Your aerobic energy system can be thought of as your large Diesel engine. Despite contributing more energy than the Anaerobic system after approximately 30-60 seconds, it still doesn’t reach full power until approximately 90 seconds of exercise have passed. Although it can’t produce energy at the same rate as your Immediate or Anaerobic systems, it offers almost unlimited supplies of fuel.


Your Aerobic system uses Carbohydrates, Fats and sometimes Protein to produce ATP for energy use. It produces far more ATP than any other system, albeit at a much slower rate. There are 3 stages to energy production by the Aerobic system. Glycolysis (in the presence of oxygen), Krebs Cycle and Electron Transport Chain.


1: Glycolysis (in presence of oxygen)
When glycolysis occurs in the presence of oxygen, glycogen is once again broken down into glucose, which is then once again broken down by enzymes to produce ATP, Pyruvate and Hydrogen ions. However, because oxygen is present, the pyruvate is then converted into a substance called 'Acetyl Coenzyme A'. Acetyl coenzyme A can then be synthesized in the second and third stages of the aerobic system to create more ATP. Any lactate that had been formed due to insufficient oxygen being present, can also be cleared by the aerobic system, primarily by being transformed back into pyruvate.

Glycolysis in the presence of sufficient oxygen.

2: Krebs Cycle
The second and third stages of the aerobic energy system, occur in the ‘powerhouses’ of muscle cells, known as ‘Mitochondria’. It is within these powerhouses that the majority of ATP are produced by the aerobic system.

The second stage is known as the ‘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 into ATP, carbon dioxide and hydrogen.

Glycolysis and the Krebs cycle.

3: Electron Transport Chain
The hydrogen produced in the Krebs cycle, plus the hydrogen produced during glycolysis, left un-dealt with, would lead to cells becoming too acidic, causing fatigue. To control this acidity, hydrogen combines with two enzymes and is then transported to the electron transport chain, enabling the aerobic system to keep creating ATP.

Glycolysis, the Krebs cycleI and Electron Transport Chain.

The ‘Electron Transport Chain’ is the most complex, yet most efficient aerobic pathway, in terms of the number of ATP produced for every molecule of glucose. Where glycolysis and the Krebs cycle each produce only 2 ATP for every molecule - the electron transport chain produces 34.

With a total of 38 ATP produced from only 1 molecule of glucose, the aerobic system is 16 times more efficient than the anaerobic system, which produces only 2 ATP for every glucose molecule.