HOW CELLS RELEASE STORED ENERGY

1. HOW CELLS RELEASE STORED ENERGY Chapter 7

2. Energy-releasing Pathways • Sun  producers  consumers (heterotrophs) • Producers harvest the sun’s energy to make glucose • Consumers eat the producers (and other consumers) to obtain energy (glucose) • Consumers break down glucose to convert its energy to ATP using aerobic (with O2) or anaerobic (without O2) respiration • The chemical equation for aerobic respiration shows its relationship to photosynthesis • C6H12O6 + 6O2 6CO2 + 6H2O + ATP • See the small figure on page 120 showing the relationship between photosynthesis and aerobic respiration

3. Energy-releasing Pathways • Both aerobic and anaerobic respiration start with glycolysis in the cytoplasm • Produces a small amount of ATP • Aerobic respiration continues in the mitochondria with the Kreb’s cycle and electron transfer phosphorylation (This is the same as electron transfer chain. The book uses phosphorylation, I will refer to it as the electron transfer chain.) • Produces a large amount of ATP

4. Fig. 7-2b, p.108

5. Fig. 7-3, p.109

6. Questions What are heterotrophs? Which type of respiration occurs in the presence of oxygen? What metabolic pathway is used by both aerobic and anaerobic respiration? Where does glycolysis occur? Where does the Krebs cycle occur? Which type of respiration produces the most ATP?

7. Glycolysis • Glycolysis is a metabolic pathway that occurs in the cytoplasm • Glucose is broken down through a series of intermediates to two pyruvate molecules • Study unit 7.3 and figure 7.5 • The following slide highlights some important steps of the pathway

8. Glycolysis • Two ATP molecules are used to energize the rearrangement of glucose into two 3-carbon molecules called PGAL • For glycolysis you have to spend some energy to earn some energy • Both PGAL molecules are rearranged through several intermediates • Electrons are stripped from PGAL producing NADH • ATP is produced when phosphate groups are transferred to ADP (phosphorylation) • The final product is two pyruvate molecules

10. Glycolysis • Results per glucose molecule • 2 ATP total • Used 2 ATP • Made 4 ATP • 2 NADH molecules • Co-enzymes carrying electrons and H+ • Will be shuttled to the electron transport chain • 2 pyruvate molecules

11. Questions What starting molecule is broken down during glycolysis? How much energy is required to get glycolysis started? After the input of energy what 3-carbon intermediate is formed? What is the final product of glycolysis? How much ATP is generated (gross and net)? How much NADH is generated?

12. Krebs Cycle • The Krebs cycle is a cyclic metabolic pathway that occurs in the mitochondria • Pyruvate is rearranged to form Acetyl-CoA which then enters the Krebs cycle where it is broken down in a series of steps to CO2 • Study unit 7.4 and figure 7.7 • The following slide highlights some important steps of the pathway

13. Krebs Cycle • Pyruvate enters the mitochondrion (remember there are two pyruvate molecules for every glucose molecule) • Pyruvate is changed to Acetyl-CoA by the following reactions • Electrons and H+ are stripped to make NADH • CO2 is released • Co-enzyme A is attached • Acetyl-CoA combines with oxaloacetate to form citrate • Several rearrangements and intermediates result in • Two more CO2 molecules are released • Three more NADH (and one FADH2, which is another electron carrying co-enzyme) form by stripping electrons and H+ from the intermediates • One molecule of ATP is formed • Ultimately oxaloacetate is reformed to start the cycle again

14. Fig. 7-6a, p.113

15. Krebs Cycle • Results per glucose molecule (remember there are two pyruvates produced per molecule of glucose) • Six CO2 molecules released • Accounts for all six carbons found in glucose C6H12O6 • Eight NADH plus two FADH2 molecules • Co-enzymes carrying electrons and H+ • Will be shuttled to the electron transport chain • Two ATP molecules

16. Questions Where does the Krebs cycle occur? Prior to starting the Krebs cycle pyruvate is changed to what molecule? Acetyl-CoA is bound to ______ to form _____. During the Krebs cycle electrons and hydrogen atoms are stripped and carried by what two co-enzymes? How many ATP molecules are formed (per one glucose)? How many NADH molecules are formed (per one glucose)?

17. Electron Transfer Chain(aka: electron transfer phosphorylation) • Electron transfer is an energy producing process that occurs over the inner mitochondrial membrane • Study unit 7.5 and figure 7.8 • The following slides highlight some important steps of the transfers

19. Electron Transfer Chain(aka: electron transfer phosphorylation) NADH and FADH2 carry the electrons and H+ to the electron transfer chain As the electrons move through the chain they release small amounts of energy allowing the transfer chain to shuttle H+ over the membrane

20. Fig. 7-7b, p.114

21. Electron Transfer Chain(aka: electron transfer phosphorylation) • An H+ gradient forms in the outer mitochondrial compartment (in between the two membranes) • The resulting gradient propels H+ across the mitochondrial membrane through ATP synthases • The flow has enough force to cause the synthases to attach phosphate to ADP, forming ATP • The process is called chemiosmosis or H+ electrochemical gradient

22. Fig. 7-7b, p.114

23. Electron Transfer Chain(aka: electron transfer phosphorylation) • Once the electrons have moved through the electron transfer chain, they are accepted by O2 which is the terminal electron acceptor • O2 combines the electrons and H+ to form water • O2 + H+ + electrons  H2O

24. Fig. 7-7b, p.114

25. Electron Transfer Chain(aka: electron transfer phosphorylation) • Results per glucose molecule • 32 molecules of ATP! • Depending on the needs of the cell the amounts can fluctuate • Shifting concentrations of reactant, intermediates and products • Shuttling mechanisms for moving NADH may use some ATP

26. Questions Where is the electron transfer chain? What carries the electrons to the chain? What happens when energy is released by electrons moving through the electron transfer chain? The build up of a H+ concentration gradient provides the force to cause what important event? For aerobic respiration what is the terminal electron acceptor? How many molecules of ATP can be formed by the electron transfer chain (per glucose molecule)?

27. Energy-releasing Pathways • Total possible for one molecule of glucose (cells don’t always harvest this much as they may use the intermediates in other processes) • Glycolysis = 2 ATP • Kreb’s = 2 ATP • Electron transfer = 32 ATP • Total = 36 ATP • See figure 7.9 for a summary of aerobic respiration

28. Fig. 7-8, p.115

29. Energy-releasing Pathways • Anaerobic respiration uses glycolysis, but due to the lack of oxygen Kreb’s and electron transfer are not used • Instead of oxygen being the terminal electron acceptor, an organic substance is used for NADH to donate the electrons (NADH must be recycled to NAD+ to be used in glycolysis again) • Alcoholic fermentation (fig 7.10) • Pyruvate is converted to acetaldehyde which accepts electrons from NADH producing ethanol and CO2 • Used to produce yeast breads and alcoholic beverages • Lactate fermentation (figure 7.11) • Pyruvate accepts electrons from NADH producing lactate • Muscle cells use this pathway when they are not receiving enough O2 • Anaerobic pathways are referred to as fermentation pathways

30. Fig. 7-9b, p.116

31. Fig. 7-10a, p.117

32. Fig. 7-10b, p.117

33. Fig. 7-10c, p.117

34. Fig. 7-9c, p.116

35. Fig. 7-11, p.117

36. Alternative Energy Sources • Glucose is a type of carbohydrate • Carbohydrates are an important part of the diet to provide energy • Proteins and lipids (fats) can also be used for energy • The molecules are broken down to form PGAL or Krebs cycle intermediates • They can then be used to produce ATP • See figure 7.12

37. Fig. 7-12, p.119

38. Questions Anaerobic respiration can also be called _____. During anaerobic respiration what type of molecule becomes the electron accepter for NADH? What pathway is used by yeast in bread making? What pathway can be used by some muscle fibers? What other molecules can be used for energy? What are alternate energy sources generally broken down to?

39. Summary • Aerobic respiration • Glycolysis • Kreb’s • Electron transfer chain • Anaerobic respiration/fermentation • Ethanol • Lactic acid • Alternate energy sources