1. ATP of Skeletal Muscles
Energy of Skeletal Muscles
Lecture -3
ATP of Skeletal Muscles

2. Objectives
Recognize the importance of ATP as energy source in skeletal muscle.
Understand how skeletal muscles derive and utilize ATP for energy.
Differentiate between energy metabolism in red and white muscle fibers.

3. ATP
ATP or adenosine triphosphate is the energy currency used by our body everyday to perform a number of tasks:
Maintain body temperature
Repair damaged cells
Digestion of food
Mechanical work – movement
ATP ↔ ADP +
Energy

4. ATP ATP is the most important form of chemical energy stored in cells
Breakdown of ATP into ADP+PO4 releases energy.
Muscles typically store limited amounts of ATP – enough to power 4-6 seconds of activity
So resting muscles must have energy stored in other ways.

5. Systems of generation of ATP
1- ATP-Ctreatine Phosphate system
2- Glycolytic system
3- Oxidative system
The process that facilitates muscular contraction is entirely dependent on body’s ability to provide & rapidly replenish ATP

6. INTERACTION OF ENERGY SYSTEMS
Immediate
Short-term
Long-term

7. Energy Transfer Systems and Exercise
100%
% Capacity of Energy System
Anaerobic Glycolysis
Aerobic Energy System
ATP – Creatine Phosphate
10 sec
30 sec
2 min
5+ min
Exercise Time

8. Energy systems for muscular exercise
Mole of ATP/min
Time to Fatigue
Immediate: ATP-CP
(ATP & creatine phosphate) 4
sec
Short Term: Glycolytic
(Glycogen-Lactic Acid)
2.5
1 - 2 min
Long Term: Oxidative 1
Unlimited time

9. Energy requirements
The three energy systems often operate simultaneously
during physical activity
HOWEVER
Relative contribution of each system to total energy requirement differs depending on exercise intensity & duration
Magnitude of energy from anaerobic sources
depends on person’s capacity and tolerance for lactic acid accumulation (Athletes are trained so that they will have better tolerance for lactic acid)
As exercise intensity diminishes & duration extends beyond 4 minutes,
energy more dependent on aerobic metabolism

10. Fatigued muscle no longer contracts due to:
Muscle Fatigue
Fatigued muscle no longer contracts due to:
1- Accumulation of lactic acid (low pH of sarcoplasm)
2- Exhaustion of energy resources ( ADP &  ATP)
3- Ionic imbalance: muscle cells is less responsive to
motor neuronal stimulation

11. What are the mechanisms by which muscle fibers obtain energy to power contractions?

12. Muscles and fiber types
White muscle : (glycolytic)
mostly fast fibers
pale (e.g. chicken breast)
Red muscle: (oxidative)
mostly slow fibers
dark (e.g. chicken legs)
Most human muscles are:
Mixed fibers
pink

13. Fast Vs. Slow Fibers
Type I
Type II

14. Slow fibers Abundant mitochondria Extensive capillary supply
High concentrations of myoglobin
Can contract for long periods of time
Fatigue resistant
Obtain their ATP mainly from fatty acids oxidation
by TCA cycle & ETC (oxidative phosphorylation)

15. Fast fibers Large glycogen reserves Relatively few mitochondria
Produce rapid & powerful contractions
BUT of short duration
Easily fatigued
Obtain their ATP mainly from anaerobic glycolysis

16. ENERGY REQUIREMENTS AND SOURCE OF ENERGY FOR SKELETAL MUSCLE ( Resting vs. Working)

17. ATP use in the resting muscle cell
During periods of muscular rest ATP is required for:
1- Glycogen synthesis (glycogenesis)
i.e. storage form of glucose to be used
during muscular exercise
2- Creatine phosphate production
i.e. energy storage compound to be used at the beginning of muscular contraction

18. Source of ATP in resting muscle fibers
Resting muscle fibers takes up free fatty acids from blood.
Fatty acids are oxidized (in the mitochondria) to produce acetyl CoA & molecules of NADH & FADH2
Acetyl CoA will then enter the citric acid cycle (in the mitochondria) ATP, NADH, FADH2 & CO2
NADH & FADH2 will enter the electron transport chain.
 synthesis of ATP

19. RESTING MUSCLE FIBERS METABOLISM
Figure 10–20a

20. ATP sources in working muscle
At the beginning of exercise, muscle fibers immediately use stored ATP
For the next 15 seconds, muscle fibers turn to the creatine phosphate.
This system dominates in events such as the 100m dash or lifting weights.

21. ATP sources in working muscle cont.
After the phosphagen system is depleted, the process of anaerobic glycolysis can maintain ATP supply for about 45-60s
Glycogen stored in muscles  Glucose  2 pyruvic acid + 2 ATPs
2 Pyruvic acid  2 lactic acid
Lactic acid diffuses out of muscles blood  liver  Glucose (by gluconeogenesis) blood  muscles
* It usually takes a little time for the respiratory and cardiovascular systems to catch up with the muscles and supply O2 for aerobic metabolism.

22. WORKING MUSCLE FIBERS METABOLISM
Figure 10–20c

23. ATP sources in working muscle cont.
Anaerobic metabolism is inefficient:
1- Large amounts of glucose are used for very small ATP returns.
2- Lactic acid is produced leading to muscle fatigue
Type of sports uses anaerobic metabolism?
Sports that requires bursts of speed and activity e.g. basketball.

24. ATP sources in working muscle cont.
Aerobic metabolism
occurs when the respiratory & cardiovascular
systems have “caught up with” the working muscles.
During rest and light to moderate exercise, aerobic metabolism contributes 95% of the necessary ATP.
Compounds which can be aerobically metabolized include:
Fatty acids, pyruvic acid (made from glucose via glycolysis) & amino acids.

25. WORKING MUSCLE FIBERS METABOLISM

26. Aerobic Vs. Anaerobic sources of energy
No requires oxygen
Source of energy:
ONLY Carbohydrate (anaerobic glycolysis)
End Products:
Lactate & ATP
Aerobic
Require oxygen
Source of energy:
mainly fatty acids, then carbohydrate
End Products:
CO2, H2O & ATP

27. The Cori cycle & The glucose-alanine cycle

28. The Cori cycle Liver converts lactate into glucose via gluconeogenesis
The newly formed glucose is transported to muscle to be used for energy again

29. The glucose-alanine cycle
Muscles produce:
1- Pyruvate from glycolysis during exercise
2- NH2 produced from normal protein degradation
Pyruvate + NH2  Alanine
Alanine is transported through the blood to liver
Liver converts alanine back to pyruvate
Alanine – NH2 = Pyruvate
Pyruvate is converted to glucose (gluconeogenesis).
Glucose is transported to muscle to be used for energy again.
Liver converts NH2 to urea for excretion (urea cycle)