Chapter 7 How Cells Release Chemical Energy

1. Chapter 7How Cells Release Chemical Energy

2. A Mitochondrion

3. Two Main Metabolic Pathways • Aerobic metabolic pathways (using oxygen) are used by most eukaryotic cells • Anaerobicmetabolic pathways (which occur in the absence of oxygen) are used by prokaryotes and protists in anaerobic habitats

4. Aerobic Respiration • In modern eukaryotic cells, most of the aerobic respiration pathway takes place inside mitochondria • Like chloroplasts, mitochondria have an internal folded membrane system that allows them to make ATP efficiently • Electron transfer chains in this membrane set up hydrogen ion gradients that power ATP synthesis • At the end of these chains, electrons are transferred to oxygen molecules

5. INTERACTION: Structure of a mitochondrion To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

6. 7.2 Overview of Carbohydrate Breakdown Pathways • Photoautotrophs make ATP during photosynthesis and use it to synthesize glucose and other carbohydrates • Most organisms, including photoautotrophs, make ATP by breaking down glucose and other organic compounds

7. energy Photosynthesis glucose CO2 H2O O2 Aerobic Respiration energy Figure 7-2 p118

8. Overview of Aerobic Respiration • Three stages • Glycolysis • Acetyl-CoA formation and Krebs cycle • Electron transfer phosphorylation (ATP formation) C6H12O6 (glucose) + O2 (oxygen) → CO2 (carbon dioxide) + H2O (water) • Coenzymes NADH and FADH2 carry electrons and hydrogen

9. Aerobic Respiration glucose In the Cytoplasm 4 ATP (2 net) Glycolysis 2 ATP 2 pyruvate 2 NADH Krebs Cycle 6 CO2 2 ATP 8 NADH, 2 FADH2 In the Mitochondrion Electron Transfer Phosphorylation H2O oxygen 32 ATP Figure 7-3 p119

10. Aerobic Respiration vs. Anaerobic Fermentation • Aerobic respiration and fermentation both begin with glycolysis, which converts one molecule of glucose into two molecules of pyruvate • After glycolysis, the two pathways diverge • Fermentation is completed in the cytoplasm, yielding 2 ATP per glucose molecule • Aerobic respiration is completed in mitochondria, yielding 36 ATP per glucose molecule

11. Carbohydrate breakdown pathways start in the cytoplasm, with glycolysis. Glycolysis Fermentation concludes in cytoplasm. In eukaryotes, aerobic respiration concludes inside mitochondria. Figure 7-4 p119

12. ANIMATED FIGURE: Where pathways start and finish To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

13. Take-Home Message: How do cells access the chemical energy in carbohydrates? • Most cells convert the chemical energy of carbohydrates to chemical energy of ATP by aerobic respiration or fermentation • Aerobic respiration and fermentation pathways start in cytoplasm, with glycolysis • Fermentation is anaerobic and ends in the cytoplasm • Aerobic respiration requires oxygen. In eukaryotes, it ends in mitochondria

14. 7.3 Glycolysis – Glucose Breakdown Starts • The reactions of glycolysis convert one molecule of glucose to two molecules of pyruvate for a net yield of two ATP • An energy investment of ATP is required to start glycolysis

15. Glycolysis • Two ATP are used to split glucose and form 2 PGAL, each with one phosphate group • Enzymes convert 2 PGAL to 2 PGA, forming 2 NADH • Four ATP are formed by substrate-level phosphorylation(net 2 ATP) • Glycolysis ends with the formation of two three-carbon pyruvate molecules

16. ATP-Requiring Steps An enzyme (hexokinase) transfers a phosphate group from ATP to glucose, forming glucose-6-phosphate. 1 A phosphate group from a second ATP is transferred to the glucose-6phosphate. The resulting molecule is unstable, and it splits into two three carbon molecules. The molecules are interconvertible, so we will call them both PGAL (phosphoglyceraldehyde). Two ATP have now been invested in the reactions. 2 ATP-Generating Steps Enzymes attach a phosphate to the two PGAL, and transfer two electrons and a hydrogen ion from each PGAL to NAD+. Two PGA (phosphoglycerate) and two NADH are the result. 3 4 Enzymes transfer a phosphate group from each PGA to ADP. Thus, two ATP have formed by substrate-level phosphorylation. The original energy investment of two ATP has now been recovered. Enzymes transfer a phosphate group from each of two intermediates to ADP. Two more ATP have formed by substrate-level phosphorylation. Two molecules of pyruvate form at this last reaction step. 5 6 Summing up, glycolysis yields two NADH, two ATP (net), and two pyruvate for each glucose molecule. Depending on the type of cell and environmental conditions, the pyruvate may enter the second stage of aerobic respiration or it may be used in other ways, such as in fermentation. Stepped Art Figure 7-5 p121

17. Take-Home Message:What is glycolysis? • Glycolysis is the first stage of carbohydrate breakdown in both aerobic respiration and fermentation • The reactions of glycolysis occur in the cytoplasm • Glycolysis converts one molecule of glucose to two molecules of pyruvate, with a net energy yield of two ATP; two NADH also form

18. 7.4 Second Stage of Aerobic Respiration • The second stage of aerobic respiration completes the breakdown of glucose that began in glycolysis • Occurs in mitochondria • Includes two sets of reactions: acetyl CoA formation and the Krebs cycle (each occurs twice in the breakdown of one glucose molecule)

19. The Krebs Cycle • Krebs cycle • A sequence of enzyme-mediated reactions that break down 1 acetyl CoA into 2 CO2 • Oxaloacetate is used and regenerated • 3 NADH and 1 FADH2 are formed • 1 ATP is formed

20. Second Stage of Aerobic Respiration cytoplasm outer membrane inner membrane The breakdown of 2 pyruvate to 6 CO2 yields 2 ATP and 10 reduced coenzymes (8 NADH, 2 FADH2). The coenzymes will carry their cargo of electrons and hydrogen ions to the third stage of aerobic respiration. matrix

21. An enzyme splits a pyruvate coenzyme A NAD+ molecule into a two-carbon acetyl group and CO2. Coenzyme A binds the acetyl group (forming acetyl–CoA). NAD+ combines with released hydrogen ions and electrons, forming NADH. 1 The Krebs cycle starts as one carbon atom is transferred from acetyl–CoA tooxaloacetate. Citrate forms, and coenzyme A is regenerated. 2 The final steps of the Krebs cycle regenerate oxaloacetate. 8 A carbon atom is removed from an intermediate and leaves the cell as CO2. NAD+ combines with released hydrogen ions and electrons, forming NADH. 3 NAD+ combines with hydrogen ions and electrons, forming NADH. 7 The coenzyme FAD combines with hydrogen ions and electrons, forming FADH2. 6 A carbon atom is removed from another intermediate and leaves the cell as CO2, and another NADH forms. 4 One ATP forms by substrate-level phosphorylation. 5 Pyruvate’s three carbon atoms have now exited the cell, in CO2. Acetyl–CoA Formation and the Krebs Cycle Krebs Cycle Stepped Art Figure 7-7 p123

22. Take-Home Message: What happens during the second stage of aerobic respiration? • The second stage of aerobic respiration, acetyl–CoA formation and the Krebs cycle, occurs in the inner compartment (matrix) of mitochondria • The pyruvate that formed in glycolysis is converted to acetyl–CoA and CO2; the acetyl–CoA enters the Krebs cycle, which breaks it down to CO2 • For two pyruvate molecules broken down in the second-stage reactions, two ATP form, and ten coenzymes (eight NAD+; two FAD) are reduced

23. Electron Transfer Phosphorylation • Coenzymes NADH and FADH2 donate electrons and H+ to electron transfer chains • Active transport forms a H+ concentration gradient in the outer mitochondrial compartment • H+ follows its gradient through ATP synthase, which attaches a phosphate to ADP • Finally, oxygen accepts electrons and combines with H+, forming water

24. Electron Transfer Phosphorylation

25. Summary: The Energy Harvest • Typically, the breakdown of one glucose molecule yields 36 ATP • Glycolysis: 2 ATP • Acetyl CoA formation and Krebs cycle: 2 ATP • Electron transfer phosphorylation: 32 ATP

26. Figure 7-9 p125

27. 7.6 Fermentation • Fermentation pathways break down carbohydrates without using oxygen • The final steps in these pathways regenerate NAD+ but do not produce ATP

28. Two Fermentation Pathways • Alcoholic fermentation • Pyruvate is split into acetaldehyde and CO2 • Acetaldehyde receives electrons and hydrogen from NADH, forming NAD+ and ethanol • Lactate fermentation • Pyruvate receives electrons and hydrogen from NADH, forming NAD+ and lactate

29. Red and White Muscle Fibers • Red muscle fibers make ATP by aerobic respiration • Have many mitochondria • Myoglobin stores oxygen • Sustain prolonged activity • White muscle fibers make ATP by lactate fermentation • Have few mitochondria and no myoglobin • Sustain short bursts of activity

30. Figure 7-11b p127

31. Figure 7-11c p127

32. Take-Home Message:What is fermentation? • ATP can form by carbohydrate breakdown in fermentation pathways, which are anaerobic • The end product of lactate fermentation is lactate. The end product of alcoholic fermentation is ethanol • Both pathways have a net yield of two ATP per glucose molecule; the ATP forms during glycolysis • Fermentation reactions regenerate the coenzyme NAD+, without which glycolysis (and ATP production) would stop

33. 7.7 Alternative Energy Sources in Food • Aerobic respiration can produce ATP from the breakdown of complex carbohydrates, fats, and proteins • As in glucose metabolism, many coenzymes are reduced, and the energy of the electrons they carry ultimately drives the synthesis of ATP in electron transfer phosphorylation

34. Energy From Fats • Enzymes cleave fats into glycerol and fatty acids • Glycerol products enter glycolysis • Fatty acids are converted to acetyl Co-A and enter the Krebs cycle • Compared to carbohydrates, fatty acid breakdown yields more ATP per carbon atom • When blood glucose level is high, acetyl CoA is diverted from the Krebs cycle and into a pathway that makes fatty acids

35. Energy from Proteins • Enzymes split dietary proteins into amino acid subunits, which are used to build proteins or other molecules • The amino group is removed and converted into ammonia (NH3), a waste product eliminated in urine • Acetyl–CoA, pyruvate, or an intermediate of the Krebs cycle forms, depending on the amino acid

36. glucose starch (a complex carbohydrate) A Complex carbohydrates are broken down to their monosaccharide subunits, which can enter glycolysis. 1 Figure 7-12a p128

37. a triglyceride (fat) glycerol head fatty acid tails Figure 7-12b p128

38. Food Fats Complex Carbohydrates Proteins glycerol glucose, other simple sugars amino acids fatty acids 2 3 1 4 acetyl–CoA acetyl–CoA PGAL Glycolysis pyruvate NADH intermediate of Krebs cycle Krebs Cycle NADH, FADH2 Electron Transfer Phosphorylation Figure 7-12b p128

39. Take-Home Message: Can organic molecules other than glucose be used for energy? • Complex carbohydrates, fats, and proteins can be oxidized in aerobic respiration to yield ATP • First the digestive system and then individual cells convert molecules in food into intermediates of glycolysis or the Krebs cycle