Three Energy Systems
The basic unit of energy within the human body is adenosine
triphosphate (ATP). To make things simple, think of a molecule of ATP as an
“energy dollar bill.” Each of us has millions of molecules of ATP in our body,
providing us with energy. We are constantly using and replenishing ATP, even
when we are not exercising. Based on this cash analogy, ATP utilization and
production can be seen as similar to the daily scenario in which we spend and
earn cash to maintain our lifestyle.
The molecular structure of ATP is shown in figure 1.1. ATP is
made up of three unique subunits: (1) adenine, (2) ribose, and (3) the phosphate
groups. Rather than memorize the structure of ATP, focus your attention on the
wavy lines that connect the three phosphate groups. Each of these wavy lines
represents a high-energy bond.
Figure 1.2 shows the basic
biochemical reaction whereby ATP produces energy. A single molecule of ATP is
represented on the left side of the reaction. When ATP comes in contact with
water and the enzyme ATPase, one of its high-energy bonds is broken or cleaved,
which releases a burst of chemical energy. This burst of chemical energy can be
used for all of the important physiological functions, including nerve
transmission, blood circulation, tissue synthesis, glandular secretion,
digestion, and skeletal muscle contraction (which we will focus on later in this
chapter). When this reaction breaks the bond in ATP, it creates a molecule of
adenosine diphosphate (ADP) and a phosphate molecule (Pi).
Now that you understand what energy is, let’s take a look at how
it is produced. The body has three energy-producing systems (see figure 1.3): the immediate
(ATP-CP), short-term (glycolysis), and long-term (oxidative
phosphorylation) systems. The three energy systems are similar in that they all
produce ATP, but they differ in how quickly they produce ATP and in the amount
of ATP produced. Two of the three energy systems—the immediate and short-term
systems—are anaerobic energy systems. In other words, these two energy systems
do not require oxygen to produce ATP. In contrast, the long-term energy system
is aerobic and requires oxygen to produce ATP.
The technical name for the immediate energy system is the ATP-CP
system (ATP stands for adenosine triphosphate, and CP stands for creatine
phosphate). The biochemical reactions involved in the immediate energy system
are shown in figure
1.4. Notice that the first reaction is the same one that was described
earlier for the conversion of ATP to chemical energy. Again, one of the
high-energy bonds is cleaved (broken) in that reaction. As a result, ATP, which
contains three phosphate groups, is converted to adenosine diphosphate (ADP),
which contains two phosphate groups. As shown in figure 1.4, ADP is not simply
thrown away after the initial reaction. Rather, it goes through a recycling
process with CP (which has one phosphate group). The CP donates its phosphate
group to ADP (two phosphate groups) to produce a new molecule of ATP (three
phosphate groups), leaving a molecule of creatine (CR), which will later bond
with another molecule of phosphate.
Using our cash analogy, the immediate energy system is similar
to the cash in a person’s wallet:
- The person can access and use the cash immediately.
- However, the person has a very limited amount of cash.
Similarly, the immediate energy system has the advantage of
producing ATP very quickly, but it has the disadvantage of producing a very
limited supply of ATP. In terms of athletic performance, the immediate energy
system is the dominant energy system during very high-intensity, short-duration
exercise lasting approximately 10 seconds or less. Examples of athletic events
in which the immediate energy system is dominant would include the 100-meter
sprint in track, a 10-meter diving event, and weightlifting events.
Like the immediate energy system, the short-term energy system
is anaerobic. The technical name for the short-term energy system is glycolysis
because the first of several biochemical reactions in this energy system
involves the conversion of glycogen (stored glucose) to free
glucose. A simplified version of the short-term energy system is shown in figure 1.5. One
molecule of glucose is converted to two molecules of pyruvic acid; then, in the
absence of oxygen, the two molecules of pyruvic acid are converted to two
molecules of lactic acid. Most important, notice that two molecules of ATP are
also produced.
Using our cash analogy, the short-term energy system is similar
to the money that a person has in a checking account:
- The person has a larger amount of money available (compared to the cash in the person’s wallet).
- However, accessing this money in order to transfer it into cash form takes a little longer.
Similarly, the short-term energy system has the advantage of
producing more ATP than the immediate energy system, but it has the disadvantage
of taking a little more time to do so. Another disadvantage is that the
short-term energy system produces lactic acid, which is quickly converted to
lactate and positively charged hydrogen ions (H+) (refer
to figure 1.5).
High concentrations of H+ create the acidic burning
sensation in exercising skeletal muscle and contribute (along with other
biochemical, neural, and biomechanical factors) to premature fatigue. In terms
of athletic performance, the short-term energy system is the dominant energy
system during high-intensity, moderate-duration exercise lasting approximately
30 to 120 seconds. Examples include the 400-meter sprint in track, the 100-meter
sprint in swimming, and the 1,000-meter track event in cycling.
The long-term energy system is aerobic in nature and requires
oxygen to produce ATP. The technical name for this energy system is oxidative
phosphorylation. A simplified version of this relatively complex energy system
is shown in figure
1.6. Notice that the long-term energy system starts out the same way as the
short-term energy system—that is, a single molecule of glucose is converted to
two molecules of pyruvic acid. However, because oxygen is available, pyruvic
acid is not converted to lactic acid as in the short-term energy system. Rather,
pyruvic acid enters several mitochondria in the cell (see figure 1.7) and is converted to
acetyl coenzyme A (acetyl CoA); it then goes through a series of biochemical
reactions (Krebs cycle and electron transport system [ETS]) that ultimately
produce 32 molecules of ATP.
Using our cash analogy, the long-term energy system is similar
to the money that a person has placed in long-term investments such as mutual
funds, stocks, bonds, or IRAs:
1. The person has a significantly larger amount of money compared to the money in
a checking account or the cash in a wallet. .
2. However, the person must go through several more steps and must wait longer to
access the funds, liquefy them, and turn them into cash.
Similarly, the long-term energy system has the advantage of producing very large amounts of ATP compared with the other energy systems; however, this system has the disadvantage of taking more time than the other energy systems to produce that large amount of ATP. The long-term energy system takes longer because it uses oxygen to produce ATP. The only place in the cell where oxygen can be used to produce ATP is in the mitochondrion, which is essentially a very large ATP factory with several “stops on the assembly line.” This ultimately increases the time needed for the final production of ATP.
In terms of athletic performance, the long-term energy system is
the dominant energy system in low- to moderate-intensity, long-duration exercise
lasting longer than 5 minutes. Examples of this type of activity include the
marathon, the 800-meter swim, and road events in cycling. So the long-term
energy system is the dominant energy system used during endurance-based sporting
events. However, athletes need to understand that the long-term system is not
the only energy system used in endurance sports.
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