Exercise Physiology
Chapter 2
Fig. 2-5. Breakdown of ATP
Note that the three phosphate groups in the ATP molecule are connected by
high-energy bonds. Energy is released, allowing the cell to perform work, when
the last phosphate group breaks off. The remaining compound is now called
adenosine diphosphate (ADP) since only two phosphate groups are left. New
ATP can be resynthesized from the ADP, the remaining phosphate, and the
addition of more energy. This process of ATP breakdown and ATP resynthesis
occurs nearly simultaneously in a coupled reaction. But where does the additional energy necessary to resynthesize ATP come from? Muscle cells have three
methods of resynthesizing ATP, utilizing the three metabolic pathways or
energy systems:
1. The phosphagen (ATP-PC) system
2. The anaerobic glycolytic (lactic acid) system
3. The aerobic (oxidative) system
These energy pathways will be discussed beginning with the simplest and
moving toward the most complex. The phosphagen system is a simple system
of coupled reactions; the anaerobic glycolytic system is more complex
involving a sequence of reactions; and finally, the aerobic system is a complex
and intricate combination of several pathways. In terms of their significance in
everyday life, however, the order of discussion would be reversed. Most energetic needs throughout the day (and night) are met by the aerobic system
alone. In fact, most cells primarily function aerobically, in particular the cells of
the heart and brain. Anaerobic pathways are used primarily by muscle cells, as
in activities that require a significant amount of muscular effort, such as
moderate to intense exercise, heavy manual labor, climbing several flights of
stairs, carrying a baby, or changing a tire. These types of activities require the
recruitment of the anaerobic pathways: the anaerobic glycolytic and phosphagen systems.
Anaerobic
Energy Systems
There are two energy systems in the body that can operate without oxygen—
the phosphagen system and the anaerobic glycolytic system. Because of this,
they are referred to as anaerobic energy systems. It is probably more important,
however, to identify them as systems that can resynthesize ATP at a high rate.
As you will see, the aerobic energy system is simply too slow to produce ATP at
the speed required for short, intense activities. For these activities, the
anaerobic energy systems are necessary.
Phosphagen (ATP-PC) System
The phosphagen system supplies energy very quickly and is the primary source
of energy for very high-intensity exercise. Biochemically, the phosphagen
system is by far the simplest of the three systems. Energy for the resynthesis of
Personal Fitness Training: Theory & Practice
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Chapter 2
Exercise Physiology
ATP comes by way of a coupled reaction involving the breakdown of creatine
phosphate (CP). The compound CP, also known as phosphocreatine (PC), is
similar to ATP. Because of this similarity, CP and ATP are referred to
collectively as phosphagens. CP consists of a creatine molecule connected to a
phosphate group by a high-energy bond. When CP is split into creatine (C)
and phosphate (P), enough energy is released to resynthesize ATP.
Fig. 2-6. Phosphagen System
The enzyme most responsible for helping to speed up the chemical reaction
in the phosphagen system is called creatine kinase (CK). An enzyme is a
protein molecule that speeds up a chemical reaction in the body; each energy
system has its own specific regulatory enzymes. Any condition that stimulates
or speeds CK will increase the rate at which the phosphagen system produces
energy. Conversely, any condition that inhibits or slows CK will reduce the rate
of energy production. The most significant stimulatory factor is the rapid
accumulation of ADP within the muscle cell. This is a signal to the muscle that
ATP is being consumed rapidly. In an attempt to maintain the concentration of
ATP, creatine kinase is activated, and CP is rapidly broken down. The energy
released from CP is used to replace the ATP being consumed.
The ATP-PC, or phosphagen, system is extremely important in activities that
require energy very quickly for a very short period of time. Such activities
include sprinting, jumping, throwing, kicking, and lifting heavy weights.
Sports that include these activities rely at least in part on the phosphagen
system. If the activity can be sustained for no more than 15–20 seconds, the
phosphagen system is the primary source of energy. Good examples of specific
events that primarily rely on the ATP-PC system include 100- and 200-meter
running sprints, 50-meter swimming sprints, high jump and long jump, shot
put and discus, and power lifting. With the limited supply of CP (remember,
PC is the same thing) in muscle cells, the capacity of the phosphagen system is
limited to approximately 1 mole or less of ATP (a mole is a unit of measurement used for counting extremely large numbers of atoms—1 mole of a
substance is 6.022 x 1023 units of that substance). This is equivalent to about 10
kcal or less of energy, which is a very small amount; barely enough to sprint
200 meters (20–30 sec.) before it is exhausted. To summarize the phosphagen
system: it can produce very small amounts of ATP extremely quickly, it is the
primary energy source for very short, highly intense activities, and it is limited
by the small supplies of CP stored within the cells.
Anaerobic Glycolytic (Lactic Acid) System
The body’s other anaerobic pathway, the glycolytic system, also provides a rapid
source of energy. Instead of CP, however, the fuel source for this system is
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