two pyruvate generated by glycolysis

44. 9-44 Krebs cycle

45. 9-45 Summary of Krebs Cycle Results in: 3 CO2 (including the one from pre-Krebs cycle conversion of pyruvate to acetyl-CoA) 1 ATP per turn by substrate phosphorylation Most of chemical is transferred during redox rxns to NAD+ and FAD. The reduced coenzymes, NADH and FADH2, shuttle the energy-electrons to electron transport chain, which uses the energy to synthesize ATP by oxidative-phosphorylation TOTAL x 2: 3 CO2 x 2 = 6 CO2 1 ATP x 2 = 2 ATP 4 NADH x 2 = 8 NADH 1 FADH2 x 2 = 2 FADH2Results in: 3 CO2 (including the one from pre-Krebs cycle conversion of pyruvate to acetyl-CoA) 1 ATP per turn by substrate phosphorylation Most of chemical is transferred during redox rxns to NAD+ and FAD. The reduced coenzymes, NADH and FADH2, shuttle the energy-electrons to electron transport chain, which uses the energy to synthesize ATP by oxidative-phosphorylation TOTAL x 2: 3 CO2 x 2 = 6 CO2 1 ATP x 2 = 2 ATP 4 NADH x 2 = 8 NADH 1 FADH2 x 2 = 2 FADH2

46. 9-46 Electron Transport Glycolysis and Krebs cycle have produced four ATP per glucose molecule by substrate phosphorylation NADH and FADH2 account for most of the energy extracted from the food These electron escorts link glycolysis and Krebs cycle to the machinery for oxidative phosphorylation

47. 9-47 The energy released by the electron transport chain is used to power ATP synthesis by oxidative phosphorylation Electron transport chain is a collection of molecules embedded in inner membrane of mitochondrion Folding of inner membrane to form cristae increases its surface area; provides space for thousands of chains

48. 9-48 Most components of chain are proteins. Bound to proteins are prosthetic groups, non-protein components essential for catalytic functions of certain enzymes During electron transport along chain, prosthetic groups alternate between reduced and oxidized states as they accept and donate electrons

49. 9-49 Free energy change during electron transport Shows sequence of electron carriers in electron transport chain and the drop in free energy as electrons travel down the chain Electrons removed from food during glycolysis and Krebs cycle are transferred by NADH to first molecule of electron transport chain: Flavoprotein (has prosthetic group called flavin mononucleotide – FMN) In next redox rxn, FMN returns to its oxidized form as it passes electrons to an iron sulfur protein (FeS) Which then passes electrons to ubiquinone (Q – lipid, only one not a protein) Most of remaining carriers are cytochromes (Cyt) Prosthetic group called heme group – has four organic rings surrounding a single iron atoms Similar to iron-containing prosthetic group found in hemoglobin Iron of cytochromes transfers electrons, no oxygen Several types of cytochromes Last one is cyt a3, passes its electrons to oxygen, which also picks of pair of H atoms to form water Shows sequence of electron carriers in electron transport chain and the drop in free energy as electrons travel down the chain Electrons removed from food during glycolysis and Krebs cycle are transferred by NADH to first molecule of electron transport chain: Flavoprotein (has prosthetic group called flavin mononucleotide – FMN) In next redox rxn, FMN returns to its oxidized form as it passes electrons to an iron sulfur protein (FeS) Which then passes electrons to ubiquinone (Q – lipid, only one not a protein) Most of remaining carriers are cytochromes (Cyt) Prosthetic group called heme group – has four organic rings surrounding a single iron atoms Similar to iron-containing prosthetic group found in hemoglobin Iron of cytochromes transfers electrons, no oxygen Several types of cytochromes Last one is cyt a3, passes its electrons to oxygen, which also picks of pair of H atoms to form water

50. 9-50 Electrons removed from food during glycolysis and Krebs cycle are transferred by NADH to the first molecule of the electron transport chain These molecules include: Flavin mononucleotide (FMN) FeS (iron-sulfur) Ubiquinone (Q) – only carrier that is a lipid Cytochromes (Crt) – most are these, has iron (heme) group that transfers electrons

51. 9-51 Last cytochrome of the chain, cyt a3, passes its electron to oxygen Oxygen also picks up pair of H ions to form water For every two NADH, one O2 molecule is reduced to two molecules of water FADH2 also adds electrons, but at a lower energy level Electron transport chain does not directly make ATP, it functions to ease the “fall” of electrons from breakdown of food to oxygen

52. 9-52 Chemiosmosis and ATP Synthase Electron transport is coupled to ATP synthesis by chemiosmosis ATP synthase is a protein complex, makes ATP from ADP and inorganic phosphate Uses energy of an existing ion gradient to power ATP synthesis -- H+ gradient drives oxidative phosphorylation

53. 9-53 ATP Synthase ATP synthase protein complex functions as “mill” and is powered by flow of H+ Located in mitochondrial and chloroplast membranes of eukaryotes and in plasma membranes of prokaryotes Has four parts consisting of polypeptide subunits Rotor – rotates clockwise when H+ flows past it down H+ gradient Stator- anchored in membrane, holds knob stationary Rod – “stalk” extends into knob, also spins, activates catalytic sites in knob Knob – has catalytic sites that join inorganic phosphate to ADP to make ATP ATP synthase protein complex functions as “mill” and is powered by flow of H+ Located in mitochondrial and chloroplast membranes of eukaryotes and in plasma membranes of prokaryotes Has four parts consisting of polypeptide subunits Rotor – rotates clockwise when H+ flows past it down H+ gradient Stator- anchored in membrane, holds knob stationary Rod – “stalk” extends into knob, also spins, activates catalytic sites in knob Knob – has catalytic sites that join inorganic phosphate to ADP to make ATP

54. 9-54 Power source for ATP synthase is difference in [H+] on opposite sides of the inner mitochondrial membrane At certain steps along electron transport chain, electron transfer causes electron-carrying protein complexes to move H+ from matrix to intermembrane space Stores energy as a proton-motive force (H+ gradient)

55. 9-55 As H+ diffuses back into the matrix through ATP synthase, its exergonic passage drives the endergonic phosphorylation of ADP to ATP

56. 9-56 Chemiosmosis couples the electron transport chain to ATP synthesis

57. 9-57 Review of Cellular Respiration

58. 9-58 Related Metabolic Processes

59. 9-59 Fermentation Enables some cells to produce ATP without help from oxygen Glycolysis generates two ATP whether oxygen is present or not, aerobic or anaerobic Anaerobic catabolism of organic nutrients can occur by fermentation Fermentation can generate ATP solely by substrate-level phosphorylation

60. 9-60 Fermentation consists of glycolysis plus reactions that regenerate NAD+ by transferring electrons from NADH to pyruvate NAD+ can then be reused to oxidize sugar by glycolysis, netting two ATP by substrate-phosphorylation Two common types are alcohol fermentation and lactic acid fermentation

61. 9-61 Pyruvate is converted to ethanol Step 1: pyruvate is converted to two carbon acetaldehyde, releasing CO2 Step 2: acetaldehyde is reduced by NADH to ethanol Regenerates the supply of NAD+ needed for glycolysis Used in brewing and winemaking,

62. 9-62 Alcohol Fermentation

63. 9-63 Lactic Acid Fermentation Pyruvate is reduced directly by NADH to form lactate as a waste product, with no release of CO2 Used to make cheese, yogurt, acetone, methanol. Human muscle cells make ATP by lactic acid fermentation when low in oxygen

64. 9-64 Lactic Acid Fermentation

65. 9-65 Pyruvate as a key junction in catabolism Some organisms (yeasts and many bacteria), can make enough ATP to survive using either fermentation or respiration - called facultative anaerobes At cellular level, muscle cells behave as facultative anaerobes. Pyruvate is a fork in the road to two catabolic routes. Some organisms (yeasts and many bacteria), can make enough ATP to survive using either fermentation or respiration - called facultative anaerobes At cellular level, muscle cells behave as facultative anaerobes. Pyruvate is a fork in the road to two catabolic routes.

66. 9-66 Glycolysis occurs in nearly all organisms and probably evolved in ancient prokaryotes before there was O2 in the atmosphere

67. 9-67 Catabolism of various food molecules Carbohydrates, fats, and proteins can all be used as fuel for cellular respiration. Monomers of these food molecules enter glycolysis or the Krebs cycle at various points. Glycolysis and the Krebs cycle are catabolic funnels through which electrons from all kinds of food molecules flow on their exergonic fall to oxygen. Carbohydrates, fats, and proteins can all be used as fuel for cellular respiration. Monomers of these food molecules enter glycolysis or the Krebs cycle at various points. Glycolysis and the Krebs cycle are catabolic funnels through which electrons from all kinds of food molecules flow on their exergonic fall to oxygen.

68. 9-68 Feedback mechanisms control cellular respiration Cellular respiration is controlled by allosteric enzymes at key points in glycolysis and the Krebs cycle. This helps the cell strike a moment-to-moment balance between catabolism and analolism.

69. 9-69 Control of Cellular Respiration Allosteric enzymes at certain points in the respiratory pathway respond to inhibitors and activators that help set the pace of glycolysis and the Krebs cycle. Example: Phosphofructokinase, enzyme that catalyzes step 3 of glycolysis. It is stimulated by AMP (derived from ADP), but it is inhibited by ATP and by citrate. This feedback regulation adjusts the rate of respiration as the cell’s catabolic and anabolic demands change. Allosteric enzymes at certain points in the respiratory pathway respond to inhibitors and activators that help set the pace of glycolysis and the Krebs cycle. Example: Phosphofructokinase, enzyme that catalyzes step 3 of glycolysis. It is stimulated by AMP (derived from ADP), but it is inhibited by ATP and by citrate. This feedback regulation adjusts the rate of respiration as the cell’s catabolic and anabolic demands change.