Download The ATP-PCr energy system can operate with or without oxygen but

The ATP-PCr System
ATP and creatine phosphate (phosphocreatine or PCr) make up the ATP-PCr system. PCr
is broken down releasing a phosphate and energy, which is then used to rebuild ATP. ATP
is resynthesized by adding a phosphate to ADP in a process called phosphorylation. The
enzyme that controls the break down of PCr is called creatine kinase.
The ATP-PCr energy system can operate with or without oxygen but because it doesnt
rely on the presence of oxygen = anaerobic. During the first 5 – 10 seconds of
exercise regardless of intensity, the ATP-PCr is relied on almost exclusively. The ATP-PCr
system can sustain all-out exercise for about 10 seconds and during this time, the
potential rate for power output is at its greatest.
If activity continues beyond this initial period, the body must rely on another energy
system to produce ATP
The Glycolytic System
Glycolysis means the breakdown (lysis) of glucose and consists of a series of reactions
catalysed by enzymes. The carbohydrates we eat supply the body with glucose, which can
be stored as glycogen in the muscles or liver for later use.
Glycolysis is the predominant energy system used for all-out exercise lasting from 30
seconds to about 2 minutes and is the second-fastest way to resynthesize ATP. During
glycolysis, carbohydrates—in blood glucose (sugar) or muscle glycogen (the stored form of
glucose) are broken down through a series of chemical reactions to form pyruvate (glycogen
is first broken down into glucose through a process called glycogenolysis).
For every molecule of glucose broken down to pyruvate through glycolysis, two molecules
of usable ATP are produced. Therefore, very little energy is produced through this pathway,
but the trade-off is that you get the energy quickly. Once pyruvate is formed, it can go 2
ways: be converted into lactate or be converted into a metabolic intermediary molecule
called acetyl coenzyme A (acetyl-CoA), which enters the mitochondria for oxidation and the
production of more ATP.
Conversion to lactate occurs when the demand for oxygen is greater than the supply (i.e.
during anaerobic exercise). When there is enough oxygen available to meet the muscles
needs (i.e. during aerobic exercise), pyruvate (via acetyl-CoA) enters the mitochondria and
goes through aerobic metabolism.
When oxygen is not supplied fast enough to meet the muscles needs (anaerobic glycolysis),
there is an increase in hydrogen ions (which causes the muscle pH to decrease  acidosis)
and other metabolites (ADP, P and potassium ions). Acidosis and the accumulation of these
other metabolites cause a number of problems inside the muscles, especially the reduction
of the ability of specific enzymes involved in metabolism and muscle contraction. As a result
of these changes, muscles lose their ability to contract effectively, and muscle force
production and exercise intensity will eventually decrease.
The Aerobic System
The aerobic system, which is dependent on oxygen, is the most complex of the three energy
systems. The metabolic reactions that take place in the presence of oxygen are responsible
for most of the cellular energy produced by the body. However, aerobic metabolism is the
slowest way to resynthesize ATP.
The aerobic system includes the Krebs cycle (also called the citric acid cycle or TCA cycle)
and the electron transport chain uses blood glucose, glycogen and fat as fuels to
resynthesize ATP in the mitochondria of muscle cells. Carbohydrates, glucose and glycogen
are first metabolized through glycolysis which results in pyruvate being used to form acetylCoA, which will then enter the Krebs cycle. The electrons produced in the Krebs cycle are
then transported through the electron transport chain, where ATP and water are produced
(oxidative phosphorylation).
Complete oxidation of glucose via glycolysis, the Krebs cycle and the electron transport
chain produces 36 molecules of ATP for every molecule of glucose broken down.