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1. Fuel for Exercise:
Bioenergetics and Muscle
Metabolism

2. TerminologyTerminology
• Substrates
– Fuel sources from which we make energy
(adenosine triphosphate [ATP])
– Carbohydrate, fat, protein
• Bioenergetics
– Process of converting substrates into energy
– Performed at cellular level
• Metabolism: chemical reactions in the body

3. Measuring Energy ReleaseMeasuring 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)

4. CarbohydrateCarbohydrate
• 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

5. FatFat
• 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

6. Table 2.1Table 2.1

7. ProteinProtein
• 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

8. Figure 2.1Figure 2.1

9. Stored Energy:Stored Energy:
High-Energy PhosphatesHigh-Energy Phosphates
• ATP stored in small amounts until needed
• Breakdown of ATP to release energy
– ATP + water + ATPase  ADP + Pi + energy
– ADP: lower-energy compound, less useful
• Synthesis of ATP from by-products
– ADP + Pi + energy  ATP (via phosphorylation)
– Can occur in absence or presence of O2

10. Figure 2.4Figure 2.4

11. Bioenergetics: Basic Energy SystemsBioenergetics: 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)

12. ATP-PCr SystemATP-PCr System
• Anaerobic, substrate-level metabolism
• ATP yield: 1 mol ATP/1 mol PCr
• Duration: 3 to 15 s
• Because ATP stores are very limited, this
pathway is used to reassemble ATP

13. ATP-PCr SystemATP-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)

14. Figure 2.5Figure 2.5

15. Figure 2.6Figure 2.6

16. Glycolytic SystemGlycolytic System
• Anaerobic
• ATP yield: 2 to 3 mol ATP/1 mol substrate
• Duration: 15 s to 2 min
• Breakdown of glucose via glycolysis

19. Glycolytic SystemGlycolytic System
• Uses glucose or glycogen as its substrate
– Must convert to glucose-6-phosphate
– Costs 1 ATP for glucose, 0 ATP for glycogen
• Pathway starts with glucose-6-phosphate,
ends with pyruvic acid
– 10 to 12 enzymatic reactions total
– All steps occur in cytoplasm
– ATP yield: 2 ATP for glucose, 3 ATP for glycogen

20. Glycolytic SystemGlycolytic 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

21. Glycolytic SystemGlycolytic System
• Phosphofructokinase (PFK)
– Rate-limiting enzyme
 ATP ( ADP)   PFK activity
 ATP   PFK activity
– Also regulated by products of Krebs cycle
• Glycolysis = ~2 min maximal exercise
• Need another pathway for longer durations

22. Oxidative SystemOxidative 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

23. Oxidation of CarbohydrateOxidation of Carbohydrate
• Stage 1: Glycolysis
• Stage 2: Krebs cycle
• Stage 3: Electron transport chain

24. Figure 2.8Figure 2.8

25. Oxidation of Carbohydrate:Oxidation of Carbohydrate:
Glycolysis RevisitedGlycolysis 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

26. Oxidation of Carbohydrate:Oxidation of Carbohydrate:
Krebs CycleKrebs Cycle
• 1 Molecule glucose  2 acetyl-CoA
– 1 molecule glucose  2 complete Krebs cycles
– 1 molecule glucose  double ATP yield
• 2 Acetyl-CoA  2 GTP  2 ATP
• Also produces NADH, FADH, H+
– Too many H+
in the cell = too acidic
– H+
moved to electron transport chain

27. Figure 2.9Figure 2.9

28. Oxidation of Carbohydrate:Oxidation of Carbohydrate:
Electron Transport ChainElectron Transport Chain
• H+
, electrons carried to electron transport
chain via NADH, FADH molecules
• H+
, electrons travel down the chain
– H+
combines with O2 (neutralized, forms H2O)
– Electrons + O2 help form ATP
– 2.5 ATP per NADH
– 1.5 ATP per FADH

29. Oxidation of Carbohydrate:Oxidation of Carbohydrate:
Energy YieldEnergy Yield
• 1 glucose = 32 ATP
• 1 glycogen = 33 ATP
• Breakdown of net totals
– Glycolysis = +2 (or +3) ATP
– GTP from Krebs cycle = +2 ATP
– 10 NADH = +25 ATP
– 2 FADH = +3 ATP

30. Figure 2.11Figure 2.11

31. Oxidation of FatOxidation 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

32. ββ-Oxidation of Fat-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

33. Oxidation of Fat:Oxidation of Fat:
Krebs Cycle, Electron Transport ChainKrebs Cycle, Electron Transport Chain
• Acetyl-CoA enters Krebs cycle
• From there, same path as glucose oxidation
• Different FFAs have different number of
carbons
– Will yield different number of acetyl-CoA molecules
– ATP yield will be different for different FFAs
– Example: for palmitic acid (16 C): 129 ATP net yield

36. Table 2.2Table 2.2

37. Oxidation of ProteinOxidation 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

38. Figure 2.12Figure 2.12

39. Interaction Among Energy SystemsInteraction 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

40. Figure 2.13Figure 2.13

41. Table 2.3Table 2.3

42. Oxidative Capacity of MuscleOxidative 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

43. Enzyme ActivityEnzyme Activity
• Not all muscles exhibit optimal activity of
oxidative enzymes
• Enzyme activity predicts oxidative potential
• Representative enzymes
– Succinate dehydrogenase
– Citrate synthase
• Endurance trained versus untrained

44. Fiber Type CompositionFiber Type Composition
and Endurance Trainingand 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

45. Oxygen Needs of MuscleOxygen 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