Energy of Skeletal Muscles
Lecture -3
ATP of Skeletal Muscles
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.
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
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.
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
INTERACTION OF ENERGY SYSTEMS
Immediate
Short-term
Long-term
% Capacity of Energy System
100%
Energy Transfer Systems and Exercise
Anaerobic
Glycolysis
Aerobic
Energy
System
ATP – Creatine Phosphate
10 sec
30 sec
2 min
Exercise Time
5+ min
Energy systems for muscular exercise
Energy Systems
Immediate: ATP-CP
(ATP & creatine phosphate)
Short Term: Glycolytic
(Glycogen-Lactic Acid)
Long Term: Oxidative
Mole of
ATP/min
Time to
Fatigue
4
5 - 10 sec
2.5
1 - 2 min
1
Unlimited time
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
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
What are the mechanisms by which
muscle fibers obtain energy to
power contractions?
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
Fast Vs. Slow Fibers
Type I
Type II
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)
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
ENERGY REQUIREMENTS AND SOURCE OF
ENERGY FOR SKELETAL MUSCLE
( Resting vs. Working)
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
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
RESTING MUSCLE FIBERS METABOLISM
Figure 10–20a
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.
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.
WORKING MUSCLE FIBERS METABOLISM
Figure 10–20c
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.
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.
WORKING MUSCLE FIBERS METABOLISM
Aerobic Vs. Anaerobic sources of energy
Aerobic
Anaerobic
Require oxygen
NOT Requires oxygen
Source of energy:
mainly fatty acids, then
carbohydrate
End Products:
CO2, H2O & ATP
Source of energy:
ONLY Carbohydrate
(anaerobic glycolysis)
End Products:
Lactate & ATP
The Cori cycle
&
The glucose-alanine cycle
The Cori cycle
• Liver converts lactate into glucose via gluconeogenesis
• The newly formed glucose is transported to muscle to
be used for energy again
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)