1. 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.

2. Adenosine Triphosphate (ATP)
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.

3. 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.

4. 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.

5. 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.

6. 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.

7. 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 by-product) and large amounts of ATP (32 molecules)
Large amounts of oxygen are required at this stage (thus it is aerobic energy production).

8. 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.

9. 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.

10. 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.

11. 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:
Phosphocreatine (PC) energy system (or ATP-PC system)
Lactate anaerobic energy system

12. 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 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.

13. 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.

14. How/Why is Lactic Acid Produced?
Hydrogen is released during both glycolysis and the Kreb’s cycle (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.

15. Summary of the 3 Energy Systems
ATP-PC System
Lactic Acid System
Aerobic System
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