Download Chapter 2. Fuel for Exercising Muscle

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