When we exercise the levels of energy requirements becomes very intense from the muscles. This demand outstrips the oxygen supply, and anaerobic breakdown occurs. This level of activity can only be sustained for a short period of time because an accumulation of waste occurs, and a reduced efficiency of the energy production process.

Glycolysis is the first stage of glucose catabolism. This happens in the cytoplasm of the cell. A glucose molecule is converted to two molecules of ATP. Energy that has been locked up by the original molecule is only released if there is enough oxygen for the pyruvic acid molecules to enter the biochemical roundabout called the citric acid cycle. All this takes place in the mitochondria of the cell which is dependant of oxygen.

Within the citric cycle new formation takes place. This happens when two molecules of pyruvic acid enters it, thus forming two molecules of ATP. A total of 38 ATP molecules can be formed altogether. 34 molecules of ATP come from the 3^rd energy – generating process, oxidative phosphorylation. This process is dependant on hydrogen atoms which are released when in the earlier stages of glucose breakdown. Oxidative phosphorylation can only occur in the presence of oxygen and in the mitochondria.

 

Glycolysis is an anaerobic process, and when oxygen levels are low Glycolysis takes place. A molecule of glucose is split into two molecules of pyruvic acid. This process differs from the previous one by means of the pyruvic acid does not enter the citric acid cycle or progress to oxidate phosphorylation. It is converted anaerobically to lactic acid. When we exercise excessively and develop stomach cramp, or a ‘stitch’, this is the build up of lactic acid. When the oxygen levels are eventually restored the lactic acid is converted into pyruvic, which can then enter the citric acid cycle.