SECTION A –
CHAPTER 1
ENERGY SYSTEMS
CHAPTER 1 – ENERGY SYSTEMS
Energy definitions
Energy is the capacity to do work, and work has a mechanical definition, namely work = force x distance moved in the
direction of the force. Energy and work are measured in joules (J).
Chemical energy is energy that is produced by a complex series of chemical reactions, which can then be made available as
kinetic energy (energy due to movement which results from muscular contractions), or potential energy which is stored
energy due to gravity.
Power is the rate at which energy is used, or the energy used per second which is measured in watts (W).
Power can be calculated using the formula:
power = energy (in joules)
answer in watts
time (in seconds)
Energy sources and systems
We derive our energy from food, namely carbohydrates (CHO), fats, and to a lesser extent proteins. Table 1.1 summarises the
details of the major contributions from our diet to the provision of energy for exercise, how these contributions are absorbed,
and where this energy is stored within the body.
Table 1.1 – summary of carbohydrate, fat and protein utilisation
type of food /
sources
function as a food fuel - how it is used
energy content
(kJ g-1)
percentage in
a balanced diet
carbohydrate
(CHO)
sugars, rice,
potatoes, pasta
Main energy supply. Absorbed as glucose in small intestine.
Transported around body as blood glucose. Available for
immediate energy. Excess stored as muscle and liver glycogen and
as fat.
17
60 %
fats
butter, oil,
pastry,
fried food
Secondary energy supply. Absorbed as fatty acids and glycerol
(called free fatty acids, FFAs) in the small intestine. Stored as
triglycerides in adipose tissue. Triglycerides conveyed to the liver
via the circulatory system. In the liver they are converted to
glucose. Available as delayed (20 minutes delay) energy source for
long duration low intensity aerobic exercise.
39
20-25 %
proteins
meat, eggs, milk,
cheese, nuts
Absorbed as amino acids in the small intestine. Used for growth
and repair by all tissues. Used as an energy source when the body
is depleted of CHO and fat. Excess protein not needed for tissue
repair is broken down and used as an energy supply. Protein can
supply between 5 and 10% of energy needed to sustain prolonged
exercise.
23
10-15 %
The energy derived from carbohydrates, fats and proteins is stored in
bodily tissues in the form of a high energy compound called adenosine
triphosphate (ATP).
figure 1.1 – all muscle action uses ATP
ATP - adenosine triphosphate
ATP is the compound which stores energy and is therefore the energy currency linked
to intensity and duration of physical activity. ATP exists in every living tissue and its
breakdown gives energy for all life functions - this includes the action of the liver and the
brain for example, as well as the contraction of muscle tissue. All muscular activity requires
the availability and breakdown of ATP (figure 1.1).
12
2
APPLIED PHYSIOLOGY TO OPTIMISE PERFORMANCE
The energy released during tissue respiration is stored in the chemical bonds in ATP, and this energy is released during the
reaction: ATP è ADP + Pi + energy
where ADP is adenosine diphosphate, and Pi is a free phosphate radical. ATPase is an enzyme which facilitates this reaction,
which is exothermic - it releases energy.
The energy stored within ATP is only available as long as ATP is retained within the cells using it. In muscle cells during intense
(flat-out) exercise, the stored ATP only lasts about 2 seconds. Therefore the ATP must be replaced as soon as possible so that
exercise can continue. There are three processes by which this can happen:
• The ATP-PC system (also called the alactic anaerobic system).
• The lactic acid system (which is also anaerobic).
• The aerobic system.
Resynthesis of ATP from ADP uses the reaction: energy + ADP + Pi è ATP
This is an endothermic reaction since energy is given to the molecule to enable the reaction to happen. This energy will be
derived from food fuels.
Anaerobic energy systems
The ATP-PC system
This system of replenishing of ATP from ADP is the predominant one for activity which lasts between 3 and 10 seconds, which
means for high intensity maximum work, for example, flat out sprinting - the 100m sprint.
% of resting value
No oxygen is needed - the process is anaerobic. The chemical reactions within this system are a coupled reaction in which
ATP is resynthesised via phosphocreatine (PC) stored in muscle cell
sarcoplasm.
figure 1.2 – changes in muscle ATP and PC
The following reactions take place: PC è Pi + C + energy
energy + ADP + Pi è ATP
100
The two reactions together are called a coupled reaction and are
ATP level
exhaustion
facilitated by the enzyme creatine kinase (CK).
80
The net effect of these two coupled reactions is:
PC + ADP è ATP + C
60
PC is re-created in muscle cells during the recovery process, which
requires energy and is an endothermic reaction.
40
During intense exercise, peak anaerobic power is attained within the first
5 seconds, and depletion of PC occurs between 7 and 9 seconds.
20
STUDENT NOTE
This process does not directly require glucose as an energy source but the re-creation of PC during recovery will do so.
0
muscle PC
0
2
4
6
8
level
10 12 14
time / seconds
Look at the graph in figure 1.2 showing changes in muscle ATP and PC. After an initial small fall, the ATP level is maintained,
then falls as the PC is used up because the energy from PC is being used to resynthesise ATP.
This causes PC levels to fall rapidly to zero after about 10 seconds. The capacity to maintain ATP production at this point
depends on the lactic acid system.
The lactic acid system
This system depends on a chemical process called glycolysis which is the incomplete breakdown of sugar. Figure 1.3 (see
overleaf) shows the schematic layout of glycolysis.
Anaerobic energy systems 13
3
SECTION A –
CHAPTER 1
ENERGY SYSTEMS
The lactic acid system
figure 1.3 – the lactic acid system
• Carbohydrate from food you have eaten is stored as glycogen (in the
glucose
muscles and liver).
C6H12O6
• This glycogen is converted into glucose by the hormone glucagon released
glycolytic
when blood glucose levels fall (when glucose is used during tissue respiration).
enzymes (GPP, PFK)
2ATP
• The breakdown of glucose provides the energy to rebuild ATP from ADP.
pyruvic
• This is facilitated by enzymes such as glycogen phosphorylase (GPP) and
acid
phosphofructokinase (PFK).
LDH
• The whole process produces pyruvic acid.
lactic
• Glycolysis is anaerobic.
acid
• It takes place in the sarcoplasm of the muscle cell.
• No oxygen is needed, and the end product of this reaction (in the absence of oxygen) is lactic acid.
• The enzyme facilitating the conversion from pyruvic acid to lactic acid is lactate dehydrogenase (LDH).
As work intensity increases, lactic acid starts to accumulate above resting values, which produces muscle fatigue and pain. The
resultant low pH inhibits enzyme action and cross-bridge formation, hence muscle action is inhibited and physical performance
deteriorates.
The lactic acid system is the predominant one used to resynthesise ATP in sport or activities in which the flat-out effort lasts up
to 30-60 seconds. For example, a 400m run or a 100m swim.
After exercise stops, extra oxygen is taken up to remove lactic acid by changing it back
into pyruvic acid - this is the EPOC (Excess Post-exercise Oxygen Consumption,
figure 1.4 – the aerobic system
sometimes called the oxygen debt), see page 18 below for
1 molecule of glucose
the details of EPOC.
The aerobic system
glycolysis
The aerobic energy system releases stored energy from
muscle glycogen, fats and proteins.
pyruvic acid
2ATP
lactic aci d
acetyl coA
Figure 1.4 is a graphic of some of the details of the aerobic
system showing how between 32 and 34 ATP molecules
are resynthesised from one molecule of glucose - which is
the food fuel created from the food we eat. This process
will continue indefinitely until energy stores run out - or the
exercise stops.
oxaloacetic acid
citric acid
2ATP
Kreb's cycle
protein
Stage one - glycolysis
This process takes place in the muscle cell sarcoplasm and
is identical to the lactic acid system (anaerobic).
ATP regenerated = 2ATP per molecule of glucose.
CO2
fats
H+ ions
and
electrons
Stage two - Kreb’s cycle (citric acid cycle)
n
rt
po
ns
tr a
n
ai
ch
4
14
ro
The enzymatic catabolism (breakdown) of fat within the muscle cell mitochondria
is termed b-oxidation. Energy derived from the breakdown of FFAs are the preferred
food fuel as the duration of exercise increases.
up to
32 ATP
t
ec
el
This stage occurs in the presence of oxygen, and takes place
in the muscle cell mitochondria within the inner fluid filled
matrix. Here, 2 molecules of pyruvic acid combine with
oxaloacetic acid (4 carbons) and acetyl coA (2 carbons) to
form citric acid (6 carbons). The cycle produces H+ and electron
pairs, and CO2 and 2 ATP. Also, free fatty acids (from body fat)
facilitated by the enzyme lipoprotein lipase, or protein (keto acids from muscle), act as the fuel for this stage.
O2
H 2O
APPLIED PHYSIOLOGY TO OPTIMISE PERFORMANCE
Stage three - the electron transport chain
The electron transport chain occurs in the presence of oxygen within the cristae (inner part) of the muscle cell
mitochondria. Hydrogen ions and electrons have potential energy which is used to produce the ATP which is then released
in a controlled step-by-step manner. Oxygen combines with the final H+ ions to produce water and 32 ATP.
Aerobic respiration
In summary, the total effect of aerobic respiration is that it is an endothermic reaction:
glucose + 36ADP + 36Pi + 6O2 è 6CO2 + 36ATP + 6H2O
figure 1.5 – change in muscle glycogen
Fat fuels produce 2 ATP less than glucose.
during low intensity exercise
The aerobic system requires carbohydrate in the form of glucose which
is derived from glycogen stored in muscle cells (mostly slow twitch - SO
type I) or in the liver.
The graph in figure 1.5 shows how the rate of usage of muscle glycogen
is high during the first 30 minutes of steady exercise - which has to
be replaced if a sportsperson is to continue at the same rate. Hence
consumption of energy drinks and bananas during a long tennis match.
Endurance athletes can utilise FFAs during prolonged exercise sooner than
untrained people. This training adaptation enables the trained athlete to
not use glycogen up immediately, but save it for later on in an exercise
effort, or when the intensity of exercise increases. This is called glycogen
sparing and is illustrated in figure 1.6.
100
% of initial muscle glycogen
Short-term response to aerobic activity
80
60
40
mus
cle
20
0
0
1
2
glyc
oge
n
3
time / hours
The energy continuum
Table 1.2 – percentage contribution of the aerobic and anaerobic energy
systems to different sports
sport or event
aerobic %
anaerobic (all) %
100m sprint
0
100
200m sprint
10
90
100m swim
20
80
boxing
30
70
800m run
40
60
hockey
50
50
2000m rowing
60
40
4000m cycle pursuit
70
30
3000m run
80
20
cross country run
90
10
marathon
100
0
rate of use of energy
This describes the process by which ATP is regenerated via the different
figure 1.6 – contribution of CHO and fats
energy systems depending on the intensity and duration of exercise.
for trained and untrained people
Although all the systems contribute to ATP regeneration during any
activity, one or other of the energy systems usually provides the major
contribution for a given activity. Table 1.2 shows approximate proportions of ATP
resynthesised via aerobic and anaerobic pathways for some sporting activities.
light to
moderate
exercise
low
intensity
exercise
untrained
person
trained
person
high
intensity
endurance
exercise
trained
person
CHO
55%
CHO
20%
CHO
35%
fats
45%
fats
80%
fats
65%
The graph in figure 1.7 overleaf shows how the different energy systems contribute resynthesis of ATP during flat-out exercise.
Obviously, at reduced intensity of exercise, the contributions will be slightly different. But note that all systems are contributing
from the start of exercise, only it takes some time for the lactic acid and aerobic systems to get going.
The energy continuum 5
15
ENERGY SYSTEMS
ce
an
rm
ste
sy
m
cid
ta
c
la
2s
o
f
It is found that thresholds are delayed by training, so that the trained
individual has a greater capacity for ATP-PC, has a greater lactic acid
toleration, and more efficient ATP regeneration than the untrained
person.
TP-
pe
r
system
PC
Long-term training effects - thresholds
over
a ll
ore
Hence the threshold between ATP-PC and lactic acid systems occurs
between 7 and 9 seconds after the start of an exercise period.
figure 1.7 – variation in contribution of energy
systems
st
ATP
The concept of a threshold applies to the time at which one particular
system of ATP regeneration takes over from another as the major
regenerator of ATP during flat out exercise - marked as T in figure 1.7.
• For example, ATP muscle stores are depleted within 2 seconds,
and towards the end of this period the ATP-PC system has risen
enough to be able to provide the ATP necessary for the exercise.
• Peak anaerobic power is attained within the first 5 seconds of flatout exercise, but depletion of PC occurs between 7 and 9 seconds.
• At this point, the lactic acid system has risen enough to be able to
provide the ATP required for the next 40 seconds or so.
A
Short-term responses - thresholds
% of maximum rate of energy production
SECTION A –
CHAPTER 1
aerobic system
base rate
T 60s
2hrs time
T 10s
T = threshold point
Other factors affecting the proportions of energy systems
• The level of fitness (whether adaptations to training have included enhancement of relevant enzymes - which would for
example postpone levels of lactate accumulation).
• The availability of O2 and food fuels. For example, a high CHO diet would assist replenishment of glycogen stores which
would then be available for glycolysis.
Practice questions
1) Define energy, and briefly describe how energy is released from food in the body.
5 marks
2) a) Identify the only form of usable energy in the body. 1 mark
b) What is meant by an exothermic reaction? Illustrate this definition with an example. 2 marks
c) What is meant by an endothermic reaction? Illustrate this definition with an example. 2 marks
3) Explain the specialist role of mitochondria in energy production. 4 marks
4) a) An elite cyclist wants to discover her maximum output when sprinting. A load of 85 newtons is applied to the wheel of a cycle ergometer. The cyclist then sprints for 10 seconds and records a distance travelled of 200m. Assuming that no other force is acting on the cycle, calculate the power output of the cyclist. Show each stage of your calculation.
4 marks
b) Name the predominant energy system that would be utilised during this activity and describe how ATP is re-created within this system. 4 marks
5) a) An elite swimmer performs a flat-out 100 metre freestyle swim in 50 seconds. Describe how most of the ATP is regenerated during the swim. 4 marks
Sketch a graph, which shows the use of the appropriate energy systems against time during the swim. 4 marks
b) 6) a) Taking part in a triathlon involves swimming, cycling and running. Briefly describe how the aerobic energy system within the cell mitochondria supports this endurance event. 6 marks
b) Construct a graph, which illustrates the food fuel usage against time during a triathlon race lasting 2 hours.
2 marks
7) Compare the relative efficiency of ATP production via the aerobic and anaerobic routes. Explain your answer. 3 marks
6
16
APPLIED PHYSIOLOGY TO OPTIMISE PERFORMANCE
8) Describe the predominant energy system being used in the following activities: shot put, 200 metres breaststroke, a game
of hockey, 100 metres hurdles race, gymnastics vault, modern pentathlon.
6 marks
Table 1.3 – table of activities
activity
energy system
shot-put
200 metres breaststroke
a game of hockey
100 metres hurdles race
gymnastics vault
modern pentathlon
figure 1.8 – energy blocks for a variety of sports
9) Figure 1.8 represents the energy systems used during the
following athletic events: 100 metres, 400 metres, 1500 metres
and marathon, but not necessarily in that order.
a) Identify each event with an energy block and comment briefly on the reasons for your choice. 8 marks
b) Using the same key, complete a block to show the approximate proportions of aerobic/anaerobic work during a basketball game. 2 marks
A
B
C
D
aerobic
ATP-PC
lactic acid
10) The diagram in figure 1.9 is an energy continuum in relation to a variety of sports
activities.
a) Explain the concept ‘the energy continuum’.
b) At each end of the continuum examples of sporting activities have been omitted. Give one example of a sporting activity that is predominantly anaerobic and one example of a sporting activity that is predominantly aerobic.
2 marks
c) Suggest two factors that need to be considered in evaluating sports activities on the basis of their relative position on the energy continuum. 2 marks
d) Explain, using specific examples, why a game of hockey has aerobic and anaerobic components.
4 marks
figure 1.9 – variation in
contribution of energy system
aerobic
anaerobic
10
90
20
80
30
70
0
2 marks
100m swim
boxing
100
40
60
hockey
50
50
rowing (2000m)
60
40
70
30
80
20
90
10
100
0
3000m run
crosscountry run
Practice questions 7
17