How Cells Release Chemical Energy

1. How Cells Release Chemical Energy Chapter 7 Biology Concepts and Applications, Eight Edition, by Starr, Evers, Starr. Brooks/Cole, Cengage Learning 2011.

2. 7.1 Overview of Carbohydrate Breakdown Pathways • All organisms (including photoautotrophs) convert chemical energy of organic compounds to chemical energy of ATP • ATP is a common energy currency that drives metabolic reactions in cells

3. Pathways of Carbohydrate Breakdown • Photoautotrophs  Photosynthetic autotrophs • Produce sugar • Fermentation pathways  anaerobic pathway • End in cytoplasm, do not use oxygen, yield 2 ATP per molecule of glucose • Aerobic respiration  oxygen-requiring pathway that breaks down carbohydrates to produce ATP • Ends in mitochondria, uses oxygen, yields up to 36 ATP per glucose molecule • **Occurs in the presence of OXYGEN**

4. Pathways of Carbohydrate Breakdown

5. Overview of Aerobic Respiration • Three main stages of aerobic respiration: • Glycolysis in the cytoplasm • Convert glucose and other sugars to (2) pyruvate and(2) ATP • Pyruvate is 3-carbon end product of glycolysis 2. Krebs cycle 3. Electron transfer phosphorylation Summary equation: C6H12O6 + 6O2 → 6CO2 + 6 H2O

6. Overview of Aerobic Respiration

7. Key Concepts: ENERGY FROM CARBOHYDRATE BREAKDOWN • All organisms produce ATP by degradative pathways that extract chemical energy from glucose and other organic compounds • Aerobic respiration yields the most ATP from each glucose molecule • In eukaryotes, aerobic respiration is completed inside mitochondria

8. 7.3 Glycolysis – Glucose Breakdown Starts • Enzymes of glycolysis use two ATP to convert one molecule of glucose to two molecules of three-carbon pyruvate • Reactions transfer electrons and hydrogen atoms to two NAD+ (reduces to NADH) • 4 ATP form by substrate-level phosphorylation • Transfers a phosphate group directly from a substrate to ADP to form ATP

9. Products of Glycolysis • Net yield of glycolysis: • 2 pyruvate, 2 ATP, and 2 NADH per glucose • Pyruvate may: • Enter fermentation pathways in cytoplasm • Enter mitochondria and be broken down further in aerobic respiration

10. Glycolysis

11. Glycolysis

12. ENERGY REQUIRING PHASE Glycolysis glucose ATP ADP glucose–6–phosphate ATP ADP DHAP P P fructose–1,6–bisphosphate Fig. 7.4c1, p.111

13. ENERGY PRODUCING PHASE 2 PGAL 2 NAD+ + 2 Pi NADH 2 reduced coenzymes 2 PGA 2 ADP ATP 2 ATP produced by substrate-level phosphorylation 2 PEP 2 ADP ATP 2 ATP produced by substrate-level phosphorylation 2 pyruvate Net 2 ATP + 2 NADH to second stage Fig. 7.4c2, p.111

14. Key Concepts:GLYCOLYSIS • Glycolysis is the first stage of aerobic respiration and of anaerobic routes (fermentation pathways) • As enzymes break down glucose to pyruvate, the coenzyme NAD+ picks up electrons and hydrogen atoms • Net energy yield is two ATP

15. 7.4 Second Stage of Aerobic Respiration • The second stage of aerobic respiration takes place in the inner compartment of mitochondria • It starts with acetyl-CoA formation and proceeds through the Krebs cycle • Kreb cycle  cyclic pathway that, along with acetyl-CoA formation, breaks down pyruvate to carbon dioxide

16. Second Stage of Aerobic Respiration

17. Acetyl-CoA Formation • Two pyruvates from glycolysis are converted to two acetyl-CoA • Two CO2 leave the cell • Acetyl-CoA enters the Krebs cycle

18. Krebs Cycle • Each turn of the Krebs cycle, one acetyl-CoA is converted to two molecules of CO2 • After two cycles • Two pyruvates are dismantled • Glucose molecule that entered glycolysis is fully broken down

19. Energy Products • Reactions transfer electrons and hydrogen atoms to NAD+ and FAD • Reduced to NADH and FADH2 • ATP forms by substrate-level phosphorylation • Direct transfer of a phosphate group from a reaction intermediate to ADP

20. Net Results • Second stage of aerobic respiration results in • Six CO2, two ATP, eight NADH, and two FADH2 for every two pyruvates • Adding the yield from glycolysis, the total is • Twelve reduced coenzymes and four ATP for each glucose molecule • Coenzymes deliver electrons and hydrogen to the third stage of reactions

21. Second Stage Reactions

22. Acetyl–CoA Formation pyruvate coenzyme A NAD+ NADH CO2 acetyl–CoA coenzyme A Krebs Cycle oxaloacetate citrate CO2 NAD+ Krebs Cycle NADH NADH NAD+ CO2 NAD+ FADH2 FAD NADH ADP + Pi ATP Fig. 7.6a, p.113

23. Fig. 7.6b, p.113

24. 7.5 Third Stage:Aerobic Respiration’s Big Energy Payoff • Coenzymes deliver electrons and hydrogen ions to electron transfer chains in the inner mitochondrial membrane • Energy released by electrons flowing through the transfer chains moves H+ from the inner to the outer compartment

25. Hydrogen Ions and Phosphorylation • H+ ions accumulate in the outer compartment, forming a gradient across the inner membrane • H+ ions flow by concentration gradient back to the inner compartment through ATP synthases (transport proteins that drive ATP synthesis)

26. The Aerobic Part of Aerobic Respiration • Oxygen combines with electrons and H+ at the end of the transfer chains, forming water • Overall, aerobic respiration yields up to 36 ATP for each glucose molecule

27. Electron Transfer Phosphorylation

28. Fig. 7.7a, p.114

29. H+ INNER COMPARTMENT NADH FADH2 H2O ADP + Pi H+ H+ H+ ATP INNER MITOCHONDRIAL MEMBRANE H+ H+ H+ H+ H+ H+ H+ H+ 1/2 O2 H+ OUTER COMPARTMENT Fig. 7.7b, p.114

30. Key Concepts:HOW AEROBIC RESPIRATION ENDS • In the Krebs cycle (and a few steps before) • Pyruvate is broken down to carbon dioxide • Coenzymes pick up electrons and hydrogen atoms • In electron transfer phosphorylation • Coenzymes deliver electrons to transfer chains that set up conditions for ATP formation • Oxygen accepts electrons at end of chains

31. glucose 2 ATP Glycolysis ATP (2 net) 2 NAD+ 2 pyruvate 2 NADH CYTOPLASM OUTER MITOCHONDRIAL COMPARTMENT INNER MITOCHONDRIAL COMPARTMENT 2 acetyl-CoA 2 NADH 2 CO2 2 NADH 4 CO2 6 NADH Krebs Cycle 2 ATP 2 FADH2 ADP + Pi Electron Transfer Phosphorylation water 32 ATP H+ H+ H+ H+ H+ oxygen Fig. 7.8, p.115

32. 7.6 Anaerobic Energy-Releasing Pathways • Different fermentation pathways begin with glycolysis and end in the cytoplasm • Do not use oxygen or electron transfer chains • Final steps do not produce ATP; only regenerate oxidized NAD+ required for glycolysis to continue

33. Anaerobic Pathways • Lactate fermentation • End product: ATP & Lactate • Bacteria break down lactose in milk  produce buttermilk, cheese, and yogurt • Yeast  preserve pickles, cored beef, and sauerkraut • Alcoholic fermentation • End product: ATP & Ethyl alcohol (or ethanol) • Yeast to make bread  dough rises as CO2 forms bubbles • Both pathways have a net yield of 2 ATP per glucose (from glycolysis) and NAD+

34. Fig. 7.9a, p.116

35. Fig. 7.9b, p.116

36. Glycolysis glucose 2 NAD+ 2 ATP NADH 2 4 ATP pyruvate Lactate Fermentation NADH 2 2 NAD+ lactate Fig. 7.9c, p.116

37. Alcoholic Fermentation

38. Animal Skeletal Muscle • Red fibers (legs of chicken) • A lot of mitochondria and Myoglobin (stores oxygen) • Produce ATP by aerobic respiration • Sustain prolonged activity (marathon runs) • White fibers (wings of chicken) • Few mitochondria and no myoglobin  can not carry out a lot of aerobic respiration • Most ATP produced by lactate fermentation • ATP produced quick by not for long • Short strenuous activity (Sprinting and weight lifting) • Humans  Mixed fibers

39. Muscles and Lactate Fermentation

40. Key Concepts: HOW ANAEROBIC PATHWAYS END • Fermentation pathways start with glycolysis • Substances other than oxygen are the final electron acceptor • Compared with aerobic respiration, net yield of ATP is small

41. 7.7 Alternative Energy Sources in the Body • In humans and other mammals, foods enter aerobic respiration at various steps • Simple sugars from carbohydrates • Glycerol and fatty acids from fats • Carbon backbones of amino acids from proteins

42. Disposition of Organic Compounds

43. Alternative Energy Sources

44. FOOD fats PROTEINS COMPLEX CARBOHYDRATES glycerol amino acids glucose, other simple sugars fatty acids acetyl-coA acetyl-coA PGAL Glycolysis NADH pyruvate oxaloacetate or another intermediate of the Krebs Krebs Cycle NADH, FADH2 Electron Transfer Phosphorylation Fig. 7.12a, p.119

45. Key Concepts:OTHER METABOLIC PATHWAYS • Molecules other than glucose are common energy sources • Different pathways convert lipids and proteins to substances that may enter glycolysis or the Krebs cycle

46. Life’s Unity • Photosynthesis and aerobic respiration are interconnected on a global scale • In its organization, diversity, and continuity through generations, life shows unity at the bioenergetic and molecular levels

47. Energy, Photosynthesis, and Aerobic Respiration

48. Key Concepts:PERSPECTIVE AT UNIT’S END • Life shows unity in its molecular and cellular organization and in its dependence on a one-way flow of energy

49. Animation: Alternative energy sources

50. Animation: Fermentation pathways