Fuel for Exercise: Bioenergetics and Muscle Metabolism 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 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 Pros 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