How Cells Harvest Energy

1. How Cells Harvest Energy Chapter 7

2. Respiration • Organisms can be classified based on how they obtain energy: • autotrophs: are able to produce their own organic molecules through photosynthesis • heterotrophs: live on organic compounds produced by other organisms • All organisms use cellular respiration to extract energy from organic molecules.

3. Respiration • Respiration is a metabolic pathway of Redox Reactions • Respiration typically oxidizes carbohydrates • The type of molecule that is reduced determines the type of respiration • The energy produced is in the form of ATP 3

4. Respiration • Cellular respiration a metabolic pathway of redox reactions: -oxidation – loss of electrons • dehydrogenations: loss of hydrogen e-’s -reduction - gain of electrons • gain on hydrogen e-’s • Oxidized molecules actually loose a hydrogen atom (1 electron, 1 proton) • Both the protons and electrons are used by cellular respiration to produce ATP

5. Respiration • During respiration, high energy electrons are passed along chains of molecules - Electron Transport Chains • Energy is released as molecules in the electron transport chains are oxidized • The energy released is used to power the production of ATP

7. Three Types of Respiration • Respiration type is determined by the final electron acceptors: • Aerobic Respiration: final electron receptor is oxygen (O2) • Anaerobic Respiration: final electron acceptor is an inorganic molecule other than O2 • Fermentation: final electron acceptor is an organic molecule

8. Aerobic Respiration • Glucose contains chemical energy that can be transferred and stored as ATP • Aerobic Respiration is a metabolic pathway that oxidizes glucose and transfers the energy to produce ATP • Oxygen is the final electron acceptor: • Recall: C6H12O6 + 6 O2 6 H2O + 6 CO2 + Energy Glucose Oxygen Water Carbon Dioxide • The Energy is in the form of ATP

9. Aerobic Respiration C6H12O6 + 6 O2 6 H2O + 6 CO2 + Energy -Now- C6H12O6 + 6O2 + 38 ADP + 38 P 6 H2O + 6CO2 + 38 ATP

10. Aerobic Respiration • Aerobic Respiration is a three stage process: Stage 1: Glycolysis Stage 2: The Krebs Cycle Stage 3: Oxidative Phosphorylation • Each of these stages produce ATP • At the end of all three stages, there is a net gain of 38 ATP molecules (profit) • recall: cells are very efficient because of enzymes

12. Stage 1: Glycolysis • Glycolysis is a 10 step metabolic pathway that cleaves glucose • Glyo-lysis = “splitting glucose” • Glycolysis occurs in the cell’s cytoplasm • that’s where the enzymes for glycolysis are located

13. Stage 1: Glycolysis Glycolysis converts glucose to pyruvate (pyruvic acid). - a 10-step biochemical pathway - occurs in the cytoplasm - 2 molecules of pyruvate are formed from each glucose - net production of 2 ATP molecules -2 NADH produced by reduction of 2 NAD+ • recall NADH just is an electron carrier • (see ch. 6)

14. Stage 1: Glycolysis • During Glycolysis, glucose (a 6 carbon molecule) is chopped up into 2Pyruvates (each pyruvate is a 3 carbon molecule) • As glucose is cleaved, it is also being • oxidized - loosing electrons (hydrogens)

15. Figure 6_07 • Glucose is cut up into 2 Pyruvates in 10 steps

16. Figure 6_07 • In step 1 ATP is used to phosphorylate glucose to make • G-6-P • Phosphorylation destabilizes the glucose molecule so it can be cleaved • Phosphorylation reactions are carried out by enzymes known as Kinases - see chapter 6 and slide 38 • 2 ATP must be invested during • first two steps of glycolysis

17. Figure 6_07 • In step 2, G-6-P is converted to F-6-P • This step is carried out by an isomerase enzyme • recall isomers from ch. 3 and slide 32 • 2 ATP must be invested during • first two steps of glycolysis

18. Figure 6_07 • In step 3, 1 ATP is used to phosphorylate F-6-P to become F-1,6-bP • this step is carried out by another kinase • 2 ATP must be invested during • first two steps of glycolysis

19. Figure 6_07 • In steps 4 and 5, the six-carbon molecule, F-1,3-bP is cleaved into 2 three-carbon molecules • G-3-P and Dihydroxyacetone phosphate (DHAP) • Dihydroxyacetone phosphate is immediately converted into another G-3-P

20. Figure 6_07 • Very Important! • Steps 6-10 occur twice for every glucose that enters glycolysis • because there are now two G-3P’s

21. Figure 6_07 • In step 6, G-3-P’s are oxidized • one NAD+ is reduced to produce one NADH • Also in step 6, G-3-P’s are phosphorylated to produce 1,3-BPG

22. Figure 6_07 • One ATP is produced in step 7 • 1,3-BPG is dephosphorylated to become 3-BPG • ADP is phosphorylated to ATP

23. Figure 6_07 • Steps 8 and 9 involve structural changes of 3-BPG to become Phosphoenolpyruvate

24. Figure 6_07 • In step 10, one more ATP is produced as Phosphoenolpyruvate is dephosphorylated to become Pyruvic Acid • another kinase

25. Fig. 7.6-1

26. Fig. 7.6-2

27. Fig. 7.6-3

28. Fig. 7.7-1

29. Fig. 7.7-2

30. Fig. 7.7-3

31. GlycolysisTotals Per Glucose Costs • 2 ATP Yield • 2 NADH • 4 ATP Net Gain from Glycolysis • 2 ATP • 2 NADH

32. Glucose Pyruvate1 Pyruvate 2 2 ATP 4 ATP, 2 NADH

33. NAD+, NADH • During redox reactions, electrons carry energy from one molecule to another • NAD+ is an electron carrier • NAD+ functions to carry electrons by carrying Hydrogen atoms • NAD+ accepts 2 electrons and 1 proton to become NADH • The reaction is reversible • NAD+ + 2e-’s + 1p+ NADH

34. NAD+

35. NAD+ Reduced to NADH

38. FAD • FAD is very similar to NAD+ • It has the same function of collecting and carrying Hydrogen atoms from one molecule to another • FAD can carry 2 Hydrogen atoms • FAD is Reduced to FADH2

39. Stage 2: The Krebs Cycle • Also known as The Citric Acid Cycle • citrate is the first molecule produced in this cycle • The Krebs cycle is a metabolic pathway that further cleaves and oxidizes pyruvate • The Krebs Cycle occurs in the cell membrane of Prokaryotic Cells and in the mitochondria of Eukaryotic Cells • In mitochondria, a multienzyme complex called pyruvate dehydrogenase catalyzes the reaction

41. Stage 2: The Krebs Cycle • The Krebs cycle is fueled with pyruvates from glycolysis • recall, there are 2 Pyruvates made from each Glucose, so there are 2 Krebs Cycles for every glucose molecule • Prep Step - before pyruvate enters the mitochondria for the Krebs cycle it is cleaved, oxidized and converted to become Acetyl-Coenzyme A (Acetyl-CoA) Pyruvate CO2 Acetyl-CoA Krebs Cycle

42. Prep Step: Pyruvate Oxidation • Pyruvates are oxidized to form Acetyl-CoA • a CO2 moiety of pyruvate is exchanged for a Coenzyme A(CoA) moiety • The products of pyruvate oxidation include: • 1 CO2 • 1 NADH • 1 acetyl-CoA which consists of 2 carbons from pyruvate attached to coenzyme A • Acetyl-CoA proceeds to the Krebs cycle

43. Prep Step: Pyruvate Oxidation • Prep Step: Pyruvates are converted • to Acetyl-CoA with the release of CO2 • in a preparation step

45. Stage 2: The Krebs Cycle • The Krebs cyclefurther oxidizes the acetyl group from pyruvate. • Occurs in the matrix of the mitochondria • Biochemical pathway of 5 steps • First Step: Each Acetyl-CoA (2 carbon per molecule) is bonded to an Oxaloacetate (a 4 carbon molecule) to produce Citrate acetyl group + oxaloacetate citrate (2 carbons) (4 carbons) (6 carbons)

46. Stage 2: The Krebs Cycle First Step of Krebs Cycle: • Each Acetyl CoA (2 carbon per molecule) is bonded to an Oxaloacetate (a 4 carbon molecule) • The new molecule made is Citrate (a 6 carbon molecule) acetyl-CoA + oxaloacetate citrate (2 carbons) (4 carbons) (6 carbons)

47. Stage 2: The Krebs Cycle • Citrate undergoes a five step cycle that builds additional ATPS • During the Krebs Cycle additional NADH’s and FADH2’s are produced • Citrate is eventually converted back into oxaloacetate and the cycle continues

49. Fig. 7.12-2 In step 1, acetyl-CoA enters the mitochondria and combines with oxaloacetate to form citrate

50. Fig. 7.12-2 Citrate is further oxidized to produce 3 NADH and and FADH2