Aerobic and Anaerobic
Energy Systems
Learning Objectives:
• To be able to explain how energy is produced
when oxygen is present.
• To be able to name and describe the two
anaerobic energy systems.
Adenosine Triphosphate (ATP)
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The body transforms the food we eat into ATP.
When ATP is broken down it releases ADP + P +
energy.
The body can resynthesise ATP by the reverse
reaction: ADP + P + energy = ATP.
The body cannot store much ATP (only enough
for about 2-3s of intense activity) so any energy
required needs to be produced immediately.
Our Fuel
• We create energy from the food we eat.
• ATP can be produced using carbohydrates, fats
or protein.
• These cannot be used until they have been
broken down into glucose, fatty acids and
amino acids, respectively.
• Excess glucose is stored in
muscles and liver as glycogen.
Aerobic Energy Production from
Glucose
• When oxygen is present the complete breakdown of
glucose is possible.
• This occurs in the mitochondria and produces CO2,
H2O, and energy.
• The advantages of aerobic energy production is that
there are no fatiguing by-products, the energy
sources are usually abundant and lots of ATP can be
produced.
• The breakdown of glucose into energy (ATP) involves
3 stages: glycolysis, Kreb’s cycle, and the electron
transport chain.
Glycolysis
• The initial stage of glucose
breakdown (occurs in sarcoplasm).
• This stage is identical in both the
aerobic and anaerobic systems).
• Some complicated reactions take
place but all you need to know is….
• Pyruvic acid is produced (pyruvate).
• 2 ATP are used and 4 ATP produced.
Kreb’s Cycle
• This follow’s on from glycolysis and
only occurs in the presence of
oxygen.
• Pyruvic acid from glycolysis is added
to coenzyme A to produce acetyl
coenzyme A to start the cycle.
• 8 enzyme driven reactions occur to
change acetyl coenzyme A into CO2.
• H atoms (which were part of acetyl
coenzyme A) go on to the electron
transport chain.
Electron Transport Chain
• The final stage of glucose breakdown.
• The H atoms from Kreb’s cycle are oxidised
(join with oxygen) to produce water (a byproduct) and large amounts of ATP (32
molecules)
• Large amounts of oxygen are required at this
stage (thus it is aerobic energy production).
Summary of Aerobic Energy
Production from Glucose
• At the end of the 3 stages 38 molecules of ATP
are produced and 2 are used (net production = 36
ATP).
• Glucose is broken down into pyruvic acid
(glycolysis), then acetyl coenzyme A, and this is
broken to form carbon dioxide (CO2), water (H2O)
and energy to resynthesise ATP.
• Remember that glycolysis occurs in the
sarcoplasm whilst Kreb’s cycle and ETC occur in
the mitochondria. Muscle cells therefore have a
large number of mitochondria.
Aerobic Energy production from Fat
• Fatty acids are broken down by a process called
beta-oxidation to acetyl CoA which enters the Kreb’s
cycle (and eventually electron transport chain).
• Even more ATP can produced from fat than from
glucose (during electron transport chain) but far
more O2 is required.
• The fatty acids we use are in the blood (very little is
stored in muscles).
• Anymore fatty acids needed are taken from our
adipose tissue (fat stores).
• Endurance training makes us better at releasing fat
from adipose tissue. This is why long duration, low
intensity exercise ‘burns’ fat.
So why not always just use Fat?
Fatty acids release huge amounts of energy but:
• Fatty acids cannot be broken down without oxygen
(aerobic only) so it is useless for high intensity
exercise.
• Extra fat means extra weight and this requires
energy to carry around.
• Excess fat can cause overheating.
• Kreb’s cycle will not operate without glucose being
present so fat cannot be used without at least some
glucose.
• This is known as ‘hitting the wall’, when glucose
stores totally run out and all energy production
stops.
Anaerobic Energy Systems
• When the body is unable to provide the oxygen
required to resynthesise ATP it must start to work
anaerobically.
• There are two anaerobic energy systems:
1. Phosphocreatine (PC) energy system (or ATP-PC
system)
2. Lactate anaerobic energy system
Phosphocreatine (PC) Energy System
(or ATP-PC system)
PC → P + C + Energy AND Energy + P + ADP = ATP
 For every molecule of PC broken down, one molecule of ATP
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can be resynthesised.
No oxygen is required.
Energy is released very rapidly (as almost no reactions take
place) and there are no waste products.
Stores only last for 5-8s of high intensity exercise.
It is therefore excellent for very high short intensity activities
(e.g. 100m sprint) but not for anything longer.
PC can be resynthesised quickly. 50% in 30s, 100% in less than 4
mins (this requires O2 so intensity must be reduced).
Creatine supplements allow us to work harder for longer (by
increasing PC stores), thereby improving strength.
Lactate Anaerobic Energy System
• This system involves the partial breakdown of
glucose (oxygen is required for full breakdown).
• 2 molecules of ATP are produced (18 times less than
aerobic!) as glucose only goes through the glycolysis
stage.
• Lactic acid is produced as a by-product (causes pain).
• This system can therefore only be sustained for
between 10 seconds and 3 mins.
• Few chemical reactions involved so energy can be
produced quickly.
How/Why is Lactic Acid Produced?
 Hydrogen is released during both glycolysis and the Kreb’s cycle
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(remember that both the aerobic and anaerobic systems can
operate at the same time).
These H atoms combine with oxygen (in the electron transport
chain).
At some point there becomes too many H atoms for the amount
of O2 available. Excess H atoms combine with pyruvate (from
glycolysis) to form lactic acid.
This point is the lactate threshold (2 mmol per litre of lactic
acid above resting levels).
The build up in lactate acid is a contributing factor for fatigue. It
produces an acidic environment which slows down enzyme
activity and stops the breakdown of glucose. It also effects nerve
endings causing some pain.
The fitter we are and the higher our VO2 max, the longer we can
resist lactic acid forming and so the higher our lactate
threshold.
Summary of the 3 Energy Systems
ATP-PC System
Lactic Acid System
Aerobic System
Anaerobic
Anaerobic
Aerobic
Very quick ATP production
Quick
Slow
ATP and PC
Glucose
Glucose and fat
No by-products
Lactic acid
No fatiguing by-products
Short duration (0-10s)
10s – 3mins
Up to 2 hours
High intensity (95-100% max)
High intensity (60-95%)
Low intensity (up to 60%)
Quick recovery (30s – 3min)
20 min – 2 hours
Up to 24 hours to replenish
glycogen stores