1. Fuel for Exercise: Bioenergetics and Muscle Metabolism
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

16. 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

17. 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

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

19. Figure 2.8

20. 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

21. Figure 2.9

22. Figure 2.11

23. 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

24. 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

26. 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

27. Figure 2.12

28. 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

30. Figure 2.13

31. Table 2.3

32. 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

33. 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

34. 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