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Chapter 9: Cellular Respiration

Chapter 9: Cellular Respiration. Energy Transfer In Life. Reaction Coupling Catabolic reactions: reactants act as “fuels,” broken down with the help of enzymes Fermentation: sugar degradation without oxygen (anaerobic) Cellular Respiration: most efficient and prevalent means of respiration

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Chapter 9: Cellular Respiration

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  1. Chapter 9: Cellular Respiration

  2. Energy Transfer In Life

  3. Reaction Coupling • Catabolic reactions: reactants act as “fuels,” broken down with the help of enzymes • Fermentation: sugar degradation without oxygen (anaerobic) • Cellular Respiration: most efficient and prevalent means of respiration • Energy released from respiration can be used for cellular work (endergonic reactions) or as heat • Ex: ATP regeneration from ADP + Pi • Glucose (C6H12O6) – ΔG = - 686 kcal/mol • spontaneous • ADP + Pi + 7.3 kcal/mol ATP • C6H12O6 + 6 O2 6 CO2 + 6 H2O + Energy (ATP + heat)

  4. Oxidation/Reduction Reactions (Redox) • The relocation of e- releases E from organic molecules. • Loss of is e- oxidation. • Gain of is e- reduction. LEO the lion goes GER!!!

  5. e- donor – reducing agent (is oxidized) • e- acceptor – oxidizing agent (is reduced) • Note: Redox can happen without a complete transfer of electrons – can change e- • Highly electronegative atoms are strong oxidizers (ex. Oxygen)

  6. Redox Reactions • The substance being reduced actually gets “bigger” because the increased number of electrons allows for more bonds • Glucose oxidation transfers electrons (of hydrogen) to a lower energy state as it bonds with oxygen • Energy released is used in ATP regeneration

  7. Pulling e- away from an atom – requires energy • This is the activation energy of cell respiration • e- lose energy when then move from a less electronegative atom to a more electronegative atom (ex. Oxygen) • This powers ATP regeneration, and creates water • Hydrogen – low electronegativity • Oxygen – high electronegativity • Hydrocarbons – many uphill e- (high ∆G), such as found in our foods, including lipids and carbohydrates

  8. Hydrocarbons – many uphill e- • Excellent fuel source – lots of e- to travel downhill – energy released. • Therefore, the oxidation of glucose is an exergonic reaction • The energy of glucose’s electrons is harvested in a series of stepwise reactions relying on NAD+ and the electron tranport chain www.tva.gov

  9. Glucose is broken down in steps. • Electrons are removed and transported with protons • Both are carried by NAD+ - nicotinamide adenine dinucleotide • NAD+ is an e- acceptor and a proton carrier • Dehydrogenase: removes 2 hydrogen atoms from a substrate, thereby oxidizing it

  10. e- transfer to O2 from NADH – ΔG = - 53 kcal/mol • NADH holds stored energy that can be used in the future to power the creation of ATP Glucose H+ and e- NADH ETC Relies on proteins in inner membrane of mitochondria of eukaryotes OXYGEN Final e- acceptor (because oxygen is highly electronegative)

  11. The Stages of Cellular Respiration: A Preview • Glycolysis • Turns glucose into 2 pyruvate molecules • No O2 • Occurs in the cytoplasm • Relies on Substrate Level Phosphorylation • Substrate level phosphorylation uses an enzyme to add a substrate’s phosphate group to ADP • Catabolic: -∆G • Dehydrogenases and NAD+

  12. Citric Acid Cycle • Uses oxygen • Occurs in Mitochondrial Matrix • Substrate Level Phosphorylation • Catabolic • Dehydrogenases and NAD+ used, transfer of e- to NAD+ making NADH • Oxidative Phosphorylation: accepts e- from NADH and passes them through a chain of proteins • Uses oxygen • Mitochondrial Inner Membrane • Includes the Electron Transport Chain and Chemiosmosis • Anabolic – requires an energy input • Proton Pump and ATP synthase

  13. Glycolysis • Hexose to Triose • 6C to 3C • Oxidized to Pyruvate • Energy Investment Phase • Glucose becomes G3P, requiring an input of 2 ATP • Energy Payoff Phase • G3P becomes pyruvate, substrate level phosphorylation occurs twice, 2 ATP created • 2 net ATP

  14. Energy Investment Phase • Step 1: • Hexokinase • Phosphate ‘traps’ glucose • Increases reactivity • Step 2: • Isomerases

  15. Step 3: • Activated for cleavage • Phosphofructokinase (PFK) phosphorylates glucose • Allostericallyregulated: PFK is inhibited by ATP (ATP is an allosteric inhibitor) • Step 4: • Cleavage • Creation of Structural Isomers • Step 5: • Isomerase • Active molecule G-3-P • 2 ATP have been used

  16. Energy Payoff Phase • Step 6: • G3P is oxidized • Very exergonic • Phosphorylation of oxidized sugar

  17. Step 7: • Substrate Level phosphorylation • Sugar oxidized to an organic acid

  18. Step 8: • Phosphate relocated • Step 9: • Dehydration reaction • Creation of double bond • Phosphate bond unstable • Step 10: • Substrate level phosphorylation • Net 2 ATP produced.

  19. Glycolysis – a review • ATP used: 2 • ATP produced: 4 • 2 per G3P • All via substrate level phosphorylation • NADH produced: 2 • 1 per G3P

  20. The Glycolysis/Citric Acid Intermediate • Oxygen Required • Occurs in the Mitochondrial Matrix • Active transport of pyruvate into matrix, Pyruvate is converted to Acetyl Coenzyme A Very Reactive Fully oxidized – very little E 2-C molecule Sulfur-containing

  21. The Citric Acid Cycle • Tricarboxylic Acid Cycle • Krebs Cycle – Hans Krebs – 1930s • 8 Steps • Specific enzymes • Cycle – 2 time per glucose (1 time per pyruvate) • FAD – flavin adenine dinucleotide • Electron carrier similar to NAD+

  22. Step 1: • 2-C + 4-C = 6-C • Acetyl CoA + Oxaloacetate • Coenzyme A recycled • Step 2: • Isomerase • Step 3: • CO2 released • NAD+ NADH • Step 4: • CO2 released • NAD+ NADH • Coenzyme A added

  23. Step 5: • Coenzyme A removed • GDP GTP • Substrate-level phosphorylation – ATP • Step 6: • FAD FADH2

  24. Step 7: • Hydration reaction • Bond rearrangement • Step 8: • OAA regenerated • NAD+ NADH

  25. The Citric Acid Cycle – A Review • CO2 • Per glucose: 6 • Per pyruvate: 3 • NADH • Per glucose: 6 • Per pyruvate: 3 • FADH2 • Per glucose: 2 • Per pyruvate: 1 • KREBS CYCLE ANIMATION • ATP • Per glucose: 2 • Per pyruvate: 1

  26. Pathway of the Electron Transport Chain • Inner membrane of the mitochondria: contains a chain of several complexes (some are protein, others are non-protein) • Cristae • 4 protein components I- IV • Prosthetic groups: non-protein components that help transport e- • e- carriers arranged in a ‘downhill’ formation via e- carriers such as NADH and FADH2 • E- carriers alternate between reduced and oxidized forms • NADH begins at Protein Complex I • FADH2 begins at Protein Complex II

  27. The Path of Electrons: • Protein Complex I • Flavoprotein • Flavin mononucleotide • Iron-sulfide • Ubiquinone (Coenzyme Q) • Non-protein prosthetic group • Hydrophobic • Mobile • Protein Complex II • FAD • Iron-sulfide

  28. Protein Complex III • Cytochrome b (heme) • Iron-sulfide • Cytochrome c1 (heme) • Cytochrome c • Not in a protein – prosthetic group • Protein Complex IV • Cytochrome a (heme) • Cytochrome a3 (heme) • Oxygen • Final electron acceptor

  29. The Electron Transport Chain • Makes no ATP • ΔG = -53 kcal/mol (exergonic – power the endergonic creation of ATP) • Proton gradient created

  30. Chemiosmosis – Energy coupling • Inner mitochondrial memebrane • ATP synthase • Reverse ion pump • Endergonic reaction powered by ETC • Relies on proton-motive force • Bacteria: use gradient across cell membrane • Cells use chemiosmosis to generate ATP, do active transport and rotate flagella • 32-34 ATP produced • Glucose → NADH → ETC → Proton Motive Force → ATP ETC animation

  31. Accounting 101 • 1 NADH = generates ~3 ATP • 10 H+ across membrane • 3-4 H+ = 1 ATP • 1 FADH2 = generates ~ 2 ATP • Electron Shuttle (via active transport) into Mitochondrion from Cytoplasm varies with different cells • NAD+: liver cells • FAD: brain cells • Total 36-38 ATP produced • 40% efficient (rest is lost as heat) Electron shuttles span membrane MITOCHONDRION CYTOSOL 2 NADH or 2 FADH2 2 FADH2 2 NADH 2 NADH 6 NADH Glycolysis Oxidative phosphorylation: electron transport and chemiosmosis Citric acid cycle 2 Acetyl CoA 2 Pyruvate Glucose + 2 ATP + 2 ATP + about 32 or 34 ATP by oxidative phosphorylation, depending on which shuttle transports electrons from NADH in cytosol by substrate-level phosphorylation by substrate-level phosphorylation About 36 or 38 ATP Maximum per glucose:

  32. Fermentation • No O2 – anaerobic • Substrate-level phosphorylation • As in glycolysis • NAD+ is needed at start and therefore must be regenerated

  33. Alcoholic Fermentation • Yeast – for use in brewing and baking • Bacteria • Lactic Acid Fermentation • Bacteria – used in making yogurt • Fungi • Muscle cells – use LAF when oxygen is scarce • Liver recycles lactic acid back to lactate • Former thinking: Lactic Acid build-up in muscle was the cause of muscle cramping • Facultative Anaerobes: aerobic or anerobic • Obligate Anaerobes: cannot live in the presence of oxygen

  34. Glucose Pyruvate CYTOSOL O2 present Cellular respiration No O2 present Fermentation MITOCHONDRION Ethanol or lactate Acetyl CoA Citric acid cycle Fermentation v. Aerobic Respiration • Both use glycolysis to oxidize glucose and other organic fuels to pyruvate • Fermentation yields 2 ATP via substrate level phosphorylation • Aerobic respiration yields as much as 38 ATP via oxidative phosphorylation • NAD+ is the oxidizing agent in fermentation so oxygen is not involved. In fermentation, the final e- acceptor is pyruvate • Aerobic respiration’s final e- acceptor is oxygen • In fermentation, the energy of pyruvate is still unavailable to the cell

  35. Evolutionary Significance • Glycolysis is performed by almost all living things • Glycolysis does not require organelles • Probably evolved in ancient prokaryotes before there was oxygen in the atmosphere • Oldest bacterial fossils date to 3.5 bya, while scientists believe oxygen was not present until 2.7 bya • Heterotroph Hypothesis • Anaerobic Heterotrophs → Anaerobic Autotrophs (cyanobacteria) → Aerobic Heterotrophs → Aerobic Autotrophs

  36. Metabolic Pathways - Catabolism • Glycolysis derives sugar from many sources • Carbohydrates are digested into simple sugars • Proteins: are digested into amino acids • Deamination: removal of the amino group from amino acids • Removed amino acids eventually become ammonia (then uric acid or urea) • Fats • Glycerol is converted into G-3-P • Beta oxidation: changes fatty acids to 2-C fragments which are then converted into acetyl-CoA • Hydrocarbons of fats are an excellent source of fuel • 1 g of fat oxidized yields twice the ATP of a carbohydrate

  37. Metabolic Pathways – Anabolism • Biosynthesis: food molecules are reused to make needed molecules other than ATP • Create ½ amino acids • Nonessential amino acids are made in cells. Essential amino acids must be obtained in the diet. • Acetyl CoA is created from fatty acids • Dihydroxacetone Phosphate – fat precursor for glycolysis

  38. Regulation • Feedback Inhibition • Enzyme regulation • Phosphofructokinase • Allosteric: contains sites for inhibitors and activators • Inhibitors • ATP • Citrate: synchronized the rate of the CAC and glycolysis • Activators • AMP

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