Cellular Respiration: Harvesting Chemical Energy

1. Cellular Respiration:Harvesting Chemical Energy AP Biology Ms. Haut

2. Energy Flow • Energy flows into an ecosystem as sunlight and leaves as heat • Photosynthesis generates oxygen and organic molecules, which are used in cellular respiration • Cells use chemical energy stored in organic molecules to regenerate ATP, which powers work

3. Catabolic Pathways and Production of ATP • The breakdown of organic molecules is exergonic • Fermentation is a partial degradation of sugars that occurs without oxygen • Cellular respiration consumes oxygen and organic molecules and yields ATP • Although carbohydrates, fats, and proteins are all consumed as fuel, it is helpful to trace cellular respiration with the sugar glucose: C6H12O6 + 6O2 6CO2 + 6H2O + Energy (ATP + heat)

4. Cellular Respiration • ATP-producing catabolic process in which the ultimate electron acceptor is an inorganic compound, Oxygen • Most efficient catabolic pathway • Is an exergonic process (ΔG = -686kcal/mol)

5. Oxidation Reduction Electrons “fall” from Organic Molecules to Oxygen during Cellular Respiration C6H12O6 + 6O2 6CO2 + 6H2O + Energy (ATP + Heat)

6. The Stages of Cellular Respiration: A Preview • Cellular respiration has three stages: • Glycolysis (breaks down glucose into two molecules of pyruvate) • The Krebs Cycle (citric acid cycle) (completes the breakdown of glucose) • Oxidative phosphorylation (accounts for most of the ATP synthesis) • The process that generates most of the ATP is called oxidative phosphorylation because it is powered by redox reactions

7. Glycolysis • Catabolic pathway • Occurs in the cytosol • Partially oxidizes glucose (6C) into two pyruvate (3C) molecules

8. Krebs Cycle • Catabolic pathway • Occurs in mitochondrial matrix • Completes glucose oxidation by breaking down a acetyl-CoA into CO2

9. Glycolysis and Krebs Cycle Together produce: • Small amount of ATP by substrate-level phosphorylation • NADH by transferring electrons from substrate to NAD+ • Krebs Cycle also produces FADH2 by transferring electrons to FAD+

10. Electron Transport Chain • Located near inner membrane of mitochondrion • Accepts energized electrons from reduced coenzymes (NADH and FADH2) that are harvested during glycolysis and Krebs Cycle • Oxygen pulls electrons down ETC to a lower energy state

11. Oxidative Phosphorylation • Accounts for almost 90% of the ATP generated by cellular respiration • Energy released at each step of the ETC is stored in a form the mitochondrion can use to make ATP • Powered by redox reactions that transfer electrons from food to oxygen • Small amount of ATP is produced directly by the enzymatic transfer of phosphate from an intermediate substrate in catabolism to ADP (substrate-level phosphorylation)

12. Glycolysis • Glycolysis (“splitting of sugar”) breaks down glucose into two molecules of pyruvate • Two major phases: • Energy investment phase • Energy payoff phase

13. Glycolysis

14. Glycolysis

15. Glycolysis

16. Glycolysis

17. Conversion of Pyruvate to Acetyl CoA: Pyruvate Oxidation

18. Krebs Cycle http://library.thinkquest.org/C004535/media/kreb_cycle.gif

19. Citric acid cycle Glycolysis Oxidation phosphorylation ATP ATP ATP Acetyl CoA H2O Oxaloacetate Citrate Isocitrate Citric acid cycle LE 9-12_1

20. Citric acid cycle Glycolysis Oxidation phosphorylation ATP ATP ATP Acetyl CoA H2O Oxaloacetate Citrate Isocitrate CO2 Citric acid cycle NAD+ NADH + H+ a-Ketoglutarate CO2 NAD+ NADH Succinyl CoA + H+ LE 9-12_2

21. Citric acid cycle Glycolysis Oxidation phosphorylation ATP ATP ATP Acetyl CoA H2O Oxaloacetate Citrate Isocitrate CO2 Citric acid cycle NAD+ NADH + H+ Fumarate a-Ketoglutarate FADH2 CO2 NAD+ FAD Succinate NADH P i Succinyl CoA + H+ GDP GTP ADP ATP LE 9-12_3

22. Citric acid cycle Glycolysis Oxidation phosphorylation ATP ATP ATP Acetyl CoA NADH H2O + H+ NAD+ Oxaloacetate Malate Citrate Isocitrate CO2 Citric acid cycle NAD+ H2O NADH + H+ Fumarate a-Ketoglutarate FADH2 CO2 NAD+ FAD Succinate NADH P i Succinyl CoA + H+ GDP GTP ADP ATP LE 9-12_4

24. Net: 2 molecules of pyruvate 8 NADH 2 FADH2 2 ATP

25. During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis • Following glycolysis and the citric acid cycle, NADH and FADH2 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

26. The Pathway of Electron Transport • The electron transport chain is in the cristae of the mitochondrion • The carriers alternate reduced and oxidized states as they accept and donate electrons • Electrons lose energy as they go down the chain and are finally passed to O2, forming water • The electron transport chain generates no ATP

27. NADH 50 FADH2 Multiprotein complexes I FAD 40 FMN II Fe•S Fe•S Q III Cyt b Oxidative phosphorylation: electron transport and chemiosmosis Citric acid cycle Glycolysis Fe•S 30 Cyt c1 IV Free energy (G) relative to O2 (kcal/mol) Cyt c ATP ATP ATP Cyt a Cyt a3 20 10 2 H+ + 1/2 O2 0 H2O LE 9-13

28. Chemiosmosis • 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

29. INTERMEMBRANE SPACE A rotor within the membrane spins as shown when H+ flows past it down the H+ gradient. H+ H+ H+ H+ H+ H+ H+ A stator anchored in the membrane holds the knob stationary. A rod (or “stalk”) extending into the knob also spins, activating catalytic sites in the knob. H+ Three catalytic sites in the stationary knob join inorganic phosphate to ADP to make ATP. ADP + ATP P i MITOCHONDRAL MATRIX LE 9-14

30. 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 proton-motive force, emphasizing its capacity to do work

31. 10 NADH = 30 ATP 2 FADH2 = 4 ATP 34 ATP 1 NADH = 3 ATP 1 FADH2 = 2 ATP

33. Aerobic: existing in the presence of oxygen Anaerobic: existing in the absence of oxygen

34. Fermentation • Cellular respiration requires O2 to produce ATP • Glycolysis can produce ATP with or without O2 (in aerobic or anaerobic conditions) • In the absence of O2, glycolysis couples with fermentation to produce ATP

35. Fermentation • Anaerobic catabolism of organic nutrients • After pyruvate is produced in glycolysis, it is reduced, and NAD+ is regenerated • Prevents cell from depleting the pool of NAD+, needed in glycolysis • No additional ATP is produced

36. Pyruvate loses CO2 and is converted to the acetylaldehyde (2C) • NADH is oxidized to NAD+ and acetylaldehyde is reduced to ethanol (EtOH) • Many bacteria and yeast carry out alcohol fermentation under anaerobic conditions

37. Alcohol Fermentation Actively fermenting beer. The yeast mass is converting the sugars to alcohol and carbon dioxide. Yeast during brewing process http://www.langhambrewery.co.uk/content/fermentation-yeast.jpg www.berkshirebrewingcompany.com/.../page4.html

38. NADH is oxidized to NAD+ and pyruvate is reduced to lactate (lactic acid) • Commercially important products: cheese & yogurt • Human muscle cells switch to lactic acid fermentation when O2 is scarce. Lactate accumulates, slowly carried to liver and converted back to pyruvate when O2 is available

39. Organisms Classified by Oxygen Requirements • Strict (obligate) aerobes—require O2 for growth and metabolism • Strict (obligate) anaerobes—only grow in the absence of O2 (O2 is toxic) • Facultative anaerobes—capable of growing in either aerobic or anaerobic conditions

40. Versatility of Catabolism • Starchglucose in digestive tract • Liver converts glycogenglucose • Excess amino acidspyruvate, acetyl CoA, and α-ketoglutarate • Fatsglycerol + fatty acids • Glycerolglyceraldehyde phosphate • Fatty acidsacetyl CoA (beta oxidation)

41. Biosynthesis • Some organic molecules of food provide carbon skeletons or raw materials for making new macromolecules • Some organic molecules from digestion used directly in anabolic pathways • Some precursors come from glycolysis and Krebs Cycle • Anabolic pathways require ATP produced from catabolic pathways

42. Feedback Mechanism of Control • Ratio of ATP:ADP and AMP reflects energy status and phosphofructokinase is sensitive to changes in the ratio • Citrate and ATP = allosteric inhibitors • ADP and AMP = allosteric activators