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The athlete’s energy supply
When you have completed this module you will be able to:
• Describe the structure of ATP and how energy is extracted from the ATP
molecule
• Name and describe the three ATP production mechanisms, two of which operate
without oxygen and one that requires oxygen in order to work.
The energy systems
Training is an energy intensive process. It takes energy to operate muscles, get rid of
waste, make new cells and send signals from the brain to the muscles. In some cells,
almost half the energy is used to transfer molecules across the cell membrane for the
thousands of energy hungry chemical reactions occurring every second of every day.
During training chemical reactions in the cells escalates increasing the demand for
energy.
Every cell has its own store of power. It comes from rechargeable energy carrier called
adenosine triphosphate or ATP, that works somewhat similar to any rechargeable battery
you are familiar with such as the type you might use in a remote control toy. When the
toy car runs it uses power from its battery. When the energy in that battery runs low you
simply plug it in to another source of energy to recharge it. The cell's rechargeable
battery operates the same way. In this case the battery is made from an adenosine
molecule coupled to a chain of three phosphate groups – thus the name – adenosine
triphosphate. The phosphate groups, especially the energy in the last bond, stores the
energy until it is needed.
Cells can only store a very small amount of ATP – enough to fuel about 2 seconds of
activity. Whenever the cell depletes its ATP battery it must spontaneously recharge it or
its activities will shut down. Each cell has three different recharging mechanisms.
It’s worth knowing a bit about how this rechargeable ATP battery works because it will
help you understand how training improves the way the athlete uses and recharges it. In
n this module we examine how the ATP battery works and how it is recharged.
How is the energy extracted from ATP?
When the cell needs energy as the muscle cells does when performing pull ups it comes
from the store of ATP.
ATP releases energy when the chemical bond between the second and third phosphate
groups is broken. When the energy is released the ATP becomes low on energy and is
called ADP (Adenosine diphosphate) because there are only two phosphate groups
bonded in sequence rather than three. The cell can recharge the ADP by adding the
phosphate back on so it becomes the high energy ATP again. So let’s take a look at how
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this happens.
Recharging mechanisms
ATP is highly charged – when the energy is used it becomes an uncharged ADP.
An energy source is required to reattach the phosphate back onto the ADP so it
becomes a high energy ATP again. The food we eat supplies this energy. The digestive
system breaks the food down into molecules the cells can use to produce the power it
needs for its ATP recharging mechanisms.
The recharging mechanisms are:
• The phosphocreatine (PCr) mechanism. This is a super fast system. It uses the
energy stored in creatine phosphate as its power source. The cell’s creatine
phosphate supply runs out within 6-8 seconds when it is recharging the ADT into
ATP at full capacity. The athlete uses the PCr mechanism to produce short very
high bursts of muscle action.
• The anaerobic glycolytic (lactate producing) mechanism. This is a reasonably
fast recharging mechanism but it produces a nasty acid byproduct. After a minute
operating at full capacity the acid byproduct interferes with the ability of this
recharging mechanism to operate. The athlete uses anaerobic glycolysis to
produce reasonable speed and power output for up to a 60 seconds. It can
provide energy for up to 3 mins at a slightly reduced speed.
• The aerobic mechanism. This is fairly slow ATP recharging mechanism but it can
recharge lots of ATP for a very long time. The athlete uses this system when
performing for longer than a couple of minutes.
The challenge you have as a coach is to ensure you are training the capacity of the
correct ATP production mechanism for the athlete’s performance needs.
Recall that the muscle is comprised of many short segments called sarcomeres. Each
sarcomere contains contractile proteins called actin and myosin. The signal to contract
starts with an impulse from the nervous system. Energy released from the small store of
ATP in the muscle cell powers each cycle of contraction and relaxation. The low
powered ADP must be recharged back into ATP. Two of these three recharging
mechanisms operate without oxygen and one does require oxygen.
Recharging with phosphocreatine (PC)
The phosphocreatine (PC) energy system is the fastest ATP production mechanism in
the cell. It uses a high-energy compound called creatine phosphate as the fuel source to
convert ADP back into ATP. You will often see the PC energy system combined with the
small store of ATP and referred to as the ATP-PC energy system or the “start up” energy
system. It can supply the athlete with about 7- 10 seconds of high power output. The
ATP-PCr system does not produce an acidic byproduct and this is the reason you will
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also hear of it being called the alactic energy system.
Breaking the bond in the end phosphate releases the energy. There is a small store of
phosphocreatine located in the muscle that is able to donate a phosphate to the ADP.
The enzyme creatine kinase splits the high energy phosphate from the creatine
phosphate stored in the muscle and links it to ADP to reform ATP.
When creatine phosphate stores run low the athlete will lose power. You see this
happening in 100m runners when they begin to slow down after about 60 meters. They
are slowing down because they are running short of creatine phosphate. At the end of
the 100 m run their creatine phosphate stores are very low and they will have to rest in
order to reform the creatine phosphate again. About 80 percent of the creatine
phosphate is reformed after about 90 seconds of rest. Here is the phosphate being
released for energy. Phosphocreatine reconnects it phosphate onto the ADP. This occurs
when the athlete rests.
How the body gets creatine: Creatine is a naturally occurring amino acid found in
meat and fish. The body also makes creatine in the liver, kidney and pancreas. About
98% of the creatine in the body is found in skeletal muscle.
Recharging with glucose/glycogen
The next fastest ATP recharging mechanism is glycolysis that uses glucose or glycogen
(the body's stored form of glucose) as its fuel source. We import glucose from our diet by
ingesting carbohydrate such as pasta, rice and potatoes. The digestive system breaks
down the carbohydrate into glucose molecules. The bonds in these molecules contain a
lot of energy. The cell breaks these bonds to extract the energy. This energy extraction
process is called glycolysis. Glycolysis produces more ATP than the phosphocreatine
energy system, but at a slightly slower rate because it involves more chemical reactions.
Glycolysis itself does not require oxygen and therefore it falls under the anaerobic
banner along with the ATP-PC system. However, while glycolysis does not use oxygen
in its chemical processes, it relies on the presence of oxygen to keep pyruvate from
building up and turning into lactic acid.
When glycolysis is working above the ability of the cell to clear pyruvate it builds up and
interferes with glycolysis and the production of ATP. To prevent this from happening
pyruvate is converted into lactic acid that is then broken down into lactate and hydrogen
ions. In this way glycolysis can keep working. As the acid builds up it causes the athlete
a great deal of pain, muscles tighten and eventually stop working.
When glycolysis is producing high levels of acid it is referred to as fast glycolysis, or
lactic acid producing glycolysis. Another name is anaerobic glycolysis. High speed
glycolysis is an important source of power once the cell runs low on creatine phosphate.
When glycolysis is operating at a speed at which the cell can clear the pyruvate it is
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referred to as slow glycolysis or aerobic glycolysis. It's called aerobic glycolysis to
indicate that the aerobic ATP recharging system is able to clear pyruvate.
Together the ATP-PC and lactate producing glycolysis can fuel high intensity work up to
60-to-90 seconds and up to 3 mins when the intensity of the exercise is reduced slightly.
The combination of ATP-PC and lactate producing glycolytic energy system are
important to anaerobic-power endurance athletes such as 200 – 800m runners and
400m hurdlers.
Storage: Glucose is stored in the liver and skeletal muscle in the form of glycogen.
Depletion of these stores depends on the intensity of the activity and can last between 12 hours and a moderate exercise intensity.
Aerobic ATP recharging mechanism
When the athlete's muscles need a sustainable source of ATP they turn to the third ATP
recharging system – the aerobic energy system. This system needs oxygen to work and
also requires support from the heart, lungs, and circulatory system to deliver oxygen to
the mitochondria where the aerobic ATP recharging mechanism is housed. In the
presence of oxygen pyruvate formed from glycolysis, moves into the mitochondria where
the remaining energy is extracted.
Mitochondria are amazing organelles and its worth knowing how they work.
A mitochondrion consists of two sacs. Folds on the inner sac increases the surface area
for the ATP regenerating mechanism. Training adds more surface area to the inner
membrane.
The mitochrondria can use the breakdown products of glucose and fats and sometimes
protein for the energy it needs to power their ATP recharging mechanisms. Fat contains
about twice the energy content of glucose.
Pyruvate and fats are disassembled in the inner portion of the mitochondria. NADH
scavenge about for electrons as the pyruvate and fat molecules are broken down.
There's several pumps located in the inner mitochondrial membrane. The electrons from
NADH provide the power to pump the hydrogen ions up into the space between the two
membranes. Oxygen has a powerful attraction for electrons and helps the electrons to
flow quickly from pump to pump by sucking up the electrons at the last pump.
Oxygen's powerful pull on the electrons allow the pumps to work faster to transport
hydrogen ions into the space between the two membranes. ATP synthase is the guts of
the ATP recharging mechanism. It operates a bit like a spillway on a hydroelectric dam.
As the hydrogen ions flow through ATP synthase it recharges ADP into ATP.
When you design training programs to strengthen an athlete's aerobic energy capacity
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you are developing this pumping mechanism along with the ability of the athlete's heart
to deliver more and more oxygen to the cell's mitochondria.
The carbon atoms that are left over after the energy has been extracted from the
glucose molecule combine with oxygen to form carbon dioxide that is picked up by the
blood stream and transported to the lungs for exhalation. Fats are broken down in a
similar way – electrons extracted and sent off to power the hydrogen ion pumps.
Glycolysis works with the aerobic energy system
It takes a minimum of 30 seconds of intense physical activity for the aerobic energy
system to become the main source of energy. It takes this long because the
cardiovascular system needs time to increase its speed of uptake and delivery of oxygen
so that large quantities of oxygen becomes available to the muscles. The presence of
large quantities of oxygen to the muscles allows a process called aerobic glycolysis to
take place.
Aerobic glycolysis refers to the transfer of pyruvate into the mitochondria. In this way the
aerobic energy system makes use of large stores of glycogen in the body for ATP
production. The byproducts are water and carbon dioxide that the muscles cells expel
into the blood. This expulsion of these waste products prevents muscles from becoming
quickly fatigued. With the ability of the heart and lungs to work harder to move greater
volumes of oxygenated blood around the body the aerobic energy system allows the
muscles to keep on working for extended periods of time.
In addition to using glycogen to resynthesize ATP the aerobic energy system can also
use the body fat. However, this requires large amounts of oxygen so for moderate to
intense physical activity glycogen is the preferred source of energy to synthesize ATP.
After around 2 hours of continuous aerobic activity the store of glycogen in the muscle
cells is all used up and fat takes over as the power source for ATP synthesis.
Marathon runners say they can detect when this shift between glycogen and fat occurs
in the body and refer to this point as hitting the wall. Because greater amounts of oxygen
are used in breaking down fat less is available to working muscles. The athlete must rest
or at the very least slow down.
In extreme events such as famine body fat can also become totally used up and protein
is then broken down for the energy needed to power ATP synthesis. However, in a
healthy athlete, protein is an insignificant source of fuel supply because there are
adequate supplies of fat.. Anorexic women can put their life in jeopardy because
breaking down protein from body structures can weaken the organ systems
considerably.
Energy systems work together
Many running events require quick bursts of intense physical activity particularly those
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undertaken over shorter distances. When fast explosive movement are needed the
anaerobic energy systems – that is, the ATP-PC and anaerobic glycolysis, are the main
sources of energy. The aerobic system begins to dominate after around 30 seconds.
However, here is a very important point. The energy systems don't work in isolation.
Almost all athletic activity involves an interplay between the three energy systems – they
all work together to provide ATP for the muscles.
Consider a 100 m sprint event. The three energy systems all start up at the same time
and all contribute in varying proportion for the provision of ATP.
In the first 5 seconds it's predominantly the ATP-PC system that provides the energy. But
it does not last long. The lactic acid energy system becomes the main source of ATP
production between around the 5 second mark and the end of the race.
The aerobic system is also providing energy but not to any great degree. However, in
events that last around 30 seconds or longer the aerobic energy system becomes the
main source of energy.
Quick review
ATP consists of a molecule called adenosine that is linked to three phosphate groups by
a high energy chemical bond. The transition from rest to exercise increases ATP demand
more than 100 fold. Yet despite large fluctuation in energy demand muscle ATP remains
practically constant. This is because cells have three methods for reforming ATP when
the last phosphate is broken off to release the energy that bond holds. The energy in this
last bond is the main source of energy the cell uses to perform its myriad of functions.
The first method includes the creatine phosphate system that is very important to power
athletes where creatine phosphate stored in the muscle is split to release energy so that
the third phosphate can be reattached to ADP very quickly. This energy system will
provide sufficient ATP for 6 – 8 seconds of very intense muscular activity.
The second method is glycolysis that uses carbohydrate in the form of glucose.
Glycogen, that is the storage form of glucose is stored in the liver and in muscle cells.
The drawback of this system is that when glucose is broken down at a very high rate the
pyruvate cannot be cleared and it will be converted into lactic acid that breaks down into
lactate and hydrogen ions. The hydrogen ions increase the acid conditions for the cells
and cause considerable discomfort and will force the athlete to stop so the body can
clear the hydrogen ions.
The third ATP regenerating system is the aerobic energy system. This system uses
carbohydrate, fat and sometime protein in conjunction with oxygen to fuel ATP
production mechanisms. However, protein is an insignificant source of fuel for the
aerobic energy system. The carbon dioxide produced is expelled from the body through
the lungs.
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