Energy Production
Metabolic systems that supply energy for the body:
1) Aerobic metabolism: dependent on oxygen.
2) Anaerobic metabolism: independent of oxygen.
The use of which systems depend on:
1) Duration.
2) Intensity.
3) Type of physical activity.
Adenosine Triphosphate (ATP):
It is an energy-rich compound that continuously supply fuel for the body used by energy requiring
processes found within the cells of the body. It is called the main energy currency for the cell,
however, it has limited supply thats why it must be resynthesized continuously in the body to
provide a constant energy source during exercise.
The process of energy production:
Energy is released when ATP loses a phosphate creating ADP (adenosine diphosphate). Then ADP is
enzymatically combined with a high energy phosphate from CP (creatine phosphate), thus
resynthesizing ATP.
Creatine phosphate:
An important temporary storage form of high-energy phosphate in muscle cells. Although its
concentration in the muscle is 5 times that of ATP, it has limited supply and provide energy only for
8 seconds, such as in a tennis serve or sprint.
Creatine
CP + ADP →→→→ ATP + creatine
kinase
It is the fastest and most immediate means of replenishing ATP, and it is done anaerobically.
* If the exercise proceeds for longer periods, the resynthesis of ATP will be from additional
sources within the muscle cells → anaerobic or aerobic metabolism.
Anaerobic Metabolism (Lactic acid pathway):
It supplies ATP for more than 8 seconds of physical activity through anaerobic glycolysis (without
the presence of oxygen).
The breakdown of glucose anaerobically produces lactic acid from pyruvic acid and releases energy
for 60-120 seconds, because of the limited supply of NAD (nicotinic acid dehydrogenase), a
coenzyme that catalyzes the process. Also, supply for the pathway substrate is limited to blood
glucose or muscle glycogen. Liver glycogen contributes but is limited in amount.
The amount of ATP synthesized is limited and only 30 % efficient, thus contributes energy lasting
up to 60-120 seconds e.g. a 440 yard sprint and sprint swimming events.
When exercise continues at intensities beyond the body's ability to supply oxygen and convert lactic
acid to fuel, lactic acid is removed from the muscle and is transported into the bloodstream and
starts to accumulate lowering the pH to a level that interferes with enzymatic reaction causing
fatigue.
* Anaerobic pathway provides energy for short duration, high intensity exercise such as
sprinting and high intensity moves in basketball or in football.
Aerobic Pathway:
It produces ATP to support continuos muscle activity for longer than 90-120 seconds. The
availability of oxygen through the process of respiration is vital. Without oxygen ATP cannot be
supplied.
Through glycolysis, pyruvic acid is supplied, with the presence of oxygen, it is converted to acetyl
Coenzyme A which enters the Kreb's cycle which generates 36-38 ATP for every glucose molecule.
Thus aerobic pathway breaks down glucose for energy far more efficiently than anaerobic pathway.
Additional substrates:
The aerobic pathway provides ATP by metabolizing carbohydrates, fats and protein.
1) Glucose from aerobic glycolysis.
2) -Oxidation of fatty acids derived from lipolysis and provides large amounts of acetyl CoA.
3) Acetyl CoA from protein catabolism.
4) Many other Kreb's cycle intermediates from protein catabolism.
5) Direct oxidation of protein.
Aerobic Metabolism Limiting factors:
1) Availability of substrates.
2) Availability of Coenzymes.
3) Continuous and adequate supply of oxygen through the cardiovascular system.
4) Also, the capacity for intense exercise and its duration influence oxygen capabilities.
* Exercise intensity and duration are inversely related. For example, an athlete can not perform high
power, high-intensity moves over a prolonged period. In order to prolong the exercise duration, its
intensity must be reduced. That is because at high intensities, oxygen availability is decreased and
lactic acid starts to accumulate causing fatigue.
* The main energy source for longer duration, lower intensities exercise is from fat and aerobic
metabolism.
* However, as intensity increases, energy source for the body relies increasingly on carbohydrates.
Main Sources (Substrate) of Fuel for the Contracting Muscle:
1) Glycolytic pathway: using:
a) Glucose from dietary carbohydrates.
b) Stored glycogen.
c) Glucose synthesized from carbon skeletons of certain amino acids through the process of
gluconeogenesis.
2) Kreb's cycle: using:
a) Glucose.
b) Fatty acids.
c)Amino acids, primarily alanine and branched-chain amino acids.
Factors affecting type of substrate use:
1) Exercise duration.
2) Exercise intensity.
3) Fitness level.
4) Gender.
5) Dietary intake.
* At high intensity and short duration, the contracting muscle rely on anaerobic pathway for the
production of ATP using mainly glucose and glycogen. Muscle glycogen is of limited amount, thats
why athletes who workout at such high intensities quickly run out of energy.
* Basketball, football, soccer and swimming have a high rate of glycogen use, because of their
intermittent bursts of high-intensity sprints and running drills.
* In moderate-intensity sports or exercise such as jogging, hiking, aerobic dance, gymnastics,
cycling and recreational swimming, about half of the energy comes from aerobic breakdown of
muscle glycogen, whereas the other half comes from circulating blood glucose and fatty acids.
* Moderate to low-intensity exercises such as walking is fueled entirely by the aerobic pathway.
Thus to create energy, fat production increases.
Effect of Training:
1) Improves the cardiovascular system that is involved in delivering oxygen to the exercising
muscle.
2) Increases the number of mitochondria and the levels of enzymes involved in the aerobic
synthesis of ATP.
3) Increases the capacity for fatty acid metabolism.