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Transcript
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 • 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 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 Table 2.1 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 Figure 2.1 Figure 2.4 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) 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) Figure 2.5 Figure 2.6 Glycolytic System • Anaerobic • ATP yield: 2 to 3 mol ATP/1 mol substrate • Duration: 15 s to 2 min • Breakdown of glucose via glycolysis 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 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 Oxidation of Carbohydrate • Stage 1: Glycolysis • Stage 2: Krebs cycle • Stage 3: Electron transport chain Figure 2.8 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 Figure 2.9 Figure 2.11 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 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 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 Figure 2.12 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 Figure 2.13 Table 2.3 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 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 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