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Chapter 2

Chapter 2. Fuel for Exercise: Bioenergetics and Muscle Metabolism. 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). Carbohydrate.

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Chapter 2

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

  2. 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)

  3. 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

  4. 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

  5. Table 2.1

  6. 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

  7. Figure 2.1

  8. Figure 2.4

  9. 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)

  10. 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)

  11. Figure 2.5

  12. Figure 2.6

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

  14. 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

  15. 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

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

  17. Figure 2.8

  18. 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

  19. Figure 2.9

  20. Figure 2.11

  21. 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

  22. 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

  23. 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

  24. Figure 2.12

  25. 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

  26. Figure 2.13

  27. Table 2.3

  28. 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

  29. 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

  30. 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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