Chapter 7
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What Are Nutrients?
Nutrients are chemical substances obtained from food and
used by the body for many different processes.
They are the raw materials our bodies need to supply
energy, to regulate cellular activities, and to build and
repair tissues.
• All organisms—including humans—require
nutrients to perform their life functions and to obtain
the energy necessary for survival.
The Three Key Energy Nutrients
The food we take in contains three key energy
nutrients that are broken down over the course
of digestion:
• Carbohydrates
• Protein
• Fats
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The Central Role of Carbohydrates
in Supplying Energy
Carbohydrates are the most abundant organic substances in nature,
and they are essential for human and animal life. Sugars and
starches are examples of carbohydrates.
• The main functions of carbohydrates are to provide materials
to build cell membranes and to provide energy for use by
cells.
• Glucose is the usual form in which carbohydrates are
assimilated by humans. Glucose is stored within skeletal muscle
and within the liver as glycogen.
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Approximate Energy Sources for Different
Types of Sport Activities
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ATP—The Common Energy Molecule
To be usable, nutrients in the food we eat need
to be reconstituted (or resynthesized) into a
universal form of energy—a “free energy” that
can then be used for muscle contraction and
many other physiological processes.
Adenosine Triphosphate (ATP)
The final form this free energy takes is
adenosine triphosphate, ATP—the common
energy molecule for all living things.
• ATP captures the chemical energy resulting
from the breakdown of food and is then
used to fuel the various cellular processes
in our bodies.
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The Release of Energy from ATP
Energy is released when a trailing phosphate
atom is broken from the ATP molecule.
This results in ADP (adenosine diphosphate)
plus energy, as in the formula below:
ATP —> ADP + P + Energy
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The Problem of Resynthesizing ATP
In high demand by the body, ATP energy supplies
are used up very quickly.
The problem becomes how to resynthesize new supplies of
ATP to ensure that bodily functions continue.
There are two methods for resynthesizing ATP:
• anaerobic (without oxygen) and
• aerobic (with oxygen).
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Two Energy Systems
• The anaerobic system occurs without the requirement of
oxygen. It can occur in two separate metabolic pathways, one
not involving the breakdown of glucose and the other
involving the partial breakdown of glucose.
• The aerobic system, a separate but to some extent overlapping
energy system, requires oxygen. It involves many enzymes
and several complex sub-pathways, and it leads to the
complete breakdown of glucose. (Fats and protein also enter
the cycle at this stage.)
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Three Metabolic Pathways
There are two energy “systems”
(anaerobic and aerobic), but there
are three metabolic “pathways” by
which ATP energy reserves are
restored. They are:
• ATP-PC (anaerobic alactic)
• Glycolysis (anaerobic lactic)
• Cellular respiration
In the presence of oxygen, the
second pathway (glycolysis) is also
the beginning of the third pathway
(the aerobic system).
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ATP-PC (Anaerobic Alactic)
This pathway draws on processes deep within the muscle
fibre itself.
• It allows for quick, intense muscle contraction.
• It is “alactic” — lactic acid is not a byproduct.
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Glycolysis (Anaerobic Lactic)
This pathway involves the partial breakdown of glucose, with
lactic acid as a byproduct.
• It does not involve oxygen and is therefore
“anaerobic.”
• It allows for longer bursts of energy.
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Cellular Respiration
The aerobic system (cellular respiration) is the main source
of energy during endurance events.
• It involves oxygen and the complete breakdown of
glucose.
• It yields large amounts of ATP.
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The ATP-PC Pathway
The ATP-PC pathway relies on the action of phosphocreatine,
a compound normally stored in muscle and readily
accessible, to sustain the levels of ATP required during the
initial phase of short but intense activity.
• This is the first and the simplest of the two
anaerobic energy pathways.
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ATP-CP athletes are fast, strong and explosive,
specializing in brief, single-effort activities like swinging
a golf club or baseball bat, Olympic weightlifting, highjumping, and shot-putting. Athletes in field and team
sports like soccer, lacrosse, tennis, martial arts,
basketball and other activities also rely heavily on the
ATP-CP system during the highest-effort moments of
sprinting, serving, kicking or driving to the hoop.
Amount of ATP This Pathway Yields
The ATP-PC pathway yields enough ATP (one molecule) for
about 10-15 seconds of strenuous effort.
Intense activities that are of short duration (for example, the
shotput, weightlifting, and the 100-metre sprint) rely heavily
on the ATP-PC pathway.
This system is referred to as anaerobic alactic because the
ATP-PC system does not yield lactic acid as a byproduct.
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The Chemical Equation for the ATP-PC Pathway
Phosphocreatine (PC) is a high-energy molecule in which the
phosphate can be broken off easily and which can be used
to convert ADP (adenosine diphosphate) back to ATP.
The chemical equation that represents this process is
as follows:
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The ATP-PC Energy Pathway in Sports
In sports, the ATP-PC pathway plays an important role in
such power events as the 50- and 100-metre dash, the high
jump, and Olympic weightlifting.
• Such events last only a few seconds and require a large
burst of energy.
• ATP-PC is important in these events because it provides the
highest rate of ATP resynthesis that cannot be matched by
other, more complex energy systems.
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Activities Relying on the ATP-PC Pathway
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Glycolysis (Anaerobic Lactic)
Glycolysis is the body’s second
(anaerobic) energy pathway. The
ATP produced in this pathway
allows a person to engage in a
high level of performance for an
additional 90 seconds or so.
• The pathway is described as
anaerobic lactic because lactic
acid is a byproduct of this
process. In the absence of
oxygen, the buildup of lactic acid
is painful and further activity is
hampered.
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Amount of ATP Glycolysis Yields
Like ATP-PC, this second metabolic pathway is also
capable of producing ATP fairly rapidly and without
the need for oxygen.
• Glycolysis is considerably more complex than
the ATP-PC pathway. In fact, glycolysis involves
10-12 separate biochemical reactions.
• Glycolysis yields twice as much ATP as the
ATP- PC pathway (i.e., two molecules of ATP
for every molecule of glucose).
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The Chemical Equation for Glycolysis
Through a series of chemical reactions,
glycolysis transfers energy from glucose
and rejoins phosphate to ADP (adenosine
diphosphate).
The chemical equation that represents
glycolysis is as follows:
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The Glycolysis (Anerobic Lactic) Pathway
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Glycolysis in Sports
Sports that rely heavily on the anaerobic lactic energy
pathway (glycolysis) involve short bouts of effort for longer
periods of time. Examples are sports such as mediumdistance track and speedskating events, and alternating
shifts in ice hockey.
• Such sports are eventually hampered by the buildup
of lactic acid. In the absence of adequate oxygen
supplies, pyruvic acid—the main product of
glycolysis—is converted to lactic acid and exhaustion
or painful muscle agony begins to set in quickly.
• The short shifts in a game of hockey perhaps
capture this process best.
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Activities Relying on Glycolysis
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Cellular Respiration
Cellular respiration refers to the process in
which the body’s cells use oxygen to generate
energy through the various metabolic pathways
found in the mitochondria of cells.
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Amount of ATP This Pathway Yields
The ATP produced by cellular respiration far
exceeds the ATP produced by the other two
pathways.
• Ultimately, 36 molecules of ATP are produced (or
a couple more, depending on the fuel source) for
every molecule of glucose—nearly 20 times the
number of ATP molecules produced by the
anaerobic energy system.
• This aerobic metabolic pathway leads to the
complete breakdown of glucose.
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