The aerobic energy system has 3 stages: 1) Aerobic glycolysis breaks down glucose to produce pyruvate and ATP. 2) The Krebs cycle further breaks down pyruvate, fatty acids, and amino acids to produce more ATP, carbon dioxide, and hydrogen ions. 3) The electron transport chain uses oxygen to combine hydrogen ions and produce water, while generating 34 more ATP molecules per glucose molecule. The aerobic system provides a massive but slower source of ATP production compared to anaerobic systems.

1. Aerobic System

2. Aerobic Energy System
• Accesses a massive store of virtually unlimited energy.
• It utilises fats, carbohydrate and sometimes proteins for re-
synthesising ATP for energy use.
• Produces far more ATP than either of the other energy systems but
it produces the ATP much more slowly, therefore it cannot fuel
intense exercise that demands the fast production of ATP.

3. 3 Stage of Aerobic System
1. Aerobic glycolysis (slow glycolysis)
2. Krebs cycle (also known as the citric acid cycle)
3. Electron transport chain

4. 1. Aerobic Glycolysis
 Exactly the same series of
reactions as anaerobic (fast)
glycolysis, except it just has a
different outcome because
sufficient oxygen is present.

5. Steps of Aerobic Glycolysis
1. Initially stored glycogen is converted to glucose. Glucose is then broken
down by a series of enzymes.
2. 2 ATP are used to fuel glycolysis and 4 are created so the body gains 2 ATP to
use for muscular contraction.
3. Pyruvate is created as the end product of the breakdown of glucose. As
oxygen is present pyruvate is converted into a substance called 'acetyl
coenzyme A'.
4. Acetyl coenzyme A can then be synthesized in the second and third stages of
the aerobic system to create more ATP.

6. The second and third stages of the aerobic energy system
continue the breakdown of glucose that was started by
aerobic glycolysis and result in the formation of the by-
products carbon dioxide (CO2) and water (H2O), and the
synthesis of more ATP.

7. 2. Krebs Cycle
• Fatty acids (from fats) and amino acids (from proteins) are converted to acetyl
coenzyme A through a series of complex chemical reactions. Along with the
acetyl coenzyme A from glycolysis they enter the Krebs cycle and are broken
down. This results in ATP production and the by-products of carbon dioxide
and hydrogen are produced.
• The main purpose of the Krebs cycle is to generate hydrogen to transfer to the
electron transport chain where it can be ‘dealt to’ in a way that will control
acidity and enable the aerobic system to keep synthesising ATP.

8. Steps of the Krebs Cycle
1. Acetyl-coenzyme A enters the Krebs cycle.
2. Acetyl-Coenzyme A is broken down into carbon
dioxide (a waste product which is expelled through
breathing) and hydrogen.
3. 2 more ATP are synthesised during this process
and made available to fuel further muscle
contractions.
4. Hydrogen is transferred to the electron transport
chain.

9. 3. Electron Transport Chain
• Complex and productive pathway of the aerobic energy
system.
• It produces 34 molecules of ATP for every molecule of
glucose that is used.
• Once in the electron transport chain the hydrogen ions
from the Krebs cycle undergo further chemical
reactions. Here they are combined with oxygen to form
the end product of water.

10. Steps of the Electron Transport Chain
1. Hydrogen ions from Krebs cycle are carried to the electron transport chain
by carrier molecules.
2. Hydrogen ions are transferred to carrier molecules embedded in the electron
transport chain where they go through a series of chemical reactions.
3. A hydrogen ion gradient is created. As hydrogen ions move across this
gradient another form of ATPase phosphorylates ADP (adds another phosphate
group) to form ATP.
4. Water is created as a by-product as hydrogen combines with oxygen.