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Metabolism and Bioenergetics • Nutrients from foods are the substrates for metabolism and are provided and stored as: – Carbohydrate – Fat – Protein • Each cell contains chemical pathways that convert these substrates to usable energy, a process called bioenergetics • The energy we derive from food is stored in cells in the form of adenosine triphosphase (ATP) • ATP serves as the immediate source of energy for most body functions including muscle contraction Kilocalorie • Energy in biological systems is measured in kilocalories • All energy eventually degrades to heat • One kilocalorie is the amount of heat energy needed to raise 1 kg of water from 1 °C to 15 °C Energy Sources • At rest, the body uses carbohydrate and fat almost equally for energy • Protein provides little energy for cellular activity, but serves as the building blocks for the body’s tissues • During intense short-duration muscular effort, the body relies mostly on carbohydrate to generate ATP • Longer, less intense exercise utilizes carbohydrates and fat for sustained energy production Carbohydrate • All dietary carbohydrate is ultimately converted to glucose • Glucose is taken up by muscles and liver and converted to the complex sugar molecule called glycogen • Glycogen is stored in the cytoplasm of muscle cells, where it can be quickly used to form ATP • Glycogen is also stored in the liver, where it is converted back to glucose as needed and transported by the blood to the muscles to form ATP Fat • Provides substantial energy during prolonged, low-intensity activity • Body stores of fat are larger than carbohydrate reserves • Fat is less readily available for cellular metabolism compared to carbohydrate • Fat is stored as triglycerides and must be broken down to free fatty acids (FFAs) to be used in metabolism • More energy is derived from breaking down fat (9.4 kcal/g) compared to carbohydrate (4.1 kcal/g) Protein • Protein can be used as a minor source of energy, but it must be converted to glucose via glucogenesis • Proteins can generate FFAs during starvation through lipogenesis • Protein can supply up to 5-10% of energy during prolonged exercise • Proteins must be broken down to their basic units—amino acids—to be used for energy The Lock-and-Key Action of Enzymes in the Catabolism of Compounds • Enzymes control the rate of free energy release from substrates • Enzyme names end with the suffix -ase Energy for Cellular Metabolism Key Points • We derive energy from food and store it as the highenergy compound ATP – Carbohydrate (4.1 kcal/g) – Fat (9.4 kcal/g) – Protein (4.1 kcal/g) • Carbohydrate is stored as glycogen in muscles and the liver and is more accessible than protein or fat • Glucose, directly from food or broken down from glycogen, is the usable form of carbohydrate • Fat is stored as triglycerides in adipose tissue and broken down to FFA ATP Is Generated Through 3 Energy Systems 1. ATP-PCr system 2. Glycolytic system 3. Oxidative system ATP Is Generated Through 3 Energy Systems ATP-PCr System • Cells store small amounts of ATP, and phosphocreatine (PCr), which is broken down to regenerate ATP • Release of ATP from PCr is facilitated by the enzyme creatine kinase • This process does not require oxygen (anaerobic) • ATP and PCr sustain the muscle’s energy needs for 3-15 sec during an all-out sprint • 1 mole of ATP is produced per one mole of PCr ATP-Phosphocreatine System ATP-PCr Although ATP is being used at a very high rate, the energy from PCr is used to resynthesize ATP, preventing the ATP level from decreasing. At exhaustion, both ATP and PCr concentrations are low. Glycogen Breakdown and Synthesis Glycolysis is the breakdown of glucose; it may be anaerobic or aerobic. Glycogenesis is the process by which glycogen is synthesized from glucose to be stored in the liver or muscle. Glycogenolysis is the process by which glycogen is broken down into glucose-1-phosphate to be used for energy production. The Glycolytic System • Requires 10-12 enzymatic reactions to break down glycogen to lactic acid, producing ATP • Occurs in the cytoplasm • Glycolysis does not require oxygen (anaerobic) • Without oxygen present, pyruvic acid produced by glycolysis becomes lactic acid • 1 mole of glycogen produces 3 moles of ATP; 1 mole of glucose produces 2 moles of ATP because 1 mole is used to convert glucose to glucose-6-phosphate • ATP-PCr and glycolysis provide the energy for ~2 min of all-out activity Glycolysis ATP-PCr and Glycolytic Systems Key Points • ATP-PCr system – Pi is separated from PCr by creatine kinase – Pi is combined with ADP to form ATP – Energy yield: 1 mole of ATP per 1 mole of PCr • Glycolytic system – Glucose or glycogen is broken down to pyruvic acid – Without oxygen, pyruvic acid is converted to lactic acid – 1 mole of glucose yields 2 moles of ATP Energy Sources for the Early Minutes of Intense Exercise The combined actions of the ATP-PCr and glycolytic systems allow muscles to generate force in the absence of oxygen; thus these two energy systems are the major energy contributors during the early minutes of highintensity exercise. The Oxidative System • The oxidative system uses oxygen to generate energy from metabolic fuels (aerobic) • Oxidative production of ATP occurs in the mitochondria • Can yield much more energy (ATP) than anaerobic systems • The oxidative system is slow to turn on • Primary method of energy production during endurance events Aerobic Glycolysis and the Electron Transport Chain Krebs Cycle Oxidation of Carbohydrate 1. In the presence of oxygen, pyruvic acid from glycolysis is converted to acetyl coenzyme A (acetyl CoA) 2. Acetyl CoA enters the Krebs cycle and forms 2 ATP, carbon dioxide, and hydrogen 3. Hydrogen ion created during glycolysis and through the Krebs cycle combines with two coenzymes (NAD and FAD) 4. NAD and FAD carry hydrogen ions to the electron transport chain (NAD and FAD → NADH and FADH) The Electron Transport Chain 1. The electron transport chain splits NADH and FADH, producing hydrogen ions which are recombined with oxygen to produce water 2. Electrons produced from the split of NADH and FADH provide the energy for the phosphorylation of ADP to ATP 3. One molecule of glycogen can generate up to 37-39 molecules of ATP Oxidative Phosphorylation: The Electron Transport Chain Oxidation of Fat • Lipolysis is the breakdown of triglycerides into glycerol and free fatty acids (FFAs) • FFAs travel via blood to muscle fibers and are broken down by enzymes in the mitochondria into acetic acid, which is converted to acetyl CoA through β-oxidation • Acetyl CoA enters the Krebs cycle and the electron transport chain • Fat oxidation requires more oxygen compared with glucose because a FFA molecule contains more carbon Oxidation of Protein • Body uses little protein during rest and exercise (5-10% to sustain prolonged exercise) • Some amino acids can be converted into glucose or intermediates of oxidative metabolism • Energy yield from protein is difficult to determine • The nitrogen in amino acids is converted into urea, which requires ATP Oxidative Capacity of Muscle • Oxidative capacity of muscle (QO2). is a measure of its maximal capacity to use oxygen • Representative enzymes to measure oxidative capacity – Succinate dehydrogenase – Citrate synthase Oxidative Capacity in Muscle Oxidative Metabolism Key Points • The oxidative system involves the breakdown of substrates in the presence of oxygen • Oxidation of carbohydrate involves glycolysis, the Krebs cycle, and the electron transport chain, resulting in the formation of H2O, CO2, and 38-39 molecules of ATP • Fat oxidation involves β-oxidation of free fatty acids, the Krebs cycle, and the electron transport chain to produce more ATP than carbohydrate • The maximum rate of ATP formation from fat oxidation is too low to match the rate of ATP utilization during high intensity exercise (continued) Oxidative Metabolism (continued) Key Points • Protein contributes little to energy production, and its oxidation is complex because amino acids contain nitrogen, which cannot be oxidized • The oxidative capacity of muscle fibers depends on their oxidative enzyme levels, fiber-type composition, and oxygen availability Measuring Energy Costs of Exercise Direct calorimetry measures the body’s heat production to estimate energy expenditure Indirect calorimetry calculates energy expenditure from the ratio of CO2 produced to O2 consumed A Direct Calorimeter for Human Use Respiratory Exchange Ratio . • The ratio between CO2 released (VCO2) and oxygen . consumed . (VO2) . • RER = VCO2 / VO2 • The RER value at rest is usually 0.78 to 0.80 RER: Determining Substrate Utilization Carbohydrate 6 O2 + C6H12O6 → 6 CO2 + 6 H2O + 38 ATP . . RER = VCO / VO 2 2 = 6 CO2 / 6 O2 = 1.0 Fat C16H32O2 +. 23 O2 → 16 CO + 16 H O + 129 ATP 2 2 . RER = VCO2 / VO2 = 16 CO2 / 23 O2 = 0.70 Calorimetry Key Points • Direct calorimetry involves using a large chamber to directly measure heat production by the body • Indirect calorimetry involves measuring O2 consumption and CO2 production • RER at rest = 0.78 to 0.80 • RER oxidation of fat = 0.70 • RER oxidation of carbohydrate = 1.0 • Isotopes can be used to determine metabolic rate over long periods of time Resting Metabolic Rate (RMR) RMR is the minimum amount of energy required by the body to sustain basic cellular function – Fat-free mass – Body surface area – Ranges from 1,100 to 2,500 kcal/day – When activity is added, daily caloric expenditure is 1,700 to 3,100 kcal/day Factors That Affect RMR • Age: RMR gradually decreases with age, generally because of a decrease in fat-free mass • Body temperature: RMR increases with increasing temperature • Psychological stress: Stress increases activity of the sympathetic nervous system • Hormones: Thyroxine from the thyroid gland and epinephrine from the adrenal medulla both increase RMR Metabolic Rate During Submaximal Exercise • Metabolism increases in direct proportion to the increase in exercise intensity • During exercise at a constant power output (work rate) VO2 increases from its resting value to a steady-state value within . 1-2 minutes • There is a linear increase in the VO2 with increases in power . output (work rate) Increase in Oxygen Uptake with Increasing Power Output Reprinted, by permission, from G.A. Gaesser and D.C. Poole, 1996, “The slow component of oxygen uptake kinetics in humans,” Exercise and Sport Sciences Reviews 24: 36. Increase in Oxygen Uptake with Increasing Power Output Reprinted, by permission, from G.A. Gaesser and D.C. Poole, 1996, “The slow component of oxygen uptake kinetics in humans,” Exercise and Sport Sciences Reviews 24: 36. Maximal Oxygen Uptake . • VO2max: The maximal capacity for oxygen consumption by the body during maximal exertion • Single best measurement of cardiorespiratory endurance and aerobic fitness • Increases with physical training • Generally expressed relative to body weight (ml · kg-1 · min-1) • Normally active untrained college-aged students = 38-42 -1 -1 ml . · kg · min • VO2max declines in active people after age 25-30 by ~ 1% per year Relationship Between Exercise Intensity and Oxygen Uptake in Trained and Untrained Man Estimating Anaerobic Effort • O2 consumption requires several minutes to reach the required steady state level at which the aerobic processes are fully functional • Oxygen deficit is calculated as the difference between the oxygen required for a given exercise intensity and the actual oxygen consumption • Anaerobic effort can be estimated by examining excess postexercise oxygen consumption (EPOC)—the mismatch between O2 consumption and energy requirements Oxygen Requirement During Exercise and Recovery Factors Responsible for EPOC • Rebuilding depleted ATP and PCr supplies • Clearing lactate produced by anaerobic metabolism • Replenishing O2 supplies borrowed from hemoglobin and myoglobin • Removing CO2 that has accumulated in body tissues • Increased metabolic and respiratory rates due to increased body temperature Lactate Threshold • It is the point at which blood lactate begins to accumulate substantially above resting concentrations during exercise of increasing intensity • The rate at which lactate production exceeds lactate clearance • Usually expressed as a percentage of maximal oxygen uptake • A high lactate threshold can indicate potential for better endurance performance • Lactate accumulation contributes to fatigue Relationship Between Exercise Intensity and Blood Lactate Concentration Lactate Threshold and Endurance Performance Lactate threshold. (LT), when expressed as a percentage of VO2max, is one of the best determinants of an athlete’s pace in endurance events such as running and cycling. While untrained people typically have LT around. 50% to 60% of their VO2max, elite athletes may not reach LT until around 70% to 80% VO2max. . Economy of Effort Fatigue and its Causes • Energy delivery (ATP-PCr, anaerobic glycolysis, and oxidation) • Accumulation of metabolic by-products, such as lactate and H+ • Failure of the muscle fiber’s contractile mechanism • Alteration in the nervous system Energy Systems and Fatigue • PCr depletion • Glycogen depletion (“hitting the wall”) – Pattern of glycogen depletion from Type I and II fibers depends on the duration and intensity of the activity – Glycogen depletion is selective to the muscle groups involved in the activity – Depletion of liver glycogen to increase blood glucose increases muscle glycogen utilization Decline in Muscle Glycogen Adapted, by permission, from D.L. Costill, 1986, Inside running: Basics of sports physiology (Indianapolis: Benchmark Press). Copyright 1986 Cooper Publishing Group, Carmel, IN. High Muscle Temperature Impairs Skeletal Muscle Function and Metabolism Adapted, by permission, from S.D.R. Galloway and R.J. Maughan, 1997, "Effects of ambient temperature on the capacity to perform prolonged cycle exercise in man," Medicine and Science in Sports and Exercise 29: 1240-1249. Metabolic By-Products and Fatigue • Short-duration activities depend on anaerobic glycolysis and produce lactate and H+ • Cells buffer H+ with bicarbonate (HCO3–) to keep cell pH between 6.4 (at exhaustion) and 7.1 • Intercellular pH lower than 6.9, however, slows glycolysis and ATP production • When pH reaches 6.4, H+ levels inhibit glycolysis and result in exhaustion Changes in Muscle pH During Sprint Exercise and Recovery Neuromuscular Fatigue Fatigue may involve: 1. Decreased release or synthesis of acetylcholine 2. Hyperactive acetylcholinesterase 3. Hypoactive acetylcholinesterase 4. Increased threshold for stimulation of the muscle fiber 5. Competition with ACh for the receptors on the muscle membrane 6. Potassium may leave the intracellular space, decreasing the membrane potential below resting values 7. Central nervous system fatigue Causes of Fatigue Key Points • Fatigue may result from depletion of PCr or glycogen, which impairs ATP production • The H+ generated by lactic acid leads to fatigue by decreasing muscle pH, which impairs the cellular processes of energy production and muscle contraction • Failure of neural transmission may cause some fatigue • The central nervous system may also limit exercise performance as a protective mechanism