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Chapter 2

Chapter 2. Fuel for Exercise: Bioenergetics and Muscle Metabolism. Terminology. Substrates Fuel sources from which we make energy (adenosine triphosphate [ATP]) Carbohydrate, fat, protein Bioenergetics Process of converting substrates into energy Performed at cellular level

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Chapter 2

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  1. Chapter 2 • Fuel for Exercise:Bioenergetics and Muscle Metabolism

  2. Terminology • Substrates • Fuel sources from which we make energy (adenosine triphosphate [ATP]) • Carbohydrate, fat, protein • Bioenergetics • Process of converting substrates into energy • Performed at cellular level • Metabolism: chemical reactions in the body

  3. Measuring Energy Release • Can be calculated from heat produced • 1 calorie (cal) = heat energy required to raise 1 g of water from 14.5°C to 15.5°C • 1,000 cal = 1 kcal = 1 Calorie (dietary)

  4. Carbohydrate • All carbohydrate converted to glucose • 4.1 kcal/g; ~2,500 kcal stored in body • Primary ATP substrate for muscles, brain • Extra glucose stored as glycogen in liver, muscles • Glycogen converted back to glucose when needed to make more ATP • Glycogen stores limited (2,500 kcal), must rely on dietary carbohydrate to replenish

  5. Fat • Efficient substrate, efficient storage • 9.4 kcal/g • +70,000 kcal stored in body • Energy substrate for prolonged, less intense exercise • High net ATP yield but slow ATP production • Must be broken down into free fatty acids (FFAs) and glycerol • Only FFAs are used to make ATP

  6. Table 2.1

  7. Protein • Energy substrate during starvation • 4.1 kcal/g • Must be converted into glucose (gluconeogenesis) • Can also convert into FFAs (lipogenesis) • For energy storage • For cellular energy substrate

  8. Figure 2.1

  9. Stored Energy: High-Energy Phosphates • ATP stored in small amounts until needed • Breakdown of ATP to release energy • ATP + water + ATPase  ADP + Pi + energy • ADP: lower-energy compound, less useful • Synthesis of ATP from by-products • ADP + Pi + energy  ATP (via phosphorylation) • Can occur in absence or presence of O2

  10. Figure 2.4

  11. Bioenergetics: Basic Energy Systems • ATP storage limited • Body must constantly synthesize new ATP • Three ATP synthesis pathways • ATP-PCr system (anaerobic metabolism) • Glycolytic system (anaerobic metabolism) • Oxidative system (aerobic metabolism)

  12. ATP-PCr System • Anaerobic, substrate-level metabolism • ATP yield: 1 mol ATP/1 mol PCr • Duration: 3 to 15 s • Because ATP stores are very limited, this pathway is used to reassemble ATP

  13. ATP-PCr System • Phosphocreatine (PCr): ATP recycling • PCr + creatine kinase  Cr + Pi + energy • PCr energy cannot be used for cellular work • PCr energy can be used to reassemble ATP • Replenishes ATP stores during rest • Recycles ATP during exercise until used up (~3-15 s maximal exercise)

  14. Figure 2.5

  15. Figure 2.6

  16. Glycolytic System • Anaerobic • ATP yield: 2 to 3 mol ATP/1 mol substrate • Duration: 15 s to 2 min • Breakdown of glucose via glycolysis

  17. Glycolytic System • Uses glucose or glycogen as its substrate • Must convert to glucose-6-phosphate • Costs 1 ATP for glucose, 0 ATP for glycogen • Pathway starts with glucose-6-phosphate, ends with pyruvic acid • 10 to 12 enzymatic reactions total • All steps occur in cytoplasm • ATP yield: 2 ATP for glucose, 3 ATP for glycogen

  18. Glycolytic System • Cons • Low ATP yield, inefficient use of substrate • Lack of O2 converts pyruvic acid to lactic acid • Lactic acid impairs glycolysis, muscle contraction • Pros • Allows muscles to contract when O2 limited • Permits shorter-term, higher-intensity exercise than oxidative metabolism can sustain

  19. Glycolytic System • Phosphofructokinase (PFK) • Rate-limiting enzyme ATP ( ADP)   PFK activity  ATP   PFK activity • Also regulated by products of Krebs cycle • Glycolysis = ~2 min maximal exercise • Need another pathway for longer durations

  20. Oxidative System • Aerobic • ATP yield: depends on substrate • 32 to 33 ATP/1 glucose • 100+ ATP/1 FFA • Duration: steady supply for hours • Most complex of three bioenergetic systems • Occurs in the mitochondria, not cytoplasm

  21. Oxidation of Carbohydrate • Stage 1: Glycolysis • Stage 2: Krebs cycle • Stage 3: Electron transport chain

  22. Figure 2.8

  23. Oxidation of Carbohydrate:Glycolysis Revisited • Glycolysis can occur with or without O2 • ATP yield same as anaerobic glycolysis • Same general steps as anaerobic glycolysis but, in the presence of oxygen, • Pyruvic acid  acetyl-CoA, enters Krebs cycle

  24. Oxidation of Carbohydrate:Krebs Cycle • 1 Molecule glucose  2 acetyl-CoA • 1 molecule glucose  2 complete Krebs cycles • 1 molecule glucose  double ATP yield • 2 Acetyl-CoA  2 GTP  2 ATP • Also produces NADH, FADH, H+ • Too many H+ in the cell = too acidic • H+ moved to electron transport chain

  25. Figure 2.9

  26. Oxidation of Carbohydrate:Electron Transport Chain • H+, electrons carried to electron transport chain via NADH, FADH molecules • H+, electrons travel down the chain • H+ combines with O2 (neutralized, forms H2O) • Electrons + O2 help form ATP • 2.5 ATP per NADH • 1.5 ATP per FADH

  27. Oxidation of Carbohydrate:Energy Yield • 1 glucose = 32 ATP • 1 glycogen = 33 ATP • Breakdown of net totals • Glycolysis = +2 (or +3) ATP • GTP from Krebs cycle = +2 ATP • 10 NADH = +25 ATP • 2 FADH = +3 ATP

  28. Figure 2.11

  29. Oxidation of Fat • Triglycerides: major fat energy source • Broken down to 1 glycerol + 3 FFAs • Lipolysis, carried out by lipases • Rate of FFA entry into muscle depends on concentration gradient • Yields ~3 to 4 times more ATP than glucose • Slower than glucose oxidation

  30. b-Oxidation of Fat • Process of converting FFAs to acetyl-CoA before entering Krebs cycle • Requires up-front expenditure of 2 ATP • Number of steps depends on number of carbons on FFA • 16-carbon FFA yields 8 acetyl-CoA • Compare: 1 glucose yields 2 acetyl-CoA • Fat oxidation requires more O2 now, yields far more ATP later

  31. Oxidation of Fat:Krebs Cycle, Electron Transport Chain • Acetyl-CoA enters Krebs cycle • From there, same path as glucose oxidation • Different FFAs have different number of carbons • Will yield different number of acetyl-CoA molecules • ATP yield will be different for different FFAs • Example: for palmitic acid (16 C): 129 ATP net yield

  32. Table 2.2

  33. Oxidation of Protein • Rarely used as a substrate • Starvation • Can be converted to glucose (gluconeogenesis) • Can be converted to acetyl-CoA • Energy yield not easy to determine • Nitrogen presence unique • Nitrogen excretion requires ATP expenditure • Generally minimal, estimates therefore ignore protein metabolism

  34. Figure 2.12

  35. Interaction Among Energy Systems • All three systems interact for all activities • No one system contributes 100%, but • One system often dominates for a given task • More cooperation during transition periods

  36. Figure 2.13

  37. Table 2.3

  38. Oxidative Capacity of Muscle • Not all muscles exhibit maximal oxidative capabilities • Factors that determine oxidative capacity • Enzyme activity • Fiber type composition, endurance training • O2 availability versus O2 need

  39. Enzyme Activity • Not all muscles exhibit optimal activity of oxidative enzymes • Enzyme activity predicts oxidative potential • Representative enzymes • Succinate dehydrogenase • Citrate synthase • Endurance trained versus untrained

  40. Fiber Type Composition and Endurance Training • Type I fibers: greater oxidative capacity • More mitochondria • High oxidative enzyme concentrations • Type II better for glycolytic energy production • Endurance training • Enhances oxidative capacity of type II fibers • Develops more (and larger) mitochondria • More oxidative enzymes per mitochondrion

  41. Oxygen Needs of Muscle • As intensity , so does ATP demand • In response • Rate of oxidative ATP production  • O2 intake at lungs  • O2 delivery by heart, vessels  • O2 storage limited—use it or lose it • O2 levels entering and leaving the lungs accurate estimate of O2 use in muscle

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